P. Faccioli

A golden age for heavy-quarkonium physics dawned a decade ago, initiated by the confluence of exciting advances in quantum chromodynamics (QCD) and an explosion of related experimental activity. The early years of this period were chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in 2004, which presented a comprehensive review of the status of the field at that time and provided specific recommendations for further progress. However, the broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles could only be partially anticipated. Since the release of the YR, the BESII program concluded only to give birth to BESIII; the B-factories and CLEO-c flourished; quarkonium production and polarization measurements at HERA and the Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the deconfinement regime. All these experiments leave legacies of quality, precision, and unsolved mysteries for quarkonium physics, and therefore beg for continuing investigations at BESIII, the LHC, RHIC, FAIR, the Super Flavor and/or Tau–Charm factories, JLab, the ILC, and beyond. The list of newly found conventional states expanded to include h c (1P), χ c2(2P), $B_{c}^{+}$ , and η b (1S). In addition, the unexpected and still-fascinating X(3872) has been joined by more than a dozen other charmonium- and bottomonium-like “XYZ” states that appear to lie outside the quark model. Many of these still need experimental confirmation. The plethora of new states unleashed a flood of theoretical investigations into new forms of matter such as quark–gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the spectroscopy, decays, production, and in-medium behavior of $c\bar{c}$ , $b\bar{b}$ , and $b\bar{c}$ bound states have been shown to validate some theoretical approaches to QCD and highlight lack of quantitative success for others. Lattice QCD has grown from a tool with computational possibilities to an industrial-strength effort now dependent more on insight and innovation than pure computational power. New effective field theories for the description of quarkonium in different regimes have been developed and brought to a high degree of sophistication, thus enabling precise and solid theoretical predictions. Many expected decays and transitions have either been measured with precision or for the first time, but the confusing patterns of decays, both above and below open-flavor thresholds, endure and have deepened. The intriguing details of quarkonium suppression in heavy-ion collisions that have emerged from RHIC have elevated the importance of separating hot- and cold-nuclear-matter effects in quark–gluon plasma studies. This review systematically addresses all these matters and concludes by prioritizing directions for ongoing and future efforts.

A bstract The discovery by the ATLAS and CMS experiments of a new boson with mass around 125 GeV and with measured properties compatible with those of a Standard-Model Higgs boson, coupled with the absence of discoveries of phenomena beyond the Standard Model at the TeV scale, has triggered interest in ideas for future Higgs factories. A new circular e + e − collider hosted in a 80 to 100 km tunnel, TLEP, is among the most attractive solutions proposed so far. It has a clean experimental environment, produces high luminosity for top-quark, Higgs boson, W and Z studies, accommodates multiple detectors, and can reach energies up to the $$ \mathrm{t}\overline{\mathrm{t}} $$ threshold and beyond. It will enable measurements of the Higgs boson properties and of Electroweak Symmetry-Breaking (EWSB) parameters with unequalled precision, offering exploration of physics beyond the Standard Model in the multi-TeV range. Moreover, being the natural precursor of the VHE-LHC, a 100 TeV hadron machine in the same tunnel, it builds up a long-term vision for particle physics. Altogether, the combination of TLEP and the VHE-LHC offers, for a great cost effectiveness, the best precision and the best search reach of all options presently on the market. This paper presents a first appraisal of the salient features of the TLEP physics potential, to serve as a baseline for a more extensive design study.

We highlight issues which are often underestimated in the experimental analyses on quarkonium polarization: the relation between the parameters of the angular distributions and the angular momentum composition of the quarkonium, the importance of the choice of the reference frame, the interplay between observed decay and production kinematics, and the consequent influence of the experimental acceptance on the comparison between experimental measurements and theoretical calculations. Given the puzzles raised by the available experimental results, new measurements must provide more detailed information, such that physical conclusions can be derived without relying on model-dependent assumptions. We describe a frame-invariant formalism which minimizes the dependence of the measurements on the experimental acceptance, facilitates the comparison with theoretical calculations, and probes systematic effects due to experimental biases. This formalism is a direct and generic consequence of the rotational invariance of the dilepton decay distribution and is independent of any assumptions specific to particular models of quarkonium production. The use of this improved approach, which exploits the intrinsic multidimensionality of the problem, will significantly contribute to a faster progress in our understanding of quarkonium production, especially if adopted as a common analysis framework by the LHC experiments, which will soon perform analyses of quarkonium polarization in proton-proton collisions.

Polarization measurements are usually considered as the most difficult challenge for the QCD description of quarkonium production. In fact, global data fits for the determination of the non-perturbative parameters of bound-state formation traditionally exclude polarization observables and use them as a posteriori verifications of the predictions, with perplexing results. With a change of perspective, we move polarization data to the centre of the study, advocating that they actually provide the strongest fundamental indications about the production mechanisms, even before we explicitly consider perturbative calculations. Considering ψ(2S) and ϒ(3S) measurements from LHC experiments and state-of-the-art next-to-leading order cross sections for the short-distance production of heavy quark–antiquark pairs of relevant colour and angular momentum configurations, we perform a search for a kinematic domain where quarkonium polarizations can be correctly reproduced together with the respective cross sections, by systematically scanning the phase space and accurately treating the experimental uncertainties. This strategy provides a straightforward solution to the "quarkonium polarization puzzle" and reassuring signs that the factorization of short- and long-distance effects works, at least in the high-transverse-momentum region, least affected by limitations in the current fixed-order calculations. The results expose unexpected hierarchies in the phenomenological long-distance parameters that open new paths towards the understanding of bound-state formation in QCD.

The covariance properties of angular momentum eigenstates imply the existence of a rotation-invariant relation among the parameters of the difermion decay distribution of inclusively observed vector mesons. This relation is a generalization of the Lam-Tung identity, a result specific to Drell-Yan production in perturbative QCD, here shown to be equivalent to the dynamical condition that the dilepton is always produced transversely polarized with respect to quantization axes belonging to the production plane.

This report contains the results of the Workshop on the CKM Unitarity Triangle, held at CERN on 13-16 February 2002 to study the determination of the CKM matrix from the available data of K, D, and B physics. This is a coherent document with chapters covering the determination of CKM elements from tree level decays and K and B meson mixing and the global fits of the unitarity triangle parameters. The impact of future measurements is also discussed.

We have searched for Theta+(1540) and Xi(--)(1862) pentaquark candidates in proton-induced reactions on C, Ti, and W targets at midrapidity and square root of s = 41.6 GeV. In 2 x 10(8) inelastic events we find no evidence for narrow (sigma approximately 5 MeV) signals in the Theta+ --> pK0(S) and Xi(--) --> Xi- pi- channels; our 95% C.L. upper limits (UL) for the inclusive production cross section times branching fraction B dsigma/dy/(y approximately 0) are (4-16) mub/N for a Theta+ mass between 1521 and 1555 MeV, and 2.5 mub/N for the Xi(--). The UL of the yield ratio of Theta+/Lambda(1520) < (3-12)% is significantly lower than model predictions. Our UL of B Xi(--)/Xi(1530)0 < 4% is at variance with the results that have provided the first evidence for the Xi(--).

Measurements of the kinematic distributions of J/ψ mesons produced in p–C, p–Ti and p–W collisions at $\sqrt{s}=41.6~\mathrm{GeV}$ in the Feynman-x region −0.34<x F <0.14 and for transverse momentum up to p T =5.4 GeV/c are presented. The x F and p T dependencies of the nuclear suppression parameter, α, are also given. The results are based on 2.4×105 J/ψ mesons reconstructed in both the e + e − and μ + μ − decay channels. The data have been collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The measurement explores the negative region of x F for the first time. The average value of α in the measured x F region is 0.981±0.015. The data suggest that the strong nuclear suppression of J/ψ production previously observed at high x F turns into an enhancement at negative x F .

Significant progress in understanding quarkonium production requires improved polarization measurements, fully considering the intrinsic multidimensionality of the problem. We propose a frame-invariant formalism which minimizes the dependence of the measured result on the experimental acceptance, facilitates the comparison with theoretical calculations, and provides a much needed control over systematic effects due to detector limitations and analysis biases. This formalism is a direct and generic consequence of the rotational invariance of the dilepton decay distribution and is independent of any assumptions specific to particular models of quarkonium production.

Circular Dichroism (CD) is known to be an excellent tool for the determination of protein secondary structure due to fingerprint signatures of α and β domains.However, CD spectra are also sensitive to the 3D arrangement of the chain as a result of the excitonic nature of additional signals due to the aromatic residues.This double sensitivity, when extended to time-resolved experiments, should allow protein folding to be monitored with high spatial resolution.To date, the exploitation of this very appealing idea has been limited, due to the difficulty in relating the observed spectral evolution to specific configurations of the chain.Here, we demonstrate that the combination of atomistic molecular dynamics simulations of the folding pathways with a quantum chemical evaluation of the excitonic spectra provides the missing key.This is exemplified for the folding of canine milk lysozyme protein.

We present an analysis of the existing data on charmonium hadro-production based on non-relativistic QCD (NRQCD) calculations at the next-to-leading order (NLO). All the data on J/ψ and ψ(2S) production in fixed-target experiments and on pp collisions at low energy are included. We find that the amount of color-octet contribution needed to describe the data is about 1/10 of that found at the Tevatron.

The interpretation of the J/ψ suppression patterns observed in nuclear collisions, at CERN and RHIC, as a signature of the formation of a deconfined phase of QCD matter, requires knowing which fractions of the measured J/ψ yields, in elementary collisions, are due to decays of heavier charmonium states. From a detailed analysis of the available mid-rapidity charmonium hadro-production cross sections, or their ratios, we determine that the J/ψ feed-down contributions from ψ' and χc decays are, respectively, (8.1±0.3)% and (25±5)%. These proton-proton values are derived from global averages of the proton-nucleus measurements, assuming that the charmonium states are exponentially absorbed with the length of matter they traverse in the nuclear targets.

Magnitude and "sign" of the measured J / psi polarization crucially depends on the reference frame used in the data analysis: a full understanding of the polarization phenomenon requires measurements reported in two "orthogonal" frames, such as the Collins-Soper and helicity frames. Moreover, the azimuthal anisotropy can be, in certain frames, as significant as the polar one. The seemingly contradictory results reported by the experiments E866, HERA-B, and CDF can be consistently described assuming that the most suitable axis for the measurement is along the direction of the relative motion of the colliding partons, and that directly produced J / psi's are longitudinally polarized at low momentum and transversely polarized at high momentum. We make specific predictions that can be tested on existing CDF data and by LHC measurements, which should show a full transverse polarization for direct J / psi's of p_{T} > 25 GeV / c.

Ratios of the ψ′ over the J/ψ production cross sections in the dilepton channel for C, Ti and W targets have been measured in 920 GeV proton-nucleus interactions with the HERA-B detector at the HERA storage ring. The ψ′ and J/ψ states were reconstructed in both the μ+μ- and the e+e- decay modes. The measurements covered the kinematic range -0.35≤xF≤0.1 with transverse momentum pT≤4.5 GeV/c. The angular dependence of the ratio has been used to measure the difference of the ψ′ and J/ψ polarization. All results for the muon and electron decay channels are in good agreement: their ratio, averaged over all events, is Rψ′(μ)/Rψ′(e)=1.00±0.08±0.04. This result constitutes a new, direct experimental constraint on the double ratio of branching fractions, (B′(μ)B(e))/(B(μ)B′(e)), of ψ′ and J/ψ in the two channels. The ψ′ to J/ψ production ratio is almost constant in the covered xF range and shows a slow increase with pT.

The angular distributions of the decay products in the successive decays chi_c (chi_b) to J/psi (Upsilon) gamma and J/psi (Upsilon) to l+l- are calculated as a function of the angular momentum composition of the decaying chi meson and of the multipole structure of the photon radiation, using a formalism independent of production mechanisms and polarization frames. The polarizations of the chi states produced in high energy collisions can be derived from the dilepton decay distributions of the daughter J/psi or Upsilon mesons, with a reduced dependence on the details of the photon reconstruction or simulation. Moreover, this method eliminates the dependence of the polarization measurement on the actual details of the multipole structure of the radiative transition. Problematic points in previous calculations of the chi_c decay angular distributions are identified and clarified.

The COMPASS Collaboration experiment recently discovered a new isovector resonancelike signal with axial-vector quantum numbers, the ${a}_{1}(1420)$, decaying to ${f}_{0}(980)\ensuremath{\pi}$. With a mass too close to and a width smaller than the axial-vector ground state ${a}_{1}(1260)$, it was immediately interpreted as a new light exotic meson, similar to the $X$, $Y$, $Z$ states in the hidden-charm sector. We show that a resonancelike signal fully matching the experimental data is produced by the decay of the ${a}_{1}(1260)$ resonance into ${K}^{*}(\ensuremath{\rightarrow}K\ensuremath{\pi})\overline{K}$ and subsequent rescattering through a triangle singularity into the coupled ${f}_{0}(980)\ensuremath{\pi}$ channel. The amplitude for this process is calculated using a new approach based on dispersion relations. The triangle-singularity model is fitted to the partial-wave data of the COMPASS experiment. Despite having fewer parameters, this fit shows a slightly better quality than the one using a resonance hypothesis and thus eliminates the need for an additional resonance in order to describe the data. We thereby demonstrate for the first time in the light-meson sector that a resonancelike structure in the experimental data can be described by rescattering through a triangle singularity, providing evidence for a genuine three-body effect.

A set of measurements of azimuthal asymmetries in the production of pairs of identified hadrons in deep-inelastic scattering of muons on transversely polarised 6LiD (deuteron) and NH3 (proton) targets is presented. All available data collected in the years 2003–2004 and 2007/2010 with the COMPASS spectrometer using a muon beam of 160GeV/c at the CERN SPS were analysed. The asymmetries provide access to the transversity distribution functions via a fragmentation function that in principle may be independently obtained from e+e− annihilation data. Results are presented, discussed and compared to existing measurements as well as to model predictions. Asymmetries of π+π− pairs measured with the proton target as a function of the Bjorken scaling variable are sizeable in the range x>0.032, indicating non-vanishing transversity distribution and di-hadron interference fragmentation functions. As already pointed out by several authors, the small asymmetries of π+π− measured on the 6LiD target can be interpreted as indication for a cancellation of u and d-quark transversity distributions.

The electromagnetic calorimeter of the HERA-B experiment built at the HERA proton accelerator at DESY (Hamburg) is described. The construction characteristics of the detector, of the related front-end, readout, trigger and service electronics are discussed together with the constraints and the motivations which inspired the design philosophy. The detector performance are presented as obtained from the analysis of the data acquired during the HERA-B running period, including calibration procedures and achievements and the electron identification capability exploiting a method, proposed here for the first time, based on the observation of the associated bremsstrahlung γ. Finally, some observed physical signals and a short overview of the main obtained physics results are presented.

A study of the angular distributions of leptons from decays of J/ψ's produced in p-C and p-W collisions at $\sqrt{s}=41.6\mbox{~GeV}$ has been performed in the J/ψ Feynman-x region −0.34<x F <0.14 and for J/ψ transverse momenta up to 5.4 GeV/c. The data were collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The results, based on a clean selection of 2.3×105 J/ψ's reconstructed in both the e + e − and μ + μ − decay channels, indicate that J/ψ's are produced polarized. The magnitude of the effect is maximal at low p T . For p T >1 GeV/c a significant dependence on the reference frame is found: the polar anisotropy is more pronounced in the Collins-Soper frame and almost vanishes in the helicity frame, where, instead, a significant azimuthal anisotropy arises.

The coefficients determining the dilepton decay angular distribution of vector particles obey certain positivity constraints and a rotation-invariant identity. These relations are a direct consequence of the covariance properties of angular momentum eigenstates and are independent of the production mechanism. The Lam-Tung relation can be derived as a particular case, simply recognizing that the Drell-Yan dilepton is always produced transversely polarized with respect to one or more quantization axes. The dilepton angular distribution continues to be characterized by a frame-independent identity also when the Lam-Tung relation is violated. Moreover, the violation can be easily characterized by measuring a one-dimensional distribution depending on one shape coefficient.

While non-relativistic QCD (NRQCD) foresees a variety of elementary quarkonium production mechanisms naturally leading to state-dependent kinematic patterns, the LHC cross sections and polarization measurements reveal a remarkably simple production scenario, independent of the quantum numbers and masses of the quarkonia. Surprisingly, NRQCD is able to accommodate the observed universal scenario, through a series of conspiring cancellations smoothing out its otherwise variegated hierarchy of mechanisms. This seemingly unnatural solution implies that the $\chi_{c1}$ and $\chi_{c2}$ polarizations, not yet measured, are strong and opposite, representing the only potential exception to a remarkably simple picture of quarkonium production. The observation of a large difference between $\chi_{c2}$ and $\chi_{c1}$ polarizations, which cannot be indirectly extracted from existing measurements because they mutually cancel each other in their contribution to the observed J/$\psi$ production, would be a smoking gun signal finally proving the multifaceted but mysteriously elusive structure of NRQCD. On the other hand, the measurement of two similar, small polarizations will urge improved P-wave calculations, if not a substantial revision of the NRQCD hierarchies.

We study the spin-exotic ${J}^{PC}={1}^{\ensuremath{-}+}$ amplitude in single-diffractive dissociation of $190\text{ }\text{ }\mathrm{GeV}/c$ pions into ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ using a hydrogen target and confirm the ${\ensuremath{\pi}}_{1}(1600)\ensuremath{\rightarrow}\ensuremath{\rho}(770)\ensuremath{\pi}$ amplitude, which interferes with a nonresonant ${1}^{\ensuremath{-}+}$ amplitude. We demonstrate that conflicting conclusions from previous studies on these amplitudes can be attributed to different analysis models and different treatment of the dependence of the amplitudes on the squared four-momentum transfer and we thus reconcile these experimental findings. We study the nonresonant contributions to the ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ final state using pseudodata generated on the basis of a Deck model. Subjecting pseudodata and real data to the same partial-wave analysis, we find good agreement concerning the spectral shape and its dependence on the squared four-momentum transfer for the ${J}^{PC}={1}^{\ensuremath{-}+}$ amplitude and also for amplitudes with other ${J}^{PC}$ quantum numbers. We investigate for the first time the amplitude of the ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ subsystem with ${J}^{PC}={1}^{\ensuremath{-}\ensuremath{-}}$ in the $3\ensuremath{\pi}$ amplitude with ${J}^{PC}={1}^{\ensuremath{-}+}$ employing the novel freed-isobar analysis scheme. We reveal this ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ amplitude to be dominated by the $\ensuremath{\rho}(770)$ for both the ${\ensuremath{\pi}}_{1}(1600)$ and the nonresonant contribution. These findings largely confirm the underlying assumptions for the isobar model used in all previous partial-wave analyses addressing the ${J}^{PC}={1}^{\ensuremath{-}+}$ amplitude.

New results are presented on a high-statistics measurement of Collins and Sivers asymmetries of charged hadrons produced in deep inelastic scattering of muons on a transversely polarised $^6$LiD target. The data were taken in 2022 with the COMPASS spectrometer using the 160 \gevv\ muon beam at CERN, balancing the existing data on transversely polarised proton targets. The first results from about two-thirds of the new data have total uncertainties smaller by up to a factor of three compared to the previous deuteron measurements. Using all the COMPASS proton and deuteron results, both the transversity and the Sivers distribution functions of the $u$ and $d$ quark, as well as the tensor charge in the measured $x$-range are extracted. In particular, the accuracy of the $d$ quark results is significantly improved.

The CMS detector will be upgraded for the HL-LHC to include a MIP Timing Detector (MTD). The MTD will consist of barrel and endcap timing layers, BTL and ETL respectively, providing precision timing of charged particles. The BTL sensors are based on LYSO:Ce scintillation crystals coupled to SiPMs with TOFHIR2 ASICs for the front-end readout. A resolution of 30-60 ps for MIP signals at a rate of 2.5 Mhit/s per channel is expected along the HL-LHC lifetime. We present an overview of the TOFHIR2 requirements and design, simulation results and measurements with TOFHIR2 ASICs. The measurements of TOFHIR2 associated to sensor modules were performed in different test setups using internal test pulses or blue and UV laser pulses emulating the signals expected in the experiment. The measurements show a time resolution of 24 ps initially during Beginning of Operation (BoO) and 58 ps at End of Operation (EoO) conditions, matching well the BTL requirements. We also showed that the time resolution is stable up to the highest expected MIP rate. Extensive radiation tests were performed, both with x-rays and heavy ions, showing that TOFHIR2 is not affected by the radiation environment during the experiment lifetime.

The inclusive production cross sections of the strange vector mesons K*0, K*0bar, and phi have been measured in interactions of 920 GeV protons with C, Ti, and W targets with the HERA-B detector at the HERA storage ring. Differential cross sections as a function of rapidity and transverse momentum have been measured in the central rapidity region and for transverse momenta up to pT=3.5 GeV/c. The atomic number dependence is parametrised as sigma(pA) = sigma(pN)*A**alpha, where sigma(pN) is the proton-nucleon cross section. Within the phase space accessible, alpha(K*0) = 0.86+/-0.03, alpha(K*0bar) = 0.87+/-0.03, and alpha(phi) = 0.96+/-0.02. The total proton-nucleon cross sections, determined by extrapolating the differential measurements to full phase space, are sigma(pN->K*0) = 5.06+/-0.54 mb, sigma(pN->K*0bar) = 4.02+/-0.45 mb, and sigma(pN->phi) = 1.17+/-0.11 mb. The Cronin effect is observed for the first time for vector mesons containing strange quarks; compared to the measurements of Cronin et al. for K+- mesons, the measured values of alpha for phi mesons coincide with those of K- mesons for all transverse momenta, while the enhancement for K*0 / K*0bar mesons is smaller.

The inclusive production cross sections of the charmed mesons D0,D+,Ds + and D*+ have been measured in interactions of 920 GeV protons on C, Ti, and W targets with the HERA-B detector at the HERA storage ring. Differential cross sections as a function of transverse momentum and Feynman’s x variable are given for the central rapidity region and for transverse momenta up to pT=3.5 GeV/c. The atomic mass number dependence and the leading to non-leading particle production asymmetries are presented as well.

ψ F < 0.15 is presented.Both µ + µ - and e + e -J/ψ decay channels are observed with an overall statistics of about 15000 χc events, which is by far the largest available sample in pA collisions.The result is Rχ c = 0.188 ± 0.013st +0.024 -0.022 sys averaged over the different materials, when no J/ψ and χc polarisations are considered.The χc1 to χc2 production ratio R12 = Rχ c 1 /Rχ c 2 is measured to be 1.02 ± 0.40, leading to a cross section ratio σ(χ c1 ) σ(χ c2 ) = 0.57 ± 0.23.The dependence of Rχ c on the Feynman-x of the J/ψ, x J/ψ F , and its transverse momentum, p J/ψ T , is studied, as well as its dependence on the atomic number, A, of the target.For the first time, an extensive study of possible biases on Rχ c and R12 due to the dependence of acceptance on the polarization states of J/ψ and χc is performed.By varying the polarisation parameter, λ obs , of all produced J/ψ's by two sigma around the value measured by HERA-B, and considering the maximum variation due to the possible χc1 and χc2 polarisations, it is shown that Rχ c could change by a factor between 1.02 and 1.21 and R12 by a factor between 0.89 and 1.16.

Using data collected by the HERA-B experiment, we have measured the fraction of J/ψ's produced via radiative χc decays in interactions of 920 GeV protons with carbon and titanium targets. We obtained Rχc=0.32±0.06stat±0.04sys for the fraction of J/ψ from χc decays averaged over proton–carbon and proton–titanium collisions. This result is in agreement with previous measurements and is compared with theoretical predictions.

A global analysis of ATLAS and CMS measurements reveals that, at mid-rapidity, the directly-produced $\chi_{c1}$, $\chi_{c2}$ and J/$\psi$ mesons have differential cross sections of seemingly identical shapes, when presented as a function of the mass-rescaled transverse momentum, $p_{\rm T}/M$. This identity of kinematic behaviours among S- and P-wave quarkonia is certainly not a natural expectation of non-relativistic QCD (NRQCD), where each quarkonium state is supposed to reflect a specific family of elementary production processes, of significantly different $p_{\rm T}$-differential cross sections. Remarkably, accurate kinematic cancellations among the variegated NRQCD terms (colour singlets and octets) of its factorization expansion can lead to a surprisingly good description of the data. This peculiar tuning of the NRQCD mixtures leads to a clear prediction regarding the $\chi_{c1}$ and $\chi_{c2}$ polarizations, the only observables not yet measured: they should be almost maximally different from one another, and from the J/$\psi$ polarization, a striking exception in the global panorama of quarkonium production. Measurements of the difference between the $\chi_{c1}$, $\chi_{c2}$ and J/$\psi$ polarizations, complementing the observed identity of momentum dependences, represent a decisive probe of NRQCD.

Using the HERA-B detector, the b-bbar production cross section has been measured in 920 GeV proton collisions on carbon and titanium targets. The b-bbar production was tagged via inclusive bottom quark decays into J/psi, by exploiting the longitudinal separation of J/psi->ll decay vertices from the primary proton-nucleus interaction. Both $e^+e^-$ and $\mu^+\mu^-$ channels have been reconstructed and the combined analysis yields the cross section $\sigma(b \bar b) = 32 ^{+14}_{-12} (stat) ^{+6}_{-7} (sys) nb/nucleon$.

The mid-rapidity (dσpN/dy at y=0) and total (σpN) production cross sections of Jψ mesons are measured in proton–nucleus interactions. Data collected by the HERA-B experiment in interactions of 920 GeV/c protons with carbon, titanium and tungsten targets are used for this analysis. The Jψ mesons are reconstructed by their decay into lepton pairs. The total production cross section obtained is σpNJ/ψ=663±74±46 nb/nucleon. In addition, our result is compared with previous measurements.

A measurement of the polarization of Λ and Λ¯ baryons produced in pC and pW collisions at s=41.6GeV has been performed with the HERA-B spectrometer. The measurements cover the kinematic range of 0.6GeV/c<p⊥<1.2GeV/c in transverse momentum and −0.15<xF<0.01 in Feynman-x. The polarization results from the two different targets agree within the statistical error. In the combined data set, the largest deviation from zero, +0.054±0.029, is measured for xF≲−0.07. Zero polarization is expected at xF=0 in the absence of nuclear effects. The polarization results for the Λ agree with a parametrization of previous measurements which were performed at positive xF values, where the Λ polarization is negative. Results of Λ¯ polarization measurements are consistent with zero.

We demonstrate that it is possible to use the polarization of vector quarkonia, measured from dilepton event samples, as an instrument to study the suppression of ${\ensuremath{\chi}}_{c}$ and ${\ensuremath{\chi}}_{b}$ in heavy-ion collisions, where a direct determination of signal yields involving the identification of low-energy photons is essentially impossible. A change of the observed $\mathrm{J}/\ensuremath{\psi}$ and $\ensuremath{\Upsilon}(1S)$ polarizations from proton-proton to central nucleus-nucleus collisions would directly reflect differences in the nuclear dissociation patterns of $S$ and $P$ states and may provide a strong indication for quarkonium sequential suppression in the quark-gluon plasma.

This open-access book addresses general characteristics of the angular distributions and emphasizes discussion of use-cases and methodological pitfall

The production of vector mesons in deep inelastic scattering is an interesting yet scarsely explored channel to study the transverse spin structure of the nucleon and the related phenomena. The COMPASS collaboration has performed the first measurement of the Collins and Sivers asymmetries for inclusively produced $\rho^0$ mesons. The analysis is based on the data set collected in deep inelastic scattering in $2010$ using a $160\,\,\rm{GeV}/c$ $\mu^+$ beam impinging on a transversely polarized $\rm{NH}_3$ target. The $\rho^{0}$ mesons are selected from oppositely charged hadron pairs, and the asymmetries are extracted as a function of the Bjorken-$x$ variable, the transverse momentum of the pair and the fraction of the energy $z$ carried by the pair. Indications for positive Collins and Sivers asymmetries are observed.

A complete review of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements and of the experimental methods for their determination is presented. A critical analysis of the relevant experimental results, and in particular of the most recent ones, allows to improve the accuracies of all the matrix elements. A chi-square minimization with the three-family unitarity constraint on the CKM matrix is performed to test the current interpretation of the CP violating phenomena inside the Standard Model. A complete and unambiguous solution satisfying all the imposed constraints is found. As a by-product of the fit, the precision on the values of the matrix elements is further increased and it is possible to obtain estimates for the important CP violation observables $sin 2\beta$, $sin 2\alpha$ and $\gamma$. Finally, an independent estimation of the CKM elements based on a Bayesian approach is performed. This complementary method constitutes a check of the results obtained, providing also the probability functions of the CKM elements and of the related quantities.

Abstract The p annihilation at rest in a NTP 4He-gas target has been studied by means of the Obelix spectrometer installed at the LEAR accelerator of CERN. Reactions with production of 2π−2π+, 2π−3π+, 2π−2π+p, 2π−π+2p, 2π−3p (with pπ>80 MeV/c and pp>300 MeV/c) have been selected and single- and multinucleon annihilations identified. For the first time some known mesonic and baryonic resonances have been observed in p 4He annihilations and branching ratios are evaluated for a number of reaction channels.

A new measurement of the $b\overline{b}$ production cross section in 920 GeV proton-nucleus collisions is presented by the HERA-B Collaboration. The $b\overline{b}$ production is tagged via inclusive bottom quark decays into $J/\ensuremath{\psi}$ mesons by exploiting the longitudinal separation of $J/\ensuremath{\psi}\ensuremath{\rightarrow}{l}^{+}{l}^{\ensuremath{-}}$ decay vertices from the primary proton-nucleus interaction point. Both ${e}^{+}{e}^{\ensuremath{-}}$ and ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ channels are reconstructed for a total of $83\ifmmode\pm\else\textpm\fi{}12$ inclusive $b\ensuremath{\rightarrow}J/\ensuremath{\psi}X$ events found. The combined analysis yields a $b\overline{b}$ to prompt $J/\ensuremath{\psi}$ cross section ratio of $\frac{\ensuremath{\Delta}\ensuremath{\sigma}(b\overline{b})}{\ensuremath{\Delta}{\ensuremath{\sigma}}_{J/\ensuremath{\psi}}}=0.032\ifmmode\pm\else\textpm\fi{}{0.005}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{0.004}_{\mathrm{sys}}$ measured in the ${x}_{F}$ acceptance ($\ensuremath{-}0.35&lt;{x}_{F}&lt;0.15$), extrapolated to $\ensuremath{\sigma}(b\overline{b})=14.9\ifmmode\pm\else\textpm\fi{}{2.2}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{2.4}_{\mathrm{sys}}\text{ }\text{ }\mathrm{\text{nb/nucleon}}$ in the total phase space.

Polarization measurements are the best instrument to understand how quark and antiquark combine into the different quarkonium states, but no model has so far succeeded in explaining the measured J/ψ and ϒ polarizations. On the other hand, the experimental data in proton–antiproton and proton–nucleus collisions are inconsistent, incomplete and ambiguous. New analyses will have to properly address often underestimated issues: the existence of azimuthal anisotropies, the dependence on the reference frame, the influence of the experimental acceptance on the comparison with other measurements and with theory. Additionally, a recently developed frame-invariant formalism will provide an alternative and often more immediate physical viewpoint and, at the same time, will help probing systematic effects due to experimental biases. The role of feed-down decays from heavier states, a crucial missing piece in the current experimental knowledge, will have to be investigated. Ultimately, quarkonium polarization measurements will also offer new possibilities in the study of the properties of the quark–gluon plasma.

The p¯ stopping power in helium from 1 keV kinetic energy is evaluated. Contrary to the effect observed around and below the maximum, Obelix data indicate a p¯ stopping power higher than that for proton, the difference being of the order of 15±5% at ≈700 keV. The result contributes to assert the fundamental difference between p¯ stoppings in the simplest gases (He, H2) and in solid targets below some MeV.

The di-fermion angular distribution observed in decays of inclusively produced vector particles is characterized by two frame-independent observables, reflecting the average spin alignment of the produced particle and the magnitude of parity violation in the decay. The existence of these observables derives from the rotational properties of angular momentum eigenstates and is a completely general result, valid for any $J=1$ state and independent of the production process. Rotation-invariant formulations of polarization and of the decay parity asymmetry can provide more significant measurements than the commonly used frame-dependent definitions, also improving the quality of the comparisons between the measurements and the theoretical calculations.

We report the stopping power of molecular hydrogen for antiprotons of kinetic energy above the maximum (approximately 100 keV) with the purpose of comparing with the proton one. Our result is consistent with a positive difference in antiproton-proton stopping powers above approximately 250 keV and with a maximum difference between the stopping powers of 21%+/-3% at around 600 keV.

Bioinformatics is one of the key application of this century. The attention received by all the current media is astonishing and both academics and industries are carrying on researches in many fields related to computational methods applied to life sciences. In this paper we present some results obtained within an Italian funded research project called ESCOGITARE. We present the design and implementation of a grid-based architecture for scientific workflow management that, differently from others, allows the dynamic discovery of existing Web services in combination to ad-hoc developed ones. The paper presents the main features of current scientific workflow management systems. Using a real-world case study (taken by the agricultural research domain), we show how we overcome the limitations of current approaches.

Dimensional analysis reveals general kinematic scaling rules for the momentum, mass, and energy dependence of Drell–Yan and quarkonium cross sections. Their application to mid-rapidity LHC data provides strong experimental evidence supporting the validity of the factorization ansatz, a cornerstone of non-relativistic QCD, still lacking theoretical demonstration. Moreover, data-driven patterns emerge for the factorizable long-distance bound-state formation effects, including a remarkable correlation between the S-wave quarkonium cross sections and their binding energies. Assuming that this scaling can be extended to the P-wave case, we obtain precise predictions for the not yet measured feed-down fractions, thereby providing a complete picture of the charmonium and bottomonium feed-down structure. This is crucial information for quantitative interpretations of quarkonium production data, including studies of the suppression patterns measured in nucleus-nucleus collisions.

Based on the observation of sizeable target-transverse-spin asymmetries in single-hadron and hadron-pair production in Semi-Inclusive measurements of Deep Inelastic Scattering (SIDIS), the chiral-odd transversity quark distribution functions h1q are nowadays well established. Several possible channels to access these functions were originally proposed. One candidate is the measurement of the polarisation of Λ hyperons produced in SIDIS off transversely polarised nucleons, where the transverse polarisation of the struck quark might be transferred to the final-state hyperon. In this article, we present the COMPASS results on the transversity-induced polarisation of Λ and Λ¯ hyperons produced in SIDIS off transversely polarised protons. Within the experimental uncertainties, no significant deviation from zero was observed. The results are discussed in the context of different models taking into account previous experimental results on h1u and h1d.

In presenting an up-to-date account of the experimental results concerning the CKM matrix, special emphasis is placed on the exceptional degree of consistency shown by the current Standard Model determination of the unitarity triangle; some implications in the question of how the theoretical nature of the dominant uncertainties affects the Standard Model predictions are discussed.

The cross section ratio RJ/ψ=Br(ϒ→l+l−)⋅dσ(ϒ)/dy|y=0/σ(J/ψ) has been measured with the HERA-B spectrometer in fixed-target proton–nucleus collisions at 920 GeV proton beam energy corresponding to a proton–nucleon c.m.s. energy of s=41.6GeV. The combined results for the decay channels ϒ→e+e− and ϒ→μ+μ− yield a ratio RJ/ψ=(9.0±2.1)×10−6. The corresponding ϒ production cross section per nucleon at mid-rapidity (y=0) has been determined to be Br(ϒ→l+l−)⋅dσ(ϒ)/dy|y=0=4.5±1.1pb/nucleon.

Abstract While the prompt $$\mathrm{J}/\psi $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>ψ</mml:mi></mml:mrow></mml:math> cross section and polarization have been measured with good precision as a function of transverse momentum, $$p_\mathrm{T}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>p</mml:mi><mml:mi>T</mml:mi></mml:msub></mml:math> , those of the directly produced $$\mathrm{J}/\psi $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>ψ</mml:mi></mml:mrow></mml:math> are practically unknown, given that the cross sections and polarizations of the $$\chi _{{c}1} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>χ</mml:mi><mml:mrow><mml:mi>c</mml:mi><mml:mn>1</mml:mn></mml:mrow></mml:msub></mml:math> and $$\chi _{{c}2} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>χ</mml:mi><mml:mrow><mml:mi>c</mml:mi><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math> mesons, large indirect contributors to $$\mathrm{J}/\psi $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>ψ</mml:mi></mml:mrow></mml:math> production, are only known with rather poor accuracy. The lack of precise measurements of the $$\chi _{{c}J} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>χ</mml:mi><mml:mrow><mml:mi>cJ</mml:mi></mml:mrow></mml:msub></mml:math> polarizations induces large uncertainties in the level of their feed-down contributions to the prompt $$\mathrm{J}/\psi $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>ψ</mml:mi></mml:mrow></mml:math> yield, because of the polarization-dependent acceptance corrections. The experimental panorama of charmonium production can be significantly improved through a consistent and model-independent global analysis of existing measurements of $$\mathrm{J}/\psi $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>ψ</mml:mi></mml:mrow></mml:math> , $$\psi \mathrm{(2S)}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>ψ</mml:mi><mml:mrow><mml:mo>(</mml:mo><mml:mn>2</mml:mn><mml:mi>S</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> and $$\chi _{c}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>χ</mml:mi><mml:mi>c</mml:mi></mml:msub></mml:math> cross sections and polarizations, faithfully respecting all the correlations and uncertainties. In particular, it is seen that the $$\chi _{{c}J} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>χ</mml:mi><mml:mrow><mml:mi>cJ</mml:mi></mml:mrow></mml:msub></mml:math> polarizations and feed-down fractions to $$\mathrm{J}/\psi $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>ψ</mml:mi></mml:mrow></mml:math> production have a negligible dependence on the $$\mathrm{J}/\psi $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>ψ</mml:mi></mml:mrow></mml:math> $$p_\mathrm{T}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>p</mml:mi><mml:mi>T</mml:mi></mml:msub></mml:math> , with average values $$\lambda _\vartheta ^{\chi _{{c}1}} = 0.55 \pm 0.23$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mi>λ</mml:mi><mml:mi>ϑ</mml:mi><mml:msub><mml:mi>χ</mml:mi><mml:mrow><mml:mi>c</mml:mi><mml:mn>1</mml:mn></mml:mrow></mml:msub></mml:msubsup><mml:mo>=</mml:mo><mml:mn>0.55</mml:mn><mml:mo>±</mml:mo><mml:mn>0.23</mml:mn></mml:mrow></mml:math> , $$\lambda _\vartheta ^{\chi _{{c}2}} = -0.39 \pm 0.22$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mi>λ</mml:mi><mml:mi>ϑ</mml:mi><mml:msub><mml:mi>χ</mml:mi><mml:mrow><mml:mi>c</mml:mi><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:msubsup><mml:mo>=</mml:mo><mml:mo>-</mml:mo><mml:mn>0.39</mml:mn><mml:mo>±</mml:mo><mml:mn>0.22</mml:mn></mml:mrow></mml:math> , $$R^{\chi _{{c}1}} = (18.8 \pm 1.4)\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi>R</mml:mi><mml:msub><mml:mi>χ</mml:mi><mml:mrow><mml:mi>c</mml:mi><mml:mn>1</mml:mn></mml:mrow></mml:msub></mml:msup><mml:mo>=</mml:mo><mml:mrow><mml:mo>(</mml:mo><mml:mn>18.8</mml:mn><mml:mo>±</mml:mo><mml:mn>1.4</mml:mn><mml:mo>)</mml:mo></mml:mrow><mml:mo>%</mml:mo></mml:mrow></mml:math> and $$R^{\chi _{{c}2}} = (6.5 \pm 0.5)\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi>R</mml:mi><mml:msub><mml:mi>χ</mml:mi><mml:mrow><mml:mi>c</mml:mi><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:msup><mml:mo>=</mml:mo><mml:mrow><mml:mo>(</mml:mo><mml:mn>6.5</mml:mn><mml:mo>±</mml:mo><mml:mn>0.5</mml:mn><mml:mo>)</mml:mo></mml:mrow><mml:mo>%</mml:mo></mml:mrow></mml:math> . The analysis also shows that $$(67.2 \pm 1.9)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>(</mml:mo><mml:mn>67.2</mml:mn><mml:mo>±</mml:mo><mml:mn>1.9</mml:mn><mml:mo>)</mml:mo></mml:mrow></mml:math> % of the prompt $$\mathrm{J}/\psi $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>ψ</mml:mi></mml:mrow></mml:math> yield is due to directly-produced mesons, of polarization constrained to remarkably small values, $$\lambda _\vartheta ^{\mathrm{J}/\psi } = 0.04 \pm 0.06$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mi>λ</mml:mi><mml:mi>ϑ</mml:mi><mml:mrow><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>ψ</mml:mi></mml:mrow></mml:msubsup><mml:mo>=</mml:mo><mml:mn>0.04</mml:mn><mml:mo>±</mml:mo><mml:mn>0.06</mml:mn></mml:mrow></mml:math> .

Measurements made at the LHC have shown that the production of the ${\rm J}/\psi$, $\psi$(2S), $\Upsilon$(1S) and $\Upsilon$(2S) quarkonia is suppressed in Pb-Pb collisions, with respect to the extrapolation of the pp production yields. The $\psi$(2S) and $\Upsilon$(2S) states are more strongly suppressed than the ground states and the level of the suppression changes with the centrality of the collision. We show that the measured patterns can be reproduced by a simple model, where all quarkonia are treated in a unified way, starting from the recent realisation that, in pp collisions, the probability of quarkonium formation has a universal dependence on the binding-energy of the bound state. The hot-medium suppression effect is parametrized by a penalty factor in the binding energy, identical for all (S- and P-wave) charmonium and bottomonium states, including those that indirectly contribute to the measured results through feed-down decays. This single parameter, computed through a global fit of all available suppression patterns, fully determines the hierarchy of nuclear effects, for all states and centrality bins. The resulting faithful description of the data provides convincing evidence in favour of the conjecture of sequential quarkonium suppression induced by QGP formation.

Of the J/$\psi$ mesons (inclusively) produced in pp collisions, a big fraction results from B decays, increasing with transverse momentum and exceeding 50\% for $p_{\rm T} > 20$ GeV. These events must be subtracted in measurements of the polarization of prompt J/$\psi$ mesons. While several studies have addressed the $\psi$(2S) and $\chi_c$ impact on the determination of the polarization of the directly-produced J/$\psi$ mesons, the theoretical and experimental knowledge of the non-prompt polarization is very poor. Furthermore, non-prompt J/$\psi$ polarization measurements can provide interesting information on quarkonium hadroproduction, complementing the studies of prompt production. We review the method of measuring the polarization of non-prompt J/$\psi$ mesons (produced in decays of unreconstructed B mesons and detected in the dilepton channel), in conditions typical of LHC experiments studying J/$\psi$ production. Realistic model-independent scenarios are validated with data from experiments studying $e^+e^- \to \Upsilon$(4S) interactions, converted to the high-momentum regime using B differential cross sections measured at the LHC. The non-prompt J/$\psi$ polarization measurements are seen to remain dependent on the event selection criteria, even after correcting for the dilepton acceptance and efficiencies. This implies that reproducible definitions of all relevant analysis choices must be reported with the polarization result, for rigorous comparisons with other measurements and/or theoretical calculations. We also discuss how the non-prompt J/$\psi$ polarization significantly depends on the relative importance of two complementary $\mathrm{B}\to {\rm J}/\psi$ decay topologies, two-body (reasonably dominated by singlet production) and multi-body (including octet contributions), providing, hence, valuable information for studies of the charmonium formation mechanisms.

The π+π−π0, K+K−π0 and K±K0π∓ final states produced by p̄p annihilation at rest at three different hydrogen target densities have been analyzed in the frame of a coupled channel analysis together with ππ and πK scattering data. Here we present the main results which concern masses, widths, ππ and KK partial widths of all the involved resonances (JP=0+,1−,2+), the direct determination of ΓKK/Γππ ratio for f0(1370) and f0(1500) (ΓKK/Γππ=0.91±0.20 and ΓKK/Γππ=0.25±0.03, respectively), the determination of a0(1300) parameters (M=1303±16 MeV; Γ=92±16 MeV) and the observation of two different high mass ρ signals associated to ρ(1450) and ρ(1700) (M=1182±30 MeV; Γ=389±20 MeV and M=1594±20 MeV; Γ=259±20 MeV, respectively).

We report on a search for the flavor-changing neutral current decay $D^0 \to \mu^+\mu^-$ using $50 \times 10^6$ events recorded with a dimuon trigger in interactions of 920 GeV protons with nuclei by the HERA-B experiment. We find no evidence for such decays and set a 90% confidence level upper limit on the branching fraction $Br(D^0 \to \mu^+\mu^-) <2.0 \times 10^{-6}$.

The relation between the branching ratios and direct CP asymmetries of B --> K pi decays and the angle gamma of the CKM unitarity triangle is studied numerically in the general framework of the SU(3) approach, with minimal assumptions about the parameters not fixed by flavour-symmetry arguments. Experimental and theoretical uncertainties are subjected to a statistical treatment according to the Bayesian method. In this context, the experimental limits recently obtained by CLEO, BaBar and Belle for the direct CP asymmetries are translated into the bound |gamma - 90 deg| > 21 deg at the 95% C.L.. A detailed analysis is carried out to evaluate the conditions under which measurements of the CP averaged branching ratios may place a significant constraint on gamma. Predictions for the ratios of charged (R_c) and neutral (R_n) B --> K pi decays are also presented.

The observation of unpolarized quarkonium production in high energy pp collisions, at mid rapidity, implies a significant violation of the non-relativistic QCD (NRQCD) velocity scaling rules. A precise experimental confirmation of this picture could definitely rule out the current formulation of the factorization expansion. This conclusion relies on current perturbative determinations of the short-distance kinematic factors and may be reverted if improved calculations would modify, in a very specific way, their transverse momentum dependences. That solution would result, however, in a full degeneracy in the presently assumed basis of $$^{2S+1}L_J$$ Fock states. Therefore, whatever the outcome, improved polarization measurements will challenge and improve our fundamental understanding of quarkonium production.

Inclusive doubly differential cross sections d 2 σ pA /dx F dp 2 as a function of Feynman-x (x F ) and transverse momentum (p T ) for the production of K 0 , Λ and $\bar{\varLambda}$ in proton-nucleus interactions at 920 GeV are presented. The measurements were performed by HERA-B in the negative x F range (−0.12<x F <0.0) and for transverse momenta up to p T =1.6 GeV/c. Results for three target materials: carbon, titanium and tungsten are given. The ratios of production cross sections are presented and discussed. The Cronin effect is clearly observed for all three V 0 species. The atomic number dependence is parameterized as σ pA =σ pN ⋅A α where σ pN is the proton-nucleon cross section. The measured values of α are all near one. The results are compared with EPOS 1.67 and PYTHIA 6.3. EPOS reproduces the data to within ≈20% except at very low transverse momentum.

The antiparticle-over-particle multiplicity ratio is measured in deep-inelastic scattering for negatively and positively charged kaons and, for the first time, for antiprotons and protons. The data were obtained by the COMPASS Collaboration using a 160 GeV muon beam impinging on an isoscalar 6LiD target. The regime of deep-inelastic scattering is ensured by requiring Q2 > 1 (GeV/c)2 for the photon virtuality and W>5 GeV/c2 for the invariant mass of the produced hadronic system. Bjorken-x is restricted to the range 0.01 to 0.40. Protons and antiprotons are identified in the momentum range from 20 GeV/c to 60 GeV/c and required to carry a large fraction of the virtual-photon energy, z>0.5. In the whole studied z-region, the p¯ over p multiplicity ratio is found to be below the lower limit expected from calculations based on leading-order perturbative Quantum Chromodynamics (pQCD). Kaons were previously analysed in the momentum range 12 GeV/c to 40 GeV/c. In the present analysis this range is extended up to 55 GeV/c, whereby events with larger virtual-photon energies are included in the analysis and the observed K− over K+ ratio becomes closer to the expectation of next-to-leading order pQCD. The results of both analyses strengthen our earlier conclusion that at COMPASS energies the phase space available for single-hadron production in deep-inelastic scattering should be taken into account in the standard pQCD formalism.

The annihilation frequency of the reaction $\overline{p}\stackrel{\ensuremath{\rightarrow}}{p}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ at rest in liquid hydrogen has been measured by the Obelix experiment by using different apparatus configurations and trigger conditions. The value obtained is $f({\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0},$ $\mathrm{LH})=(2.8\ifmmode\pm\else\textpm\fi{}{0.1}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{0.4}_{\mathrm{syst}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}.$ With the same data samples, the ${\ensuremath{\pi}}^{0}\ensuremath{\eta}$ annihilation frequency has been determined to be $f({\ensuremath{\pi}}^{0}\ensuremath{\eta},$ $\mathrm{LH})=(0.9\ifmmode\pm\else\textpm\fi{}{0.2}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{0.1}_{\mathrm{syst}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}.$ The results are discussed within the frame of the present experimental situation.

A detailed description of an original method used to measure the luminosity accumulated by the HERA-B experiment for a data sample taken during the 2002-2003 HERA running period is reported. We show that, with this method, a total luminosity measurement can be achieved with a typical precision, including overall systematic uncertainties, at a level of 5% or better. We also report evidence for the detection of delta-rays generated in the target and comment on the possible use of such delta rays to measure luminosity.

We present the study of the production of double J/ψ mesons using COMPASS data collected with a 190 GeV/c π− beam scattering off NH3, Al and W targets. Kinematic distributions of the collected double J/ψ events are analysed, and the double J/ψ production cross section is estimated for each of the COMPASS targets. The NH3 results are compared to predictions from single- and double-parton scattering models as well as the pion intrinsic charm and the tetraquark exotic resonance hypotheses. It is demonstrated that the single parton scattering production mechanism gives the dominant contribution that is sufficient to describe the data. An upper limit on the double intrinsic charm content of the pion is evaluated. No significant signatures that could be associated with exotic tetraquarks are found in the double J/ψ mass spectrum.

Several fixed-target experiments reported J/ψ and ϒ polarizations, as functions of Feynman x (xF) and transverse momentum (pT), in three different frames, using different combinations of beam particles, target nuclei, and collision energies. Despite the diverse and heterogeneous picture formed by these measurements, a detailed look allows us to discern qualitative physical patterns that inspire a simple empirical model. This data-driven scenario offers a good quantitative description of the J/ψ and ϒ(1S) polarizations measured in proton- and pion-nucleus collisions, in the xF≲0.5 domain: more than 80 data points (not statistically independent) are well reproduced with only one free parameter. This study sets the context for future low-pT quarkonium polarization measurements in proton- and pion-nucleus collisions, such as those to be made by the AMBER experiment, and shows that such measurements provide significant constraints on the poorly-known parton distribution functions of the pion.

The COMPASS Collaboration performed measurements of the Drell-Yan process in 2015 and 2018 using a 190 GeV/c $\pi^{-}$ beam impinging on a transversely polarised ammonia target. Combining the data of both years, we present final results on the amplitudes of the five azimuthal modulations in the dimuon production cross section. Three of these transverse-spin-dependent azimuthal asymmetries (TSAs) probe the nucleon leading-twist Sivers, transversity, and pretzelosity transverse-momentum dependent (TMD) parton distribution functions (PDFs). The other two are induced by subleading effects. These TSAs provide unique new inputs for the study of the nucleon TMD PDFs and their universality properties. In particular, the Sivers TSA observed in this measurement is consistent with the fundamental QCD prediction of a sign change of naive time-reversal-odd TMD PDFs when comparing the Drell-Yan process with semi-inclusive measurements of deep inelastic scattering. Also, within the context of model predictions, the observed transversity TSA is consistent with the expectation of a sign change for the Boer-Mulders function.

Abstract We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive $$\rho ^0$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>ρ</mml:mi> <mml:mn>0</mml:mn> </mml:msup> </mml:math> meson muoproduction at COMPASS using 160 GeV/ c polarised $$ \mu ^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>μ</mml:mi> <mml:mo>+</mml:mo> </mml:msup> </mml:math> and $$ \mu ^{-}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>μ</mml:mi> <mml:mo>-</mml:mo> </mml:msup> </mml:math> beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/ $$c^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>c</mml:mi> <mml:mn>2</mml:mn> </mml:msup> </mml:math> $$&lt; W&lt;$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>&lt;</mml:mo> <mml:mi>W</mml:mi> <mml:mo>&lt;</mml:mo> </mml:mrow> </mml:math> 17.0 GeV/ $$c^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>c</mml:mi> <mml:mn>2</mml:mn> </mml:msup> </mml:math> , 1.0 (GeV/ c ) $$^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow /> <mml:mn>2</mml:mn> </mml:msup> </mml:math> $$&lt; Q^2&lt;$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>&lt;</mml:mo> <mml:msup> <mml:mi>Q</mml:mi> <mml:mn>2</mml:mn> </mml:msup> <mml:mo>&lt;</mml:mo> </mml:mrow> </mml:math> 10.0 (GeV/ c ) $$^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow /> <mml:mn>2</mml:mn> </mml:msup> </mml:math> and 0.01 (GeV/ c ) $$^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow /> <mml:mn>2</mml:mn> </mml:msup> </mml:math> $$&lt; p_{\textrm{T}}^2&lt;$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>&lt;</mml:mo> <mml:msubsup> <mml:mi>p</mml:mi> <mml:mrow> <mml:mtext>T</mml:mtext> </mml:mrow> <mml:mn>2</mml:mn> </mml:msubsup> <mml:mo>&lt;</mml:mo> </mml:mrow> </mml:math> 0.5 (GeV/ c ) $$^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow /> <mml:mn>2</mml:mn> </mml:msup> </mml:math> . Here, W denotes the mass of the final hadronic system, $$Q^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>Q</mml:mi> <mml:mn>2</mml:mn> </mml:msup> </mml:math> the virtuality of the exchanged photon, and $$p_{\textrm{T}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>p</mml:mi> <mml:mtext>T</mml:mtext> </mml:msub> </mml:math> the transverse momentum of the $$\rho ^0$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>ρ</mml:mi> <mml:mn>0</mml:mn> </mml:msup> </mml:math> meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons ( $$\gamma ^*_T \rightarrow V^{ }_L$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mi>γ</mml:mi> <mml:mi>T</mml:mi> <mml:mo>∗</mml:mo> </mml:msubsup> <mml:mo>→</mml:mo> <mml:msubsup> <mml:mi>V</mml:mi> <mml:mi>L</mml:mi> <mml:mrow /> </mml:msubsup> </mml:mrow> </mml:math> ) indicate a violation of s -channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive $$\rho ^0$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>ρ</mml:mi> <mml:mn>0</mml:mn> </mml:msup> </mml:math> production.

Data on $\overline{p}$ slowing down and capture in helium at 1 and $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{mb}$ at room temperature are presented and compared to the corresponding previously publicated data in molecular hydrogen and deuterium. A Monte Carlo simulation containing a low-energy extrapolation of measured $\overline{p}$ electronic stopping power in helium gas, screened Rutherford collisions, and simple cascade mechanisms is able to reproduce the gross features of the data, but cannot explain some nontrivial details of the measured distributions.

The results of an investigation of the Pontecorvo reaction p̄d→φn with stopped antiprotons in a gaseous deuterium target at NTP are presented. A very selective dedicated trigger was used to collect a sample of about 8.5×106 annihilations. It is found that the annihilation rate is Y(p̄d→φn)=(3.56±0.20+0.2−0.1)×10−6. Comparison with the corresponding value for the ω-meson production demonstrates a large (a factor 40) apparent violation of the OZI rule, even larger than that found in the annihilation on a free nucleon.

The computing cluster built at Bologna to provide the LHCb Collaboration with a powerful Monte Carlo production tool is presented. It is a performance oriented Beowulf-class cluster, made of rack mounted commodity components, designed to minimize operational support requirements and to provide full and continuous availability of the computing resources. In this paper we describe the architecture of the cluster, and discuss the technical solutions adopted for each specialized sub-system.

Drell-Yan production cross sections were measured by the NA3 and E866 experiments, in p-Pt and pp collisions, as a function of the dimuon mass and xF. We compare these measurements to next-to-leading order Drell-Yan calculations, made with the CTEQ6M parton densities modified (or not) by nuclear effects, using the EPS09 parameterization. The analysis of the data allows us to evaluate the initial-state quark energy loss. Drell-Yan measurements are ideally suited to isolate the initial-state parton energy loss, given the absence of final-state effects on the produced dimuon. Our study shows that these data indicate negligible quark energy loss and allow us to derive rather strict upper limits. For completeness, our study has been repeated using the less accurate measurements of Drell-Yan cross section ratios between heavy and light nuclear targets, provided by the E772 and E866 experiments. Our results provide an additional constraint on the models trying to explain quarkonium production in proton-nucleus collisions, as a function of quarkonium rapidity and collision energy, where initial- and final-state energy loss has frequently been assumed to play an important role, convoluted with several other complex mechanisms, including final-state quarkonium breakup, formation time effects, etc.

The existing measurements of quarkonium polarization in proton-antiproton and proton-nucleus collisions are puzzling. We highlight issues which are often underestimated in the experimental analyses: the importance of the choice of the experimental acceptance on the comparison between experimental measurements and theoretical calculations. New measurements must provide more detailed information, such that physical conclusions can be derived without relying on model-dependent assumptions. We also describe a frame-invariant formalism which minimizes the dependence of the measurements on the experimental acceptance, facilitates the comparison with theoretical calculations, and probes systematic effects due to experimental biases.

The existing measurements of quarkonium polarization in proton-antiproton and proton-nucleus collisions are puzzling. We highlight issues which are often underestimated in the experimental analyses: the importance of the choice of the experimental acceptance on the comparison between experimental measurements and theoretical calculations. New measurements must provide more detailed information, such that physical conclusions can be derived without relying on model-dependent assumptions. We also describe a frame-invariant formalism which minimizes the dependence of the measurements on the experimental acceptance, facilitates the comparison with theoretical calculations, and probes systematic effects due to experimental biases.

The design of the phototube power supply for the HERA-B Electromagnetic Calorimeter is presented. A choice of the solution on the basis of the Cockcroft–Walton voltage multiplier is validated. Schematics developed are discussed. A special section describes the behaviour of the crucial components under irradiation since radiation tolerance becomes one of the most important items in the severe HERA-B radiation environment. Finally, performance achieved is presented.

Lattice QCD calculations [1] predict that, at sufficiently large-energy densities, hadronic matter undergoes a phase transition to a “plasma” of deconfined quarks and gluons (QGP). Considerable efforts have been invested since 1986 in the study of high-energy heavy-ion collisions to reveal the existence of this phase transition and to study the properties of strongly interacting matter in the new phase, in view of improving our understanding of confinement, a crucial feature of QCD. The study of quarkonium production and suppression is among the most interesting investigations in this field, because the calculations indicate that the QCD binding potential is screened in the QGP phase, the screening level increasing with the energy density of the system. Given the existence of several quarkonium states, of different binding energies, it is expected that they will be consecutively “dissolved” (into open charm or beauty mesons) above certain energy density thresholds [2,3].

A global analysis of ATLAS and CMS measurements reveals that, at mid-rapidity, the directly-produced $\chi_{c1}$, $\chi_{c2}$ and J/$\psi$ mesons have differential cross sections of seemingly identical shapes, when presented as a function of the mass-rescaled transverse momentum, $p_{\rm T}/M$. This identity of kinematic behaviours among S- and P-wave quarkonia is certainly not a natural expectation of non-relativistic QCD (NRQCD), where each quarkonium state is supposed to reflect a specific family of elementary production processes, of significantly different $p_{\rm T}$-differential cross sections. Remarkably, accurate kinematic cancellations among the variegated NRQCD terms (colour singlets and octets) of its factorization expansion can lead to a surprisingly good description of the data. This peculiar tuning of the NRQCD mixtures leads to a clear prediction regarding the $\chi_{c1}$ and $\chi_{c2}$ polarizations, the only observables not yet measured: they should be almost maximally different from one another, and from the J/$\psi$ polarization, a striking exception in the global panorama of quarkonium production. Measurements of the difference between the $\chi_{c1}$, $\chi_{c2}$ and J/$\psi$ polarizations, complementing the observed identity of momentum dependences, represent a decisive probe of NRQCD.

The HERA-B experiment has collected data on proton-induced collisions with three fixed nuclear targets at 41.6 GeV centre-of-mass energy. Preliminary results of an ongoing, detailed study of charmonium production include the determination of the nuclear dependence of the J/ψ production kinematics (extending for the first time into the negative-xF region) and precise measurements of the feeddown components due to χc, ψ′ and b-hadron decays. A new analysis of the J/ψ decay angular distribution finds significant polarization effects in the low-momentum region, marginally touched by previous experiments. Measurements of the D-meson cross sections are consistent with the absence of nuclear suppression in open charm production.

We study the spin-exotic $J^{PC} = 1^{-+}$ amplitude in single-diffractive dissociation of 190 GeV$/c$ pions into $\pi^-\pi^-\pi^+$ using a hydrogen target and confirm the $\pi_1(1600) \to \rho(770) \pi$ amplitude, which interferes with a nonresonant $1^{-+}$ amplitude. We demonstrate that conflicting conclusions from previous studies on these amplitudes can be attributed to different analysis models and different treatment of the dependence of the amplitudes on the squared four-momentum transfer and we thus reconcile their experimental findings. We study the nonresonant contributions to the $\pi^-\pi^-\pi^+$ final state using pseudo-data generated on the basis of a Deck model. Subjecting pseudo-data and real data to the same partial-wave analysis, we find good agreement concerning the spectral shape and its dependence on the squared four-momentum transfer for the $J^{PC} = 1^{-+}$ amplitude and also for amplitudes with other $J^{PC}$ quantum numbers. We investigate for the first time the amplitude of the $\pi^-\pi^+$ subsystem with $J^{PC} = 1^{--}$ in the $3\pi$ amplitude with $J^{PC} = 1^{-+}$ employing the novel freed-isobar analysis scheme. We reveal this $\pi^-\pi^+$ amplitude to be dominated by the $\rho(770)$ for both the $\pi_1(1600)$ and the nonresonant contribution. We determine the $\rho(770)$ resonance parameters within the three-pion final state. These findings largely confirm the underlying assumptions for the isobar model used in all previous partial-wave analyses addressing the $J^{PC} = 1^{-+}$ amplitude.

Abstract Quarkonium polarization data, usually considered a difficult challenge for the QCD description of quarkonium production and relegated to an a posteriori test of predictions exclusively driven by cross-section measurements, with puzzling results, provide, in reality, the most fundamental, direct and model-independent connection to the production mechanisms. We have simultaneously fitted the ψ ( 2 S ) and ϒ ( 3 S ) differential cross sections and polarizations, reliably measured at the LHC up to higher transverse momentum p T values than ever before, as a superposition of colour-singlet and colour-octet contributions perturbatively calculated up to next-to-leading order. We show that, except for the lowest p T cross-section data, where factorization between short-distance and long-distance QCD effects is not expected to be applicable, all the measurements are very well reproduced. Besides providing a straightforward solution to the “quarkonium polarization puzzle”, our study shows that quarkonium production is dominated by the unpolarized S 0 [ 8 ] 1 octet term, an observation that opens new paths towards the understanding of bound-state formation in QCD.

The process qq¯→ZZ→4ℓ, dominant background for new physics signals in the four-lepton channel, is characterized by a fully transverse polarization of the two Z bosons with respect to the q and q¯ directions. We show that the Z decay angular distributions can be described by a simple, analytical function of the event kinematics, not depending on parton distributions. Using the search for a heavy Higgs boson as an example, we show that the angular discrimination improves the sensitivity to rare signals and is especially beneficial when the background contribution is large.

We report on the status of the CMS experiment and on plans for spin physics measurements at the LHC. We focus on the short-term prospect of quarkonium polarization studies, which promise to solve longstanding puzzles on non-perturbative aspects of QCD.

The polarizations of the χ states produced in high energy collisions can be derived from the dilepton decay distributions of the daughter J/ψ or ϒ mesons, with a reduced dependence of the measurement on the details of photon reconstruction and simulation, and eliminating the dependence of the polarization measurement on the actual details of the multipole structure of the radiative transition.

The angular distribution of the two-body decay of a boson of unknown properties is strongly constrained by angular momentum conservation and rotation invariance, as well as by the nature of the detected decay particles and of the colliding ones. Knowing the border between the ``physical'' and ``unphysical'' parameter domains defined by these ``minimal constraints'' (excluding specific hypotheses on what is still subject of measurement) is a useful ingredient in the experimental determinations of angular distributions and can provide model-independent criteria for spin characterizations. In particular, analyzing the angular decay distribution with the general parametrization for the $J=2$ case can provide a model-independent discrimination between the $J=0$ and $J=2$ hypotheses for a particle produced by two real gluons and decaying into two real photons.

Measurements of the kinematic distributions of $J/\psi$ mesons produced in $p-$C, $p-$Ti and $p-$W collisions at $\sqrt{s}=41.6 \mathrm{GeV}$ in the Feynman-$x$ region $-0.34 < x_{F} < 0.14$ and for transverse momentum up to $p_T = 5.4 \mathrm{GeV}/c$ are presented. The $x_F$ and $p_T$ dependencies of the nuclear suppression parameter, $\alpha$, are also given. The results are based on $2.4 \cdot 10^{5}$ $J/\psi$ mesons in both the $e^+ e^-$ and $\mu^{+}\mu^{-}$ decay channels. The data have been collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The measurement explores the negative region of $x_{F}$ for the first time. The average value of $\alpha$ in the measured $x_{F}$ region is $0.981 \pm 0.015$. The data suggest that the strong nuclear suppression of $J/\psi$ production previously observed at high $x_F$ turns into an enhancement at negative $x_F$.

We report on atomistic simulation of the folding of a natively-knotted protein, MJ0366, based on a realistic force field. To the best of our knowledge this is the first reported effort where a realistic force field is used to investigate the folding pathways of a protein with complex native topology. By using the dominant-reaction pathway scheme we collected about 30 successful folding trajectories for the 82-amino acid long trefoil-knotted protein. Despite the dissimilarity of their initial unfolded configuration, these trajectories reach the natively-knotted state through a remarkably similar succession of steps. In particular it is found that knotting occurs essentially through a threading mechanism, involving the passage of the C-terminal through an open region created by the formation of the native beta-sheet at an earlier stage. The dominance of the knotting by threading mechanism is not observed in MJ0366 folding simulations using simplified, native-centric models. This points to a previously underappreciated role of concerted amino acid interactions, including non-native ones, in aiding the appropriate order of contact formation to achieve knotting.

A round table held during the Hadron07 Conference focusing on experimental observations of new hadronic states, on theoretical perspectives for their description, and on the role of hadronic spectroscopy in furthering our knowledge of the fundamental theory of strong interactions.

The interpretation of the J/ suppression patterns observed in nuclear collisions, at CERN and RHIC, as a signature of the formation of a deconfined phase of QCD matter, requires knowing which fractions of the measured J/ yields, in pp collisions, are due to decays of heavier charmonium states. From a detailed analysis of the available mid-rapidity charmonium hadro-production cross sections, ′ )

Spontaneous structural rearrangements play a central role in the organization and function of complex biomolecular systems. In principle, physics-based computer simulations like Molecular Dynamics (MD) enable us to investigate these thermally activated processes with an atomic level of resolution. However, rare conformational transitions are intrinsically hard to investigate with MD, because an exponentially large fraction of computational resources must be invested to simulate thermal fluctuations in metastable states. Path sampling methods like Transition Path Sampling hold the great promise of focusing the available computational power on sampling the rare stochastic transition between metastable states. In these approaches, one of the outstanding limitations is to generate paths that visit significantly different regions of the conformational space at a low computational cost. To overcome these problems we introduce a rigorous approach that integrates a machine learning algorithm and MD simulations implemented on a classical computer with adiabatic quantum computing. First, using functional integral methods, we derive a rigorous low-resolution representation of the system's dynamics, based on a small set of molecular configurations generated with machine learning. Then, a quantum annealing machine is employed to explore the transition path ensemble of this low-resolution theory, without introducing un-physical biasing forces to steer the system's dynamics. Using the D-Wave quantum computer, we validate our scheme by simulating a benchmark conformational transition in a state-of-the-art atomistic description. We show that the quantum computing step generates uncorrelated trajectories, thus facilitating the sampling of the transition region in configuration space. Our results provide a new paradigm for MD simulations to integrate machine learning and quantum computing.

Of the J/$\psi$ mesons (inclusively) produced in pp collisions, a big fraction results from B decays, increasing with transverse momentum and exceeding 50\% for $p_{\rm T} > 20$ GeV. These events must be subtracted in measurements of the polarization of prompt J/$\psi$ mesons. While several studies have addressed the $\psi$(2S) and $\chi_c$ impact on the determination of the polarization of the directly-produced J/$\psi$ mesons, the theoretical and experimental knowledge of the non-prompt polarization is very poor. Furthermore, non-prompt J/$\psi$ polarization measurements can provide interesting information on quarkonium hadroproduction, complementing the studies of prompt production. We review the method of measuring the polarization of non-prompt J/$\psi$ mesons (produced in decays of unreconstructed B mesons and detected in the dilepton channel), in conditions typical of LHC experiments studying J/$\psi$ production. Realistic model-independent scenarios are validated with data from experiments studying $e^+e^- \to \Upsilon$(4S) interactions, converted to the high-momentum regime using B differential cross sections measured at the LHC. The non-prompt J/$\psi$ polarization measurements are seen to remain dependent on the event selection criteria, even after correcting for the dilepton acceptance and efficiencies. This implies that reproducible definitions of all relevant analysis choices must be reported with the polarization result, for rigorous comparisons with other measurements and/or theoretical calculations. We also discuss how the non-prompt J/$\psi$ polarization significantly depends on the relative importance of two complementary $\mathrm{B}\to {\rm J}/\psi$ decay topologies, two-body (reasonably dominated by singlet production) and multi-body (including octet contributions), providing, hence, valuable information for studies of the charmonium formation mechanisms.

Several fixed-target experiments reported J/$\psi$ and $\Upsilon$ polarization measurements, as functions of Feynman $x$ ($x_{\rm F}$) and transverse momentum ($p_{\rm T}$), in three different polarization frames, using different combinations of beam particles, target nuclei and collision energies. The data form such a diverse and heterogeneous picture that, at first sight, no clear trends can be observed. A more detailed look, however, allows us to discern qualitative physical patterns that inspire and support a simple interpretation: the directly-produced quarkonia result from either gluon-gluon fusion or from quark-antiquark annihilation, with the former mesons being fully longitudinally polarized and the latter being fully transversely polarized. This hypothesis provides a reasonable quantitative description of the J/$\psi$ and $\Upsilon$(1S) polarizations measured in the $x_{\rm F} \lesssim 0.5$ kinematical domain. We provide predictions that can be experimentally tested, using proton and/or pion beams, and show that improved J/$\psi$ and $\psi$(2S) polarization measurements in pion-nucleus collisions can provide significant constraints on the poorly known parton distribution functions of the pion.

Abstract This chapter introduces the basic notions used in the measurement of the polarization of a particle and in its interpretation.We use, as a prototype, the case of a vector particle (the $$\text{J}/\psi$$ meson,a virtual photon, the Z boson, etc.) decaying into a lepton-antilepton pair.In particular, we address the general shape of the decay angular distribution, its relation to the particle polarization, the meaning of the relevant parameters, and the main difficulties usually encountered in polarization measurements.

Abstract This chapter introduces general frame-independent relations between the observable anisotropy parameters. Some of these relations reflect the geometrical properties of the distribution, including inequalities that delimit the allowed phase space of the anisotropy parameters, and the representation of the distribution in a “canonical” form. The most interesting relation defines a rotation-invariant parameter expressing the intrinsic nature of the polarization, independently of the reference frame.

Abstract In certain physical cases, the angular momentum of a vector particle may naturally align with a direction not exactly contained in the production plane, leading to an attenuation of the polarization magnitude observable in any experimentally defined frame. In this chapter we consider two possible causes of this phenomenon: the higher-order, non-planar processes contributing to Drell--Yan, W and Z production, and the intrinsic transverse momentum of the partons inside the colliding hadrons. We discuss the effects of these phenomena on the measured frame-dependent and frame-independent parameters of the observed angular distribution.

Abstract This chapter discusses the importance of the invariant polarization observable $$\mathcal{F}$$ (or $$\tilde{\lambda}$$ ) in certain physics scenarios, where none of the adoptable polarization frames would provide a particularly simple picture in terms of $$\lambda_\vartheta$$ , $$\lambda_\varphi$$ and $$\lambda_{\vartheta\varphi}$$ . One such case is the production of Drell--Yan dileptons, where the polarization parameters, calculated including perturbative QCD corrections, satisfy the Lam--Tung identity, a frame-independent relation maintaining its seemingly surprising simplicity even when the polar and azimuthal anisotropies have strong dependences on the particle momentum. The notion of invariant polarization allows us to reinterpret this relation in a geometrical way, explaining it as a mere consequence of helicity conservation and rotational invariance.

Abstract This chapter describes the different options for defining the polarization frame in terms of physical directions in a real experiment.We explain how the observed anisotropy parameters of the decay distribution of a vector particle depend on the definition of the polarization frame and, in general, on the particle's laboratory momentum.We also discuss how the choice of the polarization frame, while in principle arbitrary, can have an impact on the immediate physical indications provided by the measurement, as well as, in some cases, on the very reliability of the analysis results.

Abstract This chapter illustrates the general method to calculate the shape of the angular distribution for any considered two-body decay, and surveys examples for different kinds of initial particles, with integer or halfinteger J . We describe the dependences of the angular distribution shapes on J , J z and the identity of the decay products, as well as the physical domains of the observable parameters. We also discuss how the measurement of the decay angular distribution can lead to the determination of J for an unidentified decaying particle.

Abstract This chapter addresses the case of a vector particle produced indirectly from the decay or transformation of a particle having $$J = 0$$ ( $$\chi_{c0} \to \mathrm{J}/\psi \, \gamma$$ , $$\mathrm{B} \to \mathrm{J}/\psi \, \mathrm{K}$$ , $$\mathrm{H} \to \mathrm{Z} \, \gamma$$ , etc.), $$J = 1$$ or $$J = 2$$ ( $$\chi_{c1,2} \to \mathrm{J}/\psi \, \gamma$$ , $$\mathrm{Z} \to \mathrm{J}/\psi \, \gamma$$ , etc.), describing the polarization frame definitions that can be adopted to represent the two-step process, and their respective advantages. We illustrate how several measurement and kinematic conditions can lead to very different observable polarizations of the vector particle, in the special but frequent case where the intermediate decay step remains unobserved and only the final dilepton angular distribution is measured. We also discuss how the peculiar observation, made at the LHC, of almost unpolarized “directly” produced J/ $$\psi$$ mesons may be understood as the result of a two-step production mechanism.