Alexander Lanev

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 .

The HERA-B Outer Tracker is a large system of planar drift chambers with about 113000 read-out channels. Its inner part has been designed to be exposed to a particle flux of up to 2.10^5 cm^-2 s^-1, thus coping with conditions similar to those expected for future hadron collider experiments. 13 superlayers, each consisting of two individual chambers, have been assembled and installed in the experiment. The stereo layers inside each chamber are composed of honeycomb drift tube modules with 5 and 10 mm diameter cells. Chamber aging is prevented by coating the cathode foils with thin layers of copper and gold, together with a proper drift gas choice. Longitudinal wire segmentation is used to limit the occupancy in the most irradiated detector regions to about 20 %. The production of 978 modules was distributed among six different laboratories and took 15 months. For all materials in the fiducial region of the detector good compromises of stability versus thickness were found. A closed-loop gas system supplies the Ar/CF4/CO2 gas mixture to all chambers. The successful operation of the HERA-B Outer Tracker shows that a large tracker can be efficiently built and safely operated under huge radiation load at a hadron collider.

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.

In this paper we describe the operation and performance of the HERA-B Outer Tracker, a 112674 channel system of planar drift tube layers. The performance of the HERA-B Outer Tracker system fullfilled all requirements for stable and efficient operation in a hadronic environment, thus confirming the adequacy of the honeycomb drift tube technology and of the front-end readout system. The detector was stably operated with a gas gain of 30000 in an Ar/CF4/CO2 (65:30:5) gas mixture, yielding a good efficiency for triggering and track reconstruction, larger than 95 % for tracks with momenta above 5 GeV/c. The hit resolution of the drift cells was 300 to 320 micrometers and the relative momentum resolution can be described as: sigma(p)/p (in %) = (1.61 +- 0.02) + (0.0051 +- 0.0006) p. At the end of the HERA-B running no aging effects in the Outer Tracker cells were observed.

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 HERA-B Outer Tracker consists of drift tubes folded from polycarbonate foil and is operated with Ar/CF4/CO2 as drift gas. The detector has to stand radiation levels which are similar to LHC conditions. The first prototypes exposed to radiation in HERA-B suffered severe radiation damage due to the development of self-sustaining currents (Malter effect). In a subsequent extended R&D program major changes to the original concept for the drift tubes (surface conductivity, drift gas, production materials) have been developed and validated for use in harsh radiation environments. In the test program various aging effects (like Malter currents, gain loss due to anode aging and etching of the anode gold surface) have been observed and cures by tuning of operation parameters have been developed.

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.

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.

Meson rescattering corrections to K→2π decays have been calculated in chiral perturbation theory based on bosonized QCD using superpropagator regularization. It is found that ππ- and Kπ-interactions enlarge ΔT=12 respectively diminish ΔT=32 transitions, accounting quantitatively for the ΔT=12 rule.

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.

We discuss the processes and at in the momentum expansion. The calculation involves the tree-level, one-loop and two-loop diagrams of a chiral effective meson Lagrangian with structure coefficients fixed by bosonization of the four-quark NJL-type interaction. The importance of integrating out meson resonances (reduction) is pointed out. Our final results for the total cross section of are in good agreement with the experimental data of the Crystal Ball Collaboration. For the width of the decay we obtain the value 0.11 eV which has to be compared with the experimental value of eV. We present a prediction for the differential decay probability as a function of .

The HERA-B Outer Tracker is a large detector with 112 674 drift chamber channels.It is exposed to a particle flux of up to 2 • 10 5 cm -2 s -1 thus coping with conditions similar to those expected for the LHC experiments.The front-end readout system, based on the ASD-8 chip and a customized TDC chip, is designed to fulfil the requirements on low noise, high sensitivity, rate tolerance, and high integration density.The TDC system is based on an ASIC which digitizes the time in bins of about 0.5 ns within a total of 256 bins.The chip also comprises a pipeline to store data from 128 events which is required for a deadtime-free trigger and data acquisition system.We report on the development, installation, and commissioning of the front-end electronics, including the grounding and noise suppression schemes, and discuss its performance in the HERA-B experiment.

A survey of the results obtained by the CMS Collaboration in searches for physics beyond the Standard Model on the basis of a complete set of data measured at the c.m.collision energy of $\sqrt s $ = 8 TeV in the 2012 run at the Large Hadron Collider (LHC) is given.

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.

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 $Br(D^0 \to \mu^+\mu^-) <2.0 \times 10^{-6}$.

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.

We present an analysis of the enhancement of CP-violating charge asymmetries in K±→3πdecays. Calculations of decay amplitudes are performed on the basis of bosonized strong and weak lagrangians derived from QCD-motivated quark lagrangians. We show that the interplay of fourth-order contributions of chiral lagrangians for strong interactions and penguin operators in weak interactions significantly enhances the charge asymmetries.

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.

In the framework of recently proposed effective lagrangians for weak nonleptonic meson interactions the amplitudes of the decays K0 → 3π have been calculated with inclusion of isospin-breaking and meson rescattering effects. The imaginary part of the penguin diagram contribution, which determines direct CP violation in nonleptonic kaon decays, has been fixed with the help of the measured ratio ϵ′ϵ of CP-violation parameters. The modification of the Li-Wolfenstein relation for the direct CP-violation parameter in K0(K0) → π+π−π0 decays is discussed.

In this paper a method of internal alignment of HERA-B OTR PC chambers is discussed. The method is based on a simultaneous fit of the track and alignment parameters, using Millepede matrix reduction and singular value decomposition. Software implementing this idea has been developed, and the method has been studied on Monte Carlo models with different levels of simulation. A generalization of this method for the case of the nonlinear track model has been proposed.

The K → 2π and K → 3π decay amplitudes are calculated from recent effective weak meson Lagrangians taking into account meson rescattering corrections. By a simultaneous analysis of data all the relevant parameters are determined and predictions for CP-asymmetries in K± → π± π± π∓, K± → πoπoπ± decays are obtained.

The total cross section for Coulomb photoproduction of neutral pion pairs in the reaction is estimated within the effective chiral Lagrangian approach. The amplitude of with one off-shell photon is calculated at in the momentum expansion; in addition, nuclear absorption is taken into account. Besides its experimental feasibility, the results of the calculation demonstrate that the reaction is a powerful source of information on the process close to threshold.

The role of the p/sup 4/ corrections in the description of meson processes is discussed from the point of view of an experimental test of the low-energy chiral QCD theory. It is shown that the effective p/sup 4/ Lagrangian which occurs in this theory describes satisfactorily the ..pi pi.. and ..pi..K scattering lengths and the parameters of the expansion of the K..-->..3..pi.. decay amplitudes in powers of the invariant Dalitz variables. Experimental data on these processes with much greater accuracy will be required for a more detailed analysis of the p/sup 4/ corrections.

We discuss the processes $γγ\toπ^0π^0$ and $η\toπ^0 γγ$ at $O(p^6)$ in the momentum expansion. The calculation involves tree--level, one--loop and two--loop diagrams of a chiral effective lagrangian which is obtained by a bosonization of the NJL model. The importance of integrating out meson resonances (reduction) is pointed out. Our final results for the total cross section of $γγ\toπ^0π^0$ are in good agreement with the experimental data of the Crystal Ball Collaboration. For the width of the $η\toπ^0γγ$ decay we obtain the value $0.11$ eV which has to be compared with the experimental value of $(0.84 \pm 0.18)$ eV. Alternatively, taking empirical parameters from a vector--meson--dominance model the prediction for the decay width is $0.35$ eV. We present a prediction for the differential decay probability as a function of $m_{γγ}^2/m_η^2$.