Austin Ball

In response to the 2013 Update of the European Strategy for Particle Physics, the Future Circular Collider (FCC) study was launched, as an international collaboration hosted by CERN. This study covers a highest-luminosity high-energy lepton collider (FCC-ee) and an energy-frontier hadron collider (FCC-hh), which could, successively, be installed in the same 100 km tunnel. The scientific capabilities of the integrated FCC programme would serve the worldwide community throughout the 21st century. The FCC study also investigates an LHC energy upgrade, using FCC-hh technology. This document constitutes the second volume of the FCC Conceptual Design Report, devoted to the electron-positron collider FCC-ee. After summarizing the physics discovery opportunities, it presents the accelerator design, performance reach, a staged operation scenario, the underlying technologies, civil engineering, technical infrastructure, and an implementation plan. FCC-ee can be built with today’s technology. Most of the FCC-ee infrastructure could be reused for FCC-hh. Combining concepts from past and present lepton colliders and adding a few novel elements, the FCC-ee design promises outstandingly high luminosity. This will make the FCC-ee a unique precision instrument to study the heaviest known particles (Z, W and H bosons and the top quark), offering great direct and indirect sensitivity to new physics.

We study CPT-odd non-minimal Lorentz-symmetry violating couplings in the electroweak sector modifying the interactions between leptons, gauge mediators and the Higgs boson. The tree-level (differential) cross sections for three important electroweak processes are discussed: e+e−→ZH, e+e−→ZZ and γγ→W+W−. By considering next-generation e+e− colliders reaching center-of-mass energies at the TeV scale and the estimated improved precision for the measurements of the respective cross sections, we are able to project upper bounds on the purely time-like background 4-vector as strict as ≲10−5GeV−1, in agreement with previous work on similar Lorentz-violating couplings.

We review the physics opportunities of the Future Circular Collider, covering its e+e-, pp, ep and heavy ion programmes. We describe the measurement capabilities of each FCC component, addressing the study of electroweak, Higgs and strong interactions, the top quark and flavour, as well as phenomena beyond the Standard Model. We highlight the synergy and complementarity of the different colliders, which will contribute to a uniquely coherent and ambitious research programme, providing an unmatchable combination of precision and sensitivity to new physics.

In response to the 2013 Update of the European Strategy for Particle Physics (EPPSU), the Future Circular Collider (FCC) study was launched as a world-wide international collaboration hosted by CERN. The FCC study covered an energy-frontier hadron collider (FCC-hh), a highest-luminosity high-energy lepton collider (FCC-ee), the corresponding 100 km tunnel infrastructure, as well as the physics opportunities of these two colliders, and a high-energy LHC, based on FCC-hh technology. This document constitutes the third volume of the FCC Conceptual Design Report, devoted to the hadron collider FCC-hh. It summarizes the FCC-hh physics discovery opportunities, presents the FCC-hh accelerator design, performance reach, and staged operation plan, discusses the underlying technologies, the civil engineering and technical infrastructure, and also sketches a possible implementation. Combining ingredients from the Large Hadron Collider (LHC), the high-luminosity LHC upgrade and adding novel technologies and approaches, the FCC-hh design aims at significantly extending the energy frontier to 100 TeV. Its unprecedented centre of-mass collision energy will make the FCC-hh a unique instrument to explore physics beyond the Standard Model, offering great direct sensitivity to new physics and discoveries.

The performance of the ALEPH detector at the LEP e+e− collider is reviewed. The accuracy of the tracking detectors to measure the impact parameter and momentum of charged tracks is specified. Calorimeters are used to measure photons and neutral hadrons, and the accuracy obtained in energy and angle is given. An essential property of the detector is its ability to identify particles; the performance in identification of electrons, muons, neutrinos (from missing energy), charged hadrons, π0's and V0's is described.

We discuss the physics potential and the experimental challenges of an upgraded LHC running at an instantaneous luminosity of 1035 cm-2s-1. The detector R&D needed to operate ATLAS and CMS in a very high radiation environment and the expected detector performance are discussed. A few examples of the increased physics potential are given, ranging from precise measurements within the Standard Model (in particular in the Higgs sector) to the discovery reach for several New Physics processes.

In response to the 2013 Update of the European Strategy for Particle Physics (EPPSU), the Future Circular Collider (FCC) study was launched as a world-wide international collaboration hosted by CERN. The FCC study covered an energy-frontier hadron collider (FCC-hh), a highest-luminosity high-energy lepton collider (FCC-ee), the corresponding 100 km tunnel infrastructure, as well as the physics opportunities of these two colliders, and a high-energy LHC, based on FCC-hh technology. This document constitutes the third volume of the FCC Conceptual Design Report, devoted to the hadron collider FCC-hh. It summarizes the FCC-hh physics discovery opportunities, presents the FCC-hh accelerator design, performance reach, and staged operation plan, discusses the underlying technologies, the civil engineering and technical infrastructure, and also sketches a possible implementation. Combining ingredients from the Large Hadron Collider (LHC), the high-luminosity LHC upgrade and adding novel technologies and approaches, the FCC-hh design aims at significantly extending the energy frontier to 100 TeV. Its unprecedented centre-of-mass collision energy will make the FCC-hh a unique instrument to explore physics beyond the Standard Model, offering great direct sensitivity to new physics and discoveries.

We study the inclusive momentum distribution of charged particles in multihadronic events produced in e+e− annihilations at ECM∼M(Z0). We find agreement with the analytical formulae for gluon production that include the phenomena of soft gluon interference. Using data from CM energies between 14 and 91 GeV, we study the dependence of the inclusive momentum distribution on the centre of momentum energy. We find that the analytical formulae describe the data over the entire energy range. Both the momentum distribution at a fixed energy and the change with energy are described by QCD shower Monte Carlo's which include either coherent gluon branchings or string fragmentation. Simple incoherent models with independent fragmentation fail to reproduce the energy dependence and momentum spectra.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTConformational changes in adsorbed proteinsA. Ball and R. A. L. JonesCite this: Langmuir 1995, 11, 9, 3542–3548Publication Date (Print):September 1, 1995Publication History Published online1 May 2002Published inissue 1 September 1995https://doi.org/10.1021/la00009a042RIGHTS & PERMISSIONSArticle Views342Altmetric-Citations98LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (850 KB) Get e-Alerts Get e-Alerts

The longitudinal polarization, the transverse polarization, and the forward-backward asymmetry of $\Lambda$ baryons, have been measured using a sample of 4.34 million hadronic $\mathrm{Z}^0$ decays collected with the OPAL detector at LEP between 1990 and 1995. These results are important as an aid to the understanding of hadronization mechanisms. Significant longitudinal polarization has been observed at intermediate and high momentum. For $x_E$ ( $\equiv 2 E_{\Lambda}/\sqrt{s}) >$ 0.3, the longitudinal polarization has been measured to be $-32.9 \pm 5.5 $ (stat) $\pm$ 5.2 (syst)%. We have observed no transverse polarization. A significant forward-backward asymmetry has been measured and can be described by a JETSET model.

Relative production rates of multijet hadronic final states of Z0 boson decays, observed in e+e− annihilation around 91 GeV centre of mass energy, are presented. The data can be well described by analytic O(αs2) QCD calculations and by QCD shower model calaculations with parameters as determined at lower energies. A first judgement of ΛMS and of the renormalization scale μ2 in O(αs2) QCD results in values similar to those obtained in the continuum of e+e− annihilations. Significant scaling violations are observed when the 3-jet fractions are compared to the corresponding results from smaller centre of mass energies. They can be interpreted as being entirely due tot the energy dependence of αs, as proposed by the nonabelian nature of QCD, The possibility of an energy independent coupling constant can be excluded with a significance of 5.7 standard deviations.

We report on a search for elementary particles with charges much smaller than the electron charge using a data sample of proton-proton collisions provided by the CERN Large Hadron Collider in 2018, corresponding to an integrated luminosity of 37.5 fb$^{-1}$ at a center-of-mass energy of 13 TeV. A prototype scintillator-based detector is deployed to conduct the first search at a hadron collider sensitive to particles with charges ${\leq}0.1e$. The existence of new particles with masses between 20 and 4700 MeV is excluded at 95% confidence level for charges between $0.006e$ and $0.3e$, depending on their mass. New sensitivity is achieved for masses larger than $700$ MeV.

Photonic events with large missing energy have been observed in $\rm e^+e^-$ collisions at a centre-of-mass energy of 189 GeV using the OPAL detector at LEP. Results are presented for event topologies consistent with a single photon or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection, and the number of light neutrino species is measured. Cross-section results are compared with the expectations from the Standard Model process $\mathrme^+\mathrme^-\to \nu\overline{\nu}$ + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on $\sigma(\mathrme^+\mathrme^-\to\mathrm{X}\mathrm{Y})\cdot\mathrm{BR}(\mathrm{X}\to\mathrm{Y}\gamma)$ and $\sigma(\mathrme^+\mathrme^-\to\mathrm{XX})\cdot\mathrm{BR}^2(\mathrm{X}\to\mathrm{Y}\gamma)$ are derived for the case of stable and invisible $\mathrm{Y}$ . These limits apply to single and pair production of excited neutrinos ( $\mathrm{X} = \nu^*, \mathrm{Y} = \nu$ ), to neutralino production ( $\mathrm{X}={{{\tilde{\chi}}^{0}}_{2}}, \mathrm{Y}={{{\tilde{\chi}}^{0}}_{1}}$ ) and to supersymmetric models in which $\mathrm{X} ={{{\tilde{\chi}}^{0}}_{1}}$ and $\mathrm{Y}={\tilde{\mathrm{G}}}$ is a light gravitino. The case of macroscopic decay lengths of particle X is considered for $\mathrme^+\mathrme^- \to \mathrm{XX}$ , $\rm X \to Y \gamma$ , when $M_{\mathrm Y}\approx 0$ . The single-photon results are also used to place upper limits on superlight gravitino pair production as well as graviton-photon production in the context of theories with additional space dimensions.

We have searched for lepton flavour violating Z0→eμ, Z0→eτ and Z0→μτ decays in a sample of 4.0×106 visible Z0 decays collected with the OPAL detector at LEP during 1991 to 1994. No candidates are found for Z0→eμ. The samples of selected Z0→eτ and Z0→μτ candidates are consistent with the expected background. The following limits are set at 95% confidence level: $$\begin{gathered} BR(Z^0 \to e\mu ){\text{ }}< {\text{ 1}}{\text{.7 }} \times {\text{ 10}}^{ - {\text{6}}} \hfill \\ BR(Z^0 \to e\tau ){\text{ }}< {\text{ 9}}{\text{.8 }} \times {\text{ 10}}^{ - {\text{6}}} \hfill \\ BR(Z^0 \to \mu \tau ){\text{ }}< {\text{ 17}}{\text{. }} \times {\text{ 10}}^{ - {\text{6}}} . \hfill \\ \end{gathered}$$

We have studied hadronic events produced at LEP at centre-of-mass energies of 130 and 136 GeV. Distributions of event shape observables, jet rates, momentum spectra and multiplicities are presented and compared to the predictions of several Monte Carlo models and analytic QCD calculations. From fits of event shape and jet rate distributions to $${\mathcal{O}}(\alpha _s^2 ) + NLLA$$ QCD calculations, we determineα s (133 GeV)=0.110±0.005(stat.)±0.009(syst.). We measure the mean charged particle multiplicity 〈n ch〉=23.40±0.45(stat.) ±0.47(syst.) and the position ζ0 of the peak in the ζ p = ln(1/x p ) distribution ζ0=3.94±0.05(stat.)±0.11(syst.). These results are compared to lower energy data and to analytic QCD or Monte Carlo predictions for their energy evolution.

We report on a measurement of the mass of the Z0 boson, its total width, and its partial decay widths into hadrons and leptons. On the basis of 25 801 hadronic decays and 1999 decays into electrons, muons or taus, selected over eleven energy points between 88.28 GeV and 95.04 GeV, we obtain from a combined fit to hadrons and leptons a mass of Mz = 91.154 ± 0.021 (exp) ± 0.030 (LEP) GeV, and a total width of Γz = 2.536 ± 0.045 GeV. The errors on Mz have been separated into the experimental error and the uncertainty due to the LEP beam energy. The measured leptonic partial widths are Γee = 81.2 ± 2.6 MeV, Γμμ = 82.6 ± 5.8 MeV, and Γττ = 85.7 ± 7.1 MeV, consistent with lepton universality. From a fit assuming lepton universality we obtain Γℓ+ ℓ− = 81.9 ± 2.0 MeV. The hadronic partial width is Γhad = 1838 ± 46 MeV. From the measured total and partial widths a model independent value for the invisible width is calculated to be Γinv = 453 ± 44 MeV. The errors quoted include both the statistical and the systematic uncertainties.

Bose-Einstein correlations between like-sign charged track pairs have been studied in e+e− annihilation hadronic events at centre-of-mass energies around the Z0 peak as a function of Q, the four-momentum difference of the pair. The measurement was performed with the OPAL detector at LEP. Assuming the charged tracks to be pions, the observed Bose-Einstein enhancement was used to extract the values of the strength of the effect and the radius of the pion emitting source, which were found to be λ=0.866±0.032±0.140 and R0=0.928±0.019±0.150 fm, respectively, where the first error is statistical and the second is systematic. The results do not show significant variation in comparison to e+e− annihilation measurements at lower centre-of-mass energies. If non-pion track contamination is taken into account, the value of the strength λ becomes consistent with unity.

We present results on photoproduction of ϱ0 and ω in the reactions γp→π+π−p and γp→π+π−π0p by tagged photons in the energy ranges 20 to 70 GeV and 20 to 45 GeV, respectively. The production of the ϱ0 shows dominantly the characteristics of a diffractive process with respect to the Eγ and t dependence of the cross section and the spin density matrix. The ϱ0 photoproduction yields on average over the photon energy range a total cross section of σ(γp→ϱ0p) = 9.4±0.1 μb with an additional systematic error of ±1 μb, and average slope parameters of the t distribution dσ/dt≈exp(−b|t| + ct2), of b=9.1±0.1 GeV−2 and c = 3.1 ±±0.2 GeV−4. The shape of the ϱ0 peak in the π+π− invariant spectra shows a skewing similar to that observed at lower energies. The photoproduction of ω is also consistent with a diffractive process and has a cross section of σ(γp→ωp) = 1.2± 0.1 μb with an additional systematic error of ±0.2 μb. The average slope parameters of the t distribution are b=8.3 ± 1.3 GeV−2 and c = 3.4±2.6 GeV−4.

A search for the Standard Model Higgs boson has been performed with the OPAL detector at LEP based on the full data sample collected at s≈192–209 GeV in 1999 and 2000, corresponding to an integrated luminosity of approximately 426 pb−1. The data are examined for their consistency with the background-only hypothesis and various Higgs boson mass hypotheses. A lower bound of 109.7 GeV is obtained on the Higgs boson mass at the 95% confidence level. At higher masses, the data are consistent with both the background and the signal-plus-background hypotheses.

The strong coupling constant, αs, has been determined in hadronic decays of theZ 0 resonance, using measurements of seven observables relating to global event shapes, energy correlations and jet rates. The data have been compared with resummed QCD calculations, which are combined with theO( s 2 ) theory. The seven measurements agree to about 10% and the final results, based on a weighted average, is: $$\alpha _s (M_{Z^0 } ) = 0.120 \pm 0.006,$$ where the error includes both experimental and theoretical uncertainties. This value corresponds to renormalization scale $$\mu = M_{Z^0 } $$ and the error includes the uncertainty in this choice of scale. The present measurement complements previous determinations using theO( s 2 ) QCD matrix elements alone, and yields a compatible result, with comparable errors.

A measurement of the single photon production cross-section is presented based on a data-sample of 40.5 pb−1 collected with the OPAL detector at centre-of-mass energies within 3 GeV of the Z0 mass. Single photon events arise from initial state radiation and the production of an "invisible" final state consisting of neutrinos or possibly particles such as sneutrinos or photinos. The single photon topology is also sensitive to new Z0 decays such as $$Z^0 \to \bar vv* \to \bar vv\gamma $$ orZ 0→γX, X→invisible particles. A total of 447 single photon candidates were observed with energy exceeding 1.75 GeV in the polar angle region |cosθ|<0.7. The estimated background from processes with visible reaction products, mainly e+e−→e+e−γ, is 37±6 events. Interpreting the cross-sections as being solely due to Z0 decay to invisible particles and the expected W-contributions, the Z0 invisible with is determined to be 539±26±17 MeV corresponding toN v=3.23±0.16±0.10 light neutrino generations. The differential cross-section with photon energy is presented. Upper limits are set on additional invisible contributions to the Z0 width, on possible non-resconant processes, and on Z0 decays to single photons. The energy spectra are used to constrain exotic sources of high energy single photons. In particular, the radiative twobody decay of the Z0 to a new particle X, with mass below 64 GeV and an invisible signature, has a Z0 branching ratio of less than 4.3×10−6 at 95% confidence level.

The cross section of the pure QED process e+e−→γγ has been measured using data accumulated during the 1989 and 1990 scans of the Z0 resonance at LEP. Both the energy dependence and the angular distribution are in good agreement with the QED prediction. Upper limits on the branching ratios of Z0→γγ, Z0→π0γ and Z0→ηγ have been set at 1.4×10 −4, 1.4×10−4 and 2.0×10−4 respectively. Lower limits on the cutoff parameters of the modified electron propagator have been found to be Λ+ > 117 GeV and Λ− > 110 GeV. The reaction e+e− → γγγ has also been studied and was found to be consistent with the QED prediction. An upper limit on the branching ratio of Z0→γγγ has been set at 6.6 × 10−5. All the limits are given at 95% confidence level.

From a sample of about 450 000 hadronicZ 0 decays, measurements of the average $$B^0 - \bar B^0 $$ mixing parameter, χ, the branching fraction ofZ 0 bosons into hadrons containing bottom quarks, $${{\Gamma (Z^0 \to b\bar b)} \mathord{\left/ {\vphantom {{\Gamma (Z^0 \to b\bar b)} {\Gamma (Z^0 \to hadrons)}}} \right. \kern-\nulldelimiterspace} {\Gamma (Z^0 \to hadrons)}}$$ , the average semileptonic branching ratios for such hadrons,B(b→l) andB(b→c→l), and the mean scaled energy of these hadrons, 〈xE〉, are presented. The measurements were obtained using a simultaneous fit to single-lepton and dilepton events collected with the OPAL detector at LEP, including both electrons and muons. The results are where the errors are statistical and systematic, respectively, in each case. The result forB)b→c→l) excludes decays of the typeb→→ℓ.

A search has been performed for unstable charged scalar particles (S±) such as technipions or charged Higgs bosons, pair-produced in e+e− annihilation at energies near the Z0 pole. No evidence for such particles was observed in the decay modes e+e−→S+S−→(cs)(τv), (τv) (τv)and(cs)(cs). A lower limit (at the 95% CL ) of 35 Gev/c2 is obtained for the mass of the charged scalar particles, independent of the branching ratio.

We report on a search for new quarks in hadronic Z° decays. From the event shape analysis of a data sample containing 2185 multihadronic annihilation events, we observe no evidence for the top or b' quarks. We derive limits for the top and b' quark masses under the assumption of various possible standard model and non-standard model decay schemes. Our search is sensitive to quark masses larger than 23 GeV/c2; it yields the following lower limits at a 95% confidence level: 44.5 GeV/c2 for the top quark mass and 45.2 GeV/c2 for the b′ quark mass.

Measurements of the reaction γp → pπ+π−π+π− are presented, in which π+π−π+π− systems with masses up to 3 GeV are produced from fragmentation of the incident photon. The reaction is dominated by production of the large peak of the ϱ′(1600) meson and, at higher masses ≳2 GeV, y production of jet-like 4π systems. The ϱ′(1600) meson is produced by a predominantly s-channel helicity conserving mechanism. At higher masses there are also indications of ϱπ peaks, of masses 1.3 GeV (the A2 meson) and 1.75 GeV, produced with a recoiling π meson by a mechanism consistent with the Deck effect.

Using muon candidates in 133 000 hadronic decays of the Z0, recorded by the OPAL detector at LEP, we have measured the fraction of events containing semi-leptonic decays of b flavoured hadrons. An analysis based on fitting the shape of the momentum and transverse momentum distributions of the muon candidates gave the result: (Γ(Z0→bb)Γ(Z0→hadrons))×Br(b→μ)=0.0226±0.0007±0.0013, and also yielded: (Γ(Z0→cc)Γ(Z0→hadrons))×Br(c→μ)=0.0176±0.0025±0.0042. Using the charge of the muon and the angle of the event thrust axis with respect to the electron beam, in a b-enriched event sample, we measured the forward-backward asymmetry for Z0 → bb decays. Without correction for B0B0 mixing, the asymmetry was found to be AbFB = 0.072±0.042±0.010.

The production of the octet and decuplet baryons Λ, Ξ−, Σ(1385)±, Ξ(1530)0 and Ω− and the corresponding antibaryons has been measured in a sample of 485 000 hadronic Z0 decays. Results on differential and integrated cross sections are presented. The differential cross section of Λ baryons is found to be softer than the one predicted by the Jetset and Herwig Monte Carlo generators. The measured decuplet yields are found to disagree with the simple diquark picture where only one tuning parameter for spin 1 diquarks is used. Comparisons of the momentum spectra for Λ and Ξ− with the predictions of an analytical QCD formula are also presented.

Data taken with the OPAL detector at LEP during a scan of the Z0 resonance were searched for evidence of neutral heavy leptons that decay via mixing. Four different decay modes of the neutral heavy lepton are considered: L0→eW∗, L0→μW∗, L0→τW∗, and L0→vZ∗. No evidence is seen of a neutral heavy lepton signal; branching fraction limits in the range of 10−3–10−4 are set for Z0→L0L0 and for Z0→vL0 (orvL0) relative to Z0→hadrons.

The i-pipe system is a peculiar structural health monitoring system, based on Fiber Bragg Grating technology, installed on the central beam pipe of the compact Muon solenoid (CMS) experiment at CERN. In this contribution, i-pipe temperature sensors, originally conceived as thermal compensator for the strain sensors, are employed to monitor central beam pipe thermal behavior in correlation with the parameters of the particle beam travelling inside, in order to directly measure possible Beam RF induced heating effect. The i-pipe system turned out to be capable of monitoring, directly and without interference, the parameters of the particle beam circulating in the LHC ring. Hence, the results presented in this work pave the way to the use of the i-pipe as monitoring system of an accelerated high energy particle beam.

This paper describes the trigger system of the OPAL detector at the e+e− collider LEP and its performance during the first year of data taking. A high level of redundancy and fine detector segmentation at the trigger level led to a high efficiency for all considered physics reactions while the trigger rates were kept low.

We observe evidence for the production of b-flavoured baryons in decays of the Z0 boson with the OPAL detector at LEP. We find 68 Λl−, Λl+ candidates in 458 583 hadronic Z0 decays. We interpret this as a signal of 55 ± 9+0.3−3.1 events from the semi-leptonic decays of b baryons. Assuming weakly decaying b baryons produced in Z0 decays are mostly Λb particles, we measure the product branching ratio (Γbb/Γhad) f (b→Λb) B (Λb→Λl−vX), averaged over the electron and muon channels, to be (6.2±1.0±1.5)×10−4.

From the observed yield and properties of isolated energetic photons in the reaction e+e−→Z0→hadrons+γ measured with the OPAL detector at LEP, evidence for final state radiation from primary quarks is obtained. Combined with the measurement of the total hadronic width of the Z0, the observed rate allows the extraction of the electroweak coupling constants of up and down type quarks: ν132 + a132 = 1.24±0.47 and ν232 + a232 = 1.72±0.70 No evidence for additional photon production from anomalous decays of the Z0 or from decays of new particles is found. This measurement limits the contribution to the total Z0 width from such sources to be less than 8.2 MeV at the 95% confidence level.

Quark and gluon in 3-jet events from hadronic Z0 decays are identified through use of the semi-leptonic decays of charm and bottom quarks. This tagging method allows geometrically symmetric or transposed configurations of the quark and gluon jets to be selected, permitting a study of the asymmetry in the population of particles in the regions between jets, commonly called the string effect, which avoids restrictions present in previous studies. In particular, our demonstration of a population asymmetry is performed without use of a model. Our results imply that dynamical differences exist between quarks and gluons or between quark-antiquark and quark-gluon jet systems with respect to their particle production properties.

We report on a measurement of the processes e+e−→e+e−, e+e−→μ+μ−, and e+e−→τ+τ− near the Z0 pole. On the basis of 163 e+e−, 101 μ+μ− and 87 τ+τ− events we obtain Γee=89±4±4 MeV, Γμμ=85±9±6 MeV and Γττ=87±10±8 MeV, compatible with the standard model. Combining these with our previous results on hadronic Z0 decays, we find a hadronic width Γhad=1787±81±90 MeV and an invisible width Γinv=552±85±71 MeV.

A dipion enhancement of mass 1.59 GeV and width 0.23 GeV is observed in the channel γp→π+π−p. The spin-parity of the enhancement is consistent with being 1−.

We measure the mass of the τ to be 1775.1±1.6(mcnstat.)±1.0(mcnsys.) MeV using τ from Z0 decays. To test CPT invariance we compare the masses of the positively and negatively charged τ. The relative mass difference is found to be smaller than 3.0×10−3 at the 90% confidence level.

We present an update with increased statistics to our published analysis of hadronic and leptonic cross sections and of the leptonic forward-backward asymmetries ine + e − collisions. The published results were based on a total 454 000 hadronic and 58 000 leptonic events. This analysis adds 733 000 hadronic and 88 000 leptonic events recorded at theZ 0 peak in 1992 by the OPAL experiment at LEP. A model independent analysis ofZ 0 parameters based on an extension of the improved Born approximation leads to tests of lepton universality and gives an interpretation of the results within the Standard Model framework. We also present a model independent test for new physics.

Searches for the neutral Higgs bosons predicted by the Standard Model (SM) and the Minimal Supersymmetric extension of the Standard Model (MSSM) have been performed with the OPAL detector at LEP. Approximately 170 pb $^{-1}$ of $\mathrm{e}^+ \mathrm{e}^-$ collision data collected at $\sqrt{s} \approx 189$ GeV were used to search for Higgs boson production in the SM process ${\mathrm{e}}^+{\mathrm{e}}^- \rightarrow \mathrm{H}^{0}\mathrm{Z}^{0}$ and the MSSM processes ${\mathrm{e}}^+{\mathrm{e}}^- \rightarrow \mathrm{h}^{0} \mathrm{Z}^{0}$ and ${\mathrm{e}}^+ {\mathrm{e}}^- \rightarrow \mathrm{A}^{0} \mathrm{h}^{0}$ . The searches are sensitive to the $\mathrm{b}\bar{\mathrm{b}}$ and $\tau^+\tau^-$ decay modes of the Higgs bosons, and also to the MSSM decay mode $\mathrm{h}^{0} \rightarrow \mathrm{A}^{0} \mathrm{A}^{0}$ . OPAL search results at lower centre-of-mass energies have been incorporated in the limits, which are valid at the 95% confidence level. For the SM Higgs boson, a lower mass bound of 91.0 GeV is obtained. In the MSSM, the limits are $m_{\mathrm{H}} >74.8$ GeV and $m_{\mathrm{A}} >76.5$ GeV, assuming $\tan\beta > 1$ , that the mixing of the scalar top quarks is either zero or maximal, and that the soft SUSY-breaking masses are 1 TeV. For the case of zero scalar top mixing, the values of $\tan\beta$ between 0.72 and 2.19 are excluded.

This letter describes the first observation of W boson pair production at a centre-of-mass energy s=161GeV in the OPAL detector at LEP. The analysis is sensitive to all expected W+W− decay channels. A total of 28 events have been selected for an integrated luminosity of 9.89±0.06 pb−1. This is consistent with the Standard Model expectation, including signal and background contributions. The W pair production cross-section is measured to be σWW = 3.62−0.82+0.93±0.16 pb. An analysis of the predicted MW dependence of the accepted cross-section, taking into account interference in the four-fermion production processes, yields MW = 80.40−0.41−0.10+0.44+0.09±0.10 GeV, where the first and second uncertainties are statistical and systematic, respectively, and the third arises form the beam energy uncertainty.

From a data sample of 183 pb−1 recorded at a center-of-mass energy of s=189 GeV with the OPAL detector at LEP, 3068 W-pair candidate events are selected. Assuming Standard Model W boson decay branching fractions, the W-pair production cross section is measured to be σWW=16.30±0.34(stat.)±0.18(syst.) pb. When combined with previous OPAL measurements, the W boson branching fraction to hadrons is determined to be 68.32±0.61(stat.)±0.28(syst.)% assuming lepton universality. These results are consistent with Standard Model expectations.

Using data from e+e− collisions collected with the OPAL detector during the 1990 LEP run, a search was mode for a low mass Higgs boson (H0) with arbitrary decay mode. The existence of a minimal standard model H0 with mass in the range 0 ⩽ mH ⩽ 11.3 GeV/c2 is excluded at the 95% confidence level for all possible decay modes of the H0. Limits on the production of Higgs bosons predicted by the minimal supersymmetric standard model and new scalar particles that couple to the Z0 are presented as a function of the scalar mass.

In this Letter of Intent (LOI) we propose the construction of MATHUSLA (MAssive Timing Hodoscope for Ultra-Stable neutraL pArticles), a dedicated large-volume displaced vertex detector for the HL-LHC on the surface above ATLAS or CMS. Such a detector, which can be built using existing technologies with a reasonable budget in time for the HL-LHC upgrade, could search for neutral long-lived particles (LLPs) with up to several orders of magnitude better sensitivity than ATLAS or CMS, while also acting as a cutting-edge cosmic ray telescope at CERN to explore many open questions in cosmic ray and astro-particle physics. We review the physics motivations for MATHUSLA and summarize its LLP reach for several different possible detector geometries, as well as outline the cosmic ray physics program. We present several updated background studies for MATHUSLA, which help inform a first detector-design concept utilizing modular construction with Resistive Plate Chambers (RPCs) as the primary tracking technology. We present first efficiency and reconstruction studies to verify the viability of this design concept, and we explore some aspects of its total cost. We end with a summary of recent progress made on the MATHUSLA test stand, a small-scale demonstrator experiment currently taking data at CERN Point 1, and finish with a short comment on future work.

In this paper we describe the main characteristics and experimental results, recorded in more than four years of data acquisition, of a temperature and strain monitoring system, called i-pipe, based on Fiber Bragg Grating (FBG) sensors arrays applied for the first time to a sector of the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN): the central beam pipe (BP) of the Compact Muon Solenoid (CMS). The monitoring system, consisting of four arrays of 16 FBG sensors each, as better described in the text, is placed on four directives of the LHC section that passes inside the CMS experiment, namely the CMS central BP. The mechanical complexity of the central BP structure is described and the monitoring results of its thermal conditions and online unpredictable mechanical deformations are discussed in this paper. In spite of the harsh working conditions this monitoring system is operational since 2015 continuously (24/7) and the data collected are a confirmation of its reliability. This FBG sensors system represents the ideal solution to realize an accurate and robust sensing system to be used in harsh environments, like the CMS experimental facility and all the other High Energy Physics experimental infrastructures.

This work describes a detailed study of two possible options for the magnetic system of a Future Circular hadron-hadron Collider detector. The conceptual design study of a Future Circular hadron-hadron Collider (FCC-hh) to be constructed at CERN with a center-of-mass energy of the order of 100 TeV requires superconducting magnetic systems with a central magnetic flux density of an order of 4 T for the experimental detectors. The developed concept of the FCC-hh detector involves the use of an iron-free magnetic system consisting of three superconducting solenoids: the main coil of 10.9 m inner diameter and 18.954 m length with a total current of 69.6 MA-turns that give a central magnetic flux density of 4 T, and two superconducting forward coils of 5.6 m inner diameter and 3.3997 m length with a total current of 12.6 MA-turns each that give a central magnetic flux density of 3.2 T in each coil. A superconducting magnet with a minimal steel yoke is proposed as an alternative to the baseline iron-free design. This design includes the same three coils enclosed in the 22,240-ton steel flux-return yoke. In this study, both magnetic system options for the FCC-hh detector are modeled with Cobham's program TOSCA. All the main characteristics of both designs are compared and discussed.

A search for decays of the Bc meson was performed using data collected from 1990–1995 with the OPAL detector on or near the Z0 peak at LEP. The decay channels Bc+→J/ψπ+, Bc+→J/ψa1+ and Bc+→J/ψℓ+ν were investigated, where ℓ denotes an electron or a muon. Two candidates are observed in the mode Bc+→J/ψπ+, with an estimated background of (0.63±0.20) events. The weighted mean of the masses of the two candidates is (6.32±0.06) GeV/c2, which is consistent with the predicted mass of the Bc meson. One candidate event is observed in the mode Bc+→J/ψℓ+ν, with an estimated background of (0.82±0.19) events. No candidate events are observed in the Bc+→J/ψa1+ decay mode, with an estimated background of (1.10±0.22) events. Upper bounds at the 90% confidence level are set on the production rates for these processes.

The measurement of the magnetic field in the tracking volume inside the superconducting coil of the Compact Muon Solenoid (CMS) detector under construction at CERN is done with a fieldmapper designed and produced at Fermilab. The fieldmapper uses 10 3-D B-sensors (Hall probes) developed at NIKHEF and calibrated at CERN to precision 0.05% for a nominal 4 T field. The precise fieldmapper measurements are done in 33840 points inside a cylinder of 1.724 m radius and 7 m long at central fields of 2, 3, 3.5, 3.8, and 4 T. Three components of the magnetic flux density at the CMS coil maximum excitation and the remanent fields on the steel-air interface after discharge of the coil are measured in check-points with 95 3-D B-sensors located near the magnetic flux return yoke elements. Voltages induced in 22 flux-loops made of 405-turn installed on selected segments of the yoke are sampled online during the entire fast discharge (190 s time-constant) of the CMS coil and integrated offline to provide a measurement of the initial magnetic flux density in steel at the maximum field to an accuracy of a few percent. The results of the measurements made at 4 T are reported and compared with a three-dimensional model of the CMS magnet system calculated with TOSCA.

The rate of muons from LHC pp collisions reaching the surface above the ATLAS interaction point is measured as a function of the ATLAS luminosity and compared with expected rates from decays of W and Z bosons and b- and c-quark jets. In addition, data collected during periods without beams circulating in the LHC provide a measurement of the background from cosmic ray inelastic backscattering that is compared to simulation predictions. Data were recorded during 2018 in a 2.5 × 2.5 × 6.5 m3 active volume MATHUSLA test stand detector unit consisting of two scintillator planes, one at the top and one at the bottom, which defined the trigger, and six layers of RPCs between them, grouped into three (x,y)-measuring layers separated by 1.74 m from each other. Triggers selecting both upward-going tracks and downward-going tracks were used.

A new measurement of the τ lifetime is presented. It uses data collected with the Opal detector during 1994, which almost doubles the size of the Opal τ sample. Two statistically independent techniques are used: an impact parameter analysis of one-prong decay tracks and a fit to the decay length distribution of three-prong decays. The lifetime obtained from the 1994 data by combining the results of these methods is ττ = 289.7 ± 2.5 (stat)± 1.5 (sys) fs. When combined with the previous Opal τ lifetime measurement the improved τ lifetime is ττ = 289.2 ± 1.7 (stat.) ± 1.2 (sys.) fs.

Arrays of thin scintillating tiles with embedded wavelength shifting fibre readout have been installed in the OPAL endcaps to improve trigger performance, time resolution and hermeticity for experiments at LEP II. The arrays were required to have high single-particle detection efficiency, uniform response, low noise and good time resolution. Limited space for the detector, and a strong magnetic field in the endcap region, resulted in a need for high light output per unit thickness of scintillator, and remote readout. In addition, because of limited space for readout cables, a high light yield per embedded fibre was required. This paper describes the design and construction of a tile array that satisfies these requirements. A light yield of 14 photoelectrons per minimum ionizing particle and a time resolution of 3 ns were obtained during 1997 LEP operation.

Using the distance from the average primary vertex to reconstructed secondary vertices in jets, samples of events withb purity varying from about 13% to 89% have been selected. The charged particle multiplicity in the hemispheres opposite those containing these jets has been studied as a function of theb purity of the events. Extrapolating to 0% and 100%b purity, values of the hemisphere charged particle multiplicity inZ 0→ $$b\bar b$$ events and in non- $$b\bar b$$ events have been measured to be $$\begin{gathered} \bar n_b = 11.71 \pm 0.03 \pm 0.18 \pm 0.21, \hfill \\ \bar n_{udsc} = 10.32 \pm 0.01 \pm 0.07 \pm 0.19. \hfill \\ \end{gathered}$$ The first error is statistical, the second systematic and the third is a common systematic error. The difference in charged particle multiplicity betweenb quark events and light (u, d, s) quark events has been measured and found to be $$\delta _{bl} = 3.02 \pm 0.05 \pm 0.79.$$ The result is compared to the predictions of MLLA QCD calculations. By studying the impact parameter distributions of charged particles in the hemispheres opposite these jets, the charged particle decay multiplicity ofB hadrons fromZ 0 decay, including particles fromK 0 and Λ decay, has been measured to be $$\bar n^B = 5.51 \pm 0.05 \pm 0.51.$$ From the mean momentum of these decay products and separately from the number of primary charged particles perb event, the averagex E ofb flavoured hadrons has been measured to be $$\left\langle {x_E } \right\rangle _b = 0.693 \pm 0.003 \pm 0.030.$$

We report on an improved measurement of the value of the strong coupling constant σs at the Z0 peak, using the asymmetry of the energy-energy correlation function. The analysis, based on second-order perturbation theory and a data sample of about 145000 multihadronic Z0 decays, yields αs(Mz0 = 0.118±0.001(stat.)±0.003(exp.syst.)−0.004+0.0009 (theor. syst.), where the theoretical systematic error accounts for uncertainties due to hadronization, the choice of the renormalization scale and unknown higher-order terms. We adjust the parameters of a second-order matrix element Monte Carlo followed by string hadronization to best describe the energy correlation and other hadronic Z0 decay data. The αs result obtained from this second-order Monte Carlo is found to be unreliable if values of the renormalization scale smaller than about 0.15 Ecm are used in the generator.

The couplings of the Z0 to charged leptons are studied using measurements of the lepton pair cross sections and forward-backward asymmetries at centre of mass energies near to the mass of the Z0. The data are consistent with lepton universality. Using a parametrisation of the lepton pair differential cross section which assumes that the Z0 has only vector and axial couplings to leptons, the charged leptonic partial decay width of the Z0 is determined to be ГГol+ol−= 83.1±1.9MeV and the square of the product of the effective axial vector and vector coupling constants of the Z0 to charged leptons to be ǎ2olv̌2ol= 0.0039± 0.0083, in agreement with the standard model. A parametrisation in the form of the improved Born approximation gives effective leptonic axial vector and vector coupling constants ǎ2ol= 0.998±0.024 and v̌2ol= 0.0044±0.0083. In the framework of the standard model, the values of the parameters ϱz and sin2θw are found to be 0.998±0.024 and 0.233+0.045−0.012 respectively. Using the relationship in the minimal standard model between ϱz and sin2θw, the results sin2θSMw = 0.233+0.007−0.006 is obtained. Our previously published measurement of the ratio of the hadronic to the leptonic partial width of the Z0 is update: Rz = 21.72+0.71−0.65.

Excited leptons have been searched for using data recorded by the OPAL detector at LEP. No evidence for such particles has been found. From the study of e+e− → ℓ+ℓ−γγ events, lower limits on the masses of spin-12 excited leptons are found to be 44.9 GeV at 95% confidence level. From the study of e+e− →ℓ+ℓ−γ events, upper limits on their couplings are set up to ℓ∗ masses close to the mass of the Z0 boson.

An analysis is presented of the channel γp→π+π−π0π0p. Evidence is found for an ωπ0 enhancement of mass ≈1.25 GeV and width ≈0.3 GeV. The enhancement is a mixed spin-parity state with 1− dominant.

The production and decay of a D0 have been observed in a nuclear emulsion exposed to a photon beam from the CERN super proton synchroton in conjunction with the Omega spectrometer. All the decay products have been detected and measured in the emulsion and in the spectrometer. The mass of the charmed particle is 1866 ± 8 MeV/c2 and its decay time is (2.26 ± 0.05) × 10−14 s.

A study of W+W− events accompanied by hard photon radiation produced in e+e− collisions at LEP is presented. Events consistent with two on-shell W-bosons and an isolated photon are selected from 183 pb−1 of data recorded at s=189 GeV. From these data, 17 W+W−γ candidates are selected with photon energy greater than 10 GeV, consistent with the Standard Model expectation. These events are used to measure the e+e−→W+W−γ cross-section within a set of geometric and kinematic cuts, σ̂WWγ=136±37±8 fb, where the first error is statistical and the second systematic. The photon energy spectrum is used to set the first direct, albeit weak, limits on possible anomalous contributions to the W+W−γγ and W+W−γZ0 vertices:−0.070GeV−2<a0/Λ2<0.070GeV−2,−0.13GeV−2<ac/Λ2<0.19GeV−2,−0.61GeV−2<an/Λ2<0.57GeV−2,where Λ represents the energy scale for new physics.

Deep inelastic electron-photon scattering is studied using e+e− data collected by the OPAL detector at centre-of-mass energies s=MZ0. The photon structure function F2γ(x,Q2) is explored in a Q2 range of 1.1 to 6.6 GeV2 at lower x values than ever before. To probe this kinematic region events are selected with a beam electron scattered into one of the OPAL luminosity calorimeters at scattering angles between 27 and 55 mrad. A measurement is presented of the photon structure function F2γ(x,Q2) at 〈Q2〉 = 1.86 GeV2 and 3.76 GeV2 in five logarithmic x bins from 0.0025 to 0.2.

Bose-Einstein correlations between like charged track pairs have been studied using a sample of approximately 3.6 million multihadronic Z0 decays collected with the OPAL detector at LEP. The radius of the emitting sourceR and the chaoticity parameterλ were studied using two parametrisations, the Goldhaber (G) parametrisation and the one-dimensional Kopylov-Podgoretskii (KP) parametrisation. The radiiR G andR KP are found to increase linearly with the average observed charged multiplicityn ch , with changes with respect to a unit increase inn ch of $$\begin{gathered} \frac{1}{{\langle R_G \rangle }}\frac{{\Delta R_G }}{{\Delta n_{ch} }} = (3.6 \pm 0.6) \cdot 10^{ - 3} and \hfill \\ \frac{1}{{\langle R_{KP} \rangle }}\frac{{\Delta R_{KP} }}{{\Delta n_{ch} }} = (3.4 \pm 1.0) \cdot 10^{ - 3} . \hfill \\ \end{gathered} $$ where the 〈R〉 are the radius values measured in the inclusive event sample. The chaoticity parametersλ G andλ KP decrease with increasing charged multiplicity. It is shown that the increase ofR with multiplicity may be connected with differences between two- and three-jet events.

The production of neutral kaons in e+e− annihilation at centre-of-mass energies in the region of the Z0 mass and their Bose-Einstein correlations are investigated with the OPAL detector at LEP. A total of about 1.26×106 Z0 hadronic decay events are used in the analysis. The production rate of K0 mesons is found to be 1.99±0.01±0.04 per hadronic event, where the first error is statistical and the second systematic. Both the rate and the differential cross section for K0 production are compared to the predictions of Monte Carlo generators. This comparison indicates that the fragmentation is too soft in bothJetset andHerwig. Bose-Einstein correlations in K s 0 K s 0 pairs are measured through the quantityQ, the four momentum difference of the pair. A threshold enhancement is observed in K s 0 K s 0 pairs originating from a mixed sample of $$K^0 \bar K^0$$ and K0K0 ( $$\bar K^0 \bar K^0$$ ) pairs. For the strength of the effect and for the radius of the emitting source we find values of λ=1.14±0.23±0.32 andR 0=(0.76±0.10±0.11) fm respectively. The first error is statistical and the second systematic.

The lifetime of the Λb0 baryon has been measured using 3.6 million hadronic Z0 decays recorded by the OPAL detector at LEP from 1990 to 1994. A sample of Λb0 decays is obtained using partially reconstructed semileptonic decays involving Λc+ℓ− combinations, where the Λc+ is reconstructed from its decay to a pK−π+ final state. From the 69 ± 13Λc+ℓ− combinations attributed to Λb0 decays in this data sample, we measure τ(Λb0) = 1.14−0.19+0.22 ± 0.07 ps, where the errors are statistical and systematic, respectively.

A search for charged and neutral excited leptons is performed in 217 pb^-1 of e+e- collision data collected with the L3 detector at LEP at centre-of-mass energies from 202 GeV up to 209 GeV. The pair- and single-production mechanisms of excited electrons, muons and taus, as well as of excited electron-, muon- and tau-neutrinos,are investigated and no signals are detected. Combining with L3 results from searches at lower centre-of-mass energies, gives improved limits on the masses and couplings of excited leptons.

The reaction γp→K+K−p has been investigated with photons in the energy range of 20<Eγ<36 GeV and with K+K− pairs in the mass range of MK+K−<2.0 GeV. The production of the φ(1019) contributes with a cross section σ(γp → φp) × BR(φ→K+K−) = 240±6 nb with an additional systematic error of ±20 nb. In the higher mass range of 1.05<MK+K−<2.0 GeV the production of K+K− pairs yields a cross section σ(γp→K+K−p) = 160±8 nb with an additional systematic error of +40−30 nb.

Exclusive and inclusive pp photoproduction (with ⩾ 2 accompanying charged particles) have been measured in the photon energy range 44–70 GeV. The exclusive process shows no prominent features; the inclusive process shows evidence for S(1936) production as a resonant-like structure with mass 1930 ± 2 MeV and width 12 ± 7 MeV.

A search for narrow resonances in e+e− annihilation at c.m. energies between 29.90 and 31.46 GeV provides no evidence for the existence of such states. The 90% confidence upper limit on the integrated resonance cross section is 38 nb MeV, significantly below the value expected for the lowest (t,t̄) bound state.

We report on recent progress in the design of the proposed MATHUSLA Long Lived Particle (LLP) detector for the HL-LHC, updating the information in the original Letter of Intent (LoI), see CDS:LHCC-I-031, arXiv:1811.00927. A suitable site has been identified at LHC Point 5 that is closer to the CMS Interaction Point (IP) than assumed in the LoI. The decay volume has been increased from 20 m to 25 m in height. Engineering studies have been made in order to locate much of the decay volume below ground, bringing the detector even closer to the IP. With these changes, a 100 m x 100 m detector has the same physics reach for large c$\tau$ as the 200 m x 200 m detector described in the LoI and other studies. The performance for small c$\tau$ is improved because of the proximity to the IP. Detector technology has also evolved while retaining the strip-like sensor geometry in Resistive Plate Chambers (RPC) described in the LoI. The present design uses extruded scintillator bars read out using wavelength shifting fibers and silicon photomultipliers (SiPM). Operations will be simpler and more robust with much lower operating voltages and without the use of greenhouse gases. Manufacturing is straightforward and should result in cost savings. Understanding of backgrounds has also significantly advanced, thanks to new simulation studies and measurements taken at the MATHUSLA test stand operating above ATLAS in 2018. We discuss next steps for the MATHUSLA collaboration, and identify areas where new members can make particularly important contributions.

The fraction of $$Z^0 \to b\bar b$$ ; events in hadronicZ 0 decays has been measured using the data collected by OPAL in 1992 and 1993. The presence of electrons or muons from semileptonic decays of bottom hadrons and the detection of bottom hadron decay vertices were used together to obtain an event sample enriched in $$Z^0 \to b\bar b$$ decays. To reduce the systematic error on the measurement of the $$Z^0 \to b\bar b$$ fraction, the efficiency of the $$b\bar b$$ event tagging was obtained from the data by comparing the numbers of events having a bottom signature in either one or both thrust hemispheres. A value of $$\frac{{\Gamma (Z^0 \to b\bar b)}}{{\Gamma (Z^0 \to hadrons)}} = 0.2171 \pm 0.0021 \pm 0.0021$$ was obtained, where the first error is statistical and the second systematic. The uncertainty on the decay width $$\Gamma (Z^0 \to c\bar c)$$ is not included in these errors. A fractional variation of this width by ±8% about its Standard Model prediction would result in a variation of the measured $$Z^0 \to b\bar b$$ fraction of ±0.0015.

The spin composition of ΛΛ, ΛΛ and ΛΛ pairs at low invariant mass values has been measured for the first time in multihadronic Z0 decays with the OPAL detector at LEP. No single spin state has been observed in the ΛΛ sample, verifying that the low mass enhancement in this sample, attributed to local baryon number compensation, is not a resonance state. The fraction of the spin 1 contribution to the ΛΛ pairs was found to be consistent with the value 0.75, as expected from a statistical spin mixture. This may be the net effect of many different QCD processes which contribute to the hyperon anti-hyperon pair production. The spin composition of the identical ΛΛ and ΛΛ pairs, well above threshold, is found to be similar to that of the ΛΛ sample. A ΛΛ emitter dimension is estimated from the data assuming the onset of the Pauli exclusion principle near threshold.

A search for the Minimal Standard Model Higgs boson (H0) has been performed with data from e+e− collisions collected by the OPAL detector at LEP. The search was made for events of the types e+e−→(e+e−,μ+μ− or νν)H0, H0→qq̄ and was based on approximately 78 pb−1 of data taken at center-of-mass energies between 88 and 95 GeV in the years 1990–1993. The present study, combined with previous OPAL publications, excludes the existence of a Minimal Standard Model Higgs boson with mass below 56.9 GeV at the 95% confidence level.

Synchrotron radiation photons and off-energy electrons which contribute to backgrounds in the LEP experiments have been simulated. A multi-faceted background protection system has been designed and installed which permits the experiments to take data under very clean conditions. The backgrounds measured at LEP1 are in good agreement with calculations. Simulations and studies have led to a design of enhanced background protection for LEP2 where far more severe photon background is expected. Detector sensitivity to backgrounds and monitor systems are described.

A search for stable and long-lived massive particles of electric charge |Q/e|=1 or 2/3, pair-produced in e+e− collisions at centre-of-mass energies from 130 to 183 GeV, is reported by the OPAL collaboration at LEP. No evidence for production of these particles was observed in a mass range between 45 and 89.5 GeV. Model-independent upper limits on the production cross-section between 0.05 and 0.19 pb have been derived for scalar and spin-1/2 particles with charge ±1. Within the framework of the minimal supersymmetric model (MSSM), this implies a lower limit of 82.5 (83.5) GeV on the mass of long-lived right- (left-)handed scalar muons and scalar taus. Long-lived charged leptons and charginos are excluded for masses below 89.5 GeV. For particles with charge ±2/3 the upper limits on the production cross-section vary between 0.05 and 0.2 pb. All limits, on masses and on cross-sections, are valid at the 95% confidence level for particles with lifetimes longer than 10−6 s.

We report on the response of a prototype CMS hadron calorimeter module to charged particle beams of pions, muons, and electrons with momenta up to 375GeV/c. The data were taken at the H2 and H4 beamlines at CERN in 1995 and 1996. The prototype sampling calorimeter used copper absorber plates and scintillator tiles with wavelength shifting fibers for readout. The effects of a magnetic field of up to 3 T on the response of the calorimeter to muons, electrons, and pions are presented, and the effects of an upstream lead tungstate crystal electromagnetic calorimeter on the linearity and energy resolution of the combined calorimetric system to hadrons are evaluated. The results are compared with Monte Carlo simulations and are used to optimize the choice of total absorber depth, sampling frequency, and longitudinal readout segmentation.

Production of events with multihadronic and leptonic final states has been measured in e+e− collisions at centre-of-mass energies significantly above the Z0 mass, using the OPAL detector at LEP. A substantial production rate of radiative Z0γ events was observed, as expected, together with events with less energetic initial-state photons. The cross-sections and leptonic forward-backward asymmetries were measured and compared with Standard Model expectations. In a model-independent fit to the Z0 lineshape, the hadronic cross-section and lepton asymmetries presented here provide constraints on the size of the γZ0-interference term which are complementary to those afforded by LEP data accumulated at the Z0 resonance.

Using about 3.9 million hadronic Z decays from e+e− collisions recorded by the OPAL detector at LEP at centre-of-mass energies s≈MZ, the branching ratio for the decay D−s→τ−ν̄τ has been measured to be BR(D−s→τ−ν̄τ)=(7.0±2.1(stat)±2.0(syst))%. This result can be used to derive the decay constant of the D−s meson: fDs=(286±44(stat)±41(syst))MeV.

We report on a measurement of two-particle momentum correlations in hadronic decays of the Z0 at LEP. These data are compared with recent analytic QCD calculations based on the summation of leading and next-to-leading logarithms, and with QCD Monte Carlo simulations. We find that the analytic calculations show the same general features as the data, but that the overall level of the correlations is not reproduced, suggesting that higher order or hadronization effects are significant. This contrasts with the success of similar QCD calculations in describing single-particle momentum distributions. QCD Monte Carlo models are found to give a reasonable level of correlation, with parton shower models incorporating string hadronization giving the best description of the data.

In this LOI we propose a dedicated experiment that would detect milli-charged particles produced by pp collisions at LHC Point 5. The experiment would be installed during LS2 in the vestigial drainage gallery above UXC and would not interfere with CMS operations. With 300 fb$^{-1}$ of integrated luminosity, sensitivity to a particle with charge $\mathcal{O}(10^{-3})~e$ can be achieved for masses of $\mathcal{O}(1)$ GeV, and charge $\mathcal{O}(10^{-2})~e$ for masses of $\mathcal{O}(10)$ GeV, greatly extending the parameter space explored for particles with small charge and masses above 100 MeV.

The production of J/ψ mesons in multihadronic Z0 decays has been observed in the e+e− and β+β− final states. From a sample of approximately 45 reconstructed Jψ mesons, the inclusive branching fraction is measured to be Br(Z0→J/ψ + X) = (4.5±0.8±0.4±0.6) × 10−3, where the first error is statistical, the second systematic, and the third error is due to the uncertainty in the leptonic decay rate of the J/ψ. The J/ψ energy distribution is consistent with the distribution expected from B hadron decays. The average B hadron lifetime is calculated from the measured distances between the primary and J/ψ vertices, resulting in τB = 1.32−0.25+0.31±0.15 ps, where the first error is statistical and the second systematic.

The pure QED reaction e+e−→γγ has been studied at centre of mass energies around the mass of the Z0 boson using data recorded by the OPAL detector at LEP. The results are in good agreement with the QED prediction. Lower limits on the cutoff parameters of the modified electron propagator are found to be Λ+>89 GeV and Λ. The lower limit on the mass of an excited electron is 82 GeV assuming the coupling constant λ=1. Upper limits on the branching ratios of Z0→γγ, Z0→π0γ and Z0→ηγ are set at 3.7×10−4, 3.9×10−4 and 5.8×10−4 respectively. Two events from the reaction e+e−→γγγ have been observed, consistent with the QED prediction. An upper limit on the branching ratio of Z0→γγγ is set at 2.8×10−4. All the limits are given at 95% confidence level.

Measurements of ω and η production in the reaction γp→π+π−π+π−π0p are presented, showihg that the ωπ+π− channel is made up of an enhancement at ∼1.7 GeV of width ∼0.5 GeV, which is consistent with being a 1− state, together with a distribution at higher masses containing higher angular momentum terms. The ηπ+π− final state is produced with a similar cross section and also shows a low-mass enhancement.

This letter presents an updated measurement of the lifetime of the Bs0 meson using 3.6 million hadronic Z0 decays recorded by the OPAL detector at LEP from 1990 to 1994. A sample of Bs0 decays is obtained using Ds−ℓ+ combinations, where the Ds− is reconstructed in either the φπ− or K∗0K− decay mode. From 79 ± 13 Dxℓ combinations attributed to Bs0 decays in this data sample, we measure τ(Bs0) = 1.54−0.21+0.25 ± 0.06 ps, where the errors are statistical and systematic, respectively.

Bose-Einstein correlations in K0SK0S pairs have been studied for the first time in e+e− annihilations. These correlations were measured through the quantity Q, the four momentum difference of the pair. Out of about half a million Z0 hadronic decay events a total of about 6000 events with two or more identified K0S mesons were used for the analysis We observe a threshold enhancement in K0SK0S pairs originating from a mixed sample of K0K0 and K0K0 (K0K0) pairs. Although we can not presently exclude that a part of this effect is due to scalar meson decays, our analysis attributes the enhancement to Bose-Einstein correlations. We find the values for the strength of the effect and the radius of the K0S emitting source to be λ = 1.12 ± 0.33 ± 0.29 and R0 = (0.72 ± 0.17 ± 0.19) fm, respectively, where the first error is statistical and the second systematic.

Charginos and neutralinos, predicted by supersymmetric theories, have been searched for in e+e− collisions with an intergrated luminosity of 10.0 pb−1 at a centre-of-mass energy of s= 161GeV with the OPAL detector at LEP. Two candidate events are selected; consistent with the total background estimate of 0.7 ± 0.2 events. The 95% C.L. lower limit on the lightest chargino mass in the Minimal Supersymmetric Standard Model is 78.5 GeV if the universal scalar mass m0 is greater than 1 TeV, and 62.0 GeV for the smallest m0 compatible with slepton and sneutrino mass limits obtained at centre-of-mass energies near the Z peak. These limits were obtained under the conditions that the lightest chargino is heavier than the lightest neutralino by more than 10 GeV and tan β = 1.5. The new exclusion limits significantly improve on the results obtained at s= 130 and 136 GeV.

This letter describes a measurement of one of the anomalous triple gauge boson couplings using the first data recorded by the OPAL detector at LEP2. A total of 28 W-pair candidates have been selected for an integrated luminosity of 9.89±0.06 pb−1 recorded at a centre-of-mass energy of 161 GeV. We use these data to place constraints upon the coupling parameter αWφ. We analyse the predicted variation of the total cross-section for all observed channels and the distribution of kinematic variables in the semileptonic decay channels. We measure αWφ to be −0.61−0.610.73±0.35, which is consistent with the Standard Model expectation of zero.

The OPAL detector at LEP is used to measure the branching ratio of theZ 0 into invisible particles by measuring the cross section of single photon events ine + e − collisions at centre-of-mass energies near theZ 0 resonance. In a data sample of 5.3 pb−1, we observe 73 events with single photons depositing more than 1.5 GeV in the electromagnetic calorimeter, with an expected background of 8±2 events not associated with invisibleZ 0 decay. With this data we determine theZ 0 invisible width to be 0.50±0.07±0.03 GeV, where the first error is statistical and the second systematic. This corresponds to 3.0±0.4±0.2 light neutrino generations in the Standard Model.

Evidence is presented for inclusive photoproduction of F± mesons in three decay modes, ηπ±, ηπ±π+π− and ηπ±π+π+π−π−. The average mass of the F± is found to be 2.020±0.010 GeV.

Updated measurements of the B0 and B+ meson lifetimes are presented. From a data sample of 1.72 million hadronic Z0 decays recorded during the period 1991 to 1993, a sample of approximately 1000 semileptonic B meson decays containing a D0, D+ or D*+ has been isolated. From the distribution of decay times in the different samples the lifetimes of the B0 and B+ mesons are determined to be 1.53±0.12±0.08 ps and 1.52±0.14±0.09 ps, respectively, where the first error is statistical and the second systematic. The ratio of the B+ to B0 lifetimes is measured to be 0.99±0.14 −0.04 +0.05 , confirming expectations that the lifetimes are similar.

The branching ratio for the decay τ−→e−ν̄eντ has been measured using Z0 decay data collected by the OPAL experiment at LEP. In total 33073 τ−→e−ν̄eντ candidates were identified from a sample of 186197 selected τ decays, giving a branching ratio of B(τ−→e−ν̄eντ)=(17.81±0.09(stat)±0.06(syst))%. This result is combined with other measurements to test e - μ and μ - τ universality in charged-current weak interactions. Additionally, the strong coupling constant αs(mτ2) has been extracted from B(τ−→e−ν̄eντ) and evolved to the Z0 mass scale, giving αs(mZ2)=0.1204±0.0011 (exp)±0.0019 (theory).

Data collected with the OPAL detector at LEP during 1990–1993 are used to measure the time dependence of Bd0↔Bd0 mixing. From a sample of 153 ± 12 events with a charged D∗ and a lepton in the opposite hemisphere, we measure the Bd0↔Bd0 oscillation frequency to be Δmd = 0.57±0.11(stat.)±0.02(syst.) ps−1. This corresponds to an oscillation parameter of xd = ΔmdτBd0 = 0.82±0.16(stat.) ± 0.03(syst.) ± 0.09(syst. τB0d), where the second systematic uncertainty is due to the error in the measured Bd0 lifetime.

The time dependence of B meson oscillations is studied using hadronic Z0 decays with identified leptons in both thrust hemispheres. Decay times are reconstructed for each of the semileptonic B decays by forming vertices which include the lepton and by estimating the B meson momentum. The mass difference of the two mass eigenstates in the B d 0 system, Δm d, is measured to be $$0.462_{ - 0.053 - 0.035}^{ + 0.040 + 0.052} ps^{ - 1}$$ , where the first error is statistical and the second systematic. For the B s 0 system, a lower limit of Δm s >2.2 ps−1 at 95% C.L. is derived.

The lifetime and oscillation frequency of the B0 meson has been measured using B̄0→D∗+ℓ−ν̄ decays recorded on the Z0 peak with the OPAL detector at LEP. The D∗+→D0π+ decays were reconstructed using an inclusive technique and the production flavour of the B0 mesons was determined using a combination of tags from the rest of the event. The results τB0=1.541±0.028±0.023ps,Δmd=0.497±0.024±0.025ps−1 were obtained, where in each case the first error is statistical and the second is systematic.

A search for the decay B±→K±K±π∓ was performed using data collected by the OPAL detector at LEP. These decays are strongly suppressed in the Standard Model but could occur with a higher branching ratio in supersymmetric models, especially in those with R-parity violating couplings. No evidence for a signal was observed and a 90% confidence level upper limit of 1.29×10−4 was set for the branching ratio.

The rates are measured per hadronic $\mathrm{Z}^0$ decay for gluon splitting to $\mathrm{b\overline{b}}$ quark pairs, $g_{\mathrm{b\overline{b}}}$ , and of events containing two $\mathrm{b\overline{b}}$ quark pairs, $g_{\mathrm{4b}}$ , using a sample of four-jet events selected from data collected with the OPAL detector. Events with an enhanced signal of gluon splitting to $\mathrm{b\overline{b}}$ quarks are selected if two of the jets are close in phase-space and contain detached secondary vertices. For the event sample containing two $\mathrm{b\overline{b}}$ quark pairs, three of the four jets are required to have a significantly detached secondary vertex. Information from the event topology is combined in a likelihood fit to extract the values of $g_{\mathrm{b\overline{b}}}$ and $g_{\mathrm{4b}}$ , namely \begin{eqnarray*} g_{\mathrm{b\overline{b}}} & = & (3.07 \pm 0.53 \mathrm{(stat)} \pm 0.97\mathrm{(syst)})\times 10^{-3},\\ g_{\mathrm{4b}} & = & (0.36\pm 0.17 \mathrm{(stat)} \pm 0.27\mathrm{(syst)})\times 10^{-3}. \end{eqnarray*}

Results are presented on the inclusive photoproduction of λ and λ for incident photon energies between 25 and 70 GeV. The slope parameter of the pT2 distribution is found to be 2.83±0.1 GeV−2 for λ and 3.28±0.25 GeV−2 for λ. The xF distributions, measured in the range −0.2 to 0.7, show that while λ are produced centrally, λ production extends to more negative values of xF; the shapes show no energy dependence and are similar to those in pion-induced reactions. The polarization of the produced λ is less than 10%. The results are discussed in terms of vector dominance and quark fusion models.