Showing papers by "Peter Fischer published in 1995"

Enhanced production of low-mass electron pairs in 200 GeV/nucleon S-Au collisions at the CERN super proton synchrotron.

Abstract: We report on measurements of low-mass electron pairs in 450 GeV $p$-Be, $p$-Au, and 200 GeV/nucleon S-Au collisions at central rapidities. For the proton induced interactions, the low-mass spectra are, within the systematic errors, satisfactorily explained by electron pairs from hadron decays, whereas in the S-Au system an enhancement over the hadronic contributions by a factor of $5.0\ifmmode\pm\else\textpm\fi{}0.7(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}2.0(\mathrm{syst})$ in the invariant mass range $0.2lml1.5\phantom{\rule{0ex}{0ex}}\mathrm{GeV}{/c}^{2}$ is observed. The properties of the excess suggest that it arises from two-pion annihilation $\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$.

Observations of π-B charge-flavor correlations and resonant Bπ and BK production

Abstract: Evidence is presented for kinematic and charge correlations of B mesons with charged pions and kaons. Using a new technique, a sample of over 80 000 partially reconstructed B mesons is obtained in 3.5·106 hadronic Z0 decays recorded using the OPAL detector at LEP. The invariant mass distributions of B+π− and B+K− combinations show enhancements consistent with the decays of P-wave resonances of a b antiquark and a light quark. We observe an excess of 1738±195 B+π− pairs with invariant masses in the range 5.60-5.85 GeV and an excess of 149±31 B+K− pairs with invariant masses in the range 5.80–6.00 GeV. Labeling the observed enhancements generically as B** we find $$\begin{gathered} \frac{{BR(Z^0 \to \bar b \to B^{**0} \to B^{(*) + } \pi ^ - )}}{{BR(Z^0 \to \bar b \to B^ + )}} = 0.18 \pm 0.04, \hfill \\ \frac{{BR(Z^0 \to \bar b \to B_s^{**0} \to B^{(*) + } K^ - )}}{{BR(Z^0 \to \bar b \to B^ + )}} = 0.026 \pm 0.008, \hfill \\ \end{gathered} $$ where B(*)+ indicates the sum of B+ and B*+ and the errors include statistical and systematic contributions. From a study of π-B charge-flavor correlations we conclude that the production flavor of a B meson can be tagged with the charge of a pion in an appropriately chosen kinematic region, and that the performance of this flavor tag compares favorably in the\(Z^0 \to q\bar q\) environment with lepton-based tags.

A measurement of the production of D*± mesons on the Z0 resonance

Abstract: We have studied the production of D*± mesons in a sample of 1.25 million multihadronic decays of the Z0, in which 1969 candidates have been identified. We have determined the total multiplicity of charged D* mesons in multihadronic Z0 decays to be

Measurement of single photon production in e+e− collisions near the Z0 resonance

Abstract: 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.

Search for heavy charged particles and for particles with anomalous charge in e+e− collisions at LEP

Abstract: Using the OPAL data accumulated in 1991–1993 amounting to 74 pb−1 of integrated luminosity, corresponding to 1.64×106 selected multi-hadronic events, a search has been performed for charged particles with unusual mass or unusual charge. The mass was determined from a combination of momentum and ionization energy loss measurements. No isolation criteria were applied to the tracks examined, so that both isolated particles and particles produced in jets were valid candidates. For particles with chargeQ/e=−1, one candidate with a mass of approximately 4.2 GeV/c2 was found, which is compatible with the background rate expected according to a Monte Carlo simulation. The implications of this search for the mass limits of a conjectured stable or quasi-stable charged gluino composite $$(\tilde gq\bar q')^ \pm $$ are discussed. Limits are also presented for the production of fractionally-charged particles withQ/e=±2/3 and ±4/3 as well as for particles withQ/e=±2.

The production of neutral kaons in Z0 decays and their Bose-Einstein correlations

Abstract: 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 Ks0Ks0 pairs are measured through the quantityQ, the four momentum difference of the pair. A threshold enhancement is observed in Ks0Ks0 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 andR0=(0.76±0.10±0.11) fm respectively. The first error is statistical and the second systematic.

Low-mass lepton-pair production in p-Be collisions at 450 GeV/c

Abstract: We report on the production of low-mass electron pairs and muon pairs in p-Be collisions at 450 GeV/c at the CERN SPS. For both electron and muon pairs the low-mass spectrum can be explained satisfactorily by lepton pairs from hadronic decays, and there is no need to invoke any “unconventional” source. The normalisation of the major hadronic sources is set by the data. The upper limit, at 90% confidence level, on any new source of lepton pairs is ∼20% of the hadronic decay contribution for muons, and ∼40% for electrons.

Inclusive strange vector and tensor meson production in hadronic Z0 decays

Abstract: Measurements have been made in the OPAL experiment at LEP of the inclusive production of strange vector φ(1020) and K*(892)0 mesons, and the tensor meson K2*(1430)0. The overall production rates per hadronic Z0 decay have been determined to be 0.100±0.004stat.±0.007syst. φ(1020) mesons, 0.74±0.03stat.±0.03syst. K*(892)0 mesons and (forxE<0.3) 0.19±0.04stat.±0.06syst. K2*(1430)0 mesons. The measurements for the vector states update previously published results based on lower statistics, while the K2*(1430)0 rate represents the first direct measurement of a strange tensor state in Z0 decay. For the vector states, both the overall production rates and normalised differential cross sections, with respect to the scaled energy variablexE, have been compared to JETSET and HERWIG predictions. The peak positions in the ζ=ln(1/xp) distributions have been measured and compared to measurements of other hadron states.

Measurement of the tau lepton polarization and its forward - backward asymmetry from Z0 decays

Abstract: We report on a measurement of the tau lepton polarization and its forward-backward asymmetry at the Z0 resonance using the OPAL detector. This measurement is based on analyses of the $$\tau \to e\bar v_e v_\tau $$ , $$\tau \to \mu \bar v_\mu v_\tau $$ , τ→π(K)v τ and τ→ρ v τ from a sample of 30663 e+e+→τ+τ- events collected during the years 1990 to 1992. Assuming that the tau lepton decays according to V-A theory, we measure the average τ polarization to be

=(−14.9±1.9±1.3)% and the τ polarization forward-backward asymmetry to be A =(−8.9±2.2±0.9)%, where the first error is statistical and the second systematic. These results are consistent with the hypothesis of lepton universality. When combined under the assumption of universality, our results can be interpreted as a measurement of sin2 θ eff lept =0.2321±0.0023 within the context of the Standard Model.

Measurement of {Mathematical expression}using a double tagging method

Abstract: The fraction of\(Z^0 \to b\bar b\); events in hadronicZ0 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.

A measurement of the QCD colour factor ratios CA/CF and TF/CF from angular correlations in four-jet events

Abstract: From 1 105 045 hadronic Z0 decays observed with the OPAL detector at the LEP e+e− collider, 21 732 four-jet events are selected. A simultaneous fit of three selected angular variables from these events by the second order QCD matrix element calculation yieldsCA/CF=2.11±0.16(stat.)±0.28(syst.)TF/CF=0.40±0.11(stat.)±0.14(syst.) for the ratios of colour factors, in agreement with SU(3) expectations ofCA/CF=9/4 andTF/CF=3/8.

Improved measurements of the B0 and B+ meson lifetimes

Abstract: 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.

A study of charm meson production in semileptonic B decays

Abstract: The results of a study of charm meson production in semileptonic B hadron decays are presented. Based on a sample of 1.72 million hadronic Z0 decays the following product branching ratios (averaged over electrons and muons) are obtained: $$\begin{gathered} B(b \to D^ + \ell X) \cdot B(D^ + \to K^ - \pi ^ + \pi ^ + ) \hfill \\ = (1.82 \pm 0.20 \pm 0.12) \times 10^{ - 3} , \hfill \\ B(b \to D^0 \ell X) \cdot B(D^0 \to K^ - \pi ^ + ) \hfill \\ = (2.52 \pm 0.14 \pm 0.17) \times 10^{ - 3} \hfill \\ B(b \to D^* \ell X) \cdot B(D^{* + } \to D^0 \pi ^ + ) \cdot B(D^0 \to K^ - \pi ^ + ) \hfill \\ = (7.53 \pm 0.47 \pm 0.56) \times 10^{ - 4} \hfill \\ \end{gathered} $$ where the first error is statistical and the second systematic.

Measurement of the hadronic decay current in τ−→π− − + v τ

Abstract: The decay τ−→π−−+vτ has been studied using data collected with the OPAL detector at LEP during 1992 and 1993. The hadronic structure functions for this decay are measured model independently assuming G-parity invariance and neglecting scalar currents. Simultaneously the parity violating asymmetry parameter is determined to be\(\gamma VA = 1.08 _{ - 0.41 - 0.25}^{ + 0.46 + 0.14} \), consistent with the Standard Model prediction of γVA=1 for left-handed tau neutrinos. Models of Kuhn and Santamaria and of Isgur et al. are used to fit distributions of the invariant 3π mass as well as 2π mass projections of the Dalitz plot. The model dependent mass and width of thea1 resonance are measured to be\(m_{a_1 } = 1.266 \pm 0.014_{ - 0.002}^{ + 0.012} \) GeV and\(\Gamma _{a_1 } = 0.610 \pm 0.049_{ - 0.019}^{ + 0.053} \) GeV for the Kuhn and Santamaria model and\(m_{a_1 } = 1.202 \pm 0.009_{ - 0.001}^{ + 0.009} \) GeV and\(\Gamma _{a_1 } = 0.422 \pm 0.023_{ - 0.004}^{ + 0.033} \) GeV for the Isgur et al. model. The model dependent values obtained for the parity violating asymmetry parameter are γVA=0.87±0.27−0.06+0.05 for the Kuhn and Santamaria model and γVA=1.10±0.31−0.14+0.13 for the Isgur et al. model. Within the Isgur et al. model the ratio of theS-andD-wave amplitudes is measured to beD/S=−0.09±0.03±0.01.

A study of B meson oscillations using dilepton events

Abstract: 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 Bd0 system, Δmd, 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 Bs0 system, a lower limit of Δms>2.2 ps−1 at 95% C.L. is derived.

The pad readout of the CERES RICH detectors

Abstract: The CERES/NA45 electron spectrometer uses two Ring Imaging CHerenkov detectors with UV-sensitive gas counters of 2.84 m 2 and 0.42 m 2 size. The backplanes of these gas detectors consist of 53 800 (48 400) square pads in a 2.74 mm (7.62 mm) grid which pick up induced signals of ∼2 × 10 5 electrons from single-electron avalanches. Compact multi-channel preamplifier modules with low-noise VLSI chips are directly plugged onto the gas detector backplanes. The pad amplitudes are corrected for pedestal variations, digitized and transferred sequentially at 14 MHz in chains of up to 4096 pads to the control room. After zero suppression, the pad information is processed in a chain with pipeline organization, performing a transformation to xy-coordinates and branching part of the data to a trigger processor for fast ring finding. With a simple mechanical concept and a multiplexed data transfer, this readout method offers a good signal-to-noise ratio at gas gains of a few 10 5 , high spatial resolution on large areas and unambiguous reconstruction of multiple hits at a cost of ≲6 DM/channel.

Measurement of the leptonic branching ratios of the τ lepton

Abstract: The leptonic branching ratios of the tau lepton have been determined from data collected by the OPAL detector in 1991 and 1992. From a sample of 27196 e+e−→τ+τ− candidates we find 7322\(\tau \to e u \bar u\) and 7941\(\tau \to \mu u \bar u\) candidates. Using efficiency and background estimates determined from a study of Monte Carlo events and control samples of data, the branching ratios\(B(\tau \to e u \bar u )\)=(18.14±0.20±0.28)% and\(B(\tau \to \mu u \bar u )\)=(17.48±0.18±0.23)% have been obtained. These new results have been combined with the published results for the 1990 OPAL data to yield the following branching ratios for data taken between 1990 and 1992: $$\begin{gathered} B(\tau \to e u \bar u ) = (18.04 \pm 0.33)\% , \hfill \\ B(\tau \to \mu u \bar u ) = (17.36 \pm 0.27)\% . \hfill \\ \end{gathered} $$ These leptonic branching ratios are used with other properties of the muon and tau-lepton to test the universality of charged current leptonic couplings in these decays. The ratio\(R_\tau = B(\tau \to hadrons + u _\tau )/B(\tau \to e\bar u _e u _\tau )\) is calculated using our measured values of the leptonic branching fractions of the tau and tau lifetime from which a value of αs(Q2=Mτ2) is extracted. The value of αs(Q2=MZ2) is obtained byQ2 evolution and agrees with the value from the Z0 line shape analysis.

A test of CP-invariance in Z0→τ+τ- using optimal observables

Abstract: Using 27 490 Z0→τ+τ− decays, accumulated in 1991, 1992 and 1993 with the OPAL detector at LEP, a direct test ofCP-invariance in the neutral current reactione+e−→τ+τ− is performed by measuringCP-odd observables which are proportional to the weak dipole moment of the τ-lepton. A new method based on optimalCP-odd observables constructed from the τ flight and spin directions is employed. More sensitive measurements of the real and, for the first time, the imaginary part of the weak dipole moment with highest possible signal to noise ratio are obtained. No evidence for a non-zero expectation value of the considered observables and hence forCP-violation is observed. An upper limit on the weak dipole moment of |Re(dτω)|<7.8×10−18e·cm and |Im(dτω)|<4.5×10−17e·cm with 95% confidence level is obtained.

Performance of a beam telescope using double sided silicon microstrip detectors

Abstract: A beam telescope consisting of four double sided, DC coupled microstrip detectors with VLSI readout electronics has been built and tested in a 70 GeV μ− beam at CERN. A signal-to-noise ratio of 53:1 and a spatial resolution of 2.7 μm (junction side) and 4.8 μm (ohmic side) have been observed on the best detectors. A telescope performance for a particle track of σxy = 2–3 μm and σslope = 2–3 μrad on the front face of a test object was achieved.

Determination of an upper limit for the mass of the τ-neutrino at LEP

Abstract: An upper limit for theν τ mass is determined through the kinematic reconstruction of the decayτ→5π ± ν τ in the OPAL detector at LEP. The limit is obtained using a new method based on the comparison of the two-dimensional distribution of energy and invariant mass of the five-pion system with expectations from different neutrino mass hypotheses. From a sample of five events surviving the selection criteria we obtain an upper limit of 74 MeV at 95% confidence level. It is the first measurement at LEP energies, where the larger average multiplicity of $$e^ + e^ - \to q\bar q$$ events makes the suppression of this background more robust compared to lower energies.

Comparisons of the properties of final state photons in hadronic Z0 decays with predictions from matrix element calculations

Abstract: The properties of hadronic Z0 decays with final state photons, measured with OPAL at LEP, have been compared with predictions from two different matrix element calculations ofO(ααs). Two calculations, GNJETS and EEPRAD, have been investigated which use different schemes to restrict the phase space around the poles of the cross section. Assuming the E0-JADE jet definition, both calculations describe the data well in large regions of phase space forycut values around 0.06. For very large and very small jet-photon masses some deviations from the predictions have been found, indicating the importance of higher order corrections. Significant differences between the calculations are only apparent in the predicted rate of 1-jet plus photon events. The rate is higher in GNJETS which reproduces the data better than EEPRAD.

Determination of event shape distributions and αsb from {Mathematical expression} events at LEP

Abstract: In hadronic Z0 decays collected by the OPAL experiment at LEP, event shape variables such as jet rates, jet masses, thrust and the energy-energy correlation for $$Z^0 \to b\bar b$$ events are compared to those for all flavours using secondary vertex information to tag the b quarks. The measured distributions are found to be well described by anO"(α s ) calculation for heavy quarks as well as by parton shower simulations. We also determine the ratio of the strong coupling constant for b quarks and all quarks, α b /α incl , from these distributions. We find $$\alpha _s^b /\alpha _s^{incl} = 0.994 \pm 0.005_{ - 0.012}^{ + 0.010} ,$$ where the errors are the statistical and systematic errors. The result can be converted into the ratio for b quarks relative to the complementary flavours udsc α b /α udsc =0.992±0.007 -0.015 +0.013 .