A Measurement of Rb using a Double Tagging Method

The fraction of Z to bbbar events in hadronic Z decays has been measured by the OPAL experiment using the data collected at LEP between 1992 and 1995. The Z to bbbar decays were tagged using displaced secondary vertices, and high momentum electrons and muons. Systematic uncertainties were reduced by measuring the b-tagging efficiency using a double tagging technique. Efficiency correlations between opposite hemispheres of an event are small, and are well understood through comparisons between real and simulated data samples. A value of Rb = 0.2178 +- 0.0011 +- 0.0013 was obtained, where the first error is statistical and the second systematic. The uncertainty on Rc, the fraction of Z to ccbar events in hadronic Z decays, is not included in the errors. The dependence on Rc is Delta(Rb)/Rb = -0.056*Delta(Rc)/Rc where Delta(Rc) is the deviation of Rc from the value 0.172 predicted by the Standard Model. The result for Rb agrees with the value of 0.2155 +- 0.0003 predicted by the Standard Model.Comment: 42 pages, LaTeX, 14 eps figures included, submitted to European Physical Journal

Measurement of the B+ and B-0 lifetimes and search for CP(T) violation using reconstructed secondary vertices

The lifetimes of the B+ and B-0 mesons, and their ratio, have been measured in the OPAL experiment using 2.4 million hadronic Z(0) decays recorded at LEP. Z(0) --> b (b) over bar decays were tagged using displaced secondary vertices and high momentum electrons and muons. The lifetimes were then measured using well-reconstructed charged and neutral secondary vertices selected in this tagged data sample. The results aretau(B+) = 1.643 +/- 0.037 +/- 0.025 pstau(Bo) = 1.523 +/- 0.057 +/- 0.053 pstau(B+)/tau(Bo) = 1.079 +/- 0.064 +/- 0.041,where in each case the first error is statistical and the second systematic.A larger data sample of 3.1 million hadronic Z(o) decays has been used to search for CP and CPT violating effects by comparison of inclusive b and (b) over bar hadron decays, No evidence fur such effects is seen. The CP violation parameter Re(epsilon(B)) is measured to be Re(epsilon(B)) = 0.001 +/- 0.014 +/- 0.003and the fractional difference between b and (b) over bar hadron lifetimes is measured to(Delta tau/tau)(b) = tau(b hadron) - tau((b) over bar hadron)/tau(average) = -0.001 +/- 0.012 +/- 0.008

Search for anomalous production of di-lepton events with missing transverse momentum in e(+)e(-) collisions at root s = 161 and 172 GeV

Events containing a pair of charged leptons and significant missing transverse momentum are selected from a data sample corresponding to a total integrated luminosity of 20.6 pb^-1 at centre-of-mass energies of 161 GeV and 172 GeV. The observed number of events, four at 161 GeV and nine at 172 GeV, is consistent with the number expected from Standard Model processes, predominantly arising from W+W- production with each W decaying leptonically. This topology is also an experimental signature for the pair production of new particles that decay to a charged lepton accompanied by one or more invisible particles. Further event selection criteria are described that optimise the sensitivity to particular new physics channels. No evidence for new phenomena is observed and limits on the production of scalar charged lepton pairs and other new particles are presented

Tests of the standard model and constraints on new physics from measurements of fermion-pair production at 130-172GeV at LEP

Production of events with hadronic and leptonic final states has been measured in e(+)e(-) collisions at centre-of-mass energies of 130-172 GeV, using the OPAL detector at LEP. Cross-sections and leptonic forward-backward asymmetries are presented, both including and excluding the dominant production of radiative Z gamma events, and compared to Standard Model expectations. The ratio R-b of the cross-section for production to the hadronic cross-section has been measured. In a model-independent fit to the Z lineshape, the data have been used to obtain an improved precision on the measurement of gamma-Z interference. The energy dependence of alpha(em) has been investigated. The measurements have also been used to obtain limits on extensions of the Standard Model described by effective four-fermion contact interactions, to search for t-channel contributions from new massive particles and to place limits on gaugino pair production with subsequent decay of the gaugino into a light gluino and a quark pair

Measurement of the bb-bar fraction in hadronic Z0 decays with precision vertex detectors.

We have measured the fraction of bb» events in hadronic Z0 decays, Rbb», using the vertex detector system of the Mark II detector at the SLAC Linear Collider. We tag bb» events by requiring the coincidence of three or more tracks with significant impact parameters. This tag is 50% efficient and results in a sample of 85% purity. We find Rbb»=0.251+0.049+0.030, in good agreement with other measurements and the standard model prediction. © 1991 The American Physical Society

Measurement of tau branching ratios to five charged hadrons

The branching ratios of the decay of the tau lepton to five charged hadrons have been measured with the OPAL detector at LEP using data collected between 1991 and 1995 at e(+)e(-) centre-of-mass energies close to the Z(0) resonance. The branching ratios are measured to be B(tau(-) --> 3h(-)2h(+)v(tau)) = (0.091 +/- 0.014 +/- 0.006)% B(tau(-) --> 3h(-)2h(+)pi(0)v(tau)) = (0.027 +/0 0.018 +/- 0.009)% where the first error is statistical and the second systematic

Measurement of tau branching ratios to five charged hadrons

The branching ratios of the decay of the tau lepton to five charged hadrons have been measured with the OPAL detector at LEP using data collected between 1991 and 1995 at e(+)e(-) centre-of-mass energies close to the Z(0) resonance. The branching ratios are measured to beB(tau(-) --> 3h(-)2h(+)v(tau)) = (0.091 +/- 0.014 +/- 0.006)%B(tau(-) --> 3h(-)2h(+)pi(0)v(tau)) = (0.027 +/0 0.018 +/- 0.009)%where the first error is statistical and the second systematic

Search for chargino and neutralino production at √s=170 and 172 GeV at LEP

A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of 10.310.3 pb −1−1 at centre-of-mass energies of s√=s= 170 and 172 GeV with the OPAL detector at LEP. No evidence for these particles has been found. The results are combined with those from previous OPAL chargino and neutralino searches at lower energies to obtain limits. Exclusion regions at 95% C.L. of parameters of the Minimal Supersymmetric Standard Model are determined. Within this framework, for tanβ≥1.0tan⁡β≥1.0 , lower mass limits are placed on the lightest chargino and the three lightest neutralinos. The 95% C.L. lower mass limit on the lightest chargino, assuming that it is heavier than the lightest neutralino by more than 10 GeV, is 84.5 GeV for the case of a large universal scalar mass ( m0>m0> 1 TeV) and 65.7 GeV for the smallest m0m0 compatible with current limits on the sneutrino mass and slepton cross-sections. The lower limit on the lightest neutralino mass at 95% C.L. for tanβ≥1.0tan⁡β≥1.0 is 24.7 GeV for m0=1m0=1 TeV and 13.3 GeV for the minimum m0m0 scenario. These mass limits are higher for increasing values of tanβtan⁡β . The interpretation of the limits in terms of gluino and scalar quark mass limits is also given

Disentangling flow and signals of Chiral Magnetic Effect in U plus U, Au plus Au and p plus Au collisions

We present STAR measurements of the charge-dependent three-particle correlator gamma(a,b) = /v(2){2} and elliptic flow v(2){2} in U+U, Au+Au and p+Au collisions at RHIC. The difference Delta gamma = gamma(opposite sign)-gamma(same sign) measures charge separation across the reaction plane, a predicted signal of the Chiral Magnetic Effect (CME). Although charge separation has been observed, it has been argued that the measured separation can also be explained by elliptic flow related backgrounds. In order to separate the two effects we perform measurements of the gamma-correlator where background expectations differ from magnetic field driven effects. A differential measurement of gamma with the relative pseudorapidity (Delta eta)) between the first and second particles indicate that Delta gamma in peripheral A+A and p+A collisions are dominated by short-range correlations in A. However, a relatively wider component of the correlation in Delta eta tends to vanish the same way as projected magnetic field as predicted by MC-Glauber simulations