Recent results from the DELPHI Barrel Ring Imaging Cherenkov counter

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FIRST EVIDENCE OF HARD SCATTERING PROCESSES IN SINGLE TAGGED GAMMA-GAMMA-COLLISIONS

For the first time, multihadronic production from single tagged γγ collisions has been studied, where one of the scattered leptons was tagged at very low virtual photon absolute mass squared (〈Q2〉 = 0.06 (GeV/c2)2). Data collected during 1991 and 1992 in the DELPHI experiment at LEP are shown to agree well with predictions which included the non-perturbative vector meson dominance model in which the interacting photons are assumed to have converted into a vector meson (ρ{variant}, ω or π), a quark-parton model which describes direct photon interactions and a QCD-based model which considers the photon to have quark and gluon structure functions. Five different parametrizations of these structure functions were used and the predictions compared with the data. This study confirms recent results from no-tag experiments in requiring a QCD-based component to successfully describe the data, indicating that the photon has a significant partonic content. © 1995

PRODUCTION OF STRANGE B-BARYONS DECAYING INTO XI-/+-L-/+ PAIRS AT LEP

An excess of events containing, in a jet, a samesign Ξ∓-ℓ∓ pair as compared to those with an oppositesign Ξ∓-ℓ± pair has been observed in an analysis of 1.7 million hadronic Z0 decays collected by the DELPHI detector at LEP between 1991 and 1993 inclusive. The probability for this signal to come from non B-baryon decays is less than 5×10-4. The measured production fraction corresponds to:[Figure not available: see fulltext.] per lepton species, averaged for electrons and muons and assuming that the two channels have an equal contribution. Semileptonic decays of Λb baryons can account for less than 10% of these events and the major part of the signal has to originate from Ξb semileptonic decays. Using the subsample of these events where the Ξ∓ trajectory has been measured in the Vertex Detector, the lifetime of B-baryons producing a Ξ∓ in their semileptonic decay final state is found to be: {Mathematical expression} © 1995 Springer-Verlag

First evidence of hard scattering processes in single tagged gamma-gamma-collisions

For the first time, multihadronic production from single tagged gamma gamma collisions has been studied, where one of the scattered leptons was tagged at very low virtual photon absolute mass squared ([Q(2)] = 0.06 (GeV/c(2))(2)). Data collected during 1991 and 1992 in the DELPHI experiment at LEP are shown to agree well with predictions which included the non-perturbative vector meson dominance model in which the interacting photons are assumed to have converted into a vector meson (rho, omega or pi), a quark-parton model which describes direct photon interactions and a QCD-based model which considers the photon to have quark and gluon structure functions. Five different parametrizations of these structure functions were used and the predictions compared with the data. This study confirms recent results from no-tag experiments in requiring a QCD-based component to successfully describe the data, indicating that the photon has a significant partonic content

Measurement of the forward-backward asymmetry of charm and bottom quarks at the Z-pole using D-asterisk(+/-)-mesons

The forward-backward asymmetries for the processes e(+)e(-) --> c (c) over bar and e(+)e(-) --> b (b) over bar at the Z resonance are measured using identified D*(+/-) mesons. In 905,000 selected hadronic events, taken in 1991 and 1992 with the DELPHI detector at LEP, 4757 D*(+) --> D degrees pi(+) decays are reconstructed. The c and b quark forward-backward asymmetries are determined to be: A(FB)(c (c) over bar) = 0.077 +/- 0.029 (stat) +/- 0.012 (sys), A(FB)(b (b) over bar) = 0.059 +/- 0.062 (stat) +/- 0.025 (sys). Constraining the b asymmetry to the value measured by DELPHI using independent analyses, the charm asymmetry is determined to be: A(FB)(c,const) = 0.068 +/- 0.027 (stat) +/- 0.011 (sys). This result corresponds to an effective electroweak mixing angle measured using charm quark events of: sin2 theta(eff)(lept) = 0.2307 +/- 0.0062 (stat) +/- 0.0026 (sys)

MEASUREMENT OF THE GAMMA(B-B-OVER-BAR)/GAMMA(HAD) BRANCHING RATIO OF THE Z BY DOUBLE HEMISPHERE TAGGING

Two measurements of Gamma(b ($) over bar b)/Gamma(had) are presented. Both measurements use 250000 Z decays taken with the DELPHI detector in 1991 and rely mainly on the precision of the microvertex detector. One tagging method is as simple as possible so that background rates can be reliably predicted by simulation. The other one uses a more involved tagging technique and reduces the dependence on simulation as much as possible. Combining both results, Gamma(b ($) over bar b)/Gamma(had) is found to be 0.2209 +/- 0.0041 (stat.) +/- 0.0042(syst.) +/- 0.0018 (Gamma(c ($) over bar c))

First measurement of the strange quark asymmetry at the Z(0) peak

A measurement of the strange quark forward-backward asymmetry at the Z(0) peak was performed using 718,000 multihadronic Z(0) decays collected by the DELPHI detector at LEP in 1992. The s-quark was tagged by the presence of high momentum charged kaons identified by the Ring Imaging Cherenkov detector and by Lambda(0)'s decaying into p pi(-). The s-quark purity obtained was estimated for the two hadrons to be 43%. The average s-quark asymmetry was found to be 0.131 +/- 0.035 (stat.) +/- 0.013 (syst.). The forward-backward asymmetry was measured for unresolved d- and s-quarks, tagged by the detection of a high energy neutron or neutral kaon in the Hadron Calorimeter. The combined d- and s-quark purity was 69% and their asymmetry was found to be 0.112 +/- 0.031 (stat.) +/- 0.054 (syst.)

PRODUCTION CHARACTERISTICS OF K-0 AND LIGHT MESON RESONANCES IN HADRONIC DECAYS OF THE Z(0)

An analysis of inclusive production of KO and the meson resonances K*(+/-)(892), rho(0)(770), f(0)(975) and f(2)(1270) in hadronic decays of the Z(0) is presented, based on about 973,000 multihadronic events collected by the DELPHI detector at LEP during 1991 and 1992. Overall multiplicities have been determined as 1.962 +/- 0.060 K-0 mesons, 0.712 +/- 0.067 K*(+/-)(892) and 1.21 +/- 0.15 rho(0)(770) per hadronic Z(0) decay. The average multiplicities of f(0)(975) for scaled momentum, x(p) = p/p(beam), in the range 0.05 less than or equal to x(p) less than or equal to 0.6 and of f(2)(1270) for 0.05 less than or equal to x(p) less than or equal to 1.0 are 0.098 +/- 0.016 and 0.170 +/- 0.043 respectively. The f(0)(975) and rho(0)(770) x(p)-spectra have p similar shapes. The f(2)(1270)/rho(0)(770) ratio increases with x(p). The average multiplicities and the differential cross sections are compared with the JETSET Parton Shower model. The model with default parameters fails to reproduce the experimental K-0 momentum spectrum at low momentum, describes the K*(+/-)(892) and rho(0)(770) x(p)-spectrum shapes, but significantly overestimates their production rates