LHCb

Abstract:

During the LEP running periods in 1990 and 1991 DELPHI has accumulated approximately 450 000 Z0 decays into hadrons and charged leptons. The increased event statistics coupled with improved analysis techniques and improved knowledge of the LEP beam energies permit significantly better measurements of the mass and width of the Z0 resonance. Model independent fits to the cross sections and leptonic forward- backward asymmetries yield the following Z0 parameters: the mass and total width MZ = 91.187 ± 0.009 GeV, ΓZ = 2.486 ± 0.012 GeV, the hadronicf and leptonic partials widths Γhad = 1.725 ± 0.012 GeV, Γℓ = 83.01 ± 0.52 MeV, the invisible width Γinv = 512 ± 10 MeV, the ratio of hadronic to leptonic partial widths Rℓ = 20.78 ± 0.15, and the Born level hadronic peak cross section σ0 = 40.90 ± 0.28 nb. Using these results and the value of αs determined from DELPHI data, the number of light neutrino species is determined to be 3.08 ± 0.05. The individual leptonic widths are found to be: Γe = 82.93 ± 0.70 MeV, Γμ = 83.20 ± 1.11 MeV and Γτ = 82.89 ± 1.31 MeV. Using the measured leptonic forward-backward asymmetries and assuming lepton universality, the squared vector and axial-vector couplings of the Z0 to charged leptons are found to be gVℓ2 = (1.47 ± 0.51) × 10-3 and gAℓ2 = 0.2483 ± 0.0016. A full Standard Model fit to the data yields a value of the top mass mt = 115-82+52 (expt.)-24+52(Higgs) GeV, corresponding to a value of the weak mixing angle sin2 θefflept = 0.2339±0.0015 (expt.)-0.0004+0.0001 (Higgs). Values are obtained for the variables S and T, or ε{lunate}1 and ε{lunate}3 which parameterize electroweak loop effects. © 1994.

Abstract:

Strange beauty meson Bs0 have been reconstructed through exclusive hadronic final states, using the data collected in 1992 by the DELPHI experiment at the LEP collider. The analysis relies on the combined use of accurate tracking and of the hadron identification capabilities of DELPHI. Three Bs0 decay candidates have been selected over a negligible background using theDs- π+, Ds- a1+ (1260) and J Ψ φ decay modes, with identified kaons in the final state. The Bs0 meson mass is measured to be mBs = (5374± 16± 2) MeV/ c2. © 1994.

Abstract:

The average lifetime of B hadrons was measured using data collected with the DELPHI detector at the LEP collider during 1991 and 1992. The measurement was performed using two different anayses. The first method was an improvement on a previous technique, which used charged particle impact parameter distributions. This analysis measured an average lifetime for B hadrons of {Mathematical expression} The second method was based on a new technique, which used inclusively reconstructed secondary vertices to measure {Mathematical expression} Taking into account both the statistical and systematic corelations, the results of these two methods were combined to give an average lifetime for B hadrons of {Mathematical expression} © 1994 Springer-Verlag.

Abstract:

Two-particle correlations in invariant mass are studied separately for like-sign and unlike-sign pions. Strong correlations are observed only at small masses. The correlations decrease very rapidly for like-sign pions and slowly for unlike-sign pions, manifesting different particle dynamics. The like-sign correlations exhibit a power-law behaviour as a function of mass, compatible with the previously observed intermittency effect. The predictions of the parton shower model (JETSET 7.3) deviate significantly from the data. However, good agreement is obtained with the mass correlations, both for the unlike-sign and for the like-sign pairs if the production of the η′ and ρ0 mesons is reduced and Bose-Einstein correlations are included in the model. The value of the Bose-Einstein correlation strength is consistent with unity for pions which are produced directly or in the decays of short-lived resonances. © 1994 Springer-Verlag.

Abstract:

Jets in photoproduction events have been studied with the ZEUS detector for γp centre-of-mass energies ranging from 130 to 250 GeV. The inclusive jet distributions give evidence for the dominance of resolved photon interactions. In the di-jet sample the direct processes are for the first time clearly isolated. Di-jet cross sections for the resolved and direct processes are given in a restricted kinematic range. © 1994.