LHCb

Abstract:

The total and the differential cross sections for the reaction e+e- → γγ(γ) have been measured with the DELPHI detector at LEP using an integrated luminosity of 36.9 pb-1. The results agree with the QED predictions and consequently there is no evidence for non-standard channels with the same experimental signature. The lower limits obtained on the QED cutoff parameters are Λ+ > 143 GeV and Λ- > 120 GeV, and the lower bound on the mass of an excited electron with an effective coupling constant λγ = 1 is 132 GeV/c2. Upper limits on the branching ratios for the decays Z0 → γγ, Z0 → π0γ, Z0 → ηγ and Z0 → γγγ have been determined to be 5.5 × 10-5, 5.5 × 10-5, 8.0 × 10-5, and 1.7 × 10-5 respectively. All the limits are at the 95% confidence level. © 1994.

Abstract:

During the 1992 running period of the LEP e+e- collider, the DELPHI experiment accumulated approximately 24 pb- of data at the Z0 peak. The decays into hadrons and charged leptons have been analysed to give values for the cross sections and leptonic forward-backward asymmetries which are significantly improved with respect to those previously published by the DELPHI collaboration. Incorporating these new data, more precise values for the Z0 resonance parameters are obtained from model-independent fits. The results are interpreted within the framework of the Standard Model, yielding for the top quark mass mt = 157-48+36(expt.)-20+19(Higgs ) GeV, and for the effective mixing angle sin2 θefflept = 0.2328 ± 0.0013 (expt.)-0.0003+0.0001(Higgs), where (Higgs) represents the variation due to Higgs boson mass in the range 60 to 1000 GeV, with central value 300 GeV. © 1994.

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.