LHCb

Abstract:

A search for the Standard Model Higgs boson was performed in the data sample of around 1 million hadronic Z0 decays recorded in 1991 and 1992 with the DELPHI detector at LEP. The Higgs boson was searched for through its production in association with either neutrinos, electrons or muons. Four low mass events remain after the selections in the charged leptonic channels, that are likely to be due to background, especially four fermion processes. These results restrict the mass of the Standard Model Higgs boson to be larger than 55.7 GeV/c2 at the 95% confidence level. © 1994.

Abstract:

A search for charged Higgs bosons decaying into quarks is described, based on statistics of around 1.7 million hadronic Zo decays detected in DELPHI. Despite the very high background from standard hadronic decays of the Zo, masses in the range up to 43.5 GeV/c2 are excluded at the 95% confidence level. After combination with a search for leptonic decays, this mass limit is extended to cover all branching ratios. A similar analysis sets new limits on the possible production of any pair-produced heavy scalar decaying into a pair of jets, such as neutral Higgs bosons in a two doublet scheme and diquarks. © 1994 Springer-Verlag.

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.