LHCb

Abstract:

The bb̄ forward-backward asymmetry has been determined from the average charge flow measured in a sample of 3,500,000 hadronic Z decays collected with the DELPHI detector in 1992 - 1995. The measurement is performed in an enriched bb̄ sample selected using an impact parameter tag and results in the following values for the bb̄ forward-backward asymmetry: Abb̄FB (89.55 GeV) = 0.068 ± 0.018 (stat.) ± 0.0013(syst.) Abb̄FB (91.26 GeV) = 0.0982 ± 0.0047(stat.) ± 0.0016(syst.) Abb̄FB (92.94 GeV) = 0.123 ± 0.016 (stat.) ± 0.0027(syst.) The bb̄ charge separation required for this analysis is directly measured in the b tagged sample, while the other charge separations are obtained from a fragmentation model precisely calibrated to data. The effective weak mixing angle is deduced from the measurement to be: sin2θ1eff = 0.23186 ± 0.00083.

Abstract:

The cross section for dijet photoproduction at high transverse energies is presented as a function of the transverse energies and the pseudorapidities of the jets. The measurement is performed using a sample of ep-interactions corresponding to an integrated luminosity of 6.3 pb-1, recorded by the ZEUS detector. Jets are defined by applying a kT-clustering algorithm to the hadrons observed in the final state. The measured cross sections are compared to next-to-leading order QCD calculations. In a kinematic regime where theoretical uncertainties are expected to be small, the measured cross sections are higher than these calculations.

Abstract:

The e+p neutral-current deep inelastic scattering differential cross-sections dσ/dQ2, for Q2 > 400 GeV2, dσ/dx and dσ/dy, for Q2 > 400, 2500 and 10000 GeV2, have been measured with the ZEUS detector at HERA. The data sample of 47.7 pb-1 was collected at a center-of-mass energy of 300 GeV. The cross-section, dσ/dQ2, falls by six orders of magnitude between Q2 = 400 and 40000 GeV2. The predictions of the Standard Model are in very good agreement with the data. Complementing the observations of time-like Z° contributions to fermion-antifermion annihilation, the data provide direct evidence for the presence of Z° exchange in the space-like region explored by deep inelastic scattering.

Abstract:

Inclusive photoproduction of D^*± mesons has been measured for photon-proton centre-of-mass energies in the range 130 < W < 28OGeV and photon virtuality Q² < IGeV². The data sample used corresponds to an integrated luminosity of 37 pb^-1. Total and differential cross sections as functions of the D^* transverse momentum and pseudorapidity are presented in restricted kinematical regions and the data are compared with next-to-leading order (NLO) perturbative QCD calculations using the "massive charm" and "massless charm" schemes. The measured cross sections are generally above the NLO calculations, in particular in the forward (proton) direction. The large data sample also allows the study of dijet production associated with charm. A significant resolved as well as a direct photon component contribute to the cross section. Leading order QCD Monte Carlo calculations indicate that the resolved contribution arises from a significant charm component in the photon. A massive charm NLO parton level calculation yields lower cross sections compared to the measured results in a kinematic region where the resolved photon contribution is significant.

Abstract:

DELPHI results are presented on the inclusive production of the neutral mesons ρ⁰, f0(980), f2(1270), K2*⁰(1430) and f2′(1525) in hadronic Z⁰ decays. They are based on about 2 million multihadronic events collected in 1994 and 1995, using the particle identification capabilities of the DELPHI Ring Imaging Cherenkov detectors and measured ionization losses in the Time Projection Chamber. The total production rates per hadronic Z⁰ decay have been determined to be: 1.19 ± 0.10 for ρ⁰; 0.164 ± 0.021 for f0(980); 0.214 ± 0.038 for f2(1270); 0.073 ± 0.023 for K2*⁰(1430); and 0.012 ± 0.006 for f2′(1525). The total production rates for all mesons and differential cross-sections for the ρ⁰, f0(980) and f2(1270) are compared with the results of other LEP experiments and with models. © 1999 Elsevier Science B.V. All rights reserved.