LHCb

Abstract:

During 1993 and 1995 LEP was run at 3 energies near the Z⁰ peak in order to give improved measurements of the mass and width of the resonance. During 1994, LEP operated only at the Z⁰ peak. In total DELPHI accumulated data corresponding to an integrated luminosity of approximately 116 pb^-1. Analyses of the hadronic cross-sections and of the cross-sections and forward-backward asymmetries in the leptonic channels used the most precise evaluations of the LEP energies. In the dimuon channel, events with a photon radiated from the initial state have been used to probe the cross-sections and asymmetries down to PETRA energies. Model independent fits to all DELPHI lineshape and asymmetry data from 1990 to 1995 have been carried out giving values of the resonance parameters: MZ = 91.1863 ± 0.0028 GeV ΓZ = 2.4876 ± 0.0041 GeV σ0 = 41.578 ± 0.069 nb R1 = 20.730 ± 0.060 AFB⁰ = 0.0187 ± 0.0019. These values are significantly more precise than those previously published. The results are interpreted in terms of the Standard Model.

Abstract:

The probability P(c → D*+) that a charm quark fragments into a D*+ meson and the c → l+ semileptonic branching fraction were measured in Z0 decay into cc̄ events. From the analysis of 3.5 Million Z0 events collected from 1992 to 1995, a sample of charm meson decays with 81% cc̄ purity was selected. The product of the c → D*+ fragmentation probability times the D*+ → D0π+ branching fraction was measured to be: P(c → D*+) · BR(D*+ → D0π+) = 0.174 ± 0.010(stat) ± 0.004(syst). Using the world averaged value for BR(D*+ → D0π+), the fragmentation probability is inferred: P(c → D*+) = 0.255 ± 0.015(stat) ± 0.006(syst) ± 0.005(syst.BR). From the same sample, 1828 ± 51 identified leptons in the opposite hemisphere were selected. From this sample, the charm semileptonic branching fraction was measured to be: BR(c → l+) = 0.0958 ± 0.0042(stat) ± 0.0028(syst).

Abstract:

A sample of 2.2 million hadronic Z decays, selected from the data recorded by the DELPHI detector at LEP during 1994-1995 was used for an improved measurement of inclusive distributions of π+ K+ and p and their antiparticles in gluon and quark jets. The production spectra of the individual identified particles were found to be softer in gluon jets compared to quark jets, with a higher multiplicity in gluon jets as observed for inclusive charged particles. A significant proton enhancement in gluon jets is observed indicating that baryon production proceeds directly from colour objects. The maxima, £*, of the ξ -distributions for kaons in gluon and quark jets are observed to be different.

Abstract:

The production of D*± (2010) mesons in deep inelastic scattering has been measured in the ZEUS detector at HERA using an integrated luminosity of 37 pb-1. The decay channels D*+ → D0π+ (+c.c.), with D0 → K-π+ or D0 → K-π-π+π+, have been used to identify the D mesons. The e+p cross section for inclusive D*± production with 1 < Q2 < 600 GeV2 and 0.02 < y < 0.7 is 8.31±0.31(stat.)+0.30-0.50 (syst.) nb in the kinematic region 1.5 < pT(D*±) < 15 GeV and \η(D*±)\ < 1.5. Differential cross sections are consistent with a next-to-leading-order perturbative-QCD calculation when using charm-fragmentation models which take into account the interaction of the charm quark with the proton remnant. The observed cross section is extrapolated to the full kinematic region in pT(D*±) and η(D*±) in order to determine the charm contribution, Fcc̄2 (x, Q2), to the proton structure function. The ratio Fcc̄2/F2 rises from ≃10% at Q2 ≃ 1.8GeV2 to ≃30% at Q2 ≃130GeV2 for x values in the range 10-4 to 10-3.

Abstract:

The e+p charged-current deep inelastic scattering cross sections, dσ/dQ2 for Q2 between 200 and 60000 GeV2, and dσ/dx and dσ/dy for Q2 > 200 GeV2, have been measured with the ZEUS detector at HERA. A data sample of 47.7 pb-1, collected at a center-of-mass energy of 300 GeV, has been used. The double-differential cross-section dσ/dQ2 falls by a factor of about 50000 as Q2 increases from 280 to 30000 GeV2. The double differential cross section d2σ/dxdQ2 has also been measured. A comparison between the data and Standard Model (SM) predictions shows that contributions from antiquarks (ū and c̄) and quarks (d and s) are both required by the data. The predictions of the SM give a good description of the full body of the data presented here. A comparison of the charged-current cross-section dσ/dQ2 with the recent ZEUS results for neutral-current scattering shows that the weak and electromagnetic forces have similar strengths for Q2 above M2W, M2Z. A fit to the data for dσ/dQ2 with the Fermi constant GF and MW as free parameters yields GF = (1.171±0.034 (stat.)+0.026-0.032 (syst.)+0.016-0.015 (PDF)) × 10-5 Gev-2 and MW = 80.8+4.9-4.5 (stat.)+5.0-4.3(syst.)+1.4-1.3 (PDF) GeV. Results For MW, where the propagator effect alone or the SM constraint between GF and MW have been considered, are also presented.