LHCb

Abstract:

An updated analysis using about 1.5 million events recorded at √s = Mz with the DELPHI detector in 1994 is presented. Eighteen infrared and collinear safe event shape observables are measured as a function of the polar angle of the thrust axis. The data are compared to theoretical calculations in script O(α²s) including the event orientation. A combined fit of αs and of the renormalization scale xμ in script O(α²s) yields an excellent description of the high statistics data. The weighted average from 18 observables including quark mass effects and correlations is αs(M²z) = 0.1174±0.0026. The final result, derived from the jet cone energy fraction, the observable with the smallest theoretical and experimental uncertainty, is αs(M²z) = 0.1180 ± 0.0006(exp.) ± 0.0013(hadr.) ± 0.0008(scale) ± 0.0007(mass). Further studies include an αs determination using theoretical predictions in the next-to-leading log approximation (NLLA), matched NLLA and script O(α²s) predictions as well as theoretically motivated optimized scale setting methods. The influence of higher order contributions was also investigated by using the method of Padé approximants. Average αs values derived from the different approaches are in good agreement.

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, F^cc̄ 2 (x, Q²), to the proton structure function. The ratio F^cc̄ 2/F2 rises from ≃10% at Q² ≃ 1.8GeV² to ≃30% at Q² ≃130GeV² for x values in the range 10^-4 to 10^-3.