LHCb

Abstract:

Inclusive transverse momentum spectra of charged particles in photoproduction events in the laboratory pseudorapidity range -1.2<η<1.4 have been measured up to pT=8 GeV usign the ZEUS detector. Diffractive and non-diffractive reactions have been selected with an average γp centre of mass (c.m.) energy of 〈W〉=180 GeV. For diffractive reactions, the pT spectra of the photon dissociation events have been measured in two intervals of the dissociated photon mass with mean values 〈MX〉=5 GeV and 10 GeV. The inclusive transverse momentum spectra fall exponentially in the low pT region. The non-diffractive data show a pronounced high pT tail departing from the exponential shape. The pT distributions are compared to lower energy photoproduction data and to hadron-hadron collisions at a similar c.m. energy. The data are also compared to the results of a next-to-leading order QCD calculation. © 1995 Springer-Verlag.

Abstract:

A sample of Z0→τ+τ- events observed in the DELPHI detector at LEP in 1991 and 1992 is analysed to measure the τ polarisation in the exclusive decay channels {Mathematical expression}, {Mathematical expression}, πν, ρν and a1ν. The τ polarisation is also measured with an inclusive hadronic analysis which benefits from a higher efficiency and a better systematic precision than the use of the exclusive decay modes. The results have been combined with those published on the 1990 data. A measurement of the τ polarisation as a function of production angle yields the values for the mean τ polarisation 〈P〉τ=-0.148±0.022 and for the Z0 polarisation PZ=-0.136±0.027. These results are used to determine the ratio of vector to axial-vector effective couplings for taus {Mathematical expression}, and for electrons {Mathematical expression}, compatible with e-τ universality. With the assumption of lepton universality, the ratio of vector to axial-vector effective couplings for leptons {Mathematical expression} is obtained, implying a value of the effective weak mixing angle sin2θefflept=0.2320±0.0021 . © 1995 Springer-Verlag.

Abstract:

We report the first observation of charmed mesons with the ZEUS detector at HERA using the decay channel D^*+→ (D⁰→ K^-π⁺)π⁺(+c.c.). Clear signals in the mass difference ΔM=M(D^*)-M(D⁰) as well as in the M(Kπ) distribution at the D⁰mass are found. The ep cross section for inclusive D^*±production with Q²< 4 GeV²in the γp centre-of-mass energy range 115 < W < 275 GeV has been determined to be (32±7-7 ⁺⁴) nb in the kinematic regionpt(D^*) ≥ 1.7 GeV, |η(D^*)| < 1.5. Extrapolating outside this region, assuming a mass of the charm quark of 1.5 GeV, we estimate the ep charm cross section to be σ(ep → ccX) = (0.45 ± 0.11-0.22 ^+0.37) μb at s=296 GeV and 〈W〉 = 198 GeV. The average γp charm cross section σ(γp → ccX) is found to be (6.3 ± 2.2-3.0 ^+6.3) μb at 〈W〉 = 163 GeV and (16.9 ± 5.2-8.5 ^+13.9) μb at 〈W〉 = 243 GeV. The increase of the total charm photoproduction cross section by one order of magnitude with respect to low energy data experiments is well described by QCD NLO calculations using singular gluon distributions in the proton. © 1995.

Abstract:

Dijet production by almost real photons has been studied at HERA with the ZEUS detector. Jets have been identified using the cone algorithm. A cut on xγ ^OBS, the fraction of the photon energy participating in the production of the two jets of highest transverse energy, is used to define cross sections sensitive to the parton distributions in the proton and in the photon. The dependence of the dijet cross sections on pseudorapidity has been measured for xγ ^OBS ≥ 0.75 and xγ ^OBS < 0.75. The former is sensitive to the gluon momentum density in the proton. The latter is sensitive to the ginon in the photon. The cross sections are corrected for detector acceptance and compared to leading order QCD calculations. © 1995.

Abstract:

A sample of events enriched in bb̄ quark pairs was selected in the data recorded by the DELPHI experiment at LEP during 1992 and 1993, by the presence of secondary decay vertices from short-lived particles. Using this sample, the average multiplicities of Ks⁰, K^±, p(p̄), Λ(Λ) and of charged particles in bb̄ events have been measured, distinguishing the component from fragmentation and the component coming from the decay of b-hadrons. The measurement of the average charge multiplicity in bb̄ events was used to compute the mean fractional beam energy carried by the primary b-hadron, and the difference in charged particle multiplicity between bb̄ events and light quark (uū, dd̄, ss̄) events. © 1995.