LHCb

Abstract:

Inclusive charged particle and event shape distributions are measured using 321 hadronic events collected with the DELPHI experiment at LEP at effective centre of mass energies of 130 to 136 GeV. These distributions are presented and compared to data at lower energies, in particular to the precise Z data. Fragmentation models describe the observed changes of the distributions well. The energy dependence of the means of the event shape variables can also be described using second order QCD plus power terms. A method independent of fragmentation model corrections is used to determine αs from the energy dependence of the mean thrust and heavy jet mass. It is measured to be: αs(133 GeV) = 0.116 ± 0.007exp-0.004theo+0.005 from the high energy data. © Springer-Verlag 1997.

Abstract:

A search for lepton flavour number violating Z0 decays in the channels Z0 → μτ, Z0 → eτ, Z0 → eμ, using the DELPHI detector with data collected during the 1991-94 LEP runs, is described. No signal was found. Upper limits at 95% confidence level for the respective branching fractions of 1.2 × 10-5, 2.2 × 10-5, and 0.25 × 10-5, were obtained. © Springer-Verlag 1997.

Abstract:

Weak isosinglet Neutral Heavy Leptons (νm) have been searched for using data collected by the DELPHI detector corresponding to 3.3 × 10⁶ hadronic Z⁰ decays at LEP1. Four separate searches have been performed, for short-lived νm production giving monojet or acollinear jet topologies, and for long-lived νm giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio BR(Z⁰ → νmν̄) of about 1.3 × 10^-6 at 95% confidence level for νm masses between 3.5 and 50 GeV/c². Outside this range the limit weakens rapidly with the νm mass. The results are also interpreted in terms of limits for the single production of excited neutrinos. © Springer-Verlag 1997.

Abstract:

Data are presented on the reaction e+e- → γ + no other detected particle at centre-of-mass energies of 89.48, 91.26 and 93.08 GeV. The cross-section for this reaction is related directly to the number of light neutrino generations which couple to the Z0 boson, and to several other possible phenomena such as the production of excited neutrinos, the production of any invisible 'X' particle, and the magnetic moment of the tau neutrino. Based on the observed number of single photon events, the number of light neutrinos that couple to the Z0 is measured to be Nν = 2.89 ± 0.38. No evidence is found for anomalous production of energetic single photons, and upper limits at 95% confidence level are determined for excited neutrino production (BR < 4 - 8 × 10-6 depending on its mass), production of an invisible 'X' particle (σ < 0.1 pb for masses below 60 GeV), and the magnetic moment of the tau neutrino (< 5.1 × 10-6μB). © Springer-Verlag 1997.

Abstract:

The differential cross section dσ/dt for elastic ρ0 photoproduction, γp → ρ0ρ (ρ0 → π+π-), has been measured in ep interactions at HERA. The squared four-momentum exchanged at the proton vertex, t, has been determined directly by measuring the momentum of the scattered proton using the ZEUS Leading Proton Spectrometer (LPS), a large scale system of silicon micro-strip detectors operating close to the HERA proton beam. The LPS allows the measurement of the momentum of high energy protons scattered at small angles with accuracies of 0.4% for the longitudinal momentum and 5 MeV for the transverse momentum. Photoproduction of ρ0 mesons has been investigated in the interval 0.073 < |t| < 0.40 GeV2, for photon virtualities Q2 < 1 GeV2 and photon-proton centre-of-mass energies W between 50 and 100 GeV. In the measured range, the t distribution exhibits an exponential shape with a slope parameter b = 9.8 ± 0.8 (stat.) ± 1.1 (syst.) GeV-2. The use of the LPS eliminates the contamination from events with diffractive dissociation of the proton into low mass states. © Springler-Verlag 1997.