LHCb

Abstract:

Infrared and collinear safe event shape distributions and their mean values are determined using the data taken at five different centre of mass energies above MZ with the DELPHI detector at LEP. From the event shapes, the strong coupling αS is extracted in θ(αS²), NLLA and a combined scheme using hadronisation corrections evaluated with fragmentation model generators as well as using an analytical power ansatz. Comparing these measurements to those obtained at MZ, the energy dependence (running) of αS is accessible. The logarithmic energy slope of the inverse strong coupling is measured to be d αS^-1/d log (Ecm) = 1.39 + 0.34(stat) + 0.17(syst), in good agreement with the QCD expectation of 1.27. © 1999 Published by Elsevier Science B.V. All rights reserved.

Abstract:

Inclusive charged hadron distributions as obtained from the DELPHI measurements at 130, 136, 161, 172 and 183 GeV are presented as a function of the variables rapidity, ξp, p and transversal momenta. Data are compared with event generators and with MLLA calculations, in order to examine the hypothesis of local parton hadron duality. The differential momentum spectra show an indication for coherence effects in the production of soft particles. The relation between the energy dependence of the charged multiplicity and the rapidity distribution is examined. © 1999 Elsevier Science B.V. All rights reserved.

Abstract:

Knowledge of the centre-of-mass energy at LEP2 is of primary importance to set the absolute energy scale for the measurement of the W-boson mass. The beam energy above 80 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is calibrated against precise measurements of the average beam energy between 41 and 55GeV made using the resonant depolarisation technique. The linearity of the relationship is tested by comparing the fields measured by the probes with the total bending field measured by a flux loop. This test results in the largest contribution to the systematic uncertainty. Several further corrections are applied to derive the centre-of-mass energies at each interaction point. In addition, the centre-of-mass energy spread is evaluated. The beam energy has been determined with a precision of 25 MeV for the data taken in 1997, corresponding to a relative precision of 2.7 × 10-4. This is small in comparison to the present uncertainty on the W mass measurement at LEP. However, the ultimate statistical precision on the W mass with the full LEP2 data sample should be around 25 MeV, and a smaller uncertainty on the beam energy is desirable. Prospects for improvements are outlined.

Abstract:

Exclusive production of ρ⁰ and J/ψ mesons in e⁺p collisions has been studied with the ZEUS detector in the kinematic range 0.25 < Q² < 50 GeV², 20 < W < 167 GeV for the ρ⁰ data and 2 < Q² < 40 GeV², 50 < W < 150 GeV for the J/ψ data. Cross sections for exclusive ρ⁰ and J/ψ production have been measured as a function of Q², W and t. The spin-density matrix elements r⁰⁴ 00, r¹ 1-1i and Re r⁵ 10 have been determined for exclusive ρ⁰ production as well as r⁰⁴ 00 and r⁰⁴ 1-1 for exclusive J/ψ production. The results are discussed in the context of theoretical models invoking soft and hard phenomena.

Abstract:

The inclusive forward jet cross section in deep inelastic e+p scattering has been measured in the region of cursive Greek chi-Bjorken, 4.5 · 10-4 to 4.5 · 10-2. This measurement is motivated by the search for effects of BFKL-like parton shower evolution. The cross section at hadron level as a function of cursive Greek chi is compared to cross sections predicted by various Monte Carlo models. An excess of forward jet production at small cursive Greek chi is observed, which is not reproduced by models based on DGLAP parton shower evolution. The Colour Dipole model describes the data reasonably well. Predictions of perturbative QCD calculations at the parton level based on BFKL and DGLAP parton evolution are discussed in the context of this measurement.