LHCb

Abstract:

This paper presents the first analysis of diffractive photon dissociation events in deep inelastic positron-proton scattering at HERA in which the proton in the final state is detected and its momentum measured. The events are selected by requiring a scattered proton in the ZEUS leading proton spectrometer (LPS) with xL > 0.97, where xL is the fraction of the incoming proton beam momentum carried by the scattered proton. The use of the LPS significantly reduces the contamination from events with diffractive dissociation of the proton into low mass states and allows a direct measurement of t, the square of the four-momentum exchanged at the proton vertex. The dependence of the cross section on t is measured in the interval 0.073 < \t\ < 0.4 GeV² and is found to be described by an exponential shape with the slope parameter b = 7.2 ± 1.1(stat.)^+0.7 -0.9(syst.) GeV^-2. The diffractive structure function F^D(4) 2 is presented as a function of xIP, ≃ 1 - xL and β, the momentum fraction of the struck quark with respect to xIP, and averaged over the t interval 0.073 < \t\ < 0.4 GeV² and the photon virtuality range 5 < Q² < 20 GeV². In the kinematic range 4 x 10^-4 < xIP < 0.03 and 0.015 < β < 0.5, the xIP dependence of F^D(4) 2 is fitted with a form (1/xIP)^a , yielding a = 1.00 ± 0.09 (stat.)^+0.11 -0.05(syst.). Upon integration over t, the structure function F^D(3) 2 is determined in a kinematic range extending to higher xIP and lower β compared to our previous analysis; the results are discussed within the framework of Regge theory.

Abstract:

Photon diffractive dissociation, γp → Xp, has been studied at HERA with the ZEUS detector using ep interactions where the virtuality Q2 of the exchanged photon is smaller than 0.02 GeV2. The squared four-momentum t exchanged at the proton vertex was determined in the range 0.073 < \t\ < 0.40 GeV2 by measuring the scattered proton in the ZEUS Leading Proton Spectrometer. In the photonproton centre-of-mass energy interval 176 < W < 225 GeV and for masses of the dissociated photon system 4 < Mx < 32 GeV, the t distribution has an exponential shape, dN/d\t\ ∝ exp (-b\t\), with a slope parameter b = 6.8 ± 0.9 (stat.) +1.2-1.1 (syst.) GeV-2.

Abstract:

In an analysis of multihadronic events recorded at LEP by DELPHI in the years 1992 through 1994, rapidity correlations of A-A, proton-proton, and A-proton pairs are compared with each other and with the predictions of the string fragmentation model. For Ap̄ pairs, the additional correlation with respect to charged kaons is also analysed. © 1998 Elsevier Science B.V.

Abstract:

An update of the searches for charginos and neutralinos in DELPHI is presented, based mainly on recent data collected at centre-of-mass energies of 161 GeV and 172 GeV. No signal is found. For a sneutrino with mass above 300 GeV/c2 and a mass difference between the chargino and the lightest neutralino above 10 GeV/c2, the lower limit at 95% confidence level on the chargino mass ranges from 84.3 GeV/c2 to the kinematical limit (86.0 GeV/c2), depending on the mixing parameters. The limit decreases for lower chargino-neutralino mass differences. The limit in the case of a light sneutrino is 67.6 GeV/c2, provided that that there is no light sneutrino with a mass within 10 GeV/c2 below the chargino mass. Upper limits on neutralino pair production cross-sections of about a picobarn are derived. The (μ, M2) domain excluded in the MSSM-GUT scenario is determined by combining the neutralino and chargino searches. These results imply a limit on the mass of the lightest neutralino which, for a heavy sneutrino, is constrained to be above 24.9 GeV/c2 for tanβ ≥ 1. The search has also been extended to the case where the lightest neutralino is unstable and decays into a photon and a gravitino.

Abstract:

A search for neutral and charged Higgs bosons has been performed in the data collected by the DELPHI detector at centre-of-mass energies of 161 GeV and 172 GeV. The analysis assumes either the pair-production of charged Higgs bosons, H±, or the production of the lightest neutral Higgs boson, h, with either a Z or a neutral pseudoscalar Higgs boson, A. All final state topologies expected from the decay of h and A into hadrons or a pair of τ leptons, and from the decay of H± into a pair of quarks or a τντ pair have been considered. Lower limits at the 95% confidence level have been obtained on the Higgs boson masses. The limits are 66.2 GeV/c2 for h in the Standard Model, 59.5 GeV/c2 for h and 51.0 GeV/c2 for A in the minimal supersymmetric extension of the Standard Model with sensible assumptions on the model parameters and, using the 161 GeV data only, 51.5 GeV/c2 for H± in the general two-doublet scheme for H± branching fractions into hadrons below 0.8.