Guy Wilkinson

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.

Abstract:

The DELPHI experiment at LEP uses Ring Imaging Cherenkov detectors for particle identification. The good understanding of the RICH detectors allows the identification of charged pions, kaons and protons, covering the full momentum range from 0.7 < p < 45.6GeV/c. The π±, K±, p and p̄ normalised production rates, their differential cross sections, multiplicities and the maxima ξ*p of_the ξp = ln (1/Xp) distributions are measured for three event samples Z0 → qq̄, Z0 → bb̄, Z0 → uū, dd̄, ss̄, selected from ∼ 1 400 000 Z0 decays collected by DELPHI in 1994. The results are compared to the predictions of the JETSET string fragmentation model and the HERWIG cluster fragmentation model. The Modified Leading Logarithm Approximation with Local Parton-Hadron Duality is tested. The ξ*p dependence on the primary quark flavour is investigated and quantified for the different particle distributions. The π± , K±, p and p̄ multiplicities are measured with precisions from ±4% to ±6%. Forche Z0 → qq̄, and Z0 → bb̄, event samples, these improve on previous measurements. The π±, K±, p and p̄ multiplicities for Z0 → uū, dd̄, ss̄ are presented for the first time.