LHCb

Abstract:

An update of the searches for charginos and neutralinos is presented, based on a data sample corresponding to the 53.9 pb-1 recorded by the DELPHI detector in 1997, at a centre-of-mass energy of 183 GeV. No evidence for a signal was found. The lower mass limits are 4-5 GeV/c2 higher than those obtained at a centre-of-mass energy of 172 GeV. The (μ,M2) domain excluded by combining the neutralino and chargino searches implies a limit on the mass of the lightest neutralino which, for a heavy sneutrino, is constrained to be above 29.1 GeV/c2 for tan β ≥ 1. © 1999 Elsevier Science B.V. All rights reserved.

Abstract:

A search for charginos with masses close to the mass of the lightest neutralino is reported, based on the data collected with the DELPHI detector at LEP from 1995 to 1997 at centre-of-mass energies between 130 and 183 GeV. The signature of a photon at high transverse momentum radiated from the initial state reduces the two-photon background to acceptable rates, thus making the mass differences between a few hundred MeV/c2 and 3 GeV/c2 detectable. In very nearly degenerate scenarios, the lifetime of the chargino can be large enough to produce either visible secondary vertices or decays outside the detector; therefore, quasi-stable heavy charged particles and displaced decay vertices were also searched for. No excess of events with respect to the Standard Model expectations was observed, and limits in the plane of chargino-neutralino mass difference versus chargino mass are given.

Abstract:

A search for unstable heavy fermions with the DELPHI detector at LEP is reported. Sequential and non-canonical leptons, as well as excited leptons and quarks, are considered. The data analysed correspond to an integrated luminosity of about 48 pb-1 at an e+e- centre-of-mass energy of 183 GeV and about 20 pb-1 equally shared between the centre-of-mass energies of 172 GeV and 161 GeV. The search for pair-produced new leptons establishes 95% confidence level mass limits in the region between 70 GeV/c2 and 90 GeV/c2, depending on the channel. The search for singly produced excited leptons and quarks establishes upper limits on the ratio of the coupling of the excited fermion to its mass (λ/mf*) as a function of the mass.

Abstract:

A search for events with one jet and at most one isolated lepton used data taken at LEP-2 by the DELPHI detector. These data were accumulated at a center-of-mass energy of 183 GeV and correspond to an integrated luminosity of 47.7 pb-1. Production of single scalar and vector leptoquarks was searched for. Limits at 95% confidence leva were derived on the masses (ranging from 134 GeV/c2 to 171 GeV/c2 for electromagnetic type couplings) and couplings of the leptoquark states. A search for top-charm flavour changing neutral currents (e+e-→t̄c or charge conjugate) used the semileptonic decay channel. A limit on the flavour changing cross-section via neutral currents was set at 0.55pb (95% confidence level). © 1999 Elsevier Science B.V. All rights reserved.

Abstract:

Promptly decaying lightest neutralinos and long-lived staus are searched for in the context of light gravitino scenarios. It is assumed that the stau is the next to lightest supersymmetric particle (NLSP) and that the lightest neutralino is the next to NLSP (NNLSP). Data collected with the Delphi detector at centre-of-mass energies from 161 to 183 GeV are analysed. No evidence of the production of these particles is found. Hence, lower mass limits for both kinds of particles are set at 95% C.L.. The mass of gaugino-like neutralinos is found to be greater than 71.5GeV/c2. In the search for long-lived stau, masses less than 70.0 to 77.5GeV/c2 are excluded for gravitino masses from 10 to 150eV/c2. Combining this search with the searches for stable heavy leptons and minimal supersymmetric standard model staus a lower limit of 68.5GeV/c2 may be set for the stau mass independent of the mass of the gravitino.