LHCb

Abstract:

Neutral Higgs bosons were searched for in the data collected by DELPHI at a centre-of-mass energy of 183 GeV, corresponding to an integrated luminosity of 54 pb-1. The production of the lightest neutral Higgs boson with either an on-shell Z boson or a neutral pseudo-scalar Higgs boson was analysed. Lower limits at the 95% confidence level were obtained on the Higgs boson masses. The limits are 85.7 GeV/c2 for the Standard Model Higgs boson and 74.4 GeV/c2 for the scalar and 75.3 GeV/c2 for the pseudo-scalar Higgs bosons as predicted by the minimal super-symmetric extension of the Standard Model with commonly used assumptions on the model parameters, plus the assumption that the mass of the pseudo-scalar boson is greater than 20 GeV/c2. These results significantly improve the limits reached with previous data.

Abstract:

The events with two photons and missing (transverse) energy collected by the DELPHI detector at centre-of-mass energies between 130 GeV and 183 GeV have been studied to search for processes of the type e+e- → YY with the subsequent decay Y → Xγ, where X is an undetectable neutral particle. Reactions of this kind are expected in supersymmetric models, where the Y particle can be either the lightest neutralino, decaying to a photon and a gravitino, or the next-to-lightest neutralino, decaying to a photon and the lightest neutralino. To study the case of long-lived Y particles, a search for single-photon events with the reconstructed photon axis pointing far from the beam interaction region has also been performed. No evidence for a deviation from Standard Model expectations has been observed in the data and upper limits have been derived on the signal cross-section as a function of the the X and Y masses and of the Y mean decay path.

Abstract:

Data taken by DELPHI during the 1995 and 1996 LEP runs have been used to search for the supersymmetric partners of electron, muon and tau leptons and of top and bottom quarks. The observations are in agreement with standard model predictions. Limits are set on sfermion masses. Searches for long lived scalar leptons from low scale supersymmetry breaking models exclude stau masses below 55 GeV/c2 at the 95% confidence level irrespective of the gravitino mass.

Abstract:

A search for the Higgs boson in final states with one, two or three isolated photons has been performed based on data taken at LEP 2 by the DELPHI detector. The data analysed correspond to a total integrated luminosity of 67.5 pb-1 at centre-of-mass energies of 161 GeV (9.7 pb-1), 172 GeV (10.1 pb-1) and 183 GeV (47.7 pb-1). No evidence for the processes e+e- → Hγ with H → bb̄ or γγ and e+e- → Hqq̄ with H → γγ was observed. Model-independent limits on σ(e+e- → Hγ) × BR(H → bb̄), σ(e+e- → Hqq̄) × BR(H γγ) and σ(e+e- → Hγ) × BR(H → γγ) are set, as well as model-dependent limits on Higgs boson anomalous couplings to vector bosons. © 1999 Published by Elsevier Science B.V. All rights reserved.

Abstract:

The four-jet events collected by DELPHI during the special LEP run at centre-of-mass energies of 130 and 136 GeV in 1997 with an integrated luminosity of 5.9pb-1 are analysed. Their rate and the distributions of their di-jet masses, their smallest jet charges, and their di-jet charge separations all agree well with Standard Model predictions. Thus the hypothesis of pair production of a new particle with a sum of di-jet masses around 105 GeV/c2 is not supported. The combined result of the four LEP collaborations refuting this hypothesis at over 99% confidence level is also given. © 1999 Published by Elsevier Science B.V. All rights reserved.