28,562 research outputs found
The DELPHI detector at LEP
No full textDELPHI is a 4π detector with emphasis on particle identification, three-dimensional information, high granularity and precise vertex determination. The design criteria, the construction of the detector and the performance during the first year of operation at the large electron positron collider (LEP) at CERN are described
Multiplicity fluctuations in one- and two-dimensional angular intervals compared with analytic QCD calculations
No full textMultiplicity fluctuations in rings around the jet axis and in off-axis canes have been measured by the DELPHI collaboration in e(+)e(-) annihilations into hadrons at LEP energies. The measurements are compared with analytical perturbative QCD calculations for the corresponding multiparton system, using the concept of Local Parton Hadron Duality. Some qualitative features are confirmed by the data but substantial quantitative deviations are observed. (C) 1999 Published by Elsevier Science B.V. All rights reserved
Measurement of the partial decay width R(b)(0)=Gamma(b(b)over-bar)/Gamma(had) of the Z with the DELPHI detector at LEP
No full textThe partial decay width of the Z to quark pairs has been measured by the DELPHI detector at LEP. B-hadrons, containing b-quarks, were tagged by several methods using tracks with large impact parameters to the primary vertex complemented sometimes by event shape variables or using leptons with high transverse momentum relative to the hadron, In order to reduce the systematic uncertainties, in all methods the b-tagging efficiency has been extracted directly from the data. Combining all methods, the value: Gamma()/Gamma(had) = 0.2216 +/- 0.0016(stat.) +/- 0.0021(syst.) was found, where the production fraction was fixed to its Standard Model value
Measurements of the leptonic branching fractions of the tau
No full textData collected with the DELPHI detector from 1993 to 1995 combined with previous DELPHI results for data from 1991 and 1992 yield the branching fractions B(tau --> e nu) = (17.877 +/- 0.109(stat) +/- 0.110(sys))% and B(tau --> mu nu) = (17.325 +/- 0.095(stat) +/- 0.077(sys))%
Search for the Standard Model Higgs Boson at LEP
No full textThe four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have collected a total of 2461 pb-1 of e+e- collision data at centre-of-mass energies between 189 and 209 GeV. The data are used to search for the Standard Model Higgs boson. The search results of the four collaborations are combined and examined in a likelihood test for their consistency with two hypotheses: the background hypothesis and the signal plus background hypothesis. The corresponding confidences have been computed as functions of the hypothetical Higgs boson mass. A lower bound of 114.4 GeV/c2 is established, at the 95% confidence level, on the mass of the Standard Model Higgs boson. The LEP data are also used to set upper bounds on the HZZ coupling for various assumptions concerning the decay of the Higgs boson
Search for exclusive decays of the Lambda(b) baryon and measurement of its mass
No full textA search for fully reconstructed Lambda(b)(0) beauty baryons is performed using about 3 million Z decays collected with the DELPHI detector at LEP. The analysis relies on the combined use of the accurate tracking and of the hadron identification capabilities of DELPHI. A total of four events has been found, three in the Lambda(c)(+)pi(-) channel and one in the Lambda(c)(+)a(1)(-) channel over a small background. The Lambda(b)(0) beauty baryon mass is measured to be (5668 +/- 16 (stat.) +/-8 (syst.)) MeV/c(2)
Determination of the average lifetime of b-baryons
No full textThe average lifetime of b-baryons has been studied using 3 x 10(6) hadronic Z(0) decays collected by the DELPHI detector at LEP. Three methods have been used, based on the measurement of different observables: the proper decay time distribution of 206 vertices reconstructed with a Lambda, a lepton and an oppositely charged pion; the impact parameter distribution of 441 muons with high transverse momentum accompanied by a Lambda in the same jet; and the proper decay time distribution of 125 Lambda(c)-lepton decay vertices with the Lambda(c) exclusively reconstructed through its pK pi, pK(0) and Lambda 3 pi decay modes. The combined result is: [GRAPHICS] where the first systematic error is due to experimental uncertainties and the second to the uncertainties in the modelling of the b-baryon production and semi-leptonic decay, Including the measurement recently published by DELPHI based on a sample of proton-muon vertices, the average b-baryon lifetime is: [GRAPHICS] [GRAPHICS
Measurement of the charged particle multiplicity of weakly decaying B hadrons
No full textFrom the Z decays recorded in 1994 and 1995 by the DELPHI detector at LEP, the charged particle multiplicity of weakly decaying B hadrons was measured to be: 4.97 +/- 0.03 +/- 0.06 excluding the K-o and Lambda decay products. (C) 1998 Published by Elsevier Science B.V. All rights reserved
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