Showing papers by "José Salt published in 2005"

Flavour independent searches for hadronically decaying neutral Higgs bosons

Abstract: This paper describes flavour independent searches for hadronically decaying neutral Higgs bosons in the data collected by the DELPHI experiment at LEP, at centre-of-mass energies between 189 and 209 GeV. The collected data-set corresponds to an integrated luminosity of around 610 pb(-1). The e(+)e(-) -> hA and e(+)e(-) -> hZand processes are considered, with direct Higgs boson decays into hadrons. No evidence for Higgs boson production is found, and cross-section limits are set as a function of the Higgs boson masses. No explicit assumptions are made on the underlying physics beyond the Standard Model, allowing interpretation of the data in a large class of models.

Measurement of the energy dependence of hadronic jet rates and the strong coupling alpha(s) from the four-jet rate with the DELPHI detector at LEP

Abstract: Hadronic events from the data collected with the DELPHI detector at LEP within the energy range from 89 GeV to 209 GeV are selected, their jet rates are determined and compared to predictions of four different event generators. One of them is the recently developed APACIC++ generator which performs a massive matrix element calculation matched to a parton shower followed by string fragmentation. The four-jet rate is used to measure alpha_s in the next-to-leading-order approximation yielding alpha_s(M_Z^2) = 0.1175 +/- 0.0030. The running of alpha_s determined by using four-jet events has been tested. The logarithmic energy slope is measured to be d\alpha_s^{-1} / d\log E_{cm} = 1.14 +/- 0.36. Since the analysis is based on four-jet final states it represents an alternative approach to previous DELPHI alpha_s measurements using event shape distributions.

Charged Particle Multiplicity in Three-Jet Events and Two-Gluon Systems

Abstract: The charged particle multiplicity in hadronic three-jet events from Z decays is investigated. The topology dependence of the event multiplicity is found to be well described by a modified leading logarithmic prediction. A parameter fit of the prediction to the data yields a measurement of the colour factor ratio C_A/C_F with the result C_A/C_F = 2.261 +/- 0.014 (stat.) +/- 0.036 (exp.) +/- 0.066 (theo.) in agreement with the SU(3) expectation of QCD. The quark-related contribution to the event multiplicity is subtracted from the three-jet event multiplicity resulting in a measurement of the multiplicity of two-gluon colour-singlet states over a wide energy range. The ratios r=N_{gg}(s)/N_{q qbar}(s) of the gluon and quark multiplicities and r^{(1)}=N_{gg}'(s)/N_{q qbar}'(s) of their derivatives are compared with perturbative calculations. While a good agreement between calculations and data is observed for r^{(1)}, larger deviations are found for r indicating that non-perturbative effects are more important for r than for r^{(1)}.

Production of Xi(0)(c) and Xi(b) in Z decays and lifetime measurement of Xi(b)

Abstract: The charmed strange baryon Ξ0c was searched for in the decay channel Ξ0c → Ξ−π+, and the beauty strange baryon Ξb in the inclusive channel Ξb → Ξ−−¯νX, using the 3.5 million hadronic Z events collected by the DELPHI experiment in the years 1992–1995. The Ξ− was reconstructed through the decay Ξ− → Λπ−, using a constrained fit method for cascade decays. An iterative discriminant analysis was used for the Ξ0c and Ξb selection. The production rates were measured to be fΞ0c ×BR(Ξ0c → Ξ−π+) = (4.7 ± 1.4(stat.) ± 1.1(syst.)) × 10−4 per hadronic Z decay, and BR(b → Ξb)×BR(Ξb → Ξ−−X) = (3.0 ± 1.0(stat.) ± 0.3(syst.)) × 10−4 for each lepton species (electron or muon). The lifetime of the Ξb baryon was measured to be τΞb = 1.45+0.55 −0.43(stat.) ± 0.13(syst.) ps. A combination with the previous DELPHI lifetime measurement gives τΞb = 1.48+0.40 −0.31(stat.) ± 0.12(syst.) ps.