Showing papers by "Paolo Ronchese published in 2008"
TL;DR: In this paper, the effect of the heavy b-quark mass on the two, three and four-jet rates was studied using LEP data collected by the DELPHI experiment at the Z peak in 1994 and 1995.
Abstract: The effect of the heavy b-quark mass on the two, three and four-jet rates is studied using LEP data collected by the DELPHI experiment at the Z peak in 1994 and 1995. The rates of b-quark jets and light quark jets (l = uds) in events with n = 2, 3, and 4 jets, together with the ratio of two and four-jet rates of b-quarks with respect to light-quarks, R_n^bl, have been measured with a double-tag technique using the CAMBRIDGE jet-clustering algorithm. A comparison between experimental results and theory (matrix element or Monte Carlo event generators such as PYTHIA, HERWIG and ARIADNE) is done after the hadronisation phase. Using the four-jet observable R_4^bl, a measurement of the b-quark mass using massive leading-order calculations gives: m_b(M_Z) = 3.76 +/- 0.32 (stat) +/- 0.17 (syst) +/- 0.22 (had) +/- 0.90 (theo) GeV/c^2 . This result is compatible with previous three-jet determinations at the M_Z energy scale and with low energy mass measurements evolved to the M_Z scale using QCD Renormalisation Group Equations.
45 citations
TL;DR: In this paper, a measurement of the W boson mass and width has been performed by the DELPHI collaboration using the data collected during the full LEP2 programme (1996-2000).
Abstract: A measurement of the W boson mass and width has been performed by the DELPHI collaboration using the data collected during the full LEP2 programme (1996-2000). The data sample has an integrated luminosity of 660 pb^-1 and was collected over a range of centre-of-mass energies from 161 to 209 GeV. Results are obtained by applying the method of direct reconstruction of the mass of the W from its decay products in both the W+W- -> lvqq and W+W- -> qqqq channels. The W mass result for the combined data set is M_W = 80.336 +/- 0.055 (Stat.) +/- 0.028 (Syst.) +/- 0.025 (FSI) +/- 0.009 (LEP) GeV/c^2, where FSI represents the uncertainty due to final state interaction effects in the qqqq channel, and LEP represents that arising from the knowledge of the collision energy of the accelerator. The combined value for the W width is Gamma_W = 2.404 +/- 0.140 (Stat.) +/- 0.077 (Syst.) +/- 0.065 (FSI) GeV/c^2. These results supersede all values previously published by the DELPHI collaboration. This paper is dedicated to the memory of Carlo Caso.
37 citations
TL;DR: In this paper, a determination of the single W Spin Density Matrix (SDM) elements in the reaction e+e- -> W+W- -> l nu q qbar (l=e/mu) is reported at center-of-mass energies between 189 and 209 GeV.
Abstract: A determination of the single W Spin Density Matrix (SDM) elements in the reaction e+e- -> W+W- -> l nu q qbar (l=e/mu) is reported at centre-of-mass energies between 189 and 209 GeV. The data sample used corresponds to an integrated luminosity of 520 pb^{-1} taken by DELPHI between 1998 and 2000. The single W SDM elements, rho_{tau tau'}^{W+-} (tau,tau' = +/-1 or 0), are determined as a function of the W- production angle with respect to the e- beam direction and are obtained from measurements of the W decay products by the application of suitable projection operators, Lambda_{tau tau'}, which assume the V-A coupling of the W boson to fermions. The measured SDM elements are used to obtain the fraction of longitudinally polarised Ws, with the result: sigma_L/sigma_tot = 24.9 +/- 4.5(stat) +/- 2.2(syst) % at a mean energy of 198 GeV. The SDM elements are also used to determine the Triple Gauge Couplings Delta g_1^Z, Delta kappa_gamma, lambda_gamma and g_4^Z, ~kappa_Z and ~lambda_Z. For the CP-violating couplings the results of single parameter fits are: g_4^Z = -0.39 +0.19 -0.20 ~kappa_Z = -0.09 +0.08 -0.05 ~lambda_Z = -0.08 +/- 0.07 . The errors are a combination of statistical and systematic errors. All results are consistent with the Standard Model.
34 citations
Abstract: Muon bremsstrahlung photons converted in front of the DELPHI main tracker (TPC) in dimuon events at LEP1 were studied in two photon kinematic ranges: 0.2 < Eγ ≤ 1 GeV and transverse momentum with respect to the parent muon pT < 40 MeV/c, and 1 < Eγ ≤ 10 GeV and pT < 80 MeV/c. A good agreement of the observed photon rate with predictions from QED for the muon inner bremsstrahlung was found, contrary to the anomalous soft photon excess that has been observed recently in hadronic Z0 decays. The obtained ratios of the observed signal to the predicted level of the muon bremsstrahlung are 1.06 ± 0.12 ± 0.07 in the photon energy range 0.2 < Eγ ≤ 1 GeV and 1.04 ± 0.09 ± 0.12 in the photon energy range 1 < Eγ ≤ 10 GeV. The bremsstrahlung dead cone is observed for the first time in the direct photon production at LEP.
20 citations
TL;DR: In this article, the results from DELPHI on the searches for Higgs bosons in the minimal supersymmetric extension of the Standard Model (MSSM) are considered, that include CP conservation and explicit CP violation.
Abstract: This paper presents the final interpretation of the results from DELPHI on the searches for Higgs bosons in the minimal supersymmetric extension of the Standard Model (MSSM). A few representative scenarios are considered, that include CP conservation and explicit CP violation in the Higgs sector. The experimental results encompass the searches for neutral Higgs bosons at LEP1 and LEP2 in final states as expected in the MSSM, as well as LEP2 searches for charged Higgs bosons and for neutral Higgs bosons decaying into hadrons independent of the quark flavour. The data reveal no significant excess with respect to background expectations. The results are translated into excluded regions of the parameter space in the various scenarios. In the CP-conserving case, these lead to limits on the masses of the lightest scalar and pseudoscalar Higgs bosons, h and A, and on tan beta. The dependence of these limits on the top quark mass is discussed. Allowing for CP violation reduces the experimental sensitivity to Higgs bosons. It is shown that this effect depends strongly on the values of the parameters responsible for CP violation in the Higgs sector.
7 citations
TL;DR: In this paper, the production of two high-p T jets in the interactions of quasi-real photons in e + e - collisions was studied with data corresponding to an integrated e+ e - luminosity of 550 pb-1.
Abstract: The production of two high- p T jets in the interactions of quasi-real photons in e + e - collisions at sqrt{s_{ee}} from 189 GeV to 209 GeV is studied with data corresponding to an integrated e + e - luminosity of 550 pb-1. The jets reconstructed by the k ⊥ -cluster algorithm are defined within the pseudo-rapidity range -1< η<1 and with jet transverse momentum, p T , above 3 GeV/c. The differential di-jet cross-section is measured as a function of the mean transverse momentum overline{pT} of the jets and is compared to perturbative QCD calculations.
1 citations
TL;DR: The first measurement of the average polarisation P-tau of tau leptons produced in e(+)e(-) annihilation at energies significantly above the Z resonance is presented in this article.