Determination of the b quark mass at the M_Z scale with the DELPHI detector at LEP

An experimental study of the normalized three-jet rate of b quark events with respect to light quarks events (light= \ell \equiv u,d,s) has been performed using the CAMBRIDGE and DURHAM jet algorithms. The data used were collected by the DELPHI experiment at LEP on the Z peak from 1994 to 2000. The results are found to agree with theoretical predictions treating mass corrections at next-to-leading order. Measurements of the b quark mass have also been performed for both the b pole mass: M_b and the b running mass: m_b(M_Z). Data are found to be better described when using the running mass. The measurement yields: m_b(M_Z) = 2.85 +/- 0.18 (stat) +/- 0.13 (exp) +/- 0.19 (had) +/- 0.12 (theo) GeV/c^2 for the CAMBRIDGE algorithm. This result is the most precise measurement of the b mass derived from a high energy process. When compared to other b mass determinations by experiments at lower energy scales, this value agrees with the prediction of Quantum Chromodynamics for the energy evolution of the running mass. The mass measurement is equivalent to a test of the flavour independence of the strong coupling constant with an accuracy of 7 permil.Comment: 24 pages, 10 figures, Accepted by Eur. Phys. J.

Study of double-tagged gamma gamma events at LEPII

Double-tagged interactions of photons with virtualities Q^2 between 10 GeV^2 and 200 GeV^2 are studied with the data collected by DELPHI at LEPII from 1998 to 2000, corresponding to an integrated luminosity of 550 pb^{-1}. The gam* gam* -> mu+mu- data agree with QED predictions. The cross-section of the reaction gam* gam* -> hadrons is measured and compared to the LO and NLO BFKL calculations.Comment: 17 pages, 9 figures, Accepted by Eur. Phys. J.

Search for an LSP gluino at LEP with the DELPHI detector

In some supersymmetric models, the gluino is predicted to be light and stable. In that case, it would hadronize to form R-hadrons. In these models, the missing energy signature of the lightest supersymmetric particle is no longer valid, even if R-parity is conserved. Therefore, such a gluino is not constrained by hadron collider results, which looked for the decay gluino -> q qbar \tilde{\chi}_1^0. Data collected by the DELPHI detector in 1994 at 91.2 GeV have been analysed to search for (q qbar gluino gluino) events. No deviation from Standard Model predictions is observed and a gluino mass between 2 and 18 GeV/c^2 is excluded at the 95% confidence level in these models. Then, R-hadrons produced in the squark decays were searched for in the data collected by DELPHI at the centre-of-mass energies of 189 to 208 GeV, corresponding to an overall integrated luminosity of 609 pb^-1. The observed number of events is in agreement with the Standard Model predictions. Limits at 95% confidence level are derived on the squark masses from the excluded regions in the plane (m_\tilde{q_1},m_gluino): m_\tilde{t_1} > 90 GeV/c^2, and m_\tilde{b_1} > 96 GeV/c^2 for purely left squarks. m_\tilde{t_1} > 87 GeV/c^2, and m_\tilde{b_1} > 82 GeV/c^2 independent of the mixing angle.Comment: 36 pages, 19 figure

Inclusive measurements of the K+- and p / anti-p production in hadronic Z0 decays

This analysis, based on a sample of 170000 hadronic Z0 decays, provides a measurement of the K± and p/p̄ differential cross sections which is compared to string- and cluster fragmentation models. The total multiplicities for K± and p/p̄ per hadronic event were found to be: NK = 2.26 ± 0.18 and Np = 1.07 ± 0.14. The positions ξ* of the maxima of the differential cross sections as a function of ξ = In(1/xp) for K± and p/p̄ were determined to be 2.63 ± 0.07 and 2.96 ± 0.16 respectively. A comparison of the ξ* values for various identified particles measured at LEP with the prediction of the Modified Leading Logarithm Approximation with Local Parton Hadron Duality model has been performed. The measured ξ* position as a function of the hadron mass, after corrections due to particle decays, is in agreement with the model calculation.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe