Recent Results on Particle Production from OPAL

Three recent OPAL studies are presented in which the fragmentation process in quark and gluon jets and in identified up, down and strange flavour jets is studied. The first is a measurement of charged particle, pi0, eta and K0 multiplicities in quark and gluon jets. No evidence is found for a particle-species dependent multiplicity enhancement in gluon jets. In another study leading pion, kaon, K0, proton and lambda rates have been measured in up, down and strange flavour jets. The results confirm the leading particle effect in the fragmentation of light flavour jets. In addition, a direct determination of the strangeness suppression factor has been performed, yielding gamma_s=0.422 +/- 0.049(stat.) +/- 0.059(syst.). In a third study, mean charged particle multiplicities were measured for up, down and strange flavoured Z0 decays and found to be identical within the uncertainties of the measurement, as expected from the flavour independence of the strong interaction.Comment: 6 pages, LaTeX, to appear in the proceedings of the 30th International Symposium on Multiparticle Dynamics, October 9 - 15, 2000, Tihany, Hungar

Jets and multiplicities in e+e-

Recent results on jet and inclusive charged particle production in hadronic e+e- interactions are reviewed.Comment: 5 pages, to appear in the proceedings of the EPS/HEP-2001 conference, Budapest, Hungar

Tyrosine phosphorylation-dependent activation of phosphatidylinositide 3-kinase occurs upstream of Ca^(2+)-signalling induced by Fcy receptor cross-linking in human neutrophils

The effect of wortmannin on IgG-receptor (FcyR)-mediated stimulation of human neutrophils was investigated. The Ca^(2+) influx induced by clustering of both Fcy receptors was inhibited by wortmannin, as was the release of Ca^(2+) from intracellular stores. Wortmannin also inhibited, with the same efficacy, the accumulation of Ins(1,4,5)P3 observed after FcyR stimulation, but did not affect the increase in Ins(1,4,5)P3 induced by the chemotactic peptide, formyl-methionine-leucine-phenylalanine. Because wortmannin is, in the concentrations used here, an inhibitor of PtdIns 3-kinase, these results suggested a role for PtdIns 3-kinase upstream of Ca^(2+) signalling, induced by FcyR cross-linking. Support for this notion was obtained by investigating the effect of another inhibitor of PtdIns 3-kinase, LY 294002, and by studying the kinetics of PtdIns 3-kinase activation. We found translocation of PtdIns 3-kinase to the plasma membrane and increased PtdIns 3-kinase activity in the membrane as soon as 5 s after FccR cross-linking, even before the onset of the Ca^(2+) response. Moreover, the translocation of PtdIns 3-kinase to the plasma membrane was inhibited by cocross- linking of either FcyRIIa and FcyRIIIb with the tyrosine phosphatase, CD45, indicating a requirement for protein tyrosine phosphorylation in the recruitment of PtdIns 3-kinase to the plasma membrane. Taken together, our results suggest a role for PtdIns 3-kinase in early signal transduction events after FcyR cross-linking in human neutrophils

The Presampler for the Forward and Rear Calorimeter in the ZEUS Detector

The ZEUS detector at HERA has been supplemented with a presampler detector in front of the forward and rear calorimeters. It consists of a segmented scintillator array read out with wavelength-shifting fibers. We discuss its desi gn, construction and performance. Test beam data obtained with a prototype presampler and the ZEUS prototype calorimeter demonstrate the main function of this detector, i.e. the correction for the energy lost by an electron interacting in inactive material in front of the calorimeter.Comment: 20 pages including 16 figure

Measurement of inclusive D*+- and associated dijet cross sections in photoproduction at HERA

Inclusive photoproduction of D*+- mesons has been measured for photon-proton centre-of-mass energies in the range 130 < W < 280 GeV and a photon virtuality Q^2 < 1 GeV^2. The data sample used corresponds to an integrated luminosity of 37 pb^-1. Total and differential cross sections as functions of the D* transverse momentum and pseudorapidity are presented in restricted kinematical regions and the data are compared with next-to-leading order (NLO) perturbative QCD calculations using the "massive charm" and "massless charm" schemes. The measured cross sections are generally above the NLO calculations, in particular in the forward (proton) direction. The large data sample also allows the study of dijet production associated with charm. A significant resolved as well as a direct photon component contribute to the cross section. Leading order QCD Monte Carlo calculations indicate that the resolved contribution arises from a significant charm component in the photon. A massive charm NLO parton level calculation yields lower cross sections compared to the measured results in a kinematic region where the resolved photon contribution is significant.Comment: 32 pages including 6 figure

Search for Yukawa Production of a Light Neutral Higgs Boson at LEP

Within a Two-Higgs-Doublet Model (2HDM) a search for a light Higgs boson in the mass range of 4-12 GeV has been performed in the Yukawa process e+e- -> b bbar A/h -> b bbar tau+tau-, using the data collected by the OPAL detector at LEP between 1992 and 1995 in e+e- collisions at about 91 GeV centre-of-mass energy. A likelihood selection is applied to separate background and signal. The number of observed events is in good agreement with the expected background. Within a CP-conserving 2HDM type II model the cross-section for Yukawa production depends on xiAd = |tan beta| and xihd = |sin alpha/cos beta| for the production of the CP-odd A and the CP-even h, respectively, where tan beta is the ratio of the vacuum expectation values of the Higgs doublets and alpha is the mixing angle between the neutral CP-even Higgs bosons. From our data 95% C.L. upper limits are derived for xiAd within the range of 8.5 to 13.6 and for xihd between 8.2 to 13.7, depending on the mass of the Higgs boson, assuming a branching fraction into tau+tau- of 100%. An interpretation of the limits within a 2HDM type II model with Standard Model particle content is given. These results impose constraints on several models that have been proposed to explain the recent BNL measurement of the muon anomalous magnetic moment.Comment: 24 pages, 9 figures, Submitted to Euro. Phys. J.

A study of charm production in beauty decays with the OPAL detector at LEP

Using an inclusive method, BR(b -> D\bar{D}X) has been measured in hadronic Z^0 decays with the OPAL detector at LEP. The impact parameter significance of tracks opposite tagged b-jets is used to differentiate b -> D\bar{D}X decays from other decays. Using this result, the average number of charm and anti-charm quarks produced per beauty quark decay, n_c, is determined.Comment: 20 pages, 4 figure

Tests of model of color reconnection and a search for glueballs using gluon jets with a rapidity gap

Gluon jets with a mean energy of 22 GeV and purity of 95% are selected from hadronic Z0 decay events produced in e+e- annihilations. A subsample of these jets is identified which exhibits a large gap in the rapidity distribution of particles within the jet. After imposing the requirement of a rapidity gap, the gluon jet purity is 86%. These jets are observed to demonstrate a high degree of sensitivity to the presence of color reconnection, i.e. higher order QCD processes affecting the underlying color structure. We use our data to test three QCD models which include a simulation of color reconnection: one in the Ariadne Monte Carlo, one in the Herwig Monte Carlo, and the other by Rathsman in the Pythia Monte Carlo. We find the Rathsman and Ariadne color reconnection models can describe our gluon jet measurements only if very large values are used for the cutoff parameters which serve to terminate the parton showers, and that the description of inclusive Z0 data is significantly degraded in this case. We conclude that color reconnection as implemented by these two models is disfavored. The signal from the Herwig color reconnection model is less clear and we do not obtain a definite conclusion concerning this model. In a separate study, we follow recent theoretical suggestions and search for glueball-like objects in the leading part of the gluon jets. No clear evidence is observed for these objects.Comment: 42 pages, 18 figure