Showing papers by "Yang Gao published in 1994"

Heavy flavour production and decay with prompt leptons in the ALEPH detector

Abstract: In 431 000 hadronicZ decays recorded with the ALEPH detector at LEP, the yields of electrons and muons in events with one or more prompt leptons have been analysed to give information on the production and decay of heavy quarks. The fractions of $$b\bar b$$ and $$c\bar c$$ events are measured to be 0.219±0.006±0.005 and 0.165±0.005±0.020, and the corresponding forward-backward asymmetries at theZ mass are measured to be 0.090±0.013±0.003 and 0.111±0.021±0.018, after QED and QCD corrections. Measurements for the semileptonic branching ratios BR $$(b \to \ell ^ - \bar vX)$$ and BR (b→cl+ vX) yield 0.114±0.003±0.004 and 0.082±0.003±0.012, respectively. The dilepton events enable measurement of theb mixing parameter, Χ=0.114±0.014±0.008. Results are also presented for the energy variation of the $$b\bar b$$ asymmetry and the parameters required to describe heavy quark fragmentation. From the asymmetry measurements, the effective electroweak mixing angle is sin2θ eff =0.2333±0.0022.

Heavy quark tagging with leptons in the ALEPH detector

Abstract: This paper deals with sustainable development policies in the French South-Eastern Alps region (Digne-les-Bains). It aims to delineate the influence of communication and mediation practices which are used to implement such principles in a largely reluctant area. This approach combines the point of views of two sciences, media and communication on the one hand, geography , on the other hand

Production of $K^0$ and $\Lambda$ in hadronic Z decays

Abstract: Measurements of the inclusive cross-sections forK 0 and Λ production in hadronic decays of the Z are presented together with measurements of two-particle correlations within pairs of Λ andK 0. The results are compared with predictions from the hadronization models Jetset, based on string fragmentation, and Herwig, based on cluster decays. TheK 0 spectrum is found to be harder than predicted by both models, while the Λ spectrum is softer than predicted. The correlation measurements are all reproduced well by Jetset, while Herwig misses some of the qualitative features and overestimates the size of the $$\Lambda \bar \Lambda $$ correlation. Finally, the possibility of Bose-Einstein correlation in theK 0 K 0 system is discussed.

Production of charmed mesons in z decays

Abstract: The production of charmed mesons $$\mathop {D^0 }\limits^{( - )} $$ ,D ± , andD *± is studied in a sample of 478,000 hadronicZ decays. The production rates are measured to be $$\begin{gathered} \frac{{\Gamma ({\rm Z} \to D^{* \pm } X)}}{{\Gamma _{had} }} = 0.187 \pm 0.015(\exp .) \pm 0.013(BR), \hfill \\ \frac{{\Gamma ({\rm Z} \to D^ \pm X)}}{{\Gamma _{had} }} = 0.251 \pm 0.026(\exp .) \pm 0.025(BR), \hfill \\ \frac{{\Gamma ({\rm Z} \to \mathop {D^0 }\limits^{( - )} X)}}{{\Gamma _{had} }} = 0.518 \pm 0.052(\exp .) \pm 0.035(BR), \hfill \\ \end{gathered} $$ where the errors from this analysis are separated from those coming from theD branching ratios (BR). TheD *± momentum distribution is extracted separately for $$Z \to c\bar c$$ and $$Z \to b\bar b$$ events with the help of event shape variables. It is consistent with the prediction of the JETSET Monte Carlo program after adjustment of the charm fragmentation function. Constraining the shape of the $$Z \to b\bar b$$ contribution, the average fraction of the beam energy taken by aD * meson produced in the fragmentation of a charm quark is extracted by a parametric fit to be c =0.495±0.011±0.007. Evidence forD **0 (D 1(2420)0 and/orD 2 * (2460)0) production is found in the $$D^{* \pm } \pi ^ \mp $$ channel, accounting for a fraction (18±5±2)% of allD *± production. The relative production of vector and pseudoscalar mesons is dicussed, together with the possible effects ofD ** production. Thec-quark forward-backwardZ-pole asymmetry is detrmined from that of high momentumD *± to beA =(7.7±4.4)%.

Letter of Intent for the Study of CP Violation and Heavy Flavor Physics at PEP-II

Abstract: The study of CP-violating asymmetries in $B^0$ decays to CP eigen- states promises, at last, to allow a test of the Standard Model explanation of CP violation. More generally, the measurement of these CP asymmetries can provide us with a series of unique, and uniquely stringent, consistency tests of the quark sector of the Standard Model. These tests, which will be carried out by the BABAR detector at PEP-II, will provide us with perhaps the best chance to challenge the Standard Model in new and quantitatively precise ways. This challenge has come about through the interplay of a series of experimental and theoretical events....

Z-production cross-sections and lepton pair forward-backward asymmetries

Abstract: Precise values of theZ resonance cross sections and lepton pair forward-backward asymmetries are measured from a sample of 1.3 millionZ decays into hadrons and charged leptons taken with the ALEPH detector at LEP. A silicon-tungsten luminosity calorimeter was installed in ALEPH in 1992, permitting the determination of the absolute luminosity to a systematic precision of 0.15% (experimental) and 0.25% (theoretical). This is combined with a better understanding of the hadronic event selection and an improved tau pair selection. Including the ALEPH results of the past three years one obtains σh0 =(41.60±0.16) nb, andRl=20.69±0.09. The corresponding number of light neutrino species isNv =2.983±0.034. The lepton pair forward-backward asymmetry is used to derive the ratio of vector to axial-vector couplings of leptons:gV2(MZ2)/gA2(MZ2)=0.0058 ±0.0011. An application of these results to constrain one model with extraZ bosons is presented.