CLASSIFICATION OF THE HADRONIC DECAYS OF THE Z0 INTO B-QUARK AND C-QUARK PAIRS USING A NEURAL NETWORK

A classifier based on a feed-forward neural network has been used for separating a sample of about 123 500 selected hadronic decays of the Z0, collected by DELPHI during 1991, into three classes according to the flavour of the original quark pair: uuBAR + ddBAR + ssBAR (unresolved), ccBAR and bbBAR. The classification has been used to compute the partial widths of the ZO into b and c quark pairs. This gave GAMMA(ccBAR)/GAMMA(h) = 0.151 +/- 0.008 (stat.) +/- 0.041 (syst.), GAMMA(bbBAR)/GAMMA(h) = 0.232 +/- 0.005 (stat.) +/- 0.017 (syst.)

The reaction e+ e- ---> gamma gamma (gamma) at Z0 energies

The total and differential cross-sections for the reaction e+e- → γγ(γ) are measured at centre of mass energies around 91 GeV using an integrated luminosity of 4.7 pb-1. The aggreement with QED prediction is good. Consequently there is no evidence for non-standard channels which would have the same experimental signature. The lower limits on the QED cuttoff parameters are Λ+ > 113 GeV and Λ- > 95 GeV. An upper limit on the effective coupling between a possible excited electron and the gamma is derived. At 95% confidence level the branching ratios for Z0 decay into π0γ, ηψ and γγγ are below 1.5 × 10-4, 2.8 × 10-4 and 1.4 × 10-4 respectively.0info:eu-repo/semantics/publishe

MULTIPLICITY DEPENDENCE OF MEAN TRANSVERSE-MOMENTUM IN E+E- ANNIHILATIONS AT LEP ENERGIES

A strong increase of the mean transverse momentum [p(t)] with the number of charged particles n(ch) is observed in e+e- annihilations into hadrons at LEP energies, The effect resembles correlations observed in hadron-hadron interactions. In e+e- annihilations the [p(t)] and n(ch) correlations can be accounted for by gluon radiation

MEASUREMENT OF THE AVERAGE LIFETIME OF B-HADRONS

The average lifetime of B hadrons produced in hadronic Z0 decays has been measured with the DELPHI detector at LEP. The measurement is based on the analysis of the impact parameter distribution of high p(t) muons and hadrons. The resulting mean B lifetimes are tau(B) = (1.30 +/- 0.10 +/- 0.08) ps and tau(B) = (1.27 +/- 0.04 +/- 0.12) ps respectively, giving a combined value of tau(B) = (1.28 +/- 0.10) ps. The hadronic sample was also used to measure the partial Z0 width GAMMA(bbBAR)/GAMMA(h) and gave a value of 0.222(-0.031)+0.033 +/- 0.017

STUDY OF ORIENTATION OF 3-JET EVENTS IN Z(0) HADRONIC DECAYS USING THE DELPHI DETECTOR

The study of the orientation of three-jet events from e+ e- –> Z0 –> multi-hadrons is presented, in particular the polar angle distributions of the thrust axis and of the normal to the three-jet plane, and the azimuthal correlations between the hadron plane and the one defined by the beam and thrust axes. The data are compared with results at lower energy and with QCD predictions. Good agreement with QCD predictions is observed. The scalar gluon theory is excluded by the data

DETERMINATION OF Z0 RESONANCE PARAMETERS AND COUPLINGS FROM ITS HADRONIC AND LEPTONIC DECAYS

From measurements of the cross sections for e+ e - –> hadrons and the cross sections and forward-backward charge-asymmetries for e+ e- –> e+ e-, mu+ mu- and tau+ tau- at several centre-of-mass energies around the Z0 pole with the DELPHI apparatus, using approximately 150000 hadronic and leptonic events from 1989 and 1990, one determines the following Z0 parameters: the mass and total width M(Z) = 91.177 +/- 0.022 GeV, GAMMA(Z) = 2.465 +/- 0.020 GeV, the hadronic and leptonic partial widths GAMMA(h) = 1.726 +/- 0.019 GeV, GAMMA(t) = 83.4 +/- 0.8 MeV, the invisible width GAMMA(inv) = 488 +/- 17 MeV, the ratio of hadronic over leptonic partial widths R(Z)z = 20.70 +/- 0.29 and the Born level hadronic peak cross section sigma-0 = 41.84 +/- 0.45 nb. A flavour-independent measurement of the leptonic cross section gives very consistent results to those presented above (GAMMA(t) = 83.7 +/- 0.8 MeV). From these results the number of light neutrino species is determined to be N(nu) = 2.94 +/0. 10. The individual leptonic widths obtained are: GAMMA(e) = 82.4 +/- 1.2 MeV, GAMMA(mu) = 86.9 +/- 2.1 MeV and GAMMA(tau) = 82.7 +/- 2.4 MeV. Assuming universality, the squared vector and axial-vector couplings of the Z0 to charged leptons are: VBAR(t)2 = 0.0003 +/- 0.0010 and ABAR(l)2 = 0.2508 +/- 0.0027. These values correspond to the electroweak parameters: rho(eff) = 1.003 +/- 0.011 and sin2-theta(W)eff = 0.241 +/- 0.009. Within the Minimal Standard Model (MSM), the results can be expressed in terms of a single parameter: sin2-theta(W)MSBAR = 0.2338 +/- 0.0027. All these values are in good agreement with the predictions of the MSM. Fits yield 43 < m(top) < 215 GeV at the 95% level. Finally, the measured values of GAMMA(Z) and GAMMA(inv), are used to derive lower mass bounds for possible new particles