QCD studies with e

Hadronic events produced at LEP at centre-of-mass energies of 130, 136, 161 and 172 GeV have been studied and compared with QCD predictions. Distributions of eventshape observables, jet rates, momentum spectra and multiplicities are presented and compared to the predictions of several Monte Carlo models and analytic QCD calculations. From a fit of O(ff 2 s )+NLLA QCD calculations to the differential two-jet rate, ff s has been determined at various energies. The mean charged particle multiplicities and the peak positions ¸ in the ¸ = ln(1=x p ) distribution have also been determined. These results have been compared to lower energy data and to analytic QCD or Monte Carlo predictions for their energy evolution. (Submitted to the 1997 EPS-HEP conference, Jerusalem) 1 Introduction In the summer of 1996, the centre-of-mass energy of the LEP e + e \Gamma ring was increased first to 161 and then 172 GeV. In this paper preliminary analyses of hadronic events collected by the ALEPH..

Tests of QCD predictions in inclusive hadron production processes in

The inclusive momentum distributions of charged particles detected by DELPHI in the multihadronic Z 0 decays are presented. The data obtained in 1990 are used for comparison with the QCD predictions. Namely the data with the small values of hadronic momenta are compared with perturbative QCD calculations that take into account the soft gluons coherence effect. The data for larger values of momenta are compared with the predictions of QCD obtained by the extrapolation to p s = MZ region of the distribution functions found from the results of the QCD analysis of the analogous TASSO data. This extrapolation is done on the basis of the leading order Altarelli-Parisi evolution equations and with the values of parameters determined from the results of QCD analysis of the data collected at smaller energies. New DELPHI data agree well with the prediction obtained with the value of = 150 MeV, fixed from the QCD analysis of TASSO data. 1 Introduction A study of the process e + e \Gamma..

Tests of the JETSET Bose-Einstein correlation model in the

Introduction Recent interest in profound studies of the Bose-Einstein correlations (BEC in what follows) was sparked by reports on possible influence of this phenomenon on the measured value of the W boson mass in e + e \Gamma annihilation 1;2 . Estimations of the strength of this influence were done using the Monte Carlo particle generator JETSET 3 , which includes a phenomenological model for two-particle BEC simulation. JETSET (together with PYTHIA 3 ) is so far the only e + e \Gamma annihilation event generator which accounts for BEC. It is known to reproduce well majority of experimental data, including some basic features connected to BEC. However, more sophisticated studies reveal not only certain discrepancies between experimental and model results, but also self-inconsistencies in the model itsel

Threshold Top Quark Production in

Top quark pair production close to threshold at a next linear collider is discussed, with special focus on the influence of rescattering corrections. Numerical results are shown for the differential crosssection, i.e. the momentum distribution and the forward backward asymmetry, and for moments of the angular distribution of leptons arising from the semileptonic top quark decay. 1 Introduction The top quark certainly is one of the most interesting of the presently known elementary particles, partly because it is the one we know least of, but also because its large mass implies that we can use it for a variety of unprecedented QCD studies. A future 500 GeVe + e \Gamma collider will provide a clean laboratory where we can investigate the top quark's properties to a high precision by Talk presented at the XXXVI Cracow School of Theoretical Physis, Zakopane, June 1--10, 1996 1 The complete paper, including figures, is also available via anonymous ftp at ttpux2.physik.uni-karlsruh..

Vector Leptoquark Pair Production in

The cross section for vector leptoquark pair production in e + e \Gamma annihilation is calculated for the case of finite anomalous gauge boson couplings fl;Z and fl;Z . The minimal cross section is found to behave / fi 7 , leading to weaker mass bounds in the threshold range than in models studied previously. Various extensions of the Standard Model predict bosonic states carrying both lepton and quark quantum numbers, the leptoquarks. If their couplings are baryon and lepton number conserving, cf. see [1], these states may even exist in the mass range accessible at high energy colliders. Stringent bounds on the leptoquark--fermion couplings lq are derived from low energy data [2]. In the mass range M V 1 TeV they are required to be much smaller than the electromagnetic coupling. On the other hand the bosonic leptoquark couplings cannot be arbitrarily small but are determined by the respective gauge couplings and eventual anomalous couplings. In this note we extend the ana..

Top Quark Production and Decay at Next-to-leading Order in

We study the effects of QCD corrections to the process e + e \Gamma ! t ¯ t + X ! bW +¯ bW \Gamma + X above threshold. We show how to treat consistently to O(ff s ) the gluon radiation in both the production and the decay of the top quarks, while maintaining all angular correlations in the event. At this order there is an ambiguity in the event reconstruction whenever a real gluon occurs in the final state. We study the effects of this ambiguity on the top mass and helicity angle distributions. For a top mass of 175 GeV and collider energy of 400 GeV the gluon radiation is emitted predominantly in the decay of the top quarks. 1 Introduction Recently, the CDF[1] and the D0[2] collaborations at Fermilab announced the observation of the top quark in p¯p collisions at the Tevatron. Both groups saw a statistically significant excess of dilepton and lepton+jets events with the proper kinematic properties and bottom quark tags needed to indicate t ¯ t production. Furthermore, they ..

QCD corrections to the production of a heavy quark pair plus a hard photon in

We present complete results for the O(ff s ) corrections to the production of a heavy quark pair plus a hard photon, in e + e \Gamma annihilation, including both photon and Z intermediate states. Virtual corrections are calculated analytically in 4 \Gamma 2ffl dimensions, together with the soft approximation to the real emission diagrams. After cancellation of infrared divergences, real and virtual contributions are combined numerically to obtain the cross section for the production of two heavy quark jets plus a hard photon in next-to-leading order. The resulting fully differential cross section can be used to construct arbitrary distributions with the desired experimental cuts. 1 On leave from Universit`a di Torino, Italy 2 e-mail: [email protected] 1 Introduction Out of several millions of Z bosons produced by LEP1, several tens of thousands decayed into heavy quark pairs. This provides enough statistics to compare theory and experiment beyond leading order, and possibly to..