QCD effects andb-tagging at LEP I

We analyze the impact of usingb-tagged samples in studying non-Abelian effects due to QCD ine ⁺ e ^? → 4jet events at √s=M _z ⁰, using angular variable analyses and comparisons withe ⁺ e ^? → 3jetγ events. We find that QCD effects are largely enhanced inb-quark samples with respect to ‘unflavoured’ ones, where energy-ordering is used to distinguish between gluon and quark jets. We show that theb-quark mass influences the angular distributions significantly and should not be neglected.     

b-tagging in DELPHI at LEP

The numbers of events suitable for the determination of the gluon distribution function f ^p _g (x,Q ²) in a proton at the LHC for various intervals of x and Q ² are estimated. The contributions of background events of different sources are studied and estimated in the considered intervals of x and Q ². The PYTHIA event generator was used to produce physical events for this analysis.Received: 17 June 2003, Revised: 8 July 2004, Published online: 24 August 2004N.B. Skachkov: Present address: Joliot-Curie 6, JINR, 141980, Dubna, Moscow region, Russia     

Tests of QCD at LEP

The four experiments, ALEPH, DELPHI, L3 and OPAL at LEP, have performed a large number of precise measurements to test Quantum Chromodynamics. The strong coupling constant has been measured with high precision:α _s (M_z) = 0.123±0.004. The coupling constant has been found to be independent of quark flavour. The running ofα _s has been demonstrated by the data. Second order QCD matrix element calculations have been tested from several measured distributions in 3-jet and 4-jet events. These distributions give evidence of the vector nature of the gluon and provide measurements of the QCD colour factors. The hadronic distributions are well reproduced by QCD Monte Carlo programs as well as by analytical calculations with soft gluon coherence effects.     

QCD corrections to W pair production at LEP200

One-loop QCD corrections to hadronic W decay are calculated for arbitrary W polarizations. The results are applied to W pair production and decay at LEP200. We focus on the corrections to angular distributions with particular emphasis on azimuthal distributions and correlations. The relevance of our results to the experimental determination of possible non-standard triple gauge boson interactions is discussed.     

Search for evidence of compositeness at LEP I

Extensive searches for evidence of compositeness of quarks and leptons have been made in the complete set of ALEPH data from LEP running at the Z peak. These include searches for radiative and weak decays of excited fermions, produced either in pairs or singly, and a scalar partner of the Z. Coupling and branching ratio limits deduced from these searches are presented. Received: 6 February 1998 / Published online: 2 July 1998     

SUSY virtual effects at the LEP2 boundary

We examine the possibility that SUSY particles are light, i.e. have a mass just beyond the final kinematical reach of LEP2. In this case, even if light particles are not directly detected, their virtual effects are enhanced by a “close-to-threshold” resonance in the s-channel. We find that this resonant effect is absent in the case of light sfermions, while it is enhanced in the case of light gauginos, since neutralinos and charginos add coherently in some regions of the allowed parameter space. We discuss this “virtual alliance” in detail and try to examine the possibilities of its experimental verification. Received: 19 March 1999 / Published online: 12 August 1999     

Non-perturbative QCD effects at large momentum transfers

We calculate the simplest one-instanton correction to the perturbative QCD prediction for e⁺e^? annihilation to hadrons. At high centre-of-mass energies Q we find a contribution to the total cross section from a simple fermion loop of the form

$\text{δR}\text{R}\text{∝}\text{Q}{}^2\text{→∞}\text{Q}{}^{\mathrm{<mtext>?11?</mtext><mtext>Nf</mtext><mtext>3</mtext>}}\text{(1n Q}{}^2\text{)}{}^{\mathrm{<mtext>6(33?4Nf)</mtext><mtext>(33?2Nf)</mtext><mtext>or</mtext>}}\text{(1n Q}{}^2\text{)}{}^{\mathrm{<mtext>6(33?4Nf)</mtext><mtext>(33?2Nf)?1</mtext>}}$

where N_f is the number of quark flavours. The numerical value of this contribution is O(1) for Q ～ 1 to 2 GeV.     

Gluino signals in 4jet events and vertex tagging at LEP I

Heavy flavour tagging provides a broad range of possibilities in testing QCD features at LEP. We present here a study of 4jets events at LEP I where the so-called light gluinos could be directly produced. We show that microvertex techniques offer a unique chance to exploit simple kinematical distributions in order to optimise the signal coming from gluino production with respect to the background of ordinary QCD events. Our results indicate that experimental analyses along the lines suggested here can exclude or reveal the presence of a gluino for masses up to 10 GeV and lifetimes below 10 sec. We also point out that a large fraction of gluino events could decay in configurations carrying large missing energy, so to escape the usual selection criteria of 4jet samples. In our study, mass effects of quarks and gluinos have been taken into account exactly. Our results are independent from both the jet algorithm and its resolution parameter. Received: 25 July 1997 / Revised version: 16 September 1997 / Published online: 20 February 1998     

QED interconnection effects on W momentum distributions at LEP 2

The process ${? e}^{+}{? e}^{-}?ghtarrow {? W}^{+}{? W}^{-}?ghtarrow {? f}_{1}{?erline {? f}}_{2}{? f}_{3}{?erline {? f}}_{4}$ contains charges in the initial, intermediate and final stages. This gives a rich selection of possible QED interconnection effects. Coulomb interaction is the simplest of these, and can thus be used to explore consequences. We study a number of experimental observables, with emphasis on those related to the W momentum distribution. Second-order Coulomb effects are shown to be practically negligible. The limited LEP 2 statistics will not allow detailed tests, so any theory uncertainty will be reflected in the systematic error on the W mass. Currently the uncertainty from this source may be as high as 20 MeV.     

Two-photon interactions at LEP

Experimental results of investigation of two-photon interactions at LEP (CERN) are presented. Experimental conditions provided a possibility of investigation of the whole set of two-photon interactions from the production of lepton pairs in collisions of quasi-real photons to interactions of high-virtual photons.     

Bottonium production at LEP

We investigate the production of Υ(³ S ₁) and η _b (¹ S ₀) with two gluons from Z decay. We find that at LEP luminosities experimental detection will hardly be feasible.     

Particle searches at LEP

This paper is based on two invited talks, one by G. Giacomelli and one by P. Giacomelli, at the 2nd Trieste Conference on Recent Developments in the Phenomenology of Particle Physics, Trieste, 19–23 October, 1992.     

B physics at LEP

The Z factory at LEP is like a mini B factory. About 20% of the time a Z decays to a pair of b flavoured hadrons. As a result, this has provided a major boost to B physics studies. The four sophisticated LEP detectors, with millions of Z events, have provided a variety of interesting results. Only a snapshot of the major developments can be covered in one review. We have considered the results related to coupling, in particular, the measurements of R_b=Γ_b/Γ_had the forward backward charge asymmetry. The b hadron properties, like inclusive and exclusive lifetime measurements are summarised briefly. Measurements of oscillation parameter as well as the evidence for time dependent evolution have been discussed. Studies of interesting decay modes have been mentioned.     

Non-perturbative QCD effects and the top mass at the Tevatron

We present a new, universally applicable toy model of colour reconnections in hadronic final states. The model is based on hadronising strings and has one free parameter. We next present an implementation of this model in the PYTHIA event generator and provide several parameter sets (‘tunes’), constrained by fits to Tevatron minimum-bias data. Finally, we consider the sensitivity of a simplified top mass analysis to these effects, in exclusive semi-leptonic top events at the Tevatron. A first attempt at isolating the genuine non-perturbative effects gives an estimate of order δm_top ∼ ±0.5 GeV from non-perturbative uncertainties, and a further δm_top ∼ ±1 GeV from shower effects. PACS 12.38.-t; 13.85.Hd; 13.87.Fh     

Higgs production and background at LEP

We calculate the total cross section for the process at tree level for LEP energies. We take into account the Higgs production subprocess, background processes with two intermediate vector bosons and their interference. A Higgs lighter than 50GeV could be observed at LEP 1. In this case the background can easily be eliminated. For a heavier Higgs we need LEP 200 energies. Here we will get a significant background from doubleZ production.     

Looking for the LEP at LEP. The excited neutrino scenario

It is suggested to use LEP for a systematic search of excited neutrinos. These are expected to be the lightest excited particles (LEPs) in models of substructure and therefore could be the first direct signal of compositeness in the fermionic sector. Their signatures and production are presented. It is shown how one can extract their masses and couplings or else set limits on these parameters in case of a negative search. A comparison with heavy sequential neutrinos and heavy neutrals emanating from theories of grand unification and supersymmetry is briefly made.     

Chargino production at LEP 200

We study chargino production and decay in the energy region of LEP 200, exploring the relevant supersymmetry parameter space. We give numerical predictions for the cross sections and for the signaturesl ⁺+l ^?+p′, l+jet+p′, jet+jet+p′. New formulae are presented relating chargino masses and couplings to the supersymmetry parameters.