The study of the W boson

The status of the measurement of the W boson mass at LEP2 is reviewed. Properties of the W such as branching ratios into quarks and leptons and couplings to other neutral gauge bosons are reported. Four-fermion production cross sections in e⁺e⁻ collisions are also presented. To cite this article: O. Buchmüller et al., C. R. Physique 3 (2002) 1173–1181.     

Colour Reconnection in <Emphasis Type="Italic">WW</Emphasis> Events and the Models with It

The colour reconnection effects were intensively studied with LEP2 data and are a dominant sources of systematic uncertainty in the W boson mass in e⁺e^? annihilation at LEP2 and one of a dominant sources of systematic uncertainty in the top quark mass determination at hadronic colliders. With the discovery of Higgs boson, a new arena for the effects studies opened up. The effects are discussed within the existent different models and what future tests may come with a future FCC-ee.     

Single wino production ine + e − collisions

We present a detailed analysis of single wino production ine ⁺ e ^? collisions. We give total and differential cross sections at three different energies, relevant to present (DESY) and future (LEP) experiments for different choices of sparticle masses. We comment on different approximations (useful at different energy regimes) and discuss on signatures and backgrounds.     

Excited fermions ate + e − andeP colliders

We analyse the discovery potentials of future colliders with respect to excited fermions which are expected in models of fermionic substructure. After discussing the possibilities offered by LEP, we will consider in detail the Next Linear Collider in itse ⁺ e ^?,eγ and γγ modes [where the high-energy photon beams are obtained through back-scattering of laser light]. In addition, sinceeP colliders are well adapted to the search of the first generation of excited leptons, we include a study of their possible manifestation at HERA and LEP/LHC. We give complete and compact formulae for decay widths, production cross sections and angular distributions. Furthermore, we analyse in some detail the polarisation of the produced excited fermions which allows for an easy reconstruction of the correlations between the initial state and the decay products of the heavy fermions.     

Production of neutral pseudo-Goldstone bosons at LEP II and NLC in multiscale walking technicolor models

Walking technicolor (WTC) models predict the existence of heavy neutral pseudo-Goldstone bosons (PGBs), whose masses are typically expected to be larger than 100 GeV. In this paper, we investigate the production and decay of these particles at the high energy e⁺e⁻ experiments, LEP II and NLC.We find that, in WTC models, the production of neutral PGBs can be significantly enhanced, by one or two orders of magnitude, with respect to the predictions of traditional (QCD-like) TC models. The origin of such an enhancement is the existence of several low energy TC scales, that are likely to appear in WTC theories. This could allow the PGBs to be observed even at the energy and luminosity of the LEP II experiment. At LEP II, the PGBs are expected to be produced in the e⁺e⁻ → Pγ channel, and, possibly, in the e⁺e⁻ → Pe⁺e⁻ channel, with a total rate that can be of the order of several tenths per year. Due to the typical large values of PGB masses, the relative branching ratios of PGB decays, in WTC theories, are different from those predicted in traditional TC models. In particular, a large fraction of these decays can occur in the P → γγ channel. In considering the PGB production, at LEP II, we find that, in most of the final states, the distinctive signatures of WTC events should allow the Standard Model background to be reduced to a negligible level. We also find that, at a 500 GeV NLC experiment, the production of neutral PGBs can occur in several channels, and can be of the order of 10³ events per year. Instead, when we consider traditional TC models, we find that no PGB are typically predicted to be observed, both at LEP II and the NLC experiment.     

Anomalous effects ine + e − annihilation into boson pairs II.e + e −→ZZ, γZ, γγ

In application of the general procedure defined in I, we analyze various kinds of anomalous effects which could appear either in the leptonic or in the bosonic sector of the reactionse ⁺ e ^?→ZZ, γZ and γγ: excited electron, excited vector boson or isoscalar partner, anamalous three-neutral boson couplings, residual contact terms involving transverse or longitudinalZ-components. Results are given in view of SLC, LEP and future linear colliders.     

Anomalous contributions to 95-195-195-1at high energies

We analyze the processe ⁺ e ^?→e ⁺ e ^? ? ⁺ ? ^? (?=e, μ, τ-leptons) considering several nonstandard contributions in order to search for new physics beyond the standard model. We are able to test compositeness up to the TeV mass scale at LEP II and CLIC energies.     

Hard photon next-to-leading corrections to two-fermion production in e + e − collisions above the Z 0 peak

A complete next-to-leading order calculation of the initial-state O (α) photonic corrections to hadron and lepton-pair production in e ⁺ e ^?. collisions is presented. The impact of the next-to-leading order corrections on the twofermion cross section and forward-backward asymmetry is studied in detail for several event selections, with special emphasis on hard photon effects at LEP1.5 and LEP2 energies. It is shown that for typical realistic event selections, initial-state next-to-leading order corrections in the soft approximation can be safely extrapolated to energies above the Z ⁰ peak without significant loss of precision.     

Anomalous effects ine + e − annihilation into boson pairs I.e + e −→W + W −

We present a phenomenological analysis of the various kinds of anomalous effects which can apper either in the leptonic or in the bosonic sector of the reactionse ⁺ e ^?→V V′(V V′=W ^±,Z ⁰ or γ). A general descritpin of invariant amplitudes and helicity amplitudes is given. Applications are made with non standar couplings, new intermediate states, residuale ⁺ e ^? VV′ contact interactions. In this first part results are given fore ⁺ e ^?→W ⁺ W ^? in view of LEP 200 and future linear colliders in the TeV range.     

Jet analysis in Higgs search

In high energye ⁺ e ^? annihilations, events with jets dominate. Since heavy quark jets often contain undetectable neutrinos, methods of jet analysis are needed to study such events. A simple procedure is presented here and illustrated by applying to the search for the neutral Higgs bosonH ⁰ produced at LEP at the center-of-mass energy of 200 GeV.     

Scaling predictions for multiplicity distributions at LEP

This paper presents a study of the charged particle multiplicity distributions for thee ⁺ e ^? annihilation. It is shown that the KNO-G scaling, the concept which turned out to be very successful in thepp collisions, is also valid for thee ⁺ e ^?, data. Using this concept the predictions for the multiplicity distributions at LEP are given. The possible explanation of the scaling origin is postulated. It seems that the proposed mechanism, namely the Polyakov scale invariant branching, is responsible for the scaling observed in the data. It is worth to emphasize that the same mechanism was proposed to explain the intermittent behaviour of a multiparticle final state.     

Effects of additional isoscalar vector bosons ine^ + e^ - \to f\bar f processes at the TRISTAN and LEP energies

The phenomenological manifestation of the additionalY(Y_L) boson arising in the models with the composite structure of electroweak interactions is studied for the process \(e^ + e^ - \to f\bar f\) at the TRISTAN and LEP energies. It is shown that the presence of the additional isoscalar boson, which can provide the explanation of the experimentally observed deviation ofR _μ value from the Standard Model predictions at TRISTAN, yields a measurable effect in the cross-section of e⁺e^- → μ⁺μ^- (hadrons) processes at LEP. This effect in the muon and hadron cross-sections at the energies M_z±Г_z/2 can be ～1.5–2.0% for small values of the γ-Y mixing parameter. The increase of statistics for the above processes at TRISTAN and LEP will permit to set an additional limitation on the model parameters.     

Higgs search: present and future

In this talk I review theoretical bounds on mass of the Higgs scalar in the Standard Model (SM) and then summarise current experimental limits from the LEP experiments. Following this I discuss the search strategies for the SM Higgs at LEP 200 and the TeV energye ⁺ e ^? colliders which are under discussion. This will be followed by a summary of the Higgs search potential of the pp supercolliders such as SSC/LHC. I then close with a brief discussion of a ‘Dark Higgs’ whose dominant decay modes are into invisible channels.     

High-twist correction for inclusive hadron production in photon-photon collisions

The contribution of the QCD high-twist subprocess γq→πq′ to the inclusive cross section for e⁺ e^? → e⁺ e^? + π^± + X is calculated both in the PETRA/PEP and the LEP energy range, in the equivalent-photon approximation. Superposed on the Born cross section and its O(α_s) correction, it gives agreement with the data at relatively low transverse momenta. It is suggested that this high-twist contribution may be further isolated experimentally by triggering simultaneously on both a fast hadron and the opposite-side jet.     

Multiplicity fluctuations in one- and two-dimensional angular intervals compared with analytic QCD calculations

Multiplicity fluctuations in rings around the jet axis and in off-axis cones have been measured by the DELPHI collaboration in e⁺e⁻ annihilations into hadrons at LEP energies. The measurements are compared with analytical perturbative QCD calculations for the corresponding multiparton system, using the concept of Local Parton Hadron Duality. Some qualitative features are confirmed by the data but substantial quantitative deviations are observed.     

Average charged multiplicities inQ\bar Q events (Q=c,b, t) at LEP energies

Charged particle multiplicities ine ⁺ e ^? annihilation into hadronic states have been studied. Events with light and heavy initial quarks are treated separately. Particular attention is paid to multiple production int \(\bar t\) events at LEP energies.     

Light charged Higgs scalars at high-energy e+e− colliders

We study the prospects for detecting light charged Higgs scalars (H^±) at future e⁺e⁻ colliders. Various two-Higgs-doublet models (2HDM) and a general multi-Higgs-doublet model (MHDM) are considered, all of which may contain a H^± in the discovery range of LEP2. The potentially troublesome cases of M_H^± ∼ M_W and M_H^± ∼ M_Z are discussed, as well as ways of distinguishing the different models.     

New physics with three-photon events at LEP

The effect a generalZ??3?? vertex in the reactione ⁺ e ^???3?? is studied with a particular attention to LEP searches. We give exact analytical expressions including realistic cuts for the signal and present a detailed analysis based on a Monte Carlo that includes the effect of the irreducible 3?? QED cross section. As special applications we discuss the effect of heavy scalars, fermions and gauge bosons.     

Possibility of observable new behavior for multiparticle production ine + e − annihilation at LEP

We discuss the possibility that multiparticle production ine ⁺ e ^? annihilation at c.m. energies near 100 GeV begins to show some of the striking global characteristics of production in hadron-hadron collisions. Detailed examination of existinge ⁺ e ^? data up to 44 GeV suggests the possibility of the observable emergence of dynamical forward-backward correlations among produced charged hadrons, and of a steadily broadening multiplicity distribution. As a consequence of the increasing degree of forward-backward correlation, the shape of the multiplicity distribution in one hemisphere approaches that of the full distribution. These effects provide experimental signals for dynamics which differs from purely statistical behavior in each hemisphere. Analysis of data from LEP I and II is important for exploring this matter.