LEP-II Energies Research Articles

Bose-Einstein correlations in WW and Z hadronic decays at DELPHI

Analysis of the BEC done by the DELPHI experiment both at the Z 0 peak and at LEP-II energies is presented. Effects of elongation of the boson source and correlations betwee poins from different W's are discussed. Comparison with models and other experimental results is done as well.

Perturbative QCD description of mean jet and particle multiplicities in e+e− annihilation

A complete numerical solution of the evolution equation for parton multiplicities in quark and gluon jets with initial conditions at threshold is presented. Data on both hadron and jet multiplicities in e + e − annihilation are well described with a common normalization, giving further support to the picture of Local Parton Hadron Duality. Predictions for LEP-II energies are presented. Furthermore we study the sensitivity to the cutoff parameter Q 0 and the scale of α s .

Multilepton signatures of gauge mediated SUSY breaking at CERN LEP II

In the framework of gauge-mediated supersymmetry breaking models pair production of the lightest neutralinos, scalar leptons, or charginos at LEPII gives rise to interesting signals involving multilepton final states and missing energy. In the parameter space where the scalar tau, $\tilde{\tau}_1$, is the next-to-lightest supersymmetric particle, we identify three interesting regions, which give rise to distinctly different final states: (i) 2 $\tau$-leptons plus missing energy, (ii) 4 charged leptons plus missing energy where in some regions all four are $\tau$-leptons, or (iii) six charged leptons, of which four are $\tau$-leptons and the other two are electrons or muons, plus missing energy. We study in detail the size of these regions in the parameter space of gauge-mediated models and give cross section contours in these regions for various LEPII energies. We also discuss the possibility of chargino-pair production at LEPII.

WWF: a generator for e+e- → 4 fermions + γ

We present an efficient generator for the process e + e - → 4 fermions + γ through off-shell W pairs. It is based on a massless matrix element with leading O ( m 2) corrections. Only the resonant WW graphs are included. We have tested it against a matrix element without these approximations and found agreement to within ∼1% at LEPII energies.

Bounds on new contact interactions from future e +e − colliders

A systematic study of the “reach” for contact interactions as a probe for new physics at future e +e − colliders is performed. A large energy range is considered, from PETRA/PEP energies at the lower end, via the Z 0 peak (SLC/LEP-I) and LEP-II, up to the TeV regime of a potential future linear e +e − collider. Lower bounds on the compositeness scale Λ for leptons are calculated for Bhabha scattering and μ-pair production using realistic assumptions on the expected luminosities and detector performances. The impact of longitudinal and transverse polarization is studied in detail. The “reach” corresponding to high rates at the Z 0 peak is compared to the one obtained from running at higher energies. At LEP-II energies, a sensitivity to (lepton) substructure at the level of Λ ∼ 10 TeV is to be expected. At a 2 TeV e +e − collider the sensitivity can be as large as 100 TeV.

Probing the weak boson sector in e +e −→W +W −

A detailed analysis of W pair production in e +e − annihilation at LEP-II energies is presented by using helicity amplitudes for the process e +e −→W +W − with arbitrary WWγ and WWZ couplings. Expressing the complete angular distribution of W decay products in terms of these helicity amplitudes, we perform a systematic search for the most sensitive angular distributions or correlations for anomalous couplings. As a result precise tests of the gauge-theory cancellations between different diagrams are suggested. Angular distributions sensitive to W +W − rescattering effects and/or CP-violating vector boson couplings are studied as well. Complete helicity amplitudes for the process e + e − → W + W − → ( q 1 q 2 g)( q 3 q 4 g) with arbitrary quark masses and finite W width are presented in a form convenient for their direct numerical evaluation. Amplitudes for the processes e +e − → ZZ and Z γ are also included.