Jl Kneur

Trilinear couplings among the electroweak vector bosons and their determination at LEP2

Abstract We present a detailed investigation of the potential of LEP2 to measure the (trilinear) couplings of the vector bosons, Z 0 W + W − and γW + W − among one another. The restrictions imposed on non-standard trilinear couplings by symmetry requirements, thus excluding violent boson-loop divergences and a violent growth of tree amplitudes for vector-boson scattering, are discussed in detail. We elaborate on how to extract the W ± spin-density matrix and the W + , W − spin correlations from future data. Various theoretically motivated fits to artificial “data” on e + e − →W + W − are carried out in order to evaluate the potential of LEP2 for the measurement of trilinear vector boson couplings. We stress that a direct measurement of the couplings of the vector bosons with one another is indispensable for a verification of the basic SU(2) L ×U(1) Y structure of the present electroweak theory. Such measurements at LEP2 improve indirect LEP1 bounds by factors which occasionally reach a full order of magnitude.

THE POTENTIAL OF A NEW LINEAR COLLIDER FOR THE MEASUREMENT OF THE TRILINEAR COUPLINGS AMONG THE ELECTROWEAK VECTOR BOSONS

We study the accuracy to be obtained in measuring trilinear Z0W+W− and γW+W− couplings in the reaction e+e− → W+W− at “New Linear Collider” (NLC) energies of 500 GeV to 1000 GeV. We derive simple scaling laws for the sensitivity in the measurement of these couplings. For most couplings the sensitivity increases as √L·s, where L denotes the integrated luminosity and √s denotes the e+esu− center-of-mass energy. Detailed investigations based on various fits confirm these scaling laws and show that an accuracy of the order of the standard radiative corrections can be reached at the NLC for the design values of the luminosity.

Higgs Boson Production in Association with Scalar Top Quarks at Proton Colliders

We investigate the production of Higgs particles in association with the supersymmetric scalar partners of the top quark at proton colliders. In the minimal supersymmetric extension of the standard model, the cross sections for the lightest neutral Higgs boson production can be rather large, substantially exceeding the rate for the associated production with top quarks. If the lightest top squark is not much heavier than the top quark, this process will enhance the potential of the CERN Large Hadron Collider to discover the lightest neutral Higgs boson, and will open a window to the study of the Higgs-stop coupling, the potentially largest electroweak coupling in the supersymmetric theory.

On electroweak theories with an extra SU(2)v vector boson triplet

We examine an extended electroweak theory based on SU(2)L x SU(2)V x U(l)Y and constrained by rho = 1 at tree level. Various models previously considered in the literature appear as special cases of our framework. The non-decoupling of the additional neutral vector boson V0 for m(V) --> infinity is traced back to the extra point-like photon interaction of leptons and quarks contained in the Hung-Sakurai electroweak model based on gamma-W3 mixing. A comparison of the predictions of the present model with precision electroweak data (M(Z0), M(W +/-), sBAR(W)2) will allow one to quantify how firmly SU(2)L x U(1)Y symmetry of vector-boson-fermion interactions is established by these data.

Virtual effects of the supersymmetric two-Higgs doublets model on the Z resonance

Abstract We analyse virtual effects due to the minimal supersymmetric two-Higgs doublets model on top of the Z resonance. We find that the precise measurement of the Z partial decay width into b quarks and the forward-backward asymmetry for b jets which will be performed at LEP, may allow for indirect signals of supersymmetric nature. If no deviation from the standard model predictions is felt by the two observables, one can improve the existing limits on the additional parameters of the model.

Associated production of Higgs bosons with scalar quarks at future hadron and e+e− colliders

Abstract We analyze the production of neutral Higgs particles in association with the supersymmetric scalar partners of the third generation quarks at future high-energy hadron colliders (upgraded Tevatron, LHC) and e+e− linear machines (including the γγ option). In the Minimal Supersymmetric extension of the Standard Model, the cross section for the associated production of the lightest neutral h boson with the lightest top squark pairs can be rather substantial at high energies. This process would open a window for the measurement of the h t t coupling, the potentially largest coupling in the supersymmetric theory.

One-loop improved Born approximation for e+e−→W+W−

Abstract The one-loop-corrected helicity amplitudes and the cross section for e+e− → W+W− are represented in terms of invariant amplitudes related to corrections to Born-level gauge couplings as well as magnetic dipole, electric quadrupole, anapole, t-channel-exchange and contact interactions. Neglecting all but the (three) ultraviolet and infrared sensitive corrections to the gauge couplings yields an improved Born approximation which is sufficiently accurate for most practical purposes.