Angular Distribution of Decay Leptons from e^+e^- \to W^+W^- at Threshold

The reaction $e^+e^- \to W^+W^-$ produces a $W$-boson pair with a non-trivial spin correlation at threshold. This correlation leads to a characteristic angular correlation between the leptons produced in $W^{\pm} \to \ell ^{\pm} \nu$ (angles relative to the $e^-$ beam direction): $d \sigma /d \cos \theta_+ d \cos \theta_- \sim (1-\cos \theta_+) (1+ \cos \theta_-) (1+\cos \theta_- \cos \theta_+)$ . If only the $\ell^-$ is observed, its angular distribution is $d \sigma / d \cos \theta_- \sim (1+\cos \theta_-)(3-\cos \theta_-)$, implying a forward-backward asymmetry of 3/8. An analytic result is also given for the azimuthal correlation. These results are reproduced by a Monte Carlo program, that also enables us to study the effects of the $W$ decay width. The threshold behaviour, which stems from the dominance of $\nu$-exchange, is contrasted with that due to $\gamma$- and $Z$-exchange, which is relevant for annihilation in the helicity state $e^-_Re^+_L$.Comment: 10 pages, 6 figures, LaTeX, simpler derivation of main results, also added a comparison with Monte Carlo expectation

Model Independent Determination of the Top Yukawa Coupling from LHC and LC

We show how a measurement of the process pp-->t tbar H + X at the LHC and a measurement of the Higgs boson branching ratios BR(H-->b bar) and BR(H-->W+W-) at a future linear electron positron collider can be combined to extract a model-independent measurement of the top quark Yukawa coupling. We find that for 120 GeV < m_H < 200 GeV a measurement precision of 15% including systematic uncertainties can be achieved for integrated luminosities of 300 fb-1 at the LHC and 500 fb-1 at the LC at a centre-of-mass energy of 350 GeV.Comment: A contribution to the LHC / LC Study Group document, 7 pages, 3 figure

Spin--1/2 Particle in Gravitational Field of a Rotating Body

Effective Lagrangian describing gravitational source spin-particle spin interactions is given. Cosmological and astrophysical consequences of such interaction are examined. Although stronger than expected, the spin-spin interactions do not change any cosmological effect observed so far. They are important for background primordial neutrinos.Comment: 15 pages, Latex, 2 figure

Gauge boson couplings at LEP

A review is given of the measurements of triple and quartic couplings among the electroweak gauge bosons performed at LEP by the four experiments ALEPH, DELPHI, L3 and OPAL. Emphasis is placed on recently published results and on combinations of results performed by the LEP electroweak gauge-couplings group. All measurements presented are consistent with the Standard Model expectations.Comment: 6 pages, 7 figures. To be published in the proceedings of the BEACH04 conference, Chicago, June 27-July 3 200

Associated production of a light Higgs boson and a chargino pair in the MSSM at linear colliders

In the Minimal Supersymmetric Standard Model (MSSM), we study the light Higgs-boson radiation off a light-chargino pair in the process e+e- -> h chi^+ chi^- at linear colliders with \sqs=500 GeV. We analyze cross sections in the regions of the MSSM parameter space where the process can not proceed via on-shell production and subsequent decay of either heavier charginos or the pseudoscalar Higgs boson A. Cross sections up to a few fb's are allowed, according to present experimental limits on the Higgs-boson, chargino and sneutrino masses. We also show how a measurement of the process production rate could provide a determination of the Higgs-boson couplings to charginos.Comment: 26 pages, 9 figures; figure misplacement fixed; to appear in Eur.Phys.J.

ERATO: event generator for four-fermion production at LEP2 energies and beyond

This paper is a long write-up of ERATO. ERATO is a Monte Carlo event generator describing four fermion production at LEP2 and beyond. All tree-order processes leading to four fermions are included, taking into account all relevant QCD contributions. QED higher order corrections are introduced via a structure function approach, whereas weak corrections related to fermion loops are also included. Special attention has been paid in studying the trilinear gauge couplings, and all possible contributions, including the CP-violating ones, have been implemented.Comment: 16 pages, late

The reaction e+e- --> hh recomputed

We notice that the existing literature about the reaction e+e- --> hh suffers from a mistake in the relative sign between the t-channel and u-channel graphs. Correcting this lowers the crosssections by about an order of magnitude.Comment: 7 pages, 5 figure

Cosmological abundances of right-handed neutrinos

We study the equilibration of the right-helicity states of light Dirac neutrinos in the early universe by solving the momentum dependent Boltzmann equations numerically. We show that the main effect is due to electroweak gauge boson poles, which enhance thermalization rates by some three orders of magnitude. The right-helicity states of tau neutrinos will be brought in equilibrium independently of their initial distribution at a temperature above the poles if the tau neutrino mass is larger than about 10 keV.Comment: LaTeX, 18 pages, 5 PostScript figure

Tests of gauge boson couplings in polarized e−γe^-\gamma collisions

Single $W$-boson production in $e^-\gamma$ collisions with polarized beams is investigated. Helicity amplitudes for general couplings are derived and their properties are discussed. The results are applied to the Standard Model (SM) and the left-right model. In the framework of SM the updated estimates of the measurement precision of photon anomalous coupling parameters $\kappa,$ $\lambda$ at the NLC with $\sqrt{s_{ ee }}=500$GeV are obtained. The production of right-handed gauge bosons $W^-_{ 2 }$ in these collisions is also analysed.Comment: 18 pages, latex, 7 figures appended as compressed uuencoded .ps fil

Studying trilinear gauge couplings at LEP2 using optimal observables

We study the sensitivity of the processes `e+ e- -> lepton (l) neutrino (v) quark (u) antiquark (d)' at LEP2 energies on the non-standard trilinear gauge couplings (TGC), using the optimal observables method. All relevant leading logarithmic corrections to the tree-order cross section, as well as experimental resolution effects have been studied. Taking into account correlations among the different TGC parameters we show that the limits on the TGC can reach the level of 0.15~(1sd) at 161 GeV with 100 pb$^{-1}$, a challenge for the first LEP2 phase. At higher energies this can be improved drastically, reaching the level of 0.02~(1sd).Comment: 14 pages, latex, minor typos correcte