Gilles Couture

Flavor-changing top quark decay within the minimal supersymmetric standard model

We present the results of the gluino and scalar quark contribution to the flavor-changing top quark decay into a charm quark and a photon, gluon, or a {ital Z}{sup 0} boson within the minimal supersymmetric standard model. We include the mixing of the scalar partners of the left- and right-handed top quark. This mixing has several effects, the most important of which is to greatly enhance the {ital c} {ital Z} decay mode for large values of the soft SUSY-breaking scalar mass {ital m}{sub {ital S}} and to give rise to a GIM-like suppressions in the {ital c} {gamma} mode for certain combinations of parameters.

Supersymmetric QCD flavor-changing top quark decay

We present a detailed and complete calculation of the gluino and scalar quarks contribution to the flavor-changing top quark decay into a charm quark and a photon, gluon, or a Z{sup 0} boson within the minimal supersymmetric standard model including flavor-changing gluino-quark-scalar-quark couplings in the right-handed sector. We compare the results with the ones presented in an earlier paper where we considered flavor-changing couplings only in the left-handed sector. We show that these new couplings have important consequences leading to a large enhancement when the mixing of the scalar partners of the left- and right-handed top quark is included. Furthermore CP violation in the flavor-changing top quark decay will occur when a SUSY phase is taken into account. {copyright} {ital 1997} {ital The American Physical Society}

\(\mu\rightarrow e\gamma\) decay in the left-right supersymmetric model

Abstract. We calculate the dipole amplitude for the decay

Patterns in the fermion mixing matrix, a bottom-up approach

\mu\rightarrow e\gamma

Measurement of the W W gamma vertex through single photon production at e+ e- colliders

and related processes in the left-right supersymmetric model when parity breaking occurs at a considerably large scale. The low-energy flavor violation in the model originates either from the nonvanishing remnants of the left-right symmetry in the slepton mass matrix or from the direct flavor changing lepton-slepton-neutralino interaction. The result is found to be large and already accessible with current experimental accuracy for supersymmetric masses not far above the electroweak scale. It also provides nontrivial constraints on the lepton mixing in the model.

Phenomenological study of the process e+e---> micro+ micro- nu l nu -barl at high energy e+e- colliders and measurement of the ZWW and gamma WW couplings.

We first obtain the most general and compact parametrization of the unitary transformation diagonalizing any 3×3 Hermitian matrix H, as a function of its elements and eigenvalues. We then study a special class of fermion mass matrices, defined by the requirement that all of the diagonalizing unitary matrices (in the up, down, charged lepton, and neutrino sectors) contain at least one mixing angle much smaller than the other two. Our new parametrization allows us to quickly extract information on the patterns and predictions emerging from this scheme. In particular we find that the phase difference between two elements of the two mass matrices (of the sector in question) controls the generic size of one of the observable fermion mixing angles: i.e. just fixing that particular phase difference will predict the generic value of one of the mixing angles, irrespective of the value of anything else.

Canonical constraints on leptonic CP violation using ultrahigh energy cosmic ray neutrino fluxes

We perform a detailed study of the process [ital e][sup +][ital e][sup [minus]][r arrow][gamma][nu][sub [ital l]][bar [nu]][sub [ital l]] and its sensitivity to anomalous gauge boson couplings of the [gamma][ital WW] vertex. We concentrate on CERN LEP II energies, [radical][ital s] =200 GeV, and energies appropriate to the proposed Next Linear Collider (NLC), a high energy [ital e][sup +][ital e][sup [minus]] collider, with center of mass energies [radical][ital s] =500 and 1 TeV. At 200 GeV, the process offers, at best, a consistency check of other processes being considered at LEP 200. At 500 GeV, the parameters [kappa][sub [gamma]] and [lambda][sub [gamma]] can be measured to about [plus minus]0.05 and [plus minus]0.1, respectively, at 95% C.L. while at 1 TeV they can be measured to about [plus minus]0.02. At the high luminosities anticipated at high energy linear colliders precision measurements are likely to be limited by systematic rather than statistical errors.

Olber's paradox revisited in a static and finite Universe

We perform a detailed study of the process

The measurement of tri‐linear gauge boson couplings at e+e− colliders

{e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}{\ensuremath{\nu}}_{l}{\overline{\ensuremath{\nu}}}_{l}

Doubly charged Higgsino contribution to the decays {mu}{r_arrow}e{gamma} and {mu}{r_arrow}3e and to the anomalous magnetic moment of the muon {Delta}a{sub {mu}} within the left-right supersymmetric model

including all contributions. The contributions other than from real gauge boson production lead to a rich phenomenology. We explore the use of the process as a means of precision measurement of the