Cosmin Macesanu

Neutrino masses and mixings in a minimal SO(10) model

We consider a minimal formulation of SO(10) grand unified theory wherein all the fermion masses arise from Yukawa couplings involving one 126 and one 10 of Higgs multiplets. It has recently been recognized that such theories can explain, via the type-II seesaw mechanism, the large {nu}{sub {mu}}-{nu}{sub {tau}} mixing as a consequence of b-{tau} unification at the grand unified theory scale. In this picture, however, the Cabibbo-Kobayashi-Maskawa quark-mixing matrix phase {delta} lies preferentially in the second quadrant, in contradiction with experimental measurements. We revisit this minimal model and show that the conventional type-I seesaw mechanism generates phenomenologically viable neutrino masses and mixings, while being consistent with Cabibbo-Kobayashi-Maskawa quark-mixing matrix CP violation. We also present improved fits in the type-II seesaw scenario and suggest fully consistent fits in a mixed scenario.

A simple phenomenological parametrization of supersymmetry without R-parity

Abstract We present a parametrization of the supersymmetric standard model without R-parity that permits efficient phenomenological analyses of the full model without a priori assumptions. Under the parametrization, which is characterized by a single vacuum expectation value for the scalar components of the Y =−1/2 superfields, the expressions for tree-level mass matrices are quite simple. They do not involve the trilinear R-parity violating couplings; however, the bilinear μ i terms do enter and cannot be set to zero without additional assumptions. We set up a framework for doing phenomenology and show some illustrative results for fermion mass matrices and related bounds on parameters. We find in particular that large values of tan β can suppress R-parity violating effects, substantially weakening experimental constraints.

Supersymmetry without R parity: Constraints from leptonic phenomenology

R-parity conservation is an ad hoc assumption in the most popular version of the supersymmetric standard model. Most studies of models which do allow for R-parity violation have been restricted to various limiting scenarios. The single-VEV parametrization used in this paper provides a workable framework to analyze phenomenology of the most general theory of SUSY without R parity. We perform a comprehensive study of leptonic phenomenology at the tree level. Experimental constraints on various processes are studied individually and then combined to yield regions of admissible parameter space. In particular, we show that large R-parity violating bilinear couplings are not ruled out, especially for large tan {beta}. (c) 2000 The American Physical Society.

QCD loop corrections to top quark production and decay at e + e − colliders

We present a computation of QCD next-to-leading order virtual corrections to the top production and decay process at linear colliders. The top quarks are allowed to be off-shell and the production and decay subprocesses are treated together, thus allowing for interference effects. The framework employed for our computation is the double pole approximation (DPA). We describe the implementation of this approximation for the top production and decay process and compare it with the implementation of DPA for the evaluation of QED corrections to the W pair production at LEP II. Similarities and differences between the two cases are pointed out. One result of interest is the incomplete cancelation of interference corrections. Other results include values for the total NLO top production cross section, and the impact the nonfactorizable (interference) corrections have on the top invariant mass distribution.

Gluon radiation in top quark production and decay at an e+e− collider

We study the effects of gluon radiation on top production and decay processes at an e+e− collider. The matrix elements are computed without any approximations, using spinor techniques. We use a Monte Carlo event generator which takes into account the infrared singularity due to soft gluons and differences in kinematics associated with radiation in the production versus decay process. The calculation is illustrated for several strategies of top mass reconstruction.

QCD loop corrections to e+e−→tt̄→bW+b̄W−1

This talk describes work in progress on the subject of virtual QCD corrections to top production and decay at e+e− colliders. We examine the resonant behavior of the amplitudes that dominate the cross section and discuss implementation in a Monte Carlo program. The theoretical framework is similar to that which has been successfully applied for QED corrections to W pair production at LEP II.

QCD loop corrections to top production and decay

The subject of this talk is the computation of virtual QCD corrections to top production and decay at e+e− colliders. We examine the resonant behavior of the amplitudes that dominate the cross section and discuss the double pole approximation (DPA). The theoretical framework is similar to that which has been successfully applied for QED corrections to W pair production at LEP II.

TOP PRODUCTION AND DECAY AT LINEAR COLLIDERS: QCD CORRECTIONS

We present the results of an exact calculation of gluon radiation in top production and decay at high energy electron-positron colliders. We include all spin correlations and interferences, the bottom quark mass, and finite top width effects in the matrix element calculation. We study properties of the radiated gluons and implications for top mass measurement. We also discuss virtual corrections to the process.

Lepton–chargino mixing and R-parity violating SUSY

We present a study of charged lepton mass matrix diagonalization in R-parity violating SUSY. The case in which the bilinear couplings μi have large values is given special attention.

Could the τ be substantially different from e and μ in the supersymmetric standard model

R-parity stands as an ad hoc assumption in the most popular version of the supersymmetric standard model. More than fifteen years studies of R-parity violations have been restricted to various limiting scenarios. We illustrate how the single-VEV parametrization provides a workable framework to analyze the phenomenology of the complete theory of supersymmetry without R-parity. In our comprehensive study of various aspects of the resulting leptonic phenomenology at tree-level, we find that the physical τ lepton could actually bear substantial gaugino and higgsino components, making it very different from the e and the μ.