Manuel Masip

Gluon excitations in t tbar production at hadron colliders

We argue that a relatively light massive gluon with mass <= 1 TeV, small purely axial couplings to light quarks and sizable vector and axial couplings to the top quark can reproduce the large forward-backward asymmetry observed at the Tevatron without conflicting with the t tbar and the dijet invariant mass distributions measured at the Tevatron and the LHC. We show that realistic Higgsless models with warped extra dimensions naturally fulfil all the necessary ingredients to realize this scenario. While current data is unable to discover or exclude these heavy gluons with masses 850 GeV, they should be observed at the (7 TeV) LHC with a luminosity of the order of 300 pb^{-1}.

Discrete gauge symmetries in supersymmetric grand unified models

We investigate the presence of discrete gauge symmetries in grand unification models based on SO(10) and E[sub 6]. These models include [ital flipped] and [ital unflipped] SU(5), SU(4)[times]SU(2)[sub [ital L]][times]SU(2)[sub [ital R]], SU(3)[sub [ital C]][times]SU(3)[sub [ital L]] [times]SU(3)[sub [ital R]], and SU(6)[times]SU(2). Using the Dynkin formalism we find the U(1) subalgebras contained in the unified groups, give an expression for the Higgs fields that preserves each discrete symmetry, and determine the low-energy matter content implied by chirality. We discuss two [ital Z][sub 2] and three [ital Z][sub 3] nonequivalent cases. Among the possibilities found, only the usual [ital Z][sub 2] matter parity ([ital R] parity) of supersymmetric extensions is consistent with a minimal matter content with no right-handed neutrinos, extra Higgs doublets, or nonstandard [ital down]-type quarks.

Leptophobic Z′ from superstring derived models☆

Abstract It was recently suggested that the reported anomalies in R b and R c can be interpreted as the effect of a heavy vector boson that couples to quarks and is universally decoupled from leptons. We examine how an extra gauge boson with this property can arise from superstring derived models. In a specific three generation model we show that the U (1) B - L symmetry combines with the horizontal flavor symmetries to form a universal leptophobic U (1) symmetry. In our model there is an enhancement of the color gauge group from twisted sectors. The enhancement occurs after the breaking of the unifying gauge symmetry by “Wilson lines”. The leptophobic U (1) symmetry then becomes a generator of the color SU (4) gauge group. We examine how similar symmetries may appear in other string models without the enhancement. We propose that if the current LEP anomalies persist it may be evidence for a certain class of un-unified superstring models.

Spontaneous CP violation in supersymmetric models with four Higgs doublets.

We consider supersymmetric extensions of the standard model with two pairs of Higgs doublets. We study the possibility that {ital CP} violation is generated spontaneously in the scalar sector via vacuum expectation values (VEV`s) of the Higgs fields. Using a simple geometrical interpretation of the minimum conditions we prove that the minimum of the tree-level scalar potential for these models is always real. We show that complex VEV`s can appear once radiative corrections and/or explicit {ital soft} {ital CP}-violating terms are added to the effective potential.