Eduardo Rojas

Improved constraints on Z' bosons from electroweak precision data

We analyze various models with an extra U(1) gauge symmetry in addition to the Standard Model (SM) gauge group at low energies, and impose limits on the mass of the neutral Z boson, MZ, predicted in all such models, and on the Z?Z mixing angle, ?ZZ. The precision electroweak (EW) data strongly constrain ?ZZ to very small values and for most models we find lower limits on MZ of (1TeV). In one case we obtain a somewhat better fit than in the SM (although this is only marginally statistically significant) and here we find a weak upper limit at the 90% C.L.

Z′ bosons at colliders: a Bayesian viewpoint

A bstractWe revisit the CDF data on di-muon production to impose constraints on a large class of Z′ bosons occurring in a variety of E6 GUT based models. We analyze the dependence of these limits on various factors contributing to the production cross-section, showing that currently systematic and theoretical uncertainties play a relatively minor role. Driven by this observation, we emphasize the use of the Bayesian statistical method, which allows us to straightforwardly (i) vary the gauge coupling strength, g′, of the underlying U(1)′; (ii) include interference effects with the Z′ amplitude (which are especially important for large g′); (iii) smoothly vary the U(1)′ charges; (iv) combine these data with the electroweak precision constraints as well as with other observables obtained from colliders such as LEP 2 and the LHC; and (v) find preferred regions in parameter space once an excess is seen. We adopt this method as a complementary approach for a couple of sample models and find limits on the Z′ mass, generally differing by only a few percent from the corresponding CDF ones when we follow their approach. Another general result is that the interference effects are quite relevant if one aims at discriminating between models. Finally, the Bayesian approach frees us of any ad hoc assumptions about the number of events needed to constitute a signal or exclusion limit for various actual and hypothetical reference energies and luminosities at the Tevatron and the LHC.

Dynamical mass generation in QED with magnetic fields: Arbitrary field strength and coupling constant

We study the dynamical generation of masses for fundamental fermions in quenched quantum electrodynamics, in the presence of magnetics fields of arbitrary strength, by solving the Schwinger-Dyson equation for the fermion self-energy in the rainbow approximation. We employ the Ritus eigenfunction formalism which provides a neat solution to the technical problem of summing over all Landau levels. It is well known that magnetic fields catalyze the generation of fermion mass m for arbitrarily small values of electromagnetic coupling {alpha}. For intense fields it is also well known that m{proportional_to}{radical}(eB). Our approach allows us to span all regimes of parameters {alpha} and eB. We find that m{proportional_to}{radical}(eB) provided {alpha} is small. However, when {alpha} increases beyond the critical value {alpha}{sub c} which marks the onslaught of dynamical fermion masses in vacuum, we find m{proportional_to}{lambda}, the cutoff required to regularize the ultraviolet divergences. Our method permits us to verify the results available in literature for the limiting cases of eB and {alpha}. We also point out the relevance of our work for possible physical applications.

Constraints on the mass and mixing of Z' bosons

We tested several models in which the Standard Model (SM) gauge group is extended by an additional U(1) gauge symmetry against available electroweak precision data to impose limits on the mass of the neutral Z′ boson, MZ′, predicted in all such models, and on the Z−Z′ mixing angle, θZZ′, at 95% C.L. We found lower limits on MZ′ of order 1 TeV in most cases, while θZZ′ was found to be constrained to very small values.

Flipped versions of the universal 3-3-1 and the left-right symmetric models in

By considering the 3-3-1 and the left-right symmetric models as low energy effective theories of the

[SU(3)]^3

SU(3)_Cotimes SU(3)_Lotimes SU(3)_R

: a comprehensive approach

~(for short

Minimal nonuniversal electroweak extensions of the standard model: A chiral multiparameter solution

[SU(3)]^3

Mass spectrum and decay constants of radially excited vector mesons

) gauge group, alternative versions of these models are found. The new neutral gauge bosons of the universal 3-3-1 model and its flipped versions are presented; also, the left-right symmetric model and its flipped variants are studied. Our analysis shows that there are two flipped versions of the universal 3-3-1 model, with the particularity that both of them have the same weak charges. For the left-right symmetric model we also found two flipped versions; one of them new in the literature which, unlike those of the 3-3-1, requires a dedicated study of its electroweak properties. For all the models analyzed, the couplings of the

Collisional parton energy loss in a finite size QCD medium reexamined: Off-mass-shell effects

Z