Can new generations explain neutrino masses?

In this talk we explore the possibility that the smallness of the observed neutrino masses is naturally understood in a modified version of the standard model with N extra generations of fermions and N right-handed neutrinos, in which light neutrino masses are generated at two loops. We find that with N = 1 it is not possible to fit the observed spectrum of masses and mixings while with N = 2 it is. Within this extension, we analyse the parameters which are allowed and the possible phenomenological signals of the model in future experiments. Contribution to the proceedings of Les Rencontres de Moriond EW 2011, Young Scientist Forum

OB stars at the lowest Local Group metallicity: GTC-OSIRIS observations of Sextans A

Our aim is to find and classify OB stars in Sextans A, to later determine accurate stellar parameters of these blue massive stars in this low metallicity region $(Z \sim 0.1 \rm Z_{\odot})$. Using UBV photometry, the reddening-free index Q and GALEX imaging, we built a list of blue massive star candidates in Sextans A. We obtained low resolution (R $\sim$ 1000) GTC-OSIRIS spectra for a fraction of them and carried out spectral classification. For the confirmed O-stars we derive preliminary stellar parameters. The target selection criteria and observations were successful and have produced the first spectroscopic atlas of OB-type stars in Sextans A. From the whole sample of 18 observed stars, 12 were classified as early OB-types, including 5 O-stars. The radial velocities of all target stars are in agreement with their Sextans A membership, although three of them show significant deviations. We determined the stellar parameters of the O-type stars using the stellar atmosphere code FASTWIND, and revisited the sub-SMC temperature scale. Two of the O-stars are consistent with relatively strong winds and enhanced helium abundances, although results are not conclusive. We discuss the position of the OB stars in the HRD. Initial stellar masses run from slightly below 20 up to 40 solar masses. The target selection method worked well for Sextans A, confirming the procedure developed in Garcia \& Herrero (2013). The stellar temperatures are consistent with findings in other galaxies. Some of the targets deserve follow-up spectroscopy because of indications of a runaway nature, an enhanced helium abundance or a relatively strong wind. We observe a correlation between HI and OB associations similar to the irregular galaxy IC1613, confirming the previous result that the most recent star formation of Sextans A is currently on-going near the rim of the H\,{\sc I} cavity

Unitarization effects in EFT predictions of WZ scattering at the LHC

Effective field theories are an incredibly powerful tool in order to study and understand the true nature of the symmetry breaking sector dynamics of the Standard Model. However, they can suffer from some theoretical problems such as that of unitarity violation. Nevertheless, in order to interpret experimental data correctly a fully unitary prescription is needed. To this purpose, unitarization methods are addressed, but each of them leads to a different (unitary) prediction. Because of this, there is an inherent theoretical uncertainty in the determination of the effective field theory parameters due to the choice of one unitarization scheme. In this work, we quantify this uncertainty assuming a strongly interacting electroweak symmetry breaking sector, described by the effective electroweak chiral Lagrangian. We focus on the bosonic part of this effective Lagrangian and choose in particular the WZ scattering as our main VBS channel to study the sensitivity to new physics at the LHC. We study the different predictions of various well known unitarization methods, considering the full coupled system of helicity amplitudes, and construct the 95\% confidence level exclusion regions for the most relevant electroweak chiral Lagrangian parameters, given by the two anomalous quartic gauge couplings $a_4$ and $a_5$. This provides a consistent analysis of the different constraints on EChL parameters that can be achieved by using different unitarization methods in a combined way.Comment: 25 pages, 8 figures (20 plots), this version matches the published article in Phys. Rev.

Production of vector resonances at the LHC via WZ-scattering: a unitarized EChL analysis

In the present work we study the production of vector resonances at the LHC by means of the vector boson scattering $WZ \to WZ$ and explore the sensitivities to these resonances for the expected future LHC luminosities. We are assuming that these vector resonances are generated dynamically from the self interactions of the longitudinal gauge bosons, $W_L$ and $Z_L$, and work under the framework of the electroweak chiral Lagrangian to describe in a model independent way the supposedly strong dynamics of these modes. The properties of the vector resonances, mass, width and couplings to the $W$ and $Z$ gauge bosons are derived from the inverse amplitude method approach. We implement all these features into a single model, the IAM-MC, adapted for MonteCarlo, built in a Lagrangian language in terms of the electroweak chiral Lagrangian and a chiral Lagrangian for the vector resonances, which mimics the resonant behavior of the IAM and provides unitary amplitudes. The model has been implemented in MadGraph, allowing us to perform a realistic study of the signal versus background events at the LHC. In particular, we have focused our study on the $pp\to WZjj$ type of events, discussing first on the potential of the hadronic and semileptonic channels of the final $WZ$, and next exploring in more detail the clearest signals. These are provided by the leptonic decays of the gauge bosons, leading to a final state with $\ell_1^+\ell_1^-\ell_2^+\nu jj$, $\ell=e,\mu$, having a very distinctive signature, and showing clearly the emergence of the resonances with masses in the range of 1.5-2.5 TeV, which we have explored.Comment: Revised version accepted for publication in JHEP. Enlarged analysis. References added. 44 pages, 23 figures, 3 table