J. Alberto Casas

de Sitter vacua from uplifting D-terms in effective supergravities from realistic strings

We study the possibility of using the D-term associated to an anomalous U(1) for the uplifting of AdS vacua (to dS or Minkowski vacua) in effective supergravities arising from string theories, particularly in the type IIB context put forward by Kachru, Kallosh, Linde and Trivedi (KKLT). We find a gauge invariant formulation of such a scenario (avoiding previous inconsistencies), where the anomalous D-term cannot be cancelled, thus triggering the uplifting of the vacua. Then, we examine the general conditions for this to happen. Finally, we illustrate the results by presenting different successful examples in the type IIB context.

The MSSM fine tuning problem: a way out

As is well known, electroweak breaking in the MSSM requires substantial fine-tuning, mainly due to the smallness of the tree-level Higgs quartic coupling, λtree. Hence the fine tuning is efficiently reduced in supersymmetric models with larger λtree, as happens naturally when the breaking of SUSY occurs at a low scale (not far from the TeV). We show, in general and with specific examples, that a dramatic improvement of the fine tuning (so that there is virtually no fine-tuning) is indeed a very common feature of these scenarios for wide ranges of tanβ and the Higgs mass (which can be as large as several hundred GeV if desired, but this is not necessary). The supersymmetric flavour problems are also drastically improved due to the absence of RG cross-talk between soft mass parameters.

Implications for New Physics from Fine-Tuning Arguments: II. Little Higgs Models

We examine the fine-tuning associated to electroweak breaking in Little Higgs scenarios and find it to be always substantial and, generically, much higher than suggested by the rough estimates usually made. This is due to implicit tunings between parameters that can be overlooked at first glance but show up in a more systematic analysis. Focusing on four popular and representative Little Higgs scenarios, we find that the fine-tuning is essentially comparable to that of the Little Hierarchy problem of the Standard Model (which these scenarios attempt to solve) and higher than in supersymmetric models. This does not demonstrate that all Little Higgs models are fine-tuned, but stresses the need of a careful analysis of this issue in model-building before claiming that a particular model is not fine-tuned. In this respect we identify the main sources of potential fine-tuning that should be watched out for, in order to construct a successful Little Higgs model, which seems to be a non-trivial goal.

MSSM forecast for the LHC

We perform a forecast of the MSSM with universal soft terms (CMSSM) for the LHC, based on an improved Bayesian analysis. We do not incorporate ad hoc measures of the fine-tuning to penalize unnatural possibilities: such penalization arises from the Bayesian analysis itself when the experimental value of MZ is considered. This allows to scan the whole parameter space, allowing arbitrarily large soft terms. Still the low-energy region is statistically favoured (even before including dark matter or g-2 constraints). Contrary to other studies, the results are almost unaffected by changing the upper limits taken for the soft terms. The results are also remarkable stable when using flat or logarithmic priors, a fact that arises from the larger statistical weight of the low-energy region in both cases. Then we incorporate all the important experimental constrains to the analysis, obtaining a map of the probability density of the MSSM parameter space, i.e. the forecast of the MSSM. Since not all the experimental information is equally robust, we perform separate analyses depending on the group of observables used. When only the most robust ones are used, the favoured region of the parameter space contains a significant portion outside the LHC reach. This effect gets reinforced if the Higgs mass is not close to its present experimental limit and persits when dark matter constraints are included. Only when the g-2 constraint (based on e+e− data) is considered, the preferred region (for μ > 0) is well inside the LHC scope. We also perform a Bayesian comparison of the positive- and negative-μ possibilities.

Hints on the high-energy seesaw mechanism from the low-energy neutrino spectrum

It is an experimental fact that the mass ratio for the two heavier neutrinos, h = m3/m2 6, is much smaller than the typical quark and lepton hierarchies, which are (20–300). We have explored whether this peculiar pattern of neutrino masses can be a consequence of the peculiar way they are generated through a see-saw mechanism, determining 1) How the present experimental data restrict the structure of the high-energy seesaw parameters and 2) Which choices, among the allowed ones, produce more naturally the observed pattern of neutrino masses. We have studied in particular if starting with hierarchical neutrino Yukawa couplings, as for the other fermions, one can naturally get the observed h 6 ratio. To perform the analysis we have put forward a top-down parametrization of the see-saw mechanism in terms of (high-energy) basis-independent quantities. Among the main results, we find that in most cases m2/m1 m3/m2, so m1 should be extremely tiny. Also, the VR matrix associated to the neutrino Yukawa couplings has a far from random structure, naturally resembling VCKM. In fact we show that identifying VR and VCKM, as well as neutrino and u–quark Yukawa couplings can reproduce hexp in a highly non-trivial way, which is very suggestive. The physical implications of these results are also discussed.

Implications of light charginos for Higgs observables, LHC searches and dark matter

A bstractA rather high Higgs mass, mh ≃ 126 GeV, suggests that at least a part of the supersymmetric spectrum of the MSSM may live beyond

Large mixing angles for neutrinos from infrared fixed points

\mathcal{O}\left( {1\;\mathrm{TeV}} \right)

Charge and color breaking

and hence inaccessible to the LHC. However, there are theoretical and phenomenological reasons supporting a possibility that charginos and neutralinos remain much closer to the electroweak scale. In this paper, we explore such a scenario in the light of recent Higgs measurements, mainly its di-photon decay rate, where the data might indicate a slight excess over the SM prediction. That excess could be fitted by the contribution of light charginos provided tan β is low to moderate, a possibility that is receiving much attention for other theoretical reasons. We investigate the implications of this scenario for other observables, such as dark matter constraints, electroweak observables and experimental signals at the LHC, like dilepton, tri-lepton and same-sign dilepton. An important part of the models survive all the constraints and are able to give positive signals at LHC-14TeV and/or XENON1T.

Fair scans of the seesaw. Consequences for predictions on LFV processes

Radiative amplification of neutrino mixing angles may explain the large values required by solar and atmospheric neutrino oscillations. Implementation of such mechanism in the Standard Model and many of its extensions (including the Minimal Supersymmetric Standard Model) to amplify the solar angle, the atmospheric or both requires (at least two) quasi-degenerate neutrino masses, but is not always possible. When it is, it involves a fine-tuning between initial conditions and radiative corrections. In supersymmetric models with neutrino masses generated through the Kahler potential, neutrino mixing angles can easily be driven to large values at low energy as they approach infrared pseudo-fixed points at large mixing (in stark contrast with conventional scenarios, that have infrared pseudo-fixed points at zero mixing). In addition, quasi-degeneracy of neutrino masses is not always required.

New techniques for chargino-neutralino detection at LHC

The presence of scalar fields with color and electric charge in supersymmetric theories makes feasible the existence of dangerous charge and color breaking (CCB) minima and unbounded from below directions (UFB) in the effective potential, which would make the standard vacuum unstable. The avoidance of these occurrences imposes severe constraints on the supersymmetric parameter space. We give here a comprehensive and updated account of this topic.