Relic density calculations beyond tree-level, exact calculations versus effective couplings: the ZZ final state

The inferred value of the relic density from cosmological observations has reached a precision that is akin to that of the LEP precision measurements. This level of precision calls for the evaluation of the annihilation cross sections of dark matter that goes beyond tree-level calculations as currently implemented in all codes for the computation of the relic density. In supersymmetry radiative corrections are known to be large and thus must be implemented. Full one-loop radiative corrections for many annihilation processes have been performed. It is important to investigate whether the bulk of these corrections can be parameterised through an improved Born approximation that can be implemented as a selection of form factors to a tree-level code. This paper is a second in a series that addresses this issue. After having provided these form factors for the annihilation of the neutralinos into fermions, which cover the case of a bino-like LSP (Lightest Supersymmetric Particle), we turn our attention here to a higgsino-like dark matter candidate through its annihilation into $ZZ$. We also investigate the cases of a mixed LSP. In all cases we compare the performance of the form factor approach with the result of a full one-loop correction. We also study the issue of the renormalisation scheme dependence. An illustration of the phenomenon of non decoupling of the heavy sfermions that takes place for the annihilation of the lightest neutralino into $ZZ$ is also presented.

Higgs couplings as a probe of New Physics

We consider the Higgs boson decay processes and its production and provide a parameterisation tailored for testing models of new physics. The choice of a particular parameterisation depends on a non-obvious balance of quantity and quality of the available experimental data, envisaged purpose for the parameterisation and degree of model independence. At present only simple parameterisations with a limited number of fit parameters can be performed, but this situation will improve with the forthcoming experimental LHC data. It is therefore important that different approaches are considered and that the most detailed information is made available to allow testing the different aspects of the Higgs boson physics and the possible hints beyond the Standard Model.

Effective Approaches in and Beyond the MSSM : applications to Higgs Physics and Dark Matter observables

In this thesis we use the Effective Field Theory approach for supersymmetric theories, applied to two experimental domains : the search for the Higgs bosons at colliders and the Dark Matter observables. The reason for introducing an effective approach in the Higgs physics is that the simple supersymmetric extension of the Standard Model (the MSSM) is known to have a tightly constrained Higgs sector, in particular with a lightest Higgs mass difficult to raise without introducing some fine-tuning. Many specific expansions (as the NMSSM for instance) allow for a richer Higgs sector. In this case however, the effective approach is handy since it encompasses many different specific ultraviolet completions. We have added to the Kahler and superpotential of the MSSM all possible terms of dimensions 5 and 6 including only Higgs superfields. The phenomenology of the resulting Higgs sector appears to be much richer, and we analyse our model in view of the latest LHC results. On the other side, supersymmetry is known for providing a Dark Matter candidate, so we have also worked to improve the accuracy of the computation of the relic density in the MSSM. Indeed the impressive accuracy on the experimental side (around 6%) calls for a precise computation including radiative corrections. Those one-loop computations being rather long in the MSSM, thus often overlooked, we have used again the effective approach by introducing new effective vertices that aim at accounting for the one-loop correction. We show on a specific process (neutralinos annihilation to muons) the agreement between the effective approach and the full one-loop computation.

Supersymmetric Higgses beyond the MSSM: An update with flavour and Dark Matter constraints

Spurred by the discovery of a boson resonance at the LHC as the result of the search for the Standard Model Higgs, we pursue our investigation of the properties and signatures of Higgses in an effective supersymmetric scenario that goes beyond the usual MSSM. Such scenarios were first introduced to alleviate the naturalness problem of the MSSM Higgs and are found to have a very rich phenomenology that allows departures from the Standard Model in the production rate of the Higgs in many of the search channels. We now include the constraints from flavour observables in particular the rare decays b-> s gamma and Bs -> mu+ mu- including the recent measurement from LHCb. We also address the issue of Dark Matter and its impact on Higgs physics. In particular, we incorporate the latest data from XENON100 on the spin independent direct detection rates. These turn out to be powerful constraints, especially if one also imposes that the observed thermal relic density is obtained. We also study models with a low abundance that can more easily evade the direct detection rates. We study the impact of the flavour and Dark Matter observables on the production rates of the Higgs at the LHC, and their correlations in the diphoton, diphoton+jets and 4 leptons. We also comment on the other channels.

BMSSM Higgses at 125 GeV

The BMSSM framework is an effective theory approach that encapsulates a variety of extensions beyond the MSSM with which it shares the same field content. The lightest Higgs mass can be much heavier than in the MSSM without creating a tension with naturalness or requiring superheavy stops. The phenomenology of the Higgs sector is at the same time much richer. We critically review the properties of a Higgs with mass around 125GeV in this model. In particular, we investigate how the rates in the important inclusive 2 photons channel, the 2 photons + 2 jets and the ZZ to 4 leptons (and/or WW) can be enhanced or reduced compared to the standard model and what kind of correlations between these rates are possible. We consider both a vanilla model where stops have moderate masses and do not mix and a model with large mixing and a light stop. We show that in both cases there are scenarios that lead to enhancements in these rates at a mass of 125GeV corresponding either to the lightest Higgs or the heaviest CP-even Higgs of the model. In all of these scenarios we study the prospects of finding other signatures either of the 125GeV Higgs or those of the heavier Higgses. In most cases the $oo{tau}tau$ channels are the most promising. Exclusion limits from the recent LHC Higgs searches are folded in our analyses while the tantalising hints for a Higgs signal at 125GeV are used as an example of how to constrain the BMSSM and/or direct future searches.

SUSY Higgs searches : beyond the MSSM

The recent results from the ATLAS and CMS collaborations show that the allowed range for a Standard Model Higgs boson is now restricted to a very thin region. Although those limits are presented exclusively in the framework of the SM, the searches themselves remain sensitive to other Higgs models. We recast the limits within a generic supersymmetric framework that goes beyond the usual minimal extension. Such a generic model can be parameterised through a supersymmetric effective Lagrangian with higher order operators appearing in the Kahler potential and the superpotential, an approach whose first motivation is to alleviate the fine-tuning problem in supersymmetry with the most dramatic consequence being a substantial increase in the mass of the lightest Higgs boson as compared to the minimal supersymmetic model. We investigate in this paper the constraints set by the LHC on such models. We also investigate how the present picture will change when gathering more luminosity. Issues of how to combine and exploit data from the LHC dedicated to searches for the standard model Higgs to such supersymmetry inspired scenarios are discussed. We also discuss the impact of invisible decays of the Higgs in such scenarios.

One-loop corrections, uncertainties and approximations in neutralino annihilations: Examples

The extracted value of the relic density has reached the few per-cent level precision. One can therefore no longer content oneself with calculations of this observable where the annihilation processes are computed at tree-level, especially in supersymmetry where radiative corrections are usually large. Implementing full one-loop corrections to all annihilation processes that would be needed in a scan over parameters is a daunting task. On the other hand one may ask whether the bulk of the corrections are taken into account through effective couplings of the neutralino that improve the tree-level calculation and would be easy to implement. We address this issue by concentrating in this first study on the neutralino coupling to i) fermions and sfermions and ii) Z. After constructing the effective couplings we compare their efficiency compared to the full one-loop calculation and comment on the failures and success of the approach. As a bonus we point out that large non decoupling effects of heavy sfermions could in principle be measured in the annihilation process, a point of interest in view of the latest limit on the squark masses from the LHC. We also comment on the scheme dependencies of the one-loop corrected results.