Aspects of Phenomenology and Cosmology in Hidden sector extensions of the Standard Model

Resumen

We have studied some issues concerning hidden sectors coupled to the Standard Model through the Higgs portal. In the first part of the Thesis we have explored the cosmological implications of hidden sectors, focusing on the study of the dynamics of the Electroweak Phase Transition in these scenarios, and also on dark matter from the hidden sector. In the second part, we have considered a scenario in which the hidden sector coupled to the Standard Model through the Higgs portal is conformally invariant; this kind of scenarios in which a conformal hidden sector is coupled to the SM were recently put forward by Georgi as a possible extension of the Standard Model with very unusual features, and were given the name ¿Unparticle Scenarios¿. Here we have analyzed the effects that a coupling between the conformal hidden sector and the Standard Model through the Higgs portal will have both for the Higgs sector and for the hidden conformal sector. Cosmological Aspects of Hidden Sectors. We have explored several cosmological implications of Standard Model extensions through hidden sector scalars, using a very simple hidden sector extension of the Standard Model, which consists in adding a set of N real scalar singlets Si coupled to the SM Higgs doublet H. We first performed a study of the Electroweak phase transition in this models. In Electroweak Baryogenesis scenarios, the Electroweak phase transition plays a crucial role since it can provide the necessary departure from thermal equilibrium; for Baryogenesis to occur a 1st order phase transition is needed, and it also has to be strong enough to suppress sphaleron processes in the broken phase after the phase transition, namely v(T)/T > 1. In the Standard Model, the Electroweak phase transition is of second order or a crossover for Higgs masses above the LEP experimental limit, preventing Electroweak Baryogenesis from being realized in the Standard Model. Here we find that in models with a moderate number of hidden sector scalars (NS ~ 12) the phase transition is of first order for Higgs masses of the order of the electroweak scale (or lighter) and sizable Higgs couplings to the hidden sector, ¿ ¿ 0.9. We also find that this persists in the case of classical conformal invariance, in which the electroweak scale is generated by dimensional transmutation. The study of the first order phase transition is performed identifying and computing the relevant parameters for describing the dynamics of the transition: critical temperature Tc, nucleation temperature Tn, duration of the transition fit (or, equivalently, temperature at which the transition ends Tf ), strength of the transition R ¿ v(T)/T and latent heat liberated (normalized to the energy in the plasma) ¿. In this respect we improve most previous studies of the transition and clarify the proper definition of Tn and fit, which are defined in several, inequivalent ways in the literature; we also find that the definition of these two parameters can be related to values of the tunneling action at finite temperature S3(T)/T in a model independent way, obtaining S3(Tn)/Tn ¿ 142¿148 and S3(Tf )/Tf ¿ 110¿115 (this last one is true except for very strong phase transitions). As a result of this study, we find that in a large portion of parameter space the 1st order phase transition is strong enough to avoid the erasure of the baryon asymmetry through sphaleron processes after the transition, v(Tf )/Tf > 1. Also, we consider the possibility that the hidden sector scalars account for the dark matter of the universe. We compute the relic mass density of scalar particles S and find that the required dark matter abundance DM = 0.228 ± 0.013 can be provided by hiddensector scalars in two different regimes. In the first, the hidden-sector scalars have moderate couplings but large masses MS ~ 1 TeV. In the second, the hidden-sector scalars are rather light, MS ¿ MW; in this case, the scalars cannot annihilate into W-bosons, and this fact greatly enhances the dark matter abundance. Nevertheless, neither type of scalar can contribute significantly to the previously discussed strength of the phase transition. Hence, a simultaneous solution of the dark matter and baryogenesis problems of the Standard Model in this kind of hidden sector extensions either requires a large number of scalars (in which case we found NS ~ 50), or several types of scalars in the hidden sector with non-uniform masses and/or couplings to the Higgs sector. Unparticles Coupled to the Higgs. Unparticle models have been regarded lately as hidden sector extensions of the Standard Model with very interesting and unusual features. After giving a brief introduction on the Unparticle idea and describing Stephanov deconstructed version Unparticles, we have investigated the possibility of coupling the Higgs to the conformally invariant hidden sector (Unparticle sector), through a ¿Higgs portal¿ type of coupling |H|2OU . The study of this scenario is important for Unparticle physics since, being |H|2 the only gauge invariant scalar operator with dimension d < 3, its coupling to Unparticles can be relevant at low energies, therefore strongly affecting the low energy phenomenology of Unparticles. A first consequence of this coupling is that Electroweak symmetry breaking introduces a divergent tadpole for the Unparticle operator OU; once this divergence is cured (we have achieved this by introducing new interactions in a deconstructed model for Unparticles, that keep the VEV for the Unparticle operator finite) a mass gap mg is generated for the Unparticle sector, implying the breaking of the conformal symmetry of the Unparticles. Within the Stephanov deconstruction formalism, We have analyzed two different ways of curing the divergent tadpole for OU, giving rise to two different types of spectra for the Higgs- Unparticles system once the Higgs and the scalar Unparticles mix. In the first scenario, the Higgs mixes with a collective state coming from the Unparticle continuum of states, giving rise to two different eigenstates in the spectrum, plus a continuum above the mass gap; the eigenstates can be embedded in the continuum or be isolated depending on if their mass lies above or below the mass gap; we call this type of scenario, ¿Plasmon scenario¿ (because the collective Unparticle state is similar to plasmon excitations in condensed matter physics). In the second scenario, the mixing between the Higgs field and the Unparticles is rather strong, resulting in the appearance of a second eigenstate (apart from the one associated with the Higgs state before the mixing) not associanted with a collective Unparticle state; the call this eigenstate the ¿Phantom Higgs¿ since both its mass and couplings to gauge bosons and fermions can be much lower than naively expected. In both cases, we analyze both the pole structure of the system and perform a spectral function analysis. Finally, we have investigated the issue of Unparticle decays, using a toy model that captures the main aspects of more realistic SM-Unparticle couplings.