Geraldine Servant

Is the lightest Kaluza-Klein particle a viable dark matter candidate?

Abstract In models with universal extra dimensions (i.e., in which all Standard Model fields, including fermions, propagate into compact extra dimensions) momentum conservation in the extra dimensions leads to the conservation of Kaluza–Klein (KK) number at each vertex. KK number is violated by loop effects because of the orbifold imposed to reproduce the chiral Standard Model with zero modes, however, a KK parity remains at any order in perturbation theory which leads to the existence of a stable lightest KK particle (LKP). In addition, the degeneracy in the KK spectrum is lifted by radiative corrections so that all other KK particles eventually decay into the LKP. We investigate cases where the Standard Model lives in five or six dimensions with compactification radius of TeV−1 size and the LKP is the first massive state in the KK tower of either the photon or the neutrino. We derive the relic density of the LKP under a variety of assumptions about the spectrum of first tier KK modes. We find that both the KK photon and the KK neutrino, with masses at the TeV scale, may have appropriate annihilation cross sections to account for the dark matter, Ω M ∼0.3 .

COSMOLOGICAL EXPANSION IN THE PRESENCE OF AN EXTRA DIMENSION

It was recently pointed out that global solutions of Einsteins equations for a 3-brane universe embedded in four spatial dimensions give rise to a Friedmann equation of the form

Discovering the top partners at the LHC using same-sign dilepton final states

H\ensuremath{\propto}\ensuremath{\rho}

Warped unification, proton stability and dark matter

on the brane, instead of the usual

First-order electroweak phase transition in the standard model with a low cutoff

H\ensuremath{\propto}\sqrt{\ensuremath{\rho}}

Baryon number in warped grand unified theories: model building and (dark matter related) phenomenology

, which is inconsistent with cosmological observations. We remedy this problem by adding cosmological constants to the brane and the bulk, as in the recent scenario of Randall and Sundrum. Our observation allows for normal expansion during nucleosynthesis, but faster than normal expansion in the very early universe, which could be helpful for electroweak baryogenesis, for example.

Science with the space-based interferometer eLISA. II: Gravitational waves from cosmological phase transitions

A natural, non-supersymmetric solution to the hierarchy problem generically requires fermionic partners of the top quark with masses not much heavier than 500GeV. We study the pair production and detection at the LHC of the top partners with electric charge Qe = 5/3 (T5/3) and Qe = −1/3 (B), that are predicted in models where the Higgs is a pseudo-Goldstone boson. The exotic T 5/3 fermion, in particular, is the distinct prediction of a LR custodial parity invariance of the electroweak symmetry breaking sector. Both kinds of new fermions decay to W t, leading to a tW final state. We focus on the golden channel with two same-sign leptons, and show that a discovery could come with less than 100pb −1 (less than 20fb −1 ) of integrated luminosity for masses M = 500GeV (M = 1TeV). In the case of the T5/3, we present a simple strategy for its reconstruction in the fully hadronic decay chain. Although no full mass reconstruction is possible for the B, we still find that the same-sign dilepton channel offers the best chances of discovery compared to other previous searches that used final states with one or two opposite-sign leptons, and hence suffered from the large tbackground.

Non-resonant new physics in top pair production at hadron colliders

We show that solving the problem of baryon-number violation in nonsupersymmetric grand unified theories (GUTs) in warped higher-dimensional spacetime can lead to a stable Kaluza-Klein particle. This exotic particle has gauge quantum numbers of a right-handed neutrino, but carries fractional baryon number and is related to the top quark within the higher-dimensional GUT. A combination of baryon number and SU(3) color ensures its stability. Its relic density can easily be of the right value for masses in the 10 GeV-few TeV range. An exciting aspect of these models is that the entire parameter space will be tested at near future dark matter direct detection experiments. Other exotic GUT partners of the top quark are also light and can be produced at high energy colliders with distinctive signatures.

Gravitational wave generation from bubble collisions in first-order phase transitions: an analytic approach

We study the possibility of a first-order electroweak phase transition (EWPT) due to a dimension-six operator in the effective Higgs potential. In contrast with previous attempts to make the EWPT strongly first-order as required by electroweak baryogenesis, we do not rely on large one-loop thermally generated cubic Higgs interactions. Instead, we augment the Standard Model (SM) effective theory with a dimension-six Higgs operator. This addition enables a strong first-order phase transition to develop even with a Higgs boson mass well above the current direct limit of 114 GeV. The phi^6 term can be generated for instance by strong dynamics at the TeV scale or by integrating out heavy particles like an additional singlet scalar field. We discuss conditions to comply with electroweak precision constraints, and point out how future experimental measurements of the Higgs self couplings could test the idea.

The stochastic gravitational wave background from turbulence and magnetic fields generated by a first-order phase transition

In the past year, a new non-supersymmetric framework for electroweak symmetry breaking (with or without Higgs) involving SU(2)L × SU(2)R × U(1)B−L in higher dimensional warped geometry has been suggested. In this work, we embed this gauge structure into a GUT such as SO(10) or Pati–Salam. We showed recently (in hep-ph/0403143) that in a warped GUT, a stable Kaluza–Klein fermion can arise as a consequence of imposing proton stability. Here, we specify a complete realistic model where this particle is a weakly interacting right-handed neutrino, and present a detailed study of this new dark matter candidate, providing relic density and detection predictions. We discuss phenomenological aspects associated with the existence of other light () KK fermions (related to the neutrino), whose lightness is a direct consequence of the top quarks heaviness. The AdS/CFT interpretation of this construction is also presented. Most of our qualitative results do not depend on the nature of the breaking of the electroweak symmetry provided that it happens near the TeV brane.