Thomas Hambye

Low energy effects of neutrino masses

While all models of Majorana neutrino masses lead to the same dimension five effective operator, which does not conserve lepton number, the dimension six operators induced at low energies conserve lepton number and differ depending on the high energy model of new physics. We derive the low-energy dimension six operators which are characteristic of generic Seesaw models, in which neutrino masses result from the exchange of heavy fields which may be either fermionic singlets, fermionic triplets or scalar triplets. The resulting operators may lead to effects observable in the near future, if the coefficients of the dimension five and six operators are decoupled along a certain pattern, which turns out to be common to all models. The phenomenological consequences are explored as well, including their contributions to μ→eγ and new bounds on the Yukawa couplings for each model.

A facility to search for hidden particles at the CERN SPS: the SHiP physics case.

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (search for hidden particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, [Formula: see text] and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the standard model and describe interactions between new particles and four different portals-scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the standard model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation.

Matching conditions and Higgs mass upper bounds revisited

Matching conditions relate couplings to particle masses. We discuss the importance of one-loop matching conditions in the Higgs boson and top quark sector as well as the choice of the matching scale. We argue for matching scales {mu}{sub 0,t}{approx_equal}m{sub t} and {mu}{sub 0,H}{approx_equal}max{l_brace}m{sub t},M{sub H}{r_brace}. Using these results, the two-loop Higgs boson mass upper bounds are reanalyzed. Previous results for {Lambda}{approx} few TeV are found to be too stringent, and a recent update is found to be wrong. For {Lambda}=10{sup 19} GeV we find M{sub H}{lt}180{plus_minus}4{plus_minus}5 GeV, the first error indicating the theoretical uncertainty and the second error reflecting the experimental uncertainty due to m{sub t}=175{plus_minus}6 GeV. Hence a Higgs boson mass of about 160{endash}170 GeV certainly satisfies both upper and lower Higgs boson mass bounds for cutoff scales up to {Lambda}=10{sup 19} GeV if m{sub t}=175 GeV. For such Higgs boson and top quark masses the renormalization group behavior of the minimal standard model does not require new physics to set in before the Planck scale. {copyright} {ital 1997} {ital The American Physical Society}

Minimal Flavour Seesaw Models

We explore realizations of minimal flavour violation (MFV) for the lepton sector. We find that it can be realized within those seesaw models where a separation of the lepton number and lepton flavour violating scales can be achieved, such as scalar mediated (type II) and inverse seesaw models. We present in particular a simple implementation of the MFV hypothesis which differs in nature from those previously discussed. It allows to reconstruct the flavour structure of the model from the values of the light neutrino masses and mixing parameters, even in the presence of CP-violating phases. Experimentally reachable predictions for rare processes such as ??e? are given.

Scalar multiplet dark matter

We perform a systematic study of the phenomenology associated to models where the dark matter consists in the neutral component of a scalar SU(2)L n-uplet, up to n = 7. If one includes only the pure gauge induced annihilation cross-sections it is known that such particles provide good dark matter candidates, leading to the observed dark matter relic abundance for a particular value of their mass around the TeV scale. We show that these values actually become ranges of values - which we determine - if one takes into account the annihilations induced by the various scalar couplings appearing in these models. This leads to predictions for both direct and indirect detection signatures as a function of the dark matter mass within these ranges. Both can be largely enhanced by the quartic coupling contributions. We also explain how, if one adds right-handed neutrinos to the scalar doublet case, the results of this analysis allow to have altogether a viable dark matter candidate, successful generation of neutrino masses, and leptogenesis in a particularly minimal way with all new physics at the TeV scale.

Electroweak symmetry breaking induced by dark matter

Abstract The mechanism behind electroweak symmetry breaking (EWSB) and the nature of dark matter (DM) are currently among the most important issues in high energy physics. Since a natural dark matter candidate is a weakly interacting massive particle or WIMP, with mass around the electroweak scale, it is clearly of interest to investigate the possibility that DM and EWSB are closely related. In the context of a very simple extension of the Standard Model, the inert doublet model, we show that dark matter could play a crucial role in the breaking of the electroweak symmetry. In this model, dark matter is the lightest component of an inert scalar doublet. The coupling of the latter with the Standard Model Higgs doublet breaks the electroweak symmetry at one-loop, a la Coleman–Weinberg . The abundance of dark matter, the breaking of the electroweak symmetry and the constraints from electroweak precision measurements can all be accommodated by imposing (an exact or approximate) custodial symmetry.

WIMP dark matter, Higgs exchange and DAMA

In the WIMP scenario, there is a one-to-one relation between the dark matter (DM) relic density and spin-independent direct detection rate if both the annihilation of DM and its elastic scattering on nuclei go dominantly through Higgs exchange. In particular, for DM masses much smaller than the Higgs boson mass, the ratio of the relevant cross sections depends only on the DM mass. Assuming DM mass and direct detection rate within the ranges allowed by the recent DAMA collaboration results—taking account of the channelling effect on energy threshold and the null results of the other direct detection experiments—gives a definite range for the relic density. For scalar DM models, like the Higgs portal models or the inert doublet model, the relic density range turns out to be in agreement with WMAP. This scenario implies that the Higgs boson has a large branching ratio to pairs of DM particles, a prediction which might challenge its search at the LHC.

Consequences of triplet seesaw for leptogenesis

Abstract We present the various leptogenesis scenarios which may occur if, in addition to the ordinary heavy right-handed neutrinos, there exists a heavy scalar SU (2) L triplet coupled to leptons. We show that the contributions of the right-handed neutrinos and the triplet to the lepton asymmetry are proportional to their respective contributions to the neutrino mass matrix. A consequence of the triplet contribution to the lepton asymmetry is that there is no more upper bound on the neutrino masses from leptogenesis due to the fact that the neutrino mass constraints do not necessarily induce asymmetry washout effects. We also show how such a triplet leptogenesis mechanism may emerge naturally in the framework of the left–right symmetric theories, such as Pati–Salam or SO (10).

Light scalar WIMP through the Higgs portal and CoGeNT

If dark matter (DM) simply consists in a scalar particle interacting dominantly with the Higgs boson, the ratio of its annihilation cross section---which is relevant both for the relic abundance and indirect detection---and its spin-independent scattering cross section on nuclei depends only on the DM mass. It is an intriguing result that, fixing the mass and direct detection rate to fit the annual modulation observed by the DAMA experiment, one obtains a relic density in perfect agreement with its observed value. In this article we update this result and confront the model to the recent CoGeNT data, tentatively interpreting the excess of events in the recoil energy spectrum as being due to DM. CoGeNT, as DAMA, points toward a light DM candidate, with somewhat different (but not necessarily incompatible) masses and cross sections. For the CoGeNT region too, we find an intriguing agreement between the scalar DM relic density and direct detection constraints. We give the

Type-III seesaw mechanism at CERN LHC

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