Lorenzo Calibbi

Naturalness of the Non-Universal MSSM in the light of the recent Higgs results

A bstractWe analyse the naturalness of the Minimal Supersymmetric Standard Model (MSSM) in the light of recent LHC results from the ATLAS and CMS experiments. We study non-universal boundary conditions for the scalar and the gaugino sector, with fixed relations between some of the soft breaking parameters, and find a significant reduction of fine-tuning for non-universal gaugino masses. For a Higgs mass of about 125 GeV, as observed recently, we find parameter regions with a fine-tuning of O (10), taking into account experimental and theoretical uncertainties. These regions also survive after comparison with simplified model searches in ATLAS and CMS. For a fine-tuning less than 20 the lightest neutralino is expected to be lighter than about 400 GeV and the lighter stop can be as heavy as 3.5TeV. On the other hand, the gluino mass is required to be above 1.5 TeV. For non-universal gaugino masses, we discuss which fixed GUT scale ratios can lead to a reduced fine-tuning and find that the recent Higgs results have a strong impact on which ratio is favoured. We also discuss the naturalness of GUT scale Yukawa relations, comparing the non-universal MSSM with the CMSSM.

Universal constraints on low-energy flavour models

A bstractIt is pointed out that in a general class of flavour models one can identify certain universally present FCNC operators, induced by the exchange of heavy flavour messengers. Their coefficients depend on the rotation angles that connect flavour and fermion mass basis. The lower bounds on the messenger scale are derived using updated experimental constraints on the FCNC operators. The obtained bounds are different for different operators and in addition they depend on the chosen set of rotations. Given the sensitivity expected in the forthcoming experiments, the present analysis suggests interesting room for discovering new physics. As the highlights emerge the leptonic processes, μ → eγ, μ → eee and μ → e conversion in nuclei.

Phenomenology of SUSY SU(5) with type I+III seesaw

We consider a supersymmetric SU(5) model where two neutrino masses are obtained via a mixed type I+ III seesaw mechanism induced by the component fields of a single SU(5) adjoint. We have analyzed the phenomenology of the model paying particular attention to flavour violating processes and dark matter relic density, assuming universal boundary conditions. We have found that, for a seesaw scale larger than 1012÷13 GeV, BR(μ→eγ) is in the reach of the MEG experiment in sizable regions of the parameter space. On the other side, current bounds on it force BR(τ→μγ) to be well below the reach of forthcoming experiments, rendering thus the model disprovable if a positive signal is found. The same bounds still allow for a sizable positive contribution to ϵK, while the CP violation in the Bs mixing turns out to be too small to account for the di-muon anomaly reported by the D0 collaboration. Finally, the regions where the neutralino relic density is within the WMAP bounds can be strongly modified with respect to the constrained MSSM case. In particular, a peculiar coannihilation region, bounded from above, can be realized, which allows us to put an upper bound on the dark matter mass for certain set-ups of the parameters.

The messenger sector of SUSY flavour models and radiative breaking of flavour universality

A bstractThe flavour messenger sectors and their impact on the soft SUSY breaking terms are investigated in SUSY flavour models. In the case when the flavour scale M is below the SUSY breaking mediation scale MS , the universality of soft terms, even if assumed at MS , is radiatively broken. We estimate this effect in a broad class of models. In the CKM basis that effect gives flavour off-diagonal soft masses comparable to the tree-level estimate based on the flavour symmetry.

Light Neutralino in the MSSM: a playground for dark matter, flavor physics and collider experiments

We investigate the constraints to the light neutralino dark matter scenario in the minimal supersymmetric standard model from available experimental observations such as decays of B and K meson, relic dark matter abundance, and the search for neutralino and Higgs production at colliders. We find that two regions of the MSSM parameter space fulfill all the constraints: a fine-tuned strip with large tanβ where the lightest neutralino can be a slight as 8xa0GeV, and alow tanβ region providing a neutralino mass larger than 16xa0GeV. The large tan β strip, which can be compatible with recently reported signals from direct detection experiments, can be fully tested by means of low-energy observables and, in particular, by Bsu2009→u2009μμ and Higgs bosons searches at the LHC within the upcoming months.

Status of supersymmetric type-I seesaw in SO(10) inspired models

A bstractWe report on the status of supersymmetric seesaw models in the light of recent experimental results on μ → e + γ, θ13 and the light Higgs mass at the LHC. SO(10)-like relations are assumed for neutrino Dirac Yukawa couplings and two cases of mixing, one large, PMNS-like, and another small, CKM-like, are considered. It is shown that for the large mixing case, only a small range of parameter space with moderate tan β is still allowed. This remaining region can be ruled out by an order of magnitude improvement in the current limit on BR(μ → e + γ). We also explore a model with non-universal Higgs mass boundary conditions at the high scale. It is shown that the renormalization group induced flavor violating slepton mass terms are highly sensitive to the Higgs boundary conditions. Depending on the choice of the parameters, they can either lead to strong enhancements or cancellations within the flavor violating terms. Such cancellations might relax the severe constraints imposed by lepton flavor violation compared to mSUGRA. Nevertheless for a large region of parameter space the predicted rates lie within the reach of future experiments once the light Higgs mass constraint is imposed. We also update the potential of the ongoing and future experimental searches for lepton flavor violation in constraining the supersymmetric parameter space.

From flavour to SUSY flavour models

Abstract If supersymmetry (SUSY) will be discovered, successful models of flavour not only have to provide an explanation of the flavour structure of the Standard Model fermions, but also of the flavour structure of their scalar superpartners. We discuss aspects of such “SUSY flavour” models, towards predicting both flavour structures, in the context of supergravity (SUGRA). We point out the importance of carefully taking into account SUSY-specific effects, such as 1-loop SUSY threshold corrections and canonical normalisation, when fitting the model to the data for fermion masses and mixings. This entangles the flavour model with the SUSY parameters and leads to interesting predictions for the sparticle spectrum. We demonstrate these effects by analyzing an example class of flavour models in the framework of an SU ( 5 ) Grand Unified Theory with a family symmetry with real triplet representations. For flavour violation through the SUSY soft breaking terms, the class of models realises a scheme we refer to as “Trilinear Dominance”, where flavour violation effects are dominantly induced by the trilinear terms.

Postcards from oases in the desert: phenomenology of SUSY with intermediate scales

A bstractThe presence of new matter fields charged under the Standard Model gauge group at intermediate scales below the Grand Unification scale modifies the renormalization group evolution of the gauge couplings. This can in turn significantly change the running of the Minimal Supersymmetric Standard Model parameters, in particular the gaugino and the scalar masses. In the absence of new large Yukawa couplings we can parameterise all the intermediate scale models in terms of only two parameters controlling the size of the unified gauge coupling. As a consequence of the modified running, the low energy spectrum can be strongly affected with interesting phenomenological consequences. In particular, we show that scalar over gaugino mass ratios tend to increase and the regions of the parameter space with neutralino Dark Matter compatible with cosmological observations get drastically modified. Moreover, we discuss some observables that can be used to test the intermediate scale physics at the LHC in a wide class of models.

BRIDGING FLAVOUR VIOLATION AND LEPTOGENESIS IN SU(3) FAMILY MODELS

Were consider basic Pati-Salam ×SU(3) family models implementing the Type I see-saw mechanism to reproduce the observed fermion masses and mixing. Our goal is to explore a model with minimal field content which accounts for the observed baryon asymmetry through the thermal decay of the lightest right-handed neutrino consistently with the expected experimental lepton flavour violation sensitivity. This kind of models have been previously considered but it was not possible to achieve a compatibility among all of the ingredients mentioned above. We emphasize how different SU(3) messengers, the heavy fields that decouple and produce the right form of the Yukawa couplings together with the scalars breaking the SU(3) symmetry, can lead to different Yukawa couplings. This in turn implies different consequences for flavour violation couplings and conditions for realizing the right amount of baryon asymmetry through the decay of the lightest right-handed neutrino. Another highlight of the present work is a new fit of the Yukawa textures traditionally embedded in SU(3) family models.

Light neutralino in the MSSM: An update with the latest LHC results

We discuss the scenario of light neutralino dark matter in the minimal supersymmetric standard model, which is motivated by the results of some of the direct detection experiments — DAMA/LIBRA, CoGeNT, and CRESST. We update our previous analysis with the latest results of the LHC. We show that new LHC constraints disfavour the parameter region that can reproduce the results of DAMA/LIBRA and CoGeNT.