• Expert ANR

• Physique/Physique des Hautes Energies - Phénoménologie
  
• Physique/Physique des Hautes Energies - Théorie
  
• Physique/Relativité Générale et Cosmologie Quantique
  
• Physique/Physique Quantique
  

Tree-level dynamical stability of scalar field potentials in renormalizable theories can in principle be expressed in terms of positivity conditions on quartic polynomial structures. However, these conditions cannot always be cast in a fully analytical resolved form, involving only the couplings and being valid for all field directions. In this paper we consider such forms in three physically motivated models involving SU (2) triplet scalar fields: the Type-II seesaw model, the Georgi-Machacek model, and a generalized two-triplet model. A detailed analysis of the latter model allows to establish the full set of necessary and sufficient boundedness from below conditions. These can serve as a guide, together with unitarity and vacuum structure constraints, for consistent phenomenological (tree-level) studies. They also provide a seed for improved loop-level conditions, and encompass in particular the leading ones for the more specific Georgi-Machacek case. Incidentally, we present complete proofs of various properties and also derive general positivity conditions on quartic polynomials that are equivalent but much simpler than the ones used in the literature.

The standard lore has it that supersymmetry breaking in the visible sector is always soft when mediated by gravity from a hidden sector of Supergravity.Correspondingly, the associated generic forms of the superpotential and Kahler potential have long been the basis for the construction of consistent low energy susy models. In this talk we report on the existence of a whole class of new forms, equally consistent for low energy susy model building, but exhibiting hard susy breaking on top of the usual soft one. Moreover, these new forms require Standard Model gauge singlet chiral superfields displaying specific structures and flat directions. We discuss briefly the possible phenomenological spin-offs regarding the 125GeV Higgs fine-tuning issues and the (so far) non-discovery of susy particles, as well as next-to-minimal susy inflationary model-building. (based on G. Moultaka, M.R. de Traubenberg & D. Tant, Int.J.Mod.Phys. A34 (2019) no.01, 1950004, and work in progress).

General forms of the Kähler and superpotenials that lead to consistent low energy brokenSupersymmetry originating from N = 1 Supergravity have been classified and used formodel building since more than three decades. We point out the incompleteness of thisclassification when hidden sector vacuum expectation values are of the order of the Planckmass. Focusing in this paper mainly on the case of minimal Kähler potential, we adopta rigorous approach that retrieves on the one hand the known forms, and demonstrateon the other hand the existence of a whole set of new forms for the superpotential ofwhich we give a complete classification. The latter forms involve a new type of chiralsuperfields having the unusual property of belonging neither to the hidden sector nor tothe conventional observable sector. Comparing the obtained forms with the conventionalones, we argue how new possibilities for model building can arise, and discuss the gravitymediation of soft as well as additional hard (but parametrically small) Supersymmetrybreaking, in the presence of the new type of chiral superfields. In the simplest case,we study the vacuum structure, characterize the masses and couplings of the scalarcomponents to the hidden and observable sectors and discuss briefly the physical rolethey could play. In the generic case, we estimate the magnitude and possible consequencesof the hard breaking of Supersymmetry in terms of the interplay between hidden andvisible sectors mass scales.

While the presence of a light stop is increasingly disfavored by the experimental limits set on R-parity conserving scenarios, the naturalness of supersymmetry could still be safely concealed in the more challenging final states predicted by the existence of non-null R-parity violating couplings. Although R-parity violating signatures are extensively looked for at the Large Hadron Collider, these searches mostly assume 100% branching ratios for the direct decays of supersymmetric particles into Standard Model ones. In this paper we scrutinize the implications of relaxing this assumption by focusing on one motivated scenario where the lightest stop is heavier than a chargino and a neutralino. Considering a class of R-parity baryon number violating couplings, we show on general grounds that while the direct decay of the stop into Standard Model particles is dominant for large values of these couplings, smaller values give rise, instead, to the dominance of a plethora of longer decay chains and richer final states that have been so far barely analyzed at the LHC, thus weakening the impact of the present experimental stop mass limits. We characterize the case for R-parity baryon number violating couplings in the 10^−7 − 10^−1 range, in two different benchmark points scenarios within the model-independent setting of the low-energy phenomenological Minimal Supersymmetric Standard Model. We identify the different relevant experimental signatures from stop pair production and decays, estimate the corresponding proton–proton cross sections at sqrt(s) = 14 TeV and discuss signal versus background issues.