Constraints on mSUGRA and SUSY particle production at future e+e−e^+e^- linear colliders

We perform a complete analysis of the supersymmetric particle spectrum in the Minimal Supergravity (mSUGRA) model. We show that present constraints on the Higgs boson and superparticle masses from collider searches and precision measurements still allow for large regions of the mSUGRA parameter space where some sparticles as well as the heavier Higgs particles, are light enough to be produced at the next generation of $e^+e^-$ linear colliders. An important part of this parameter space remains even when we require that the density of the lightest neutralinos left over from the Big Bang falls in the range favored by current determinations of the Dark Matter density in the Universe.Comment: 6 pages, latex, JHEPstyle, 2 psfigures. Contribution to International Europhysics Conference on High Energy Physics, July 2001, Budapes

The 2d Gross-Neveu Model at Finite Temperature and Density with Finite Corrections

We use the linear $\delta$ expansion, or optimized perturbation theory, to evaluate the effective potential for the two dimensional Gross-Neveu model at finite temperature and density obtaining analytical equations for the critical temperature, chemical potential and fermionic mass which include finite $N$ corrections. Our results seem to improve over the traditional large-N predictions.Comment: 7 pages, 8 figure

Revisiting No-Scale Supergravity Inspired Scenarios: Updated Theoretical and Phenomenological Constraints

We consider no-scale inspired supergravity scenarios, where the gravitino mass and related soft supersymmetry-breaking parameters are determined dynamically by radiative corrections to an essentially flat tree-level potential in the supersymmetry breaking hidden sector. We examine the theoretical and phenomenological viability of such a mechanism, when including up-to-date calculations of the low energy sparticle spectrum and taking into account the latest LHC results and other experimental constraints. We (re)emphasize the role of the scale-dependent vacuum energy contribution to the effective potential, in obtaining realistic no-scale electroweak minima, examining carefully the impact of boundary conditions and of variants of the minimization procedure. We also discuss and implement the B_0 (soft breaking Higgs mixing parameter) input boundary condition at high scale, therefore fixing tan beta(B_0) at low scales. For general high scale boundary conditions with non-vanishing B_0, m_0..., our analysis provides theoretical correlations among the supersymmetric, soft and vacuum energy parameters and related phenomenological consequences at the LHC. For instance, a zero vacuum energy at the GUT scale would lead to a decoupled supersymmetric spectrum, together with a light standard model-like Higgs boson at the electroweak scale. Given the experimental exclusion limits, a substantial class of the boundary conditions, and in particular the strict no-scale with m_0=A_0=B_0=0, are only compatible with a stau being the lightest MSSM particle. Then an enlarged allowed parameter space emerges when assuming a gravitino LSP to account for the observed dark matter relic density.Comment: 37 pages, 13 figures, 2 tables. v2: some references adde

Holographic models of composite Higgs in the Veneziano limit. Part I. Bosonic sector

We study strongly-coupled, approximately scale-invariant gauge theories, which develop a mass gap in the infrared. We argue that a large number of fermion flavours is most suitable to provide an ultraviolet completion for the composite Higgs scenario. The holographic approach allows to describe the qualitative features of the non-perturbative dynamics in the Veneziano limit. We introduce new bottom-up holographic models, which incorporate the backreaction of flavour on the geometry, and show that this can correlate the mass gap to the scale of flavour-symmetry breaking. We compute the mass spectrum for the various composite bosonic states, and study its dependence on the scaling dimension of the symmetry-breaking operators, as well as on the number of flavours. The different regions with a light dilaton are critically surveyed. We carefully assess the domain of validity of the holographic approach, and compare it with lattice simulations and the Nambu--Jona-Lasinio model.Comment: Version published in JHEP. Additional discussion of the Veneziano limit and of the dilaton lightnes

Renormalon disappearance in Borel sum of the 1/N expansion of the Gross-Neveu model mass gap

The exact mass gap of the O(N) Gross-Neveu model is known, for arbitrary $N$, from non-perturbative methods. However, a "naive" perturbative expansion of the pole mass exhibits an infinite set of infrared renormalons at order 1/N, formally similar to the QCD heavy quark pole mass renormalons, potentially leading to large ${\cal O}(\Lambda)$ perturbative ambiguities. We examine the precise vanishing mechanism of such infrared renormalons, which avoids this (only apparent)contradiction, and operates without need of (Borel) summation contour prescription, usually preventing unambiguous separation of perturbative contributions. As a consequence we stress the direct Borel summability of the (1/N) perturbative expansion of the mass gap. We briefly speculate on a possible similar behaviour of analogous non-perturbative QCD quantities.Comment: 16 pp., 1 figure. v2: a few paragraphs and one appendix added, title and abstract slightly changed, essential results unchange

Lightest-neutralino decays in R_p-violating models with dominant lambda^{prime} and lambda couplings

Decays of the lightest neutralino are studied in R_p-violating models with operators lambda^{prime} L Q D^c and lambda L L E^c involving third-generation matter fields and with dominant lambda^{prime} and lambda couplings. Generalizations to decays of the lightest neutralino induced by subdominant lambda^{prime} and lambda couplings are straightforward. Decays with the top-quark among the particles produced are considered, in addition to those with an almost massless final state. Phenomenological analyses for examples of both classes of decays are presented. No specific assumption on the composition of the lightest neutralino is made, and the formulae listed here can be easily generalized to study decays of heavier neutralinos. It has been recently pointed out that, for a sizable coupling lambda^{prime}_{333}, tau-sleptons may be copiously produced at the LHC as single supersymmetric particles, in association with top- and bottom-quark pairs. This analysis of neutralino decays is, therefore, a first step towards the reconstruction of the complete final state produced in this case.Comment: 40 pages, 11 figures, version to appear in JHE

Neutralino Dark Matter in mSUGRA: reopening the light Higgs pole window

The requirement that the lightest neutralino $\tilde\chi_1^0$ has the right thermal relic density to explain all Dark Matter in the universe strongly constrains the parameter space of supersymmetric models in general, and of the mSUGRA model in particular. Recently improved calculations of the mass of the light CP-even Higgs boson $h$ present in this model, and the increased central value of the mass of the top quark, have re--opened the possibility that 2 \mlsp \lsim m_h. In this ``$h-$pole region'' the LSP annihilation cross section is enhanced by near-resonant $h$ exchange in the $s-$channel, reducing the relic density to acceptable values. We delineate the corresponding region of mSUGRA parameter space, and explore its phenomenology. In particular, we find strong upper bounds on the masses of the gluino, lighter chargino and LSP.Comment: 16 pages, latex, 4 eps figures; v2: references adde

Calculs perturbatifs variationnellement améliorés en théorie des champs; calcul du spectre et contraintes sur le modèle supersymétrique standard

Je discute quelques activités récentes en deux parties indépendantes. La première partie décrit une technique de resommation particulière des séries perturbatives qui peut donner des résultats non-perturbatifs dans certains cas. On détaille quelques applications en théorie des champs et de la matière condensée. Dans la deuxième partie, on développe quelques activités récentes en phénoménologie du modèle supersymétrique standard minimal, concernant le calcul précis du spectre de masse des particules supersymétriques, le calcul de la densité relique de matière noire supersymétrique, et les contraintes théoriques et expérimentales actuelles qu'on peut en déduire sur certains modèles