Mariano Quiros

Analytical expressions for radiatively corrected Higgs masses and couplings in the MSSM

We propose, for the computation of the Higgs mass spectrum and couplings, a renormalization-group improved leading-log approximation, where the renormalization scale is fixed to the top-quark pole mass. For the case mA ∼ MSUSY, our leading-log approximation differs by less than 2 GeV from previous results on the Higgs mass computed using a nearly scale independent renormalization-group improved effective potential up to nextto-leading order. Moreover, for the general case mA < MSUSY, we provide analytical formulae (including two-loop leading-log corrections) for all the masses and couplings in the Higgs sector. For MSUSY < 1.5 TeV and arbi

Effective potential methods and the Higgs mass spectrum in the MSSM

Abstract We generalize the analytical expressions for the two-loop leading-log neutral Higgs boson masses and mixing angles to the case of general left- and right-handed soft supersymmetry breaking stop and sbottom masses and left-right mixing mass parameters ( m Q , m U , m D , A t , A b ). This generalization is essential for the computation of Higgs masses and couplings in the presence of light stops. At high scales we use the minimal supersymmetric standard model effective potential, while at low scales we consider the two-Higgs doublet model (renormalization group improved) effective potential, with general matching conditions at the thresholds where the squarks decouple. We define physical (pole) masses for the top quark, by including QCD self-energies, and for the neutral Higgs bosons, by including the leading one-loop electroweak self-energies where the top/stop and bottom/sbottom sectors propagate. For m Q = m U = m D and moderate left-right mixing mass parameters, for which the mass expansion in terms of renormalizable Higgs quartic couplings is reliable, we find excellent agreement with previously obtained results.

The Standard model from extra dimensions

Abstract We present a simple N=1 five-dimensional model where the fifth dimension is compactified on the orbifold S 1 / Z 2 . Non-chiral matter lives in the bulk of the fifth dimension (five dimensions) while chiral matter lives on the fixed points of the orbifold (four-dimensional boundaries). The massless sector constitutes the Minimal Supersymmetric Standard Model while the massive modes rearrange in N=2 supermultiplets. After supersymmetry breaking by the Scherk-Schwarz mechanism the zero modes can be reduced to the non-supersymmetric Standard Model.

Low-energy behaviour of realistic locally-supersymmetric grand unified theories

The recently proposed cosmologically acceptable N=1 supergravity models based on the SU(5) unification group define unambigously the minimal particle content of the theory. This fact allows us to determine quite precisely their low-energy behaviour. The SU(2)×U(1) breaking to U(1)e.m. is a consequence of radiative corrections of the supergravity induced soft breaking terms. The proposed mechanism (which is model independent) introduces naturally a hierarchy between the MW and MX scales. Calculating the low-energy effective potential we shot that a corrects SU(2)×U(1) breaking is obtained without any limit (except the experimental one) on the top-quark mass. The masses of the supersymmetric partners of mater and gauge fermions can be low and consequently accessible experimentally (sleptons, s quarks, gauginos ⋍ 20–50 GeV). A neutral Higgs is also predicted wirth a mass mH⋍O(5) GeV. In addition, we show that if mt≲45 GeV, the gravitino and gluino masses are bounded from below by 10GeV ≲ m32 and 15 GeV ≲ mgluino. The values of sin2 θW (in the two-loop approximation) and the mbmτ ratio predicted are in very good agreement with the experimentally measured values.

The lightest Higgs boson mass in the Minimal Supersymmetric Standard Model

We compute the upper bound on the mass of the lightest Higgs boson in the Minimal Supersymmetric Standard Model in a model-independent way, including leading (one-loop) and next-to-leading order (two-loop) radiative corrections. We find that (contrary to some recent claims) the two-loop corrections are negative with respect to the one-loop result and relatively small (⪅ 3%). After defining physical (pole) top quark mass Mt, by including QCD self-energies, and physical Higgs mass MH, by including the electroweak self-energies ∏ (MH2) − ∏ (0), we obtain the upper limit on MH as a function of supersymmetric parameters. We include as supersymmetric parameters the scale of supersymmetry breaking Ms, the value of tan β and the mixing between stops Xt = At + μ cot β (which is responsible for the threshold correction on the Higgs quartic coupling). Our results do not depend on further details of the supersymmetric model. In particular, for Ms ⩽ 1 TeV, maximal threshold effect Xt2 = 6MS2 and any value of tan β, we find MH ⩽ 140 GeV for Mt ⩽ 190 GeV. In the particular scenario where the top is in its infrared fixed point we find MH ⩽ 86 GeV for Mt = 170 GeV.

Improved Higgs mass stability bound in the standard model and implications for supersymmetry

Abstract We re-examine the lower bound on the mass of the Higgs boson, Mh, from Standard Model vaccum stability including next-to-leading-log radiative corrections. This amounts to work with the full one-loop effective potential, V(θ), improved by two-loop RGE, and allows to keep control of the scale invariance of V in a wide range of the θ-field. Our results show that the bound is O (10 GeV) less stringent than in previous estimates. In addition we perform a detailed comparison between the SM lower bounds on Mh and the supersymmetric upper bounds on it. It turns out that depending on the actual value of the top mass, Mt, the eventually measured Higgs mass can discard the pure SM, the Minimal Supersymmetric Standard Model or both.

Opening the Window for Electroweak Baryogenesis

We perform an analysis of the behaviour of the electroweak phase transition in the Minimal Supersymmetric Standard Model, in the presence of light stops. We show that, in previously unexplored regions of parameter space, the order parameter v(Tc)/Tc can become significantly larger than one, for values of the Higgs and supersymmetric particle masses consistent with the present experimental bounds. This implies that baryon number can be efficiently generated at the electroweak phase transition. As a by-product of this study, we present an analysis of the problem of colour breaking minima at zero and finite temperature and we use it to investigate the region of parameter space preferred by the best fit to the present precision electroweak measurement data, in which the left-handed stops are much heavier than the right-handed ones.

Supersymmetry and Electroweak breaking from extra dimensions at the TeV-scale ∗

We analyze some features of the role that extra dimensions, of radius R in the TeV 1 range, can play in the soft breaking of supersymmetry and the spontaneous breaking of electroweak symmetry. We use a minimal model where the gauge and Higgs sector of the MSSM are living in the bulk of five dimensions and the chiral multiplets in a four-dimensional boundary. Supersymmetry is broken in the bulk by the Scherk-Schwarz mechanism and transmitted to the boundary by radiative corrections. The particle spectrum is completely predicted as a function of a unique R-charge. The massless sector corresponds to the pure Standard Model and elec- troweak symmetry is radiatively broken with a light Higgs weighing < 110 GeV. The µ-problem is solved and Higgsinos, gauginos and heavy Higgses acquire masses ∼ 1/R. Chiral sfermions acquire radiative squared-masses ∼ �i/R 2 . The effective potential is explicitly computed in the bulk of extra dimensions and some cosmolog- ical consequences can be immediately drawn from it. Gauge coupling running and unification is studied in the presence of Scherk-Schwarz supersymmetry breaking. The unification is similar to that in the supersymmetric theory.

Dynamical supersymmetry breaking with a large internal dimension

Abstract Supersymmetry breaking in string perturbation theory predicts the existence of a new dimension at the TeV scale. The simplest realization of the minimal supersymmetric standard model in the context of this mechanism has two important consequences: (i) A natural solution to the μ-problem; (ii) The absence of quadratic divergences in the cosmological constant, which leads to a dynamical determination of the supersymmetry breaking and electroweak scale. We present an explicit example in which the whole particle spectrum is given as a function of the top quark mass. A generic prediction of this mechanism is the existence of Kaluza-Klein excitations for gauge bosons and higgses. In particular the first excitation of the photon could be accessible to future accelerators and give a clear signal of the proposed mechanism.

Production of Kaluza-Klein states at future colliders

Abstract Perturbative breaking of supersymmetry in four-dimensional string theories predict in general the existence of new large dimensions at the TeV scale. Such large dimensions lie in a domain of energies accessible to particle accelerators. Their main signature is the production of Kaluzu-Klein excitations which can be detected at future colliders. We study this possibility for hadron colliders (TEVATRON, LHC) and e + e − colliders (LEP-200, NCL-500).