arXiv subjects

M. Quiros

Publications and source records attributed to M. Quiros.

71 records · Page 4Linked to original sources

arXiv1996

Sphalerons in the MSSM

We construct the sphaleron solution, at zero and finite temperature, in the Minimal Supersymmetric Standard Model as a function of the supersymmetric parameters, including the leading one-loop corrections to the effective potential in the presence of the sphaleron. At zero temperature we have included the one-loop radiative corrections, dominated by the top/stop sector. The sphaleron energy $E_{\rm MSSM}$ mainly depends on an effective Higgs mass ${\displaystyle m_h^{\rm eff}=\lim_{m_A\gg m_W} m_h}$, where $m_h$ is the lightest CP-even Higgs mass and $m_A$ the pseudoscalar mass. We have compared it with the Standard Model result, with $m_h^{\rm SM}=m_h^{\rm eff}$, and found small differences (1-2\%) in all cases. At finite temperature we have included the one-loop effective potential improved by daisy diagram resummation. The sphaleron energy at the critical temperature can be encoded in the temperature dependence of the vacuum expectation value of the Higgs field with an error $\simlt 10$\%. The light stop scenario has been re-examined and the existence of a window where baryon asymmetry is not erased after the phase transition, confirmed. Although large (low) values of $m_h$ ($m_A$) are disfavoured by the strength of the phase transition, that window (along with LEP results) allows for $m_h\simlt 80$ GeV, $m_A\simgt 110$ GeV and $A_t\simlt 0.4\; m_Q$.

hep-ph↗

arXiv1996

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 $v(T_c)/T_c$ 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.

hep-ph↗

arXiv1996

Standard Model stability bounds for new physics within LHC reach

We analyse the stability lower bounds on the Standard Model Higgs mass by carefully controlling the scale independence of the effective potential. We include resummed leading and next-to-leading-log corrections, and physical pole masses for the Higgs boson, M_H, and the top-quark, M_t. Particular attention is devoted to the cases where the scale of new physics Λis within LHC reach, i.e. Λ\leq 10 TeV, which have been the object of recent controversial results. We clarify the origin of discrepancies and confirm our earlier results within the error of our previous estimate. In particular for Λ=1 TeV we find that M_H[GeV]>52+0.64(M_t[GeV]-175)-0.50\frac{α_s(M_Z)-0.118}{0.006}. For fixed values of M_t and α_s(M_Z), the error from higher effects, as the lack of exact scale invariance of the effective potential and higher-order radiative corrections, is conservatively estimated to be \simlt 5 GeV.

hep-ph↗

arXiv1996

Higgs Physics at LEP2

In this report we review the prospects for Higgs physics at LEP2. The theoretical aspects and the phenomenology of Higgs particles are discussed within the Standard Model (SM) and the Minimal Supersymmetric Standard Model (MSSM). The experimental search techniques are described and the discovery limits for Higgs bosons in the LEP2 energy range are summarized. In addition, opportunities of detecting Higgs particles in non-minimal extensions of the SM and the MSSM are investigated.

hep-ph↗

arXiv1995

Effective potential methods and the Higgs mass spectrum in the MSSM

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.

hep-ph↗

arXiv1995

Would a Light Higgs Detection Imply New Physics?

Depending on the Higgs-boson and top-quark masses, $M_H$ and $M_t$, the effective potential of the Standard Model can develop a non-standard minimum for values of the field much larger than the weak scale. In those cases the standard minimum becomes metastable and the possibility of decay to the non-standard one arises. Comparison of the decay rate to the non-standard minimum at finite (and zero) temperature with the corresponding expansion rate of the Universe allows to identify the region, in the ($M_H$, $M_t$) plane, where the Higgs field is sitting at the standard electroweak minimum. Since that region depends on the cutoff scale $Λ$, up to which we believe the Standard Model, the discovery of the Higgs boson, mainly at LEP-200, might put an upper bound (below the Planck scale) on the scale of new physics $Λ$.

hep-ph↗

arXiv1995

ON GAUGINO CONDENSATION WITH FIELD-DEPENDENT GAUGE COUPLINGS

We study in detail gaugino condensation in globally and locally supersymmetric Yang-Mills theories. We focus on models for which gauge-neutral matter couples to the gauge bosons only through nonminimal gauge kinetic terms, for the cases of one and several condensing gauge groups. Using only symmetry arguments, the low-energy expansion, and general properties of supersymmetry, we compute the low energy Wilson action, as well as the (2PI) effective action for the composite {\it classical} superfield $U\equiv\langle \Tr\WW \rangle$, with $W_α$ the supersymmetric gauge field strength. The 2PI effective action provides a firmer foundation for the approach of Veneziano and Yankielowicz, who treated the composite superfield, $U$, as a quantum degree of freedom. We show how to rederive the Wilson action by minimizing the 2PI action with respect to $U$. We determine, in both formulations and for global and local supersymmetry, the effective superpotential, $W$, the non-perturbative contributions to the low-energy Kähler potential $K$, and the leading higher supercovariant derivative terms in an expansion in inverse powers of the condensation scale. As an application of our results we include the string moduli dependence of the super- and Kähler potentials for simple orbifold models.

hep-th↗

arXiv1995

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 $m_A\sim M_{\rm SUSY}$, 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 next-to-leading order. Moreover, for the general case $m_A\simlt M_{\rm SUSY}$, we provide analytical formulae (including two-loop leading-log corrections) for all the masses and couplings in the Higgs sector. For $M_{\rm SUSY}\simlt 1.5$ TeV and arbitrary values of $m_A$, $\tanβ$ and the stop mixing parameters, they reproduce the numerical renormalization-group improved leading-log result for the Higgs masses with an error of less than 3 GeV. For the Higgs couplings, our analytical formulae reproduce the numerical results equally well. Comparison with other methods is also performed.

hep-ph↗

arXiv1995

IMPROVED METASTABILITY BOUNDS ON THE STANDARD MODEL HIGGS MASS

Depending on the Higgs-boson and top-quark masses, $M_H$ and $M_t$, the effective potential of the Standard Model at finite (and zero) temperature can have a deep and unphysical stable minimum $\langle ϕ(T)\rangle$ at values of the field much larger than $G_F^{-1/2}$. We have computed absolute lower bounds on $M_H$, as a function of $M_t$, imposing the condition of no decay by thermal fluctuations, or quantum tunnelling, to the stable minimum. Our effective potential at zero temperature includes all next-to-leading logarithmic corrections (making it extremely scale-independent), and we have used pole masses for the Higgs-boson and top-quark. Thermal corrections to the effective potential include plasma effects by one-loop ring resummation of Debye masses. All calculations, including the effective potential and the bubble nucleation rate, are performed numerically, and so the results do not rely on any kind of analytical approximation. Easy-to-use fits are provided for the benefit of the reader. Conclusions on the possible Higgs detection at LEP-200 are drawn.

hep-ph↗

arXiv1995

GAUGINO CONDENSATES AND CHIRAL-LINEAR DUALITY: AN EFFECTIVE LAGRANGIAN ANALYSIS

We show how to formulate the phenomenon of gaugino condensation in a super-Yang-Mills theory with a field-dependent gauge coupling described with a linear multiplet. We prove the duality equivalence of this approach with the more familiar formulation using a chiral superfield. In so doing, we resolve a longstanding puzzle as to how a linear-multiplet formulation can be consistent with the dynamical breaking of the Peccei-Quinn symmetry which is thought to occur once the gauginos condense. In our approach, the composite gauge degrees of freedom are described by a real vector superfield, $V$, rather than the chiral superfield that is obtained in the traditional dual formulation. Our dualization, when applied to the case of several condensing gauge groups, provides strong evidence that this duality survives strong-coupling effects in string theory.

hep-th↗

arXiv1994

Improved Higgs Mass Stability Bound in the Standard Model and Implications for Supersymmetry

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

hep-ph↗

arXiv1994

The Linear Multiplet and Quantum 4-D String Effective Actions

In 4-D heterotic superstrings, the dilaton and antisymmetric tensor fields belong to a linear N=1 supersymmetric multiplet L. We study the lagrangian describing the coupling of one linear multiplet to chiral and gauge multiplets in global and local supersymmetry, with particular emphasis on string tree-level and loop-corrected effective actions. This theory is dual to an equivalent one with chiral multiplets only. But the formulation with a linear multiplet appears to have decisive advantages beyond string tree-level since, in particular, is the string loop-counting parameter and the duality transformation is in general not exactly solvable beyond tree-level. This formulation allows us to easily deduce some powerful non-renormalization theorems in the effective theory and to obtain explicitly some loop corrections to the string effective supergravity for simple compactifications. Finally, we discuss the issue of supersymmetry breaking by gaugino condensation using this formalism.

hep-th↗

arXiv1993

Aspects of the electroweak phase transition in the Minimal Supersymmetric Standard Model

We study the finite-temperature effective potential of the Minimal Supersymmetric Standard Model in the full (mA, tan(beta)) parameter space. As for the features of the electroweak phase transition, we identify two possible sources of significant differences with respect to the Standard Model: a stop sector with little supersymmetry breaking makes the phase transition more strongly first-order, whereas a light CP-odd neutral boson weakens its first-order nature. After including the leading plasma effects, T=0 radiative corrections due to top and stop loops, and the most important experimental constraints, we find that the danger of washing out any baryon asymmetry created at the electroweak scale is in general no less than in the Standard Model.

hep-ph↗

arXiv1993

On the nature of the electroweak phase transition

We discuss the finite-temperature effective potential of the Standard Model, $\veff$, with emphasis on the resummation of the most important infrared contributions. We compute the one-loop scalar and vector boson self-energies in the zero-momentum limit. By solving the corresponding set of gap equations, with the inclusion of subleading contributions, we find a non-vanishing magnetic mass for the $SU(2)$ gauge bosons. We comment on its possible implications for the nature of the electroweak phase transition. We also discuss the range of validity of our approximations and compare this with other approaches.

hep-ph↗

arXiv1993

On the electroweak phase transition in the Minimal Supersymmetric Standard Model

We study the finite-temperature effective potential of the Minimal Supersymmetric Standard Model, in the limit of only one light Higgs boson. Because of the large top Yukawa coupling, there can be significant differences with respect to the Standard Model case: for given values of the Higgs and top masses, little supersymmetry breaking in the stop sector can make the phase transition more strongly first-order. After including the full structure of the stop mass matrix, the most important experimental constraints and the leading plasma effects, we find that the present limits on Higgs and squark masses are still compatible with the scenario of electroweak baryogenesis, in a small region of parameter space corresponding to $m_h \simlt 70 \gev$ and $\msba \simlt 105 \gev$.

hep-ph↗

arXiv1992

Scalar--Tensor Theories of Gravity with \PHI Dependent Masses

We study new physical phenomena and constraints in generalized scalar--tensor theories of gravity with $Φ$--dependent masses. We investigate a scenario (which can arise in string theories) with two types of $Φ$--dependent masses which could correspond to visible and dark matter sectors. The parameters of this theory are constrained from post--Newtonian bounds, primordial nucleosynthesis and the age of the Universe. We present a perfect fluid formalism for the dark matter sector with variable masses and find an entropy increase effect during the matter era and, in principle, a measurable effect on the motion of the halo of spiral galaxies. For the case of string effective theories, the constancy of gauge couplings provide new bounds which are orders of magnitude stronger than the previous ones.

hep-ph↗

arXiv1992

Precision bounds on $m_H$ and $m_t

We perform a fit to precise electroweak data to determine the Higgs and top masses. Penalty functions taking into account their production limits are included. We find ${\displaystyle m_H=65^{+245}_{-4}\ GeV}$ and ${\displaystyle m_t=122^{+25}_{-20}\ GeV}$. However whereas the top $χ^2$ distribution behaves properly near the minimum, the Higgs $χ^2$ distribution does not, indicating a statistical fluctuation or new physics. In fact no significative bound on the Higgs mass can be given at present. However, if the LEP accuracy is improved and the top is discovered in the preferred range of top masses, a meaningful bound on the Higgs mass could be obtained within the standard model framework.

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