Searcharxiv⌕ Search

arXiv subjects

J. R. Espinosa

Publications and source records attributed to J. R. Espinosa.

At least 91 records · Page 5Linked to original sources
arXiv1998

What is the upper limit on the lightest supersymmetric Higgs mass?

In this talk the question of what is the upper bound on the lightest supersymmetric Higgs mass, m_h is addressed. This question is relevant since experimental lower bounds on m_h might implement, in the near future, exclusion of supersymmetry. By imposing (perturbative) unification of the gauge couplings at some high scale \simgt 10^{17} GeV, we have found that for a top-quark mass M_t=175 GeV, and depending on the supersymmetric parameters, this bound can be as high as 205 GeV.

hep-ph↗
arXiv1998

A No-Lose Theorem for Higgs Searches at a Future Linear Collider

Assuming perturbativity up to a high energy scale $\sim 10^{16}$ GeV, we demonstrate that a future $e^+e^-$ linear collider operating at $\sqrt{s} =$ 500 GeV with $\int{\cal L}=$ 500 fb$^{-1}$ per year (such as the recently proposed TESLA facility) will detect a Higgs boson signal regardless of the complexity of the Higgs sector and of how the Higgs bosons decay.

hep-ph↗
arXiv1998

The Higgs mass in the MSSM infrared fixed point scenario

In the infrared fixed point (IFP) scenario of the minimal supersymmetric model (MSSM), the top-quark mass and other physical quantities of the low-energy theory are insensitive to the values of the parameters of the theory at some high energy scale. In this framework we evaluate the light CP-even Higgs mass, $m_h$, taking into account some important effects that had not been previously considered. In particular, the supersymmetric correction to the relation between the running and the physical top-quark masses lowers the value of $\tanβ$, thereby implying a lower predicted value of $m_h$. Assuming a supersymmetric threshold of $M_S\leq 1$ TeV and $M_t=175$ GeV, we find an upper bound of $m_h\le 97\pm 2$ GeV; the most plausible value of $m_h$ lies somewhat below the upper bound. This places the Higgs boson in the IFP scenario well within the reach of the LEP-2 Higgs search.

hep-ph↗
arXiv1998

D-term Inflation in Superstring Theories

An inflationary stage dominated by a $D$-term avoids the slow-roll problem of inflation in supergravity and may emerge in theories with a non-anomalous or anomalous U(1) gauge symmetry. The most intriguing and commonly invoked possibility is that the Fayet-Iliopoulos $D$-term triggering inflation is the one emerging in superstring theories. We discuss the complications one has to face when trying to build up a successful $D$-term inflationary scenario in superstring models. In particular, we show that the ``vacuum shifting'' phenomenon of string theories is usually very efficient even in the early Universe, thus preventing inflation from taking place. On the other hand, when $D$-term inflation is free to occur, the presence of a plethora of fields and several non-anomalous additional abelian symmetries in string theories may help in reconciling the value of the Fayet-Iliopoulos $D$-term required by the COBE normalization with the value predicted by string theories. We also show that in superstring $D$-term inflation gravitinos are likely to pose no cosmological problem.

hep-ph↗
arXiv1998

Intermediate Scales, Mu Parameter, and Fermion Masses from String Models

We address intermediate scales within a class of string models. The intermediate scales occur due to the SM singlets S_i acquiring non-zero VEVs due to radiative breaking; the mass-square (m_i)^2 of S_i is driven negative at mu_{RAD} due to order(1) Yukawa couplings of S_i to exotic particles (calculable in a class of string models). The actual VEV of S_i depends on the relative magnitude of the non-renormalizable terms of the type (S_i)^{K+3}/M^K in the superpotential. We mainly consider the case in which the S_i are charged under an additional non-anomalous U(1) gauge symmetry and the VEVs occur along F- and D-flat directions. We explore various scenarios in detail, depending on the type of Yukawa couplings to the exotic particles and on the initial boundary values of the soft SUSY breaking parameters. We then address the implications of these scenarios for the mu parameter and the fermionic masses of the standard model.

hep-ph↗
arXiv1997

Classification of Flat Directions in Perturbative Heterotic Superstring Vacua with Anomalous U(1)

We develop techniques to classify D- and F-flat directions for N=1 supersymmetric string vacua of the perturbative heterotic string theory, which possess an anomalous U(1) gauge group at the tree level. Genus-one corrections generate a Fayet-Iliopoulos term for the D-term of U(1)_A, which is canceled by non-zero vacuum expectation values (VEVs) of certain massless multiplets in such a way that the anomalous U(1) is broken, while maintaining the D- and F-flatness of the effective field theory. A systematic analysis of flat directions is given for non-zero VEVs of non-Abelian singlets, and the techniques are illustrated for a specific model. The approach sets the stage to classify the D- and F-flat directions for a large class of perturbative string vacua. This classification is a prerequisite to address systematically the phenomenological consequences of these models.

hep-th↗
arXiv1997

The baryogenesis window in the MSSM

Thermal two-loop QCD corrections associated with light stops have a dramatic effect on the strength of the MSSM electroweak phase transition, making it more strongly first order as required for the viability of electroweak baryogenesis. We perform a perturbative analysis of the transition strength in this model, including these important contributions, extending previous work to arbitrary values of the pseudoscalar Higgs boson mass, m_A. We find a strong enough transition in a region with 2< tan beta < 4 and m_A > 120 GeV, a light Higgs boson with nearly standard couplings, and mass below 85 GeV within the reach of LEP II, and one stop not much heavier than the top quark. In addition, we give a qualitative discussion of the parameter space dependence of the transition strength and comment on the possibility that the transition turns to a crossover for sufficiently large Higgs masses.

hep-ph↗
arXiv1997

Perturbative Analysis of the MSSM Electroweak Phase Transition

Light stops can give a strong first order electroweak phase transition in the Minimal Supersymmetric Standard Model (MSSM) satisfying one of the necessary conditions for electroweak baryogenesis. I discuss the region of parameter space where this occurs using a perturbative analysis of the Higgs effective potential. Two-loop QCD corrections associated with stop loops are crucial to open this baryogenesis window, which corresponds to 2.25 < tan beta < 3.6, one stop not much heavier than the top quark, m_A > 120 GeV and a light Higgs boson with m_h < 85 GeV. This region will be explored by LEP II very soon.

hep-ph↗
arXiv1997

Cold Dark Matter Candidate in a Class of Supersymmetric Models with an Extra U(1)

In supersymmetric models whose gauge group includes an additional U(1) factor at the TeV scale, broken by the VEV of an standard model singlet S, the parameter space can accommodate a very light neutralino not ruled out experimentally. This higgsino-like fermion, stable if R-parity is conserved, can make a good cold dark matter candidate. We examine the thermal relic density of this particle and discuss the prospects for its direct detection if it forms part of our galactic halo.

hep-ph↗
arXiv1996

New limits on the mass of neutral Higgses in General Models

In general electroweak models with weakly coupled (and otherwise arbitrary) Higgs sector there always exists in the spectrum a scalar state with mass controlled by the electroweak scale. A new and simple recipe to compute an analytical tree-level upper bound on the mass of this light scalar is given. We compare this new bound with similar ones existing in the literature and show how to extract extra information on heavier neutral scalars in the spectrum from the interplay of independent bounds. Production of these states at future colliders is addressed and the implications for the decoupling limit in which only one Higgs is expected to remain light are discussed.

hep-ph↗
arXiv1996

Bosonic Thermal Masses in Supersymmetry

Effective thermal masses of bosonic particles in a plasma play an important role in many different phenomena. We compute them in general supersymmetric models at leading order. The origin of different corrections is explicitly shown for the formulas to be applicable when some particles decouple. The correct treatment of Boltzmann decoupling in the presence of trilinear couplings and mass mixing is also discussed. As a relevant example, we present results for the Minimal Supersymmetric Standard Model.

hep-ph↗
arXiv1996

Dominant Two-Loop Corrections to the MSSM Finite Temperature Effective Potential

We show that two-loop corrections to the finite temperature effective potential in the MSSM can have a dramatic effect on the strength of the electroweak phase transition, making it more strongly first order. The change in the order parameter $v/T_c$ can be as large as 75\% of the one-loop daisy improved result. This effect can be decisive to widen the region in parameter space where erasure of the created baryons by sphaleron processes after the transition is suppressed and hence, where electroweak baryogenesis might be successful. We find an allowed region with $\tanβ\simlt 4.5$ and a Higgs boson with standard couplings and mass below $80\ GeV$ within the reach of LEP II.

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↗
arXiv1995

THE SUPERSYMMETRIC SINGLET MAJORON MODEL AND THE GENERAL UPPER BOUND ON THE LIGHTEST HIGGS BOSON MASS

An upper bound on the tree-level mass of the lightest Higgs boson of the Supersymmetric Singlet Majoron Model is obtained. Contrary to some recent claims, it is shown to be of the same form as the general mass bound previously calculated for supersymmetric models with an extended Higgs sector. Soft-breaking masses or exotic vacuum expectation values do not enter in the tree-level bound [which is only controlled by the electroweak scale (M_Z)] and also decouple from the most important radiative corrections to the bound (the ones coming from the top-stop sector). The derivation of the upper bound for general Supersymmetric Models is reviewed in order to clarify its range of applicability.

hep-ph↗
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↗
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↗