arXiv subjects

M. Quiros

Publications and source records attributed to M. Quiros.

At least 55 records · Page 3Linked to original sources

arXiv1999

Strong coupling unification and extra dimensions

We analyze the implications of electroweak and strong coupling unification in a very general class of models extending the minimal supersymmetric standard model in $4+p$ dimensions $(p\geq 0)$. In general, electroweak precision data require the presence of large extra dimensions (low compactification scales, $M_c$) and/or low unification scale, $M_U$. In particular, the actual experimental value of the strong coupling at $M_Z$ imposes an upper bound on the compactification and unification scales. In four dimensional theories ($M_c\simeq M_U$) with canonical hypercharge assignment we find $M_U\lesssim 10^{9}$ GeV. In theories with extra dimensions ($M_c<M_U$) we find $M_c\lesssim 10^7$ GeV, for a supersymmetric spectrum at the TeV scale.

hep-ph↗

arXiv1999

Massive Neutrinos and the Higgs Mass Window

If neutrino masses are produced by a see-saw mechanism the Standard Model prediction for the Higgs mass window (defined by upper (perturbativity) and lower (stability) bounds) can be substantially affected. Actually the Higgs mass window can close completely, which settles an upper bound on the Majorana mass for the right-handed neutrinos, $M$, ranging from $10^{13}$ GeV for three generations of quasi-degenerate massive neutrinos with $m_ν\simeq 2 $ eV, to $5\times 10^{14}$ GeV for just one relevant generation with $m_ν\simeq 0.1 $ eV. A slightly weaker upper bound on $M$, coming from the requirement that the neutrino Yukawa couplings do not develop a Landau pole, is also discussed.

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arXiv1999

Electroweak Baryogenesis in the MSSM

We review the baryogenesis scenario in the MSSM at the perturbative level and, in particular, the impact of two-loop corrections on the strength of the phase transition and the amount of generated baryon asymmetry. We confirm the baryogenesis window, where m_H is less than 115GeV, for m_Q less than a few GeV, and the right-handed stop mass is constrained in the region between 100GeV and the top mass. This scenario will be tested at LEP and Tevatron colliders.

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arXiv1999

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 electroweak symmetry is radiatively broken with a light Higgs weighing $\simlt$ 110 GeV. The $μ$-problem is solved and Higgsinos, gauginos and heavy Higgses acquire masses $\sim 1/R$. Chiral sfermions acquire radiative squared-masses $\sim α_i/R^2$. The effective potential is explicitly computed in the bulk of extra dimensions and some cosmological 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.

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arXiv1998

Soft Masses in Theories with Supersymmetry Breaking by TeV-Compactification

We study the sparticle spectroscopy and electroweak breaking of theories where supersymmetry is broken by compactification (Scherk-Schwarz mechanism) at a TeV. The evolution of the soft terms above the compactification scale and the resulting sparticle spectrum are very different from those of the usual MSSM and gauge mediated theories. This is traced to the softness of the Scherk-Schwarz mechanism which leads to scalar sparticle masses that are only logarithmically sensitive to the cutoff starting at two loops. As a result, squarks and sleptons are naturally an order of magnitude lighter than gauginos. In addition, the mechanism is very predictive and the sparticle spectrum depends on just two new parameters. A significant advantage of this mechanism relative to gauge mediation is that a Higgsino mass $μ\sim M_susy$ is automatically generated when supersymmetry is broken. Our analysis applies equally well to theories where the cutoff is near a TeV or $M_{Pl}$ or some intermediate scale. We also use these observations to show how we may obtain compactification radii which are hierarchically larger than the fundamental cutoff scale.

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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

The effective potential in the presence of several mass scales

We consider the problem of improving the effective potential in mass independent schemes, as e.g. the $\MSbar$ or $\DRbar$ renormalization scheme, in the presence of an arbitrary number of fields with $ϕ$-dependent masses $M_i(ϕ_c)$. We use the decoupling theorem at the scales $μ_i=M_i(ϕ_c)$ such that the matching between the effective (low energy) and complete (high energy) one-loop theories contains no thresholds. We find that for any value of $ϕ_c$, there is a convenient scale $μ^*\equiv\min_i\{M_i(ϕ_c)\}$, at which the loop expansion has the best behaviour and the effective potential has the least $μ$-dependence. Furthermore, at this scale the effective potential coincides with the (improved) tree-level one in the effective field theory. The decoupling method is explicitly illustrated with a simple Higgs-Yukawa model, along with its relationship with other decoupling prescriptions and with proposed multi-scale renormalization approaches. The procedure leads to a nice suppression of potentially large logarithms and can be easily adapted to include higher-loop effects, which is explicitly shown at the two-loop level.

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arXiv1998

The Standard Model from extra dimensions

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.

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arXiv1998

Bubbles in the supersymmetric standard model

We compute the tunneling probability from the symmetric phase to the true vacuum, in the first order electroweak phase transition of the MSSM, and the corresponding Higgs profiles along the bubble wall. We use the resummed two-loop temperature-dependent effective potential, and pay particular attention to the light stop scenario, where the phase transition can be sufficiently strongly first order not to wipe off any previously generated baryon asymmetry. We compute the bubble parameters which are relevant for the baryogenesis mechanism: the wall thickness and $Δβ$. The two-loop corrections provide important enhancement effects, with respect to the one-loop results, in the amount of baryon asymmetry.

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arXiv1997

On the M-theory description of gaugino condensation

We present an additional test of the recent proposal for describing supersymmetry breaking due to gaugino condensation in the strong coupling regime, by a Scherk-Schwarz mechanism on the eleventh dimension of M-theory. An analysis of supersymmetric transformations in the infinite-radius limit reveals the presence of a discontinuity in the spinorial parameter, which coincides with the result found in the presence of gaugino condensation. The condensate is then identified with the quantized parameter entering the modification of the Scherk-Schwarz boundary conditions. This mechanism provides an alternative perturbative explanation of the gauge hierarchy that determines the scale of low-energy supersymmetry breaking in terms of the unification gauge coupling

hep-th↗

arXiv1997

Electroweak Baryogenesis and Higgs and Stop Searches at LEP and the Tevatron

It has been recently shown that the observed baryon number may originate at the electroweak phase transition, provided that the Higgs boson and the lightest stop are sufficiently light. In this work, we perform a detailed analysis, including all dominant two-loop finite temperature corrections to the Higgs effective potential, as well as the non-trivial effects proceeding from the mixing in the stop sector, to define the region of parameter space for which electroweak baryogenesis can happen. The limits on the stop and Higgs masses are obtained by taking into account the experimental bounds on these quantities, as well as those coming from the requirement of avoiding dangerous color breaking minima. We find for the Higgs mass $m_h \simlt 105$ GeV, while the stop mass may be close to the present experimental bound and must be smaller than, or of order of, the top quark mass. These results provide a very strong motivation for further non-perturbative analysis of the electroweak phase transition, as well as for the search for Higgs and stop particles at the LEP and Tevatron colliders.

hep-ph↗

arXiv1997

Supersymmetry breaking in M-theory

We describe the breaking of supersymmetry in M-theory by coordinate dependent (Scherk-Schwarz) compactification of the eleventh dimension. Supersymmetry is spontaneously broken in the gravitational and moduli sector and communicated to the observable sector, living at the end-point of the semicircle, by radiative gravitational interactions. This mechanism shares the generic features of non-perturbative supersymmetry breaking by gaugino condensation, in the presence of a constant antisymmetric field strength, in the weakly coupled regime of the heterotic string, which suggests that both mechanisms could be related by duality. In particular an analysis of supersymmetric transformations in the infinite-radius limit reveals the presence of a discontinuity in the spinorial parameter, which coincides with the result found in the presence of gaugino condensation, while the condensate is identified with the quantized parameter entering the boundary conditions

hep-th↗

arXiv1997

Supersymmetry breaking in M-theory and gaugino condensation

We argue that supersymmetry breaking by gaugino condensation in the strongly coupled heterotic string can be described by an analogue of Scherk-Schwarz compactification on the eleventh dimension in M-theory. The M-theory scale is identified with the gauge coupling unification mass, whereas the radius of the eleventh dimension $ρ$ is at an intermediate scale $ρ^{-1}\sim 10^{12}$ GeV. At the lowest order, supersymmetry is broken only in the gravitational and moduli sector at a scale $m_{3/2}\simρ^{-1}$, while it is mediated by gravitational interactions to the observable world. Computation of the mass splittings yields in general a hierarchy of soft masses at the TeV scale $(\simρ^{-2}/M_p)$ with matter scalars much heavier than gauginos.

hep-th↗

arXiv1997

Constraints on the Higgs boson properties from the effective potential

We review the constraints on Higgs boson properties from effective potential methods. In the Standard Model, the requirement of stability (or metastability) of the standard electroweak minimum puts an upper bound on the scale of new physics as a function of the Higgs mass. This upper bound is below the Planck scale if the Higgs weights $\simlt$ 130 GeV. In supersymmetric extensions of the Standard Model the former methods are useful to compute the Higgs mass spectrum, and couplings, after resumming higher loop effects. In particular, if the Higgs mass weights $\simgt$ 130 GeV, the minimal supersymmetric extension of the Standard Model will be ruled out, while its non-minimal supersymmetric extension containing singlets is only marginally allowed.

hep-ph↗

arXiv1997

Electroweak baryogenesis and low energy supersymmetry

Electroweak baryogenesis is an interesting theoretical scenario, which demands physics beyond the Standard Model at energy scales of the order of the weak boson masses. It has been recently emphasized that, in the presence of light stops, the electroweak phase transition can be strongly first order, opening the window for electroweak baryogenesis in the MSSM. For the realization of this scenario, the Higgs boson must be light, at the reach of the LEP2 collider. In this article, we compute the baryon asymmetry assuming the presence of non-trivial CP violating phases in the parameters associated with the left-right stop mixing term and the Higgsino mass $μ$. We conclude that a phase $|\sin ϕ_μ| > 0.01$ and Higgsino and gaugino mass parameters $|μ| \simeq M_2$, and of the order of the electroweak scale, are necessary in order to generate the observed baryon asymmetry.

hep-ph↗

arXiv1996

Squarks and sphalerons

Electric charge and color breaking minima along third generation squark directions do appear in the Minimal Supersymmetric Standard Model for particular regions of the corresponding supersymmetric parameters $A_t$, $μ$, $\tanβ$, $m_Q^2$ and $m_U^2$. We have studied possible instabilities of electroweak sphalerons along non-trivial squark configurations. We have found that instabilities along the latter lie in the region of parameter space where the electroweak minimum is not the global one. Thus charge and color conservation imply that the standard electroweak sphaleron is not destabilized along squark directions.

hep-ph↗

arXiv1996

Large radii and string unification

We study strong coupling effects in four-dimensional heterotic string models where supersymmetry is spontaneously broken with large internal dimensions, consistently with perturbative unification of gauge couplings. These effects give rise to thresholds associated to the dual theories: type I superstring or M-theory. In the case of one large dimension, we find that these thresholds appear close to the field-theoretical unification scale $\sim 10^{16}$ GeV, offering an appealing scenario for unification of gravitational and gauge interactions. We also identify the inverse size of the eleventh dimension of M-theory with the energy at which four-fermion effective operators become important.

hep-th↗

arXiv1996

Higgs bosons in the Standard Model and the Minimal Supersymmetric Standard Model

In these lectures we present a brief review of the Higgs boson sector in the ``Standard Model'', and its Minimal Supersymmetric Extension, with particular emphasis on the main mechanisms for Higgs production and decay at LEP2 and LHC, and theoretical bounds on the Higgs boson masses. In the ``Standard Model'' the effective potential can develop a non-standard minimum for values of the field much larger than the weak scale. 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 which can be accomodated by the theory. In the ``Minimal Supersymmetric Standard Model'', approximate analytical expressions for the Higgs mass spectrum and couplings are worked out. An appropriate treatment of squark decoupling allows to consider large values of the stop mixing parameters and thus fix a reliable upper bound on the mass of the lightest CP-even Higgs boson mass. The discovery of the Higgs boson at LEP2 might put an upper bound (below the Planck scale) on the scale of new physics $Λ$ and eventually disentangle between the Standard Model and the Minimal Supersymmetric Standard Model.

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