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J. A. Casas

Publications and source records attributed to J. A. Casas.

At least 55 records · Page 3Linked to original sources
arXiv1999

Nearly degenerate neutrinos, Supersymmetry and radiative corrections

If neutrinos are to play a relevant cosmological role, they must be essentially degenerate with a mass matrix of the bimaximal mixing type. We study this scenario in the MSSM framework, finding that if neutrino masses are produced by a see-saw mechanism, the radiative corrections give rise to mass splittings and mixing angles that can accommodate the atmospheric and the (large angle MSW) solar neutrino oscillations. This provides a natural origin for the $Δm^2_{sol} << Δm^2_{atm}$ hierarchy. On the other hand, the vacuum oscillation solution to the solar neutrino problem is always excluded. We discuss also in the SUSY scenario other possible effects of radiative corrections involving the new neutrino Yukawa couplings, including implications for triviality limits on the Majorana mass, the infrared fixed point value of the top Yukawa coupling, and gauge coupling and bottom-tau unification.

hep-ph↗
arXiv1999

Naturalness of nearly degenerate neutrinos

If neutrinos are to play a relevant cosmological role, they must be essentially degenerate. We study whether radiative corrections can or cannot be responsible for the small mass splittings, in agreement with all the available experimental data. We perform an exhaustive exploration of the bimaximal mixing scenario, finding that (i) the vacuum oscillations solution to the solar neutrino problem is always excluded; (ii) if the mass matrix is produced by a see-saw mechanism, there are large regions of the parameter space consistent with the large angle MSW solution, providing a natural origin for the $Δm^2_{sol} << Δm^2_{atm}$ hierarchy; (iii) the bimaximal structure becomes then stable under radiative corrections. We also provide analytical expressions for the mass splittings and mixing angles and present a particularly simple see-saw ansatz consistent with all observations.

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.

hep-ph↗
arXiv1999

F-term inflation in Superstring Theories

A supersymmetric inflationary stage dominated by an $F$-term has the problem that the flatness of the potential is spoiled by supergravity corrections, that is the slow-roll parameter $η$ gets contributions of order unity. We show that in $F$-term inflationary models based on strings there is natural way of obtaining small values of $η$. This happens in models of hybrid inflation based on orbifold constructions, in which a modulus $T$ field is responsible for the large value of the potential during inflation, and a second field $ϕ$ with appropriate modular weight is responsible for the roll-over. We illustrate the mechanism with a model in which the inflaton potential is provided by gaugino condensation, leading to succesful inflation.

hep-ph↗
arXiv1998

Anomalous U(1), gaugino condensation and supergravity

The interplay between gaugino condensation and an anomalous Fayet-Iliopoulos term in string theories is not trivial and has important consequences concerning the size and type of the soft SUSY breaking terms. In this paper we examine this issue, generalizing previous work to the supergravity context. This allows, in particular, to properly implement the cancellation of the cosmological constant, which is crucial for a correct treatment of the soft breaking terms. We obtain that the D-term contribution to the soft masses is expected to be larger than the F-term one. Moreover gaugino masses must be much smaller than scalar masses. We illustrate these results with explicit examples. All this has relevant phenomenological consequences, amending previous results in the literature.

hep-ph↗
arXiv1998

Phenomenological viability of string and M-theory scenarios

We analyze the constraints that a correct phenomenology and the absence of dangerous charge and color breaking (CCB) minima or unbounded from below (UFB) directions impose on the parameter space of different superstring and M-theory scenarios. Namely, we analyze in detail the case where supersymmetry (SUSY) breaking is driven by non-vanishing dilaton and/or moduli F-terms in weakly and strongly coupled strings, and the specific case where the mechanism of SUSY breaking is gaugino condensation with or without the participation of non-perturbative contributions to the K{ä}hler potential. The results indicate that, only in some small windows of the parameter space all the previous tests are succesfully passed. We also discuss the impact of non-universality of the soft breaking terms on CCB/UFB bounds.

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.

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

Generation of Large Lepton Asymmetries

We present here a realistic model to produce a very large lepton asymmetry L of 0.1 to 1, without producing a large baryon asymmetry. The model is based on the Affleck and Dine scenario, in which the field acquiring a large vacuum expectation value during inflation is a s-neutrino right. We require $L$ to be large enough for the electroweak symmetry to be spontaneously broken at all temperatures after inflation, with the consequent suppression of sphaleron transitions.

hep-ph↗
arXiv1998

Baryogenesis, Inflation and Superstrings

We study the conditions for successful Affleck-Dine baryogenesis in generic inflation and supergravity scenarios, finding powerful restrictions on them. String-based SUGRA models are especially interesting since they are surprisingly suitable for the implementation not only of AD baryogenesis but also inflation itself, presenting a nice solution to the $η$--problem.

hep-ph↗
arXiv1997

Charge and Color Breaking

The presence of scalar fields with color and electric charge in supersymmetric theories makes feasible the existence of dangerous charge and color breaking (CCB) minima and unbounded from below directions (UFB) in the effective potential, which would make the standard vacuum unstable. The avoidance of these occurrences imposes severe constraints on the supersymmetric parameter space. We give here a comprehensive and updated account of this topic.

hep-ph↗
arXiv1996

The Problem of the Stabilization of the Dilaton in String Theories

The crucial problem of how the dilaton field is stabilized at a phenomenologically acceptable value in string theories remains essentially unsolved. We show that the usual scenario of assuming that the dilaton is fixed by the (SUSY breaking) dynamics of just the dilaton itself (dilaton dominance scenario) is {\em inconsistent} unless the Kähler potential receives very important perturbative or non-perturbative contributions. Then, the usual predictions about soft breaking terms are lost, but still is possible to derive model-independent predictions for them.

hep-th↗
arXiv1996

The Generalized Dilaton Supersymmetry Breaking Scenario

We show that the usual dilaton dominance scenario, derived from the tree level Kähler potential, can never correspond to a global minimum of the potential at $V=0$. Similarly, it cannot correspond to a local minimum either, unless a really big conspiracy of different contributions to the superpotential $W(S)$ takes place. These results, plus the fact that the Kähler potential is likely to receive sizeable string non-perturbative contributions, strongly suggest to consider a more general scenario, leaving the Kähler potential arbitrary. In this way we obtain generalized expressions for the soft breaking terms but a predictive scenario still arises. Finally, we explore the phenomenological capability of some theoretically motivated forms for non-perturbative Kähler potentials, showing that it is easy to stabilize the dilaton at the realistic value $S\sim 2$ with just one condensate and no fine-tuning.

hep-th↗
arXiv1996

Stability Bounds on Flavor-Violating Trilinear Soft Terms in the MSSM

The stability of the standard vacuum imposes constraints on flavor violating trilinear soft terms which are stronger than the laboratory bounds coming from the absence of neutral flavor violations (FCNC). Furthermore, contrary to the FCNC bounds, these constraints persist even if the scale of supersymmetry breaking is arbitrarily large.

hep-ph↗
arXiv1996

Some Implications of Charge and Color Breaking in the MSSM

We examine some physically relevant implications of potentially dangerous charge and color breaking minima for supersymmetric models. First, we analyze the stability of the corresponding constraints with respect to variations of the initial scale for the running of the soft breaking terms, finding that the larger the scale is, the stronger the bounds become. In particular, by taking $M_P$ rather than $M_X$ for the initial scale, which is a more sensible election, we find substantially stronger and very important constraints. Second, we find general bounds on the universal gaugino mass, the universal scalar mass, $m\geq 55$ GeV, and the Higgs bilinear coefficient, $|B|\simlt 3m$. Finally, we study the infrared fixed point solution of the top quark mass. Again, the constraints on the parameter space turn out to be very important, including the analytically derivable bound $\left|M/m\right|\simlt 1$.

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

Problems for Supersymmetry Breaking by the Dilaton in Strings from Charge and Color Breaking

The general constraints on the parameter space of soft-breaking terms, in order to avoid dangerous charge and color breaking minima, are applied to the four-dimensional string scenario where the dilaton is the source of supersymmetry breaking (dilaton-dominated limit). The results indicate that the whole parameter space is excluded on these grounds after imposing the present experimental data on the top mass. The inclusion of a non-vanishing cosmological constant does not improve essentially the prospects. Possible way-outs to this situation are briefly discussed.

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