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B. de Carlos

Publications and source records attributed to B. de Carlos.

35 records · Page 2Linked to original sources
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

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

Relic Abundances and Detection Rates of Neutralinos in String-Inpired Supergravity Models

We calculate ``relic abundances'' and ``detection rates'' of the neutralino (LSP) in string-inspired supergravity models with ``dilaton-moduli'' induced supersymmetry breaking. In particular we investigate ``universal'' scenarios for the soft-supersymmetry breaking terms from Calabi-Yau compactifications, as well as from the dilaton-dominated limit. ``Non-universal'' scenarios from orbifold string theory are also incorporated into the analysis. In all cases, in the cosmologically interesting region, we find $m_{LSP}\geq 50$ GeV and direct-detection rates in the range O($10^{-3}$ events/(Kg day)) - O($10^{-4}$ events/(Kg day)). ``Indirect-detection'' rates from LSPs captured in the Sun are also calculated.

hep-ph↗
arXiv1996

R-Parity Violation and Quark Flavour Violation

We investigate the implications of $R$-Parity violation (RPV) for quark flavour violation both by constraining the sneutrino masses to be positive and by studying the processes $b\to sγ$ and $K^0-\bar K^0$ mixing. In the latter there are two major contributions, one from ``direct'' one loop diagrams involving RPV couplings, and one from the ``indirect'' contributions generated by the renormalisation group. We compare the effects and discuss the implications of our results.

hep-ph↗
arXiv1996

$R$-Parity Violation and Flavour Violation

The MSSM with $R$-Parity violation allows flavour changing neutral currents through two mechanisms, directly through one loop diagrams, and indirectly through the generation of flavour violation in the sparticle sector. We discuss the use of these mechanisms for constraining $R$-Parity violation, and show that the indirect mechanism parametrised in terms of the running of the soft masses from the unification or Planck scale typically dominates previously calculated effects. We discuss neutrino mass generation, $μ\to eγ$, $b\to sγ$, and $K^0-\bar K^0$ mixing as examples.

hep-ph↗
arXiv1996

R-parity Violation Effects through Soft Supersymmetry Breaking Terms and the Renormalisation Group

We present full renormalisation group equations for the MSSM with R-parity violation, including all soft supersymmetry breaking terms. The inclusion of dimensionless R-parity violating couplings can generate many possible low energy effects through the generation of off-diagonal soft masses violating lepton and quark flavour, and through the generation of lepton-Higgs mixing. We discuss the relation between the weak and unification scale R-parity violation and study the effects on neutrino mass generation and $μ\to eγ$.

hep-ph↗
arXiv1995

Natural flavour mixing in the MSSM and $μ--> e, γ$

In the absence of any additional assumption it is natural to conjecture that sizeable flavour-mixing mass entries, $Δm^2$, may appear in the mass matrices of the scalars of the MSSM, i.e. $Δm^2\sim O(m^2)$. This flavour violation can still be reconciled with the experiment if the gaugino mass, $M_{1/2}$, is large enough to yield (through the renormalization group running) a sufficiently small $Δm^2 / m^2$ at low energy. This leads to a gaugino dominance framework (i.e. $M_{1/2}^2\gg m^2$), which permits a remarkably model--independent analysis. We study this possibility focussing our attention on the $μ\rightarrow e,γ$ decay. In this way we obtain very strong and general constraints, in particular $\frac{M_{1/2}^2}{Δm} \simgt 34\ {\rm TeV}$.

hep-ph↗
arXiv1995

Supergravity and the $b\to s,γ$ decay

We evaluate the branching ratio BR($b\rightarrow s,γ$) in the minimal supersymmetric standard model (MSSM), determining the corresponding phenomenological restrictions on two attractive supergravity scenarios, namely minimal supergravity and a class of models with a natural solution to the $μ$ problem. We have included in the calculation some one--loop refinements that have a substantial impact on the results. It is stressed the fact that an eventual improvement of the experimental bounds of order $10^{-4}$ would strengthen the restrictions on the MSSM dramatically. This would be enough to discard these supergravity scenarios with $μ<0$ if no discrepancy is found with the standard model prediction, while for $μ>0$ there will remain low-energy windows.

hep-ph↗
arXiv1995

Constraints on Supersymmetric Theories from $μ\to e,γ$

In the absence of any additional assumption it is natural to conjecture that sizeable flavour-mixing mass entries, $Δm^2$, may appear in the mass matrices of the scalars of the MSSM, i.e. $Δm^2\sim O(m^2)$. This flavour violation can still be reconciled with the experiment if the gaugino mass, $M_{1/2}$, is large enough, leading to a {\em gaugino dominance} framework (i.e. $M_{1/2}^2\gg m^2$), which permits a remarkably model--independent analysis. We study this possibility focussing our attention on the $μ\rightarrow e,γ$ decay. In this way we obtain very strong and general constraints, in particular $\frac{M_{1/2}^2}{Δm}\simgt 34\ {\rm TeV}$. On the other hand, we show that our analysis and results remain valid for values of $m^2$ much larger than $Δm^2$, namely for $\frac{Δm^2}{m^2}\simgt \frac{m^2} {10\ {\rm TeV^2}}$, thus extending enormously their scope of application. Finally, we discuss the implications for superstring scenarios.

hep-ph↗
arXiv1995

Constraining Supergravity Scenarios through the $b\to s,γ$ Decay.

We evaluate the branching ratio BR($b\rightarrow s,γ$) in the minimal supersymmetric standard model (MSSM), determining the corresponding phenomenological restrictions on two attractive supergravity scenarios, namely minimal supergravity and a class of models with a natural solution to the $μ$ problem. We have included in the calculation some one--loop refinements that have a substantial impact on the results. The numerical results show some disagreements with part of the previous results in the literature, while they are in agreement with others. For minimal supergravity the CLEO upper and lower bounds put important restrictions on the scalar and gaugino masses in both cases $μ<0$ and $μ>0$. For the other supergravity scenarios the relevant CLEO bound is the upper one. It is stressed the fact that an eventual improvement of the experimental bounds of order $10^{-4}$ would strengthen the restrictions on the MSSM dramatically. This would be enough to discard these supergravity scenarios with $μ<0$ if no discrepancy is found with the standard model prediction, while for $μ>0$ there will remain low-energy windows.

hep-ph↗
arXiv1994

Soft Supersymmetry Breaking from Gaugino Condensation

We study the structure of soft breaking terms in the context of a gaugino condensation scenario. Assuming that the Supergravity Lagrangian is the correct quantum field theory limit, at some momentum scale $μ_{UV}$, of a more fundamental one, we demonstrate that the correct result is obtained simply by substituting, in the tree level Supergravity Lagrangian, $λλ$ (the gaugino condensate) by its vacuum expectation value $Λ^3$. In string inspired scenarios this implies, in particular, that the scalar masses are vanishing at the string tree-level and receive a contribution, at the one loop level, which is proportional to the Green Schwarz coefficient $δ_{GS}$. Our results do not agree with the ones obtained in the effective Lagrangian approach. We study in detail the origin of this discrepancy, and we argue that the use of the supertrace anomaly to determine the effective theory for the condensate does not fix its gravitational interactions, leaving the soft breaking terms and the vacua of the theory unspecified.

hep-ph↗
arXiv1993

The Fine-Tuning Problem of the Electroweak Symmetry Breaking Mechanism in Minimal SUSY Models

We calculate the region of the MSSM parameter space (i.e. $M_{1/2}$, $m_{0}$, $μ$, \ldots) compatible with a correct electroweak breaking and a realistic top-quark mass. To do so we have included {\em all} the one-loop corrections to the effective potential $V_{1}$ and checked their importance in order to obtain consistent results. We also consider the fine-tuning problem due to the enormous dependence of $M_{Z}$ on $h_{t}$ (the top Yukawa coupling), which is substantially reduced when the one-loop effects are taken into account. We also explore the reliability of the so-called "standard" criterion to estimate the degree of fine-tuning. As a consequence, we obtain a new set of upper bounds on the MSSM parameters or, equivalently, on the supersymmetric masses perfectly consistent with the present experimental bounds.

hep-ph↗
arXiv1993

Model Independent Properties and Cosmological Implications of the Dilaton and Moduli Sectors of 4-d Strings

We show that if there is a realistic 4-d string, the dilaton and moduli supermultiplets will generically acquire a small mass O(m_{3/2}), providing the only vacuum-independent evidence of low-energy physics in string theory beyond the supersymmetric standard model. The only assumptions behind this result are (i) softly broken supersymmetry at low energies with zero cosmological constant, (ii) these particles interact with gravitational strength and the scalar components have a flat potential in perturbation theory, which are well-known properties of string theories. (iii) They acquire a $vev$ of the order of the Planck scale (as required for the correct value of the gauge coupling constants and the expected compactification scale) after supersymmetry gets broken. We explore the cosmological implications of these particles. Similar to the gravitino, the fermionic states may overclose the Universe if they are stable or destroy nucleosynthesis if they decay unless their masses belong to a certain range or inflation dilutes them. For the scalar states it is known that the problem cannot be entirely solved by inflation, since oscillations around the minimum of the potential can lead to a huge entropy generation at late times. We discus some possible ways to alleviate this entropy problem, that favour low-temperature baryogenesis, and also comment on the possible role of these particles as dark matter candidates or as sources of the baryon asymmetry through their decay.

hep-ph↗
arXiv1993

SUSY Soft Breaking Terms from String Scenarios

The general SUSY soft breaking terms for a large class of phenomenologically relevant string scenarios (symmetric orbifolds) are given. They show a certain lack of universality, but not dangerous for flavor changing neutral currents. To get more quantitative results a specific SUSY breaking mechanism has to be considered, namely gaugino condensation in the hidden sector. Then, it turns out that squark and slepton masses tend to be much larger than scalar masses ($m_ϕ > 10 M_a$), which probably is a quite general fact. Experimental bounds and the requirement of a successful electroweak breaking without fine tuning impose further restrictions on the soft breaking terms. As a consequence the gluino and chargino masses should be quite close to their present experimental limits, whereas squark and slepton masses should be much higher (> 1 TeV).

hep-ph↗
arXiv1993

One-Loop Analysis of the Electroweak Breaking in Supersymmetric Models and the Fine-Tuning Problems

We examine the electroweak breaking mechanism in the minimal supersymmetric standard model (MSSM) using the {\em complete} one-loop effective potential $V_1$. First, we study what is the region of the whole MSSM parameter space (i.e. $M_{1/2},m_o,μ,...$) that leads to a succesful $SU(2)\times U(1)$ breaking with an acceptable top quark mass. In doing this it is observed that all the one-loop corrections to $V_1$ (even the apparently small ones) must be taken into account in order to get reliable results. We find that the allowed region of parameters is considerably enhanced with respect to former "improved" tree level results. Next, we study the fine-tuning problem associated with the high sensitivity of $M_Z$ to $h_t$ (the top Yukawa coupling). Again, we find that this fine-tuning is appreciably smaller once the one-loop effects are considered than in previous tree level calculations. Finally, we explore the ambiguities and limitations of the ordinary criterion to estimate the degree of fine-tuning. As a result of all this, the upper bounds on the MSSM parameters, and hence on the supersymmetric masses, are substantially raised, thus increasing the consistency between supersymmetry and observation.

hep-ph↗
arXiv1992

Soft SUSY Breaking Terms in Stringy Scenarios: Computation and Phenomenological Viability

We calculate the soft SUSY breaking terms arising from a large class of string scenarios, namely symmetric orbifold constructions, and study its phenomenological viability. They exhibit a certain lack of universality, unlike the usual assumptions of the minimal supersymmetric standard model. Assuming gaugino condensation in the hidden sector as the source of SUSY breaking, it turns out that squark and slepton masses tend to be much larger than gaugino masses. Furthermore, we show that these soft breaking terms can be perfectly consistent with both experimental and naturalness constraints (the latter comes from the absence of fine tuning in the $SU(2)\times U(1)_Y\rightarrow U(1)_{em}$ breaking process). This is certainly non--trivial and in fact imposes interesting constraints on measurable quantities. More precisely, we find that the gluino mass ($M_3$) and the chargino mass ($M_{χ^{\pm}}$) cannot be much higher than their present experimental lower bounds ($M_3\stackrel{<}{{}_\sim}285\ $GeV ; $M_{χ^\pm}\stackrel{<}{{}_\sim}80\ $GeV), while squark and slepton masses must be much larger ($\stackrel{>}{{}_\sim} 1\ $TeV). This can be considered as an observational signature of this kind of stringy scenarios. Besides, the top mass is constrained to be within a range ($80\ $GeV$\stackrel{<}{{}_\sim}m_t\stackrel{<}{{}_\sim}165\ $GeV)

hep-ph↗
arXiv1992

Supersymmetry Breaking and Determination of the Unification Gauge Coupling Constant in String Theories

We study in a systematic and modular invariant way gaugino condensation in the hidden sector as a potential source of hierarchical supersymmetry breaking and a non--trivial potential for the dilaton $S$ whose real part corresponds to the tree level gauge coupling constant (${\rm Re}\ S\sim g_{gut}^{-2}$). For the case of pure Yang--Mills condensation, we show that no realistic results (in particular no reasonable values for ${\rm Re}\ S$) can emerge, even if the hidden gauge group is not simple. However, in the presence of hidden matter (i.e. the most frequent case) there arises a very interesting class of scenarios with two or more hidden condensing groups for which the dilaton dynamically acquires a reasonable value (${\rm Re}\ S\sim 2$) and supersymmetry is broken at the correct scale ($m_{3/2}\sim 10^3\ GeV$) with no need of fine--tuning. Actually, good values for ${\rm Re}\ S$ and $m_{3/2}$ are correlated. We make an exhaustive classification of the working possibilities. Remarkably, the results are basically independent from the value of $δ^{GS}$ (the contributions from the Green--Schwarz mechanism). The radius of the compactified space also acquires an expectation value, breaking duality spontaneously.

hep-th↗