arXiv subjects

C. Pallis

Publications and source records attributed to C. Pallis.

At least 73 records · Page 4Linked to original sources

arXiv2011

Non-Minimal Higgs Inflation and non-Thermal Leptogenesis in a Supersymmetric Pati-Salam Model

We consider a supersymmetric (SUSY) Grand Unified Theory (GUT) based on the gauge group G_PS=SU(4)_C x SU(2)_L x SU(2)_R, which incorporates non-minimal chaotic inflation, driven by a quartic potential associated with the Higgs fields involved in the spontaneous breaking of G_PS. The inflationary model relies on renormalizable superpotetial terms and does not lead to overproduction of magnetic monopoles. It is largely independent of the one-loop radiative corrections and can become consistent with the current observational data on the inflationary observables, with the symmetry breaking scale of G_PS assuming its SUSY value. Within our model, the strong CP and the mu problems of the minimal supersymmetric standard model can be resolved via a Peccei-Quinn symmetry. Moreover baryogenesis occurs via non-thermal leptogenesis realized by the out-of-equilibrium decay of the right-handed neutrinos, which are produced by the inflaton's decay. We consider two versions of such a scenario, assuming that the inflaton can decay to the lightest or to the next-to-lightest right-handed neutrino. Both scenaria can become compatible with the constraints arising from the baryon asymmetry of the universe, the gravitino limit on the reheating temperature and the upper bound on the light neutrino masses, provided that the gravitino is somehow heavy. In the second scenario extra restrictions from the SU(4)_C symmetry on the heaviest Dirac neutrino mass and the data on the atmospheric neutrino oscillations can be also met.

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arXiv2011

CMSSM with Yukawa Quasi-Unification Revisited

The constrained minimal supersymmetric standard model with mu>0 supplemented by an `asymptotic' Yukawa coupling quasi-unification condition, which allows an acceptable b-quark mass, is reinvestigated. Imposing updated constraints from the cold dark matter abundance in the universe, B physics, the muon anomalous magnetic moment, and the mass mh of the lightest neutral CP-even Higgs boson, we find that the allowed parameter space is quite limited but not unnaturally small with the cold dark matter abundance suppressed only via neutralino-stau coannihilations. The lightest neutralino with mass in the range (341-677) GeV is possibly detectable in the future direct cold dark matter searches via its spin-independent cross section with nucleon. In the allowed parameter space of the model, we obtain mh=(117-122.2) GeV.

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arXiv2011

Non-Minimal Sneutrino Inflation, Peccei-Quinn Phase Transition and non-Thermal Leptogenesis

We consider a phenomenological extension of the minimal supersymmetric standard model which incorporates non-minimal chaotic inflation, driven by a quartic potential associated with the lightest right-handed sneutrino. Inflation is followed by a Peccei-Quinn phase transition based on renormalizable superpotential terms, which resolves the strong CP and mu problems of the minimal supersymmetric standard model provided that one related parameter of the superpotential is somewhat small. Baryogenesis occurs via non-thermal leptogenesis, which is realized by the inflaton decay. Confronting our scenario with the current observational data on the inflationary observables, the baryon assymetry of the universe, the gravitino limit on the reheating temperature and the upper bound on the light neutrino masses, we constrain the effective Yukawa coupling involved in the decay of the inflaton to relatively small values and the inflaton mass to values lower than 10^12 GeV.

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arXiv2010

F-Term Hybrid Inflation Followed by a Peccei-Quinn Phase Transition

We consider a cosmological set-up, based on renormalizable superpotential terms, in which a superheavy scale F-term hybrid inflation is followed by a Peccei-Quinn phase transition, resolving the strong CP and mu problems of the minimal supersymmetric standard model. We show that the field which triggers the Peccei-Quinn phase transition can remain after inflation well above the Peccei-Quinn scale thanks to (i) its participation in the supergravity and logarithmic corrections during the inflationary stage and (ii) the high reheat temperature after the same period. As a consequence, its presence influences drastically the inflationary dynamics and the universe suffers a second period of reheating after the Peccei-Quinn phase transition. Confronting our inflationary predictions with the current observational data, we find that, for about the central value of the spectral index, the grand unification scale can be identified with its supersymmetric value for the relevant coupling constant κ=0.002 and, more or less, natural values, +/-(0.01-0.1), for the remaining parameters. On the other hand, the final reheat temeperature after the Peccei-Quinn phase transition turns out to be low enough so as the gravitino problem is avoided.

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arXiv2010

Non-Minimally Gravity-Coupled Inflationary Models

We consider the non-supersymmetric models of chaotic (driven by a quadratic potential) and hybrid inflation, taking into account the minimal possible radiative corrections to the inflationary potential. We show that two simple coupling functions f(sigma) (with a parameter ck involved) between the inflaton field, sigma, and the Ricci scalar curvature ensure, for sub-Planckian values of the inflaton field, observationally acceptable values for the spectral index, ns, and sufficient reheating after inflation. In the case of chaotic inflation we consider two models with large ck's resulting to ns=0.955 or 0.967 and tensor-to-scalar ratio r=0.2 or 0.003, respectively. In the case of hybrid inflation, the selected f(sigma) assists us to obtain hilltop-type inflation. For values of the relevant mass parameter, m, less than 10^6 TeV and the observationally central value of ns, we find ck=(0.015-0.078) with the relevant coupling constants lambda=kappa and the symmetry breaking scale, M, confined in the ranges (2x 10^{-7}-0.001) and (1-16.8)x10^{17} GeV, respectively.

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arXiv2010

Cold Dark Matter in non-Standard Cosmologies, PAMELA, ATIC and Fermi LAT

We consider two non-standard cosmological scenaria according to which the universe is reheated to a low reheating temperature after the late decay of a scalar field or is dominated by the kinetic energy of a quintessence field in the context of a tracking quintessential model. In both cases, we calculate the relic density of the \emph{Weakly Interacting Massive Particles} (WIMPs) and show that it can be enhanced with respect to its value in the standard cosmology. By adjusting the parameters of the low reheating or the quintessential scenario, the cold dark matter abundance in the universe can become compatible with large values for the annihilation cross section times the velocity of the WIMPs. Using these values and assuming that the WIMPs annihilate predominantly to $e^+e^-$, $μ^+μ^-$ or $τ^+τ^-$, we calculate the induced fluxes of $e^\pm$ cosmic rays and fit the current data of PAMELA and ATIC or Fermi LAT. We achieve rather good fits especially to PAMELA and Fermi-LAT data in conjunction with a marginal fulfillment of the restriction arising from cosmic microwave background, provided that the WIMPs annihilate predominantly to $μ^+μ^-$. In both non-standard scenaria the required transition temperature to the conventional radiation dominated era turns out to be lower than about $0.7 \GeV$. In the case of the low reheating, an appreciable non-thermal contribution to the WIMP relic density is also necessitated.

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arXiv2010

Tracking Quintessence and Cold Dark Matter Candidates

We study the generation of a kination-dominated phase in the context of a quintessential model with an inverse-power-law potential and a Hubble-induced mass term for the quintessence field. The presence of kination is associated with an oscillating evolution of the quintessence field and the barotropic index. We find that, in sizeable regions of the parameter space, a tracker scaling solution can be reached sufficiently early to alleviate the coincidence problem. Other observational constraints originating from nucleosynthesis, the inflationary scale, the present acceleration of the universe and the dark-energy-density parameter can be also met. The impact of this modified kination-dominated phase on the thermal abundance of cold dark matter candidates is investigated too. We find that: (i) the enhancement of the relic abundance of the WIMPs with respect to the standard paradigm, crucially depends on the hierarchy between the freeze-out temperature and the temperature at which the extrema in the evolution of the quintessence field are encountered, and (ii) the relic abundance of e-WIMPs takes its present value close to the temperature at which the earliest extremum of the evolution of the quintessence field occurs and, as a consequence, both gravitinos and axinos arise as natural cold dark matter candidates. In the case of unstable gravitinos, the gravitino constraint can be satisfied for values of the initial temperature well above those required in the standard cosmology.

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arXiv2010

Tracking Quintessence, WIMP Relic Density, PAMELA and Fermi LAT

The generation of an early kination dominated era within a tracking quintessential model is investigated, the relic density of the Weakly Interacting Massive Particles (WIMPs) is calculated and we show that it can be enhanced with respect to its value in the standard cosmology. By adjusting the parameters of the quintessential scenario, the cold dark matter abundance in the universe can become compatible with large values for the annihilation cross section times the velocity of the WIMPs. Using these values and assuming that the WIMPs annihilate predominantly to $μ^+μ^-$, we calculate the induced fluxes of $e^\pm$ cosmic rays and fit the current PAMELA and Fermi-LAT data. We achieve rather good fits in conjunction with a marginal fulfillment of the restriction arisen from the cosmic microwave background radiation.

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arXiv2009

Quintessential Kination and Thermal Production of Gravitinos and Axinos

The impact of a kination-dominated phase generated by a quintessential exponential model on the thermal abundance of gravitinos and axinos is investigated. We find that their abundances become proportional to the transition temperature from the kination to the radiation era; since this temperature is significantly lower than the initial ("reheating") temperature, the abundances decrease with respect to their values in the standard cosmology. For values of the quintessential energy-density parameter close to its upper bound, at the eve of nucleosynthesis, we find the following: (i) for unstable gravitinos, the gravitino constraint is totally evaded; (ii) If the gravitino is stable, its thermal abundance is not sufficient to account for the cold dark matter of the universe; (iii) the thermal abundance of axinos can satisfy the cold dark matter constraint for values of the initial temperature well above those required in the standard cosmology. A novel calculation of the axino production rate by scatterings at low temperature is also presented.

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arXiv2009

Kaehler Potentials for Hilltop F-term Hybrid Inflation

We consider the basic supersymmetric (SUSY) models of F-term hybrid inflation (FHI). We show that a simple class of Kaehler potentials ensures a resolution to the ηproblem and allows for inflation of hilltop type. As a consequence, observationally acceptable values for the spectral index, ns, can be achieved constraining the coefficient c4k of the quartic supergravity correction to the inflationary potential. For about the central value of ns, in the case of standard FHI, the grand unification (GUT) scale turns out to be well below its SUSY value with the relevant coupling constant κin the range (0.0006-0.15) and c4k=-(1100-0.05). In the case of shifted [smooth] FHI, the GUT scale can be identified with its SUSY value for c4k=-16 [c4k=-1/16].

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arXiv2008

F-Term Hybrid Inflation Followed by Modular Inflation

We consider the well motivated model of the (standard) supersymmetric F-term hybrid inflation (FHI) which can be realized close to the grand unification scale. The predicted scalar spectral index $n_s$ cannot be smaller than 0.98 and can exceed unity including corrections from minimal supergravity, if the number of e-foldings corresponding to the pivot scale $k_*=0.002/{\rm Mpc}$ is around 50. These results are marginally consistent with the fitting of the Wilkinson microwave anisotropy probe data by the standard power-law cosmological model with cold dark matter and a cosmological constant. However, $n_s$ can be reduced by restricting the number of e-foldings that $k_*$ suffered during FHI. The additional e-foldings required for solving the horizon and flatness problems can be generated by a subsequent stage of fast-roll [slow-roll] modular inflation realized by a string modulus which does [does not] acquire effective mass before the onset of modular inflation.

hep-ph↗

arXiv2008

Reducing the spectral index in F-term hybrid inflation

We consider a class of well motivated supersymmetric models of F-term hybrid inflation (FHI) which can be linked to the supersymmetric grand unification. The predicted scalar spectral index n_s cannot be smaller than 0.97 and can exceed unity including corrections from minimal supergravity, if the number of e-foldings corresponding to the pivot scale k_*=0.002/Mpc is around 50. These results are marginally consistent with the fitting of the three-year Wilkinson microwave anisotropy probe data by the standard power-law cosmological model with cold dark matter and a cosmological constant. However, n_s can be reduced by applying two mechanisms: (i) The utilization of a quasi-canonical Kahler potential with a convenient choice of a sign and (ii) the restriction of the number of e-foldings that k_* suffered during FHI. In the case (i), we investigate the possible reduction of n_s without generating maxima and minima of the potential on the inflationary path. In the case (ii), the additional e-foldings required for solving the horizon and flatness problems can be generated by a subsequent stage of fast-roll [slow-roll] modular inflation realized by a string modulus which does [does not] acquire effective mass before the onset of modular inflation.

hep-ph↗

arXiv2008

Quintessential Kination and Thermal Production of SUSY e-WIMPs

The impact of a kination-dominated phase generated by a quintessential exponential model on the thermal abundance of Supersymmetric (SUSY) extremely Weekly Interacting Massive Particles (e-WIMPs) is investigated. For values of the quintessential energy-density parameter on the eve of nucleosynthesis close to its upper bound, we find that: (i) the gravitino constraint is totally evaded for unstable gravitinos; (ii) the thermal abundance of stable gravitinos is not sufficient to account for the cold dark matter of the universe; (iii) the thermal abundance of axinos can satisfy the cold dark matter constraint for values of the initial (``reheating'') temperature well above those required in the standard cosmology.

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arXiv2007

Reducing the spectral index in F-term hybrid inflation through a complementary modular inflation

We consider two-stage inflationary models in which a superheavy scale F-term hybrid inflation is followed by an intermediate scale modular inflation. We confront these models with the restrictions on the power spectrum P_R of curvature perturbations and the spectral index n_s implied by the recent data within the power-law cosmological model with cold dark matter and a cosmological constant. We show that these restrictions can be met provided that the number of e-foldings N_HI* suffered by the pivot scale k_*=0.002/Mpc during hybrid inflation is appropriately restricted. The additional e-foldings required for solving the horizon and flatness problems can be naturally generated by the subsequent modular inflation. For central values of P_R and n_s, we find that, in the case of standard hybrid inflation, the values obtained for the grand unification scale are close to its supersymmetric value M_GUT=2.86 x 10^16 GeV, the relevant coupling constant is relatively large (0.005-0.14), and N_HI* between about 10 and 21.7. In the case of shifted [smooth] hybrid inflation, the grand unification scale can be identified with M_GUT provided that N_HI*=21 [N_HI*=18].

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arXiv2006

Kination Dominated Reheating and Cold Dark Matter Abundance

We consider the decay of a massive particle under the complete or partial domination of the kinetic energy density generated by a quintessential exponential model and we impose a number of observational constraints originating from nucleosynthesis, the present acceleration of the universe and the dark-energy-density parameter. We show that the presence of kination causes a prolonged period during which the temperature is frozen to a plateau value, much lower than the maximal temperature achieved during the process of reheating in the absence of kination. The decoupling of a cold dark matter particle during this period is analyzed, its relic density is calculated both numerically and semi-analytically and the results are compared with each other. Using plausible values (from the viewpoint of particle models) for the mass and the thermal averaged cross section times the velocity of the cold relic, we investigate scenaria of equilibrium or non-equilibrium production. In both cases, acceptable results for the cold dark matter abundance can be obtained, by constraining the initial energy density of the decaying particle, its decay width, its mass and the averaged number of the produced cold relics. The required plateau value of the temperature is, in most cases, lower than about 40 GeV

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arXiv2006

CDM Abundance in non-Standard Cosmologies

The relic density of a cold dark matter (CDM) candidate is calculated in the context of three non-standard cosmological scenaria and its value is compared with the one obtained in the standard regime. In particular, we consider the decoupling of the CDM particle during: (i) A decaying-particle dominated phase or (ii) a kinetic-energy dominated phase or (iii) the decay of a massive particle under the complete or partial domination of kination. We use plausible values (from the viewpoint of supersymmetric models) for the mass and the thermal averaged cross section times the velocity of the cold relic and we investigate scenaria of equilibrium and non-equilibrium production. In the case (i) a low reheat temperature, in the range (1-20) GeV, significantly facilitates the achievement of an acceptable CDM abundance. On the other hand, the presence of kination in the case (ii) can lead to an enhancement of the CDM abundance up to three orders of magnitude. The latter enhancement can be avoided, in the case (iii). In a such situation, the temperature turns out to be frozen to a plateau value which is, mostly, lower than about 40 GeV.

hep-ph↗

arXiv2006

Supersymmetric Dark Matter, Inflation and Yukawa Quasi-Unification

The construction of supersymmetric grand unified models based on the Pati-Salam gauge group and leading to a set of Yukawa quasi-unification conditions is briefly reviewed. For each sign of mu, an appropriately chosen condition from this set can allow an acceptable b-quark mass within the constrained minimal supersymmetric standard model. The restrictions on the parameter space which arise from the cold dark matter, b-->s gamma, muon anomalous magnetic moment and collider requirements are also investigated. For mu>0, a wide and natural range of parameters is allowed. On the contrary, the mu<0 case is excluded by the combination of the cold dark matter and b-->s gamma requirements. In the mu>0 case, the predicted neutralinos are possibly detectable in the future direct cold dark matter searches. Moreover, the mu term is generated via a Peccei-Quinn symmetry. The same model gives rise to a new shifted inflationary scenario, which is based only on renormalizable terms and does not suffer from the problem of monopole overproduction at the end of inflation. Although inflation takes place at large values of the inflaton field and, thus, supergravity corrections could invalidate it, this does not happen provided that an extra gauge singlet with a superheavy vacuum expectation value from D-terms is introduced and a specific Kaehler potential is used.

hep-ph↗

arXiv2005

Quintessential Kination and Cold Dark Matter Abundance

The generation of a kination-dominated phase by a quintessential exponential model is investigated and the parameters of the model are restricted so that a number of observational constraints (originating from nucleosynthesis, the present acceleration of the universe and the dark-energy-density parameter) are satisfied. The decoupling of a thermal cold dark matter particle during the period of kination is analyzed, the relic density is calculated both numerically and semi-analytically and the results are compared with each other. It is argued that the enhancement, with respect to the standard paradigm, of the cold dark matter abundance can be expressed as a function of the quintessential density parameter at the onset of nucleosynthesis. We find that values of the latter quantity close to its upper bound require the thermal-averaged cross section times the velocity of the cold relic to be almost three orders of magnitude larger than this needed in the standard scenario so as compatibility with the cold dark matter constraint is achieved.

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