arXiv subjects

Grigoris Panotopoulos

Publications and source records attributed to Grigoris Panotopoulos.

At least 109 records · Page 6Linked to original sources

arXiv2017

Constraining the scalar singlet and inert dark matter models using neutron stars

In the present work we study the scalar singlet as well as the Two-Higgs Doublet model inert dark matter particles impact on compact objects, and we provide the first constraints of the parameter space using neutron stars. The models discussed here are characterized by two free parameters, namely the mass $M_χ$ of the scalar particle that plays the role of the dark matter in the Universe, and a dimensionless coupling constant $λ_χ$ that determines the strength of the interaction of the dark matter particles with the Standard Model Higgs boson. By considering a typical neutron star we were able to obtain constraints on scalar dark matter depending on the DM annihilation cross section and self-interaction coupling constant. Our findings show that i) for heavy DM particles neutron stars can provide us with bounds better that the current limits from direct detection searches only when the self-annihilations of DM particles are negligible and the DM self-interaction coupling constant is very small, while ii) for light DM particles the bounds obtained here are comparable to limits from Higgs invisible decays unless the DM particles are extremely light.

hep-ph↗

arXiv2017

Greybody factors for a minimally coupled massless scalar field in Einstein-Born-Infeld dilaton spacetime

We have analyzed in detail the propagation of a minimally coupled massless scalar field in the gravitational background of a four-dimensional Einstein-Born-Infeld dilaton charged black hole. We have obtained analytical expressions for the absorption cross section as well as for the decay rate for the scalar field in the aforementioned spacetime, and we have shown graphically its behavior for different values of the free parameters of the theory.

hep-th↗

arXiv2017

Strange stars in $f(R)$ theories of gravity in the Palatini formalism

In the present work we study strange stars in $f(R)$ theories of gravity in the Palatini formalism. We consider two concrete well-known cases, namely the $R+R^2/(6 M^2)$ model as well as the $R-μ^4/R$ model for two different values of the mass parameter $M$ or $μ$. We integrate the modified Tolman-Oppenheimer-Volkoff equations numerically, and we show the mass-radius diagram for each model separately. The standard case corresponding to the General Relativity is also shown in the same figure for comparison. Our numerical results show that the interior solution can be vastly different depending on the model and/or the value of the parameter of each model. In addition, our findings imply that i) for the cosmologically interesting values of the mass scales $M,μ$ the effect of modified gravity on strange stars is negligible, while ii) for the values predicting an observable effect, the modified gravity models discussed here would be ruled out by their cosmological effects.

gr-qc↗

arXiv2016

Consistency relations for sharp inflationary non-Gaussian features

If cosmic inflation suffered tiny time-dependent deviations from the slow-roll regime, these would induce the existence of small scale-dependent features imprinted in the primordial spectra, with their shapes and sizes revealing information about the physics that produced them. Small sharp features could be suppressed at the level of the two-point correlation function, making them undetectable in the power spectrum, but could be amplified at the level of the three-point correlation function, offering us a window of opportunity to uncover them in the non-Gaussian bispectrum. In this article, we show that sharp features may be analyzed using only data coming from the three point correlation function parametrizing primordial non-Gaussianity. More precisely, we show that if features appear in a particular non-Gaussian triangle configuration (e.g. equilateral, folded, squeezed), these must reappear in every other configuration according to a specific relation allowing us to correlate features across the non-Gaussian bispectrum. As a result, we offer a method to study scale-dependent features generated during inflation that depends only on data coming from measurements of non-Gaussianity, allowing us to omit data from the power spectrum.

astro-ph.CO↗

arXiv2016

Consistency relations for sharp features in the primordial spectra

We study the generation of sharp features in the primordial spectra within the framework of effective field theory of inflation, wherein curvature perturbations are the consequence of the dynamics of a single scalar degree of freedom. We identify two sources in the generation of features: rapid variations of the sound speed c_s (at which curvature fluctuations propagate) and rapid variations of the expansion rate H during inflation. With this in mind, we propose a non-trivial relation linking these two quantities that allows us to study the generation of sharp features in realistic scenarios where features are the result of the simultaneous occurrence of these two sources. This relation depends on a single parameter with a value determined by the particular model (and its numerical input) responsible for the rapidly varying background. As a consequence, we find a one-parameter consistency relation between the shape and size of features in the bispectrum and features in the power spectrum. To substantiate this result, we discuss several examples of models for which this one-parameter relation (between c_s and H) holds, including models in which features in the spectra are both sudden and resonant.

astro-ph.CO↗

arXiv2014

Nonminimal GUT inflation after Planck results

In the present work we study GUT Coleman-Weinberg inflation with a nonminimal coupling to gravity. In this kind of model one usually finds that either the nonminimal coupling is large or the inflaton self-coupling in unnaturally small. We have shown here that the model is in agreement with the recent results from Planck for natural values of the couplings.

hep-ph↗

arXiv2012

Non-perturbative effects in WIMP scattering off nuclei in the NMSSM

We explore a scenario in the Next-to-Minimal-Supersymmetric-Standard-Model (NMSSM) with both a light O(10) GeV neutralino and a CP-odd Higgs boson with significant coupling to down-type fermions, evading all current B physics, LEP and WMAP bounds. Motivated by a possible slight lepton universality breaking hinted in Upsilon decays, we consider the effect of the mixing of eta(b) resonances with the pseudoscalar Higgs on the spin-dependent scattering neutralino cross section off nucleons. We conclude that this mechanism could be relevant provided that non-perturbative effects enhance the effective eta(b)-nucleon coupling, taking over velocity/q^2 suppression factors, perhaps giving a new insight into the current controversial situation concerning direct search experiments of dark matter.

hep-ph↗

arXiv2012

Proceedings of the first workshop on Flavor Symmetries and consequences in Accelerators and Cosmology (FLASY2011)

The main goals of the first "Workshop on FLAvor SYmmetries and consequences in accelerators and cosmology" (FLASY) was to summarize the theoretical status of flavor symmetries, bringing together young researchers in the field to stimulate discussions and new collaborations, with the aim of investigating possible new physics scenarios to be tested at the LHC, as well as in future neutrino, cosmology experiments and dark matter searches.

hep-ph↗

arXiv2011

The physics of a new gauge boson in a Stueckelberg extension of the two-Higgs-doublet model

String theory constructions using D-brane physics offer a framework where ingredients like extra abelian factors in the gauge group, more than one Higgs doublet and a generalized Green-Schwarz mechanism appear at the same time. Motivated by works towards the direction of obtaining the Standard Model in orientifold constructions, we study in the present work a Stueckelberg extension of the two-Higgs-doublet model. The distinctive features of our model are i) a sharp decay width for the heavy gauge boson, and ii) a charged Higgs boson having two main decay channels at tree level with equal branching ratios.

hep-ph↗

arXiv2011

A dynamical dark energy model with a given luminosity distance

It is assumed that the current cosmic acceleration is driven by a scalar field, the Lagrangian of which is a function of the kinetic term only, and that the luminosity distance is a given function of the red-shift. Upon comparison with Baryon Acoustic Oscillations (BAOs) and Cosmic Microwave Background (CMB) data the parameters of the models are determined, and then the time evolution of the scalar field is determined by the dynamics using the cosmological equations. We find that the solution is very different than the corresponding solution when the non-relativistic matter is ignored, and that the universe enters the acceleration era at larger red-shift compared to the standard $ΛCDM$ model.

gr-qc↗

arXiv2011

Critical behavior of a supersymmetric extension of the Ginzburg-Landau model

We make a connection between quantum phase transitions in condensed matter systems, and supersymmetric gauge theories that are of interest in the particle physics literature. In particular, we point out interesting effects of the supersymmetric quantum electrodynamics upon the critical behavior of the Ginzburg-Landau model. It is shown that supersymmetry fixes the critical exponents, as well as the Landau-Ginzburg parameter, and that the model resides in the type II regime of superconductivity.

hep-th↗

arXiv2011

The degenerate scenario in the NMSSM: Direct singlino-like neutralino searches with a gravitino LSP

A two-component dark matter model is studied adopting the degenerate scenario in the R-parity conserving NMSSM. The gravitino LSP and the neutralino NLSP are extremely degenerate in mass, avoiding the BBN bounds and obtaining a high reheating temperature for thermal leptogenesis to work. In this model both gravitino (absolutely stable) and neutralino (quasi-stable) contribute to dark matter, and direct detection searches for neutralino are discussed. Points that survive all the constraints correspond to a singlino-like neutralino.

hep-ph↗

arXiv2011

Direct neutralino searches in the NMSSM with gravitino LSP in the degenerate scenario

In the present work a two-component dark matter model is studied adopting the degenerate scenario in the R-parity conserving NMSSM. The gravitino LSP and the neutralino NLSP are extremely degenerate in mass, avoiding the BBN bounds and obtaining a high reheating temperature for thermal leptogenesis. In this model both gravitino (absolutely stable) and neutralino (quasi-stable) contribute to dark matter, and direct detection searches for neutralino are discussed. Points that survive all the constraints correspond to a singlino-like neutralino.

hep-ph↗

arXiv2010

Gravitino dark matter with neutralino NLSP in the constrained NMSSM

The gravitino dark matter with neutralino NLSP hypothesis is investigated in the framework of NMSSM. We have considered both the thermal and non-thermal gravitino production mechanisms, and we have taken into account all the collider and cosmological constraints. The maximum allowed reheating temperature after inflation, as well as the maximum allowed gravitino mass are determined.

hep-ph↗

arXiv2010

Higher-order coupled quintessence

We study a coupled quintessence model in which the interaction with the dark matter sector is a function of the quintessence potential. Such a coupling can arise from a field dependent mass term for the dark matter field. The dynamical analysis of a standard quintessence potential coupled with the interaction explored here shows that the system possesses a late time accelerated attractor. In light of these results, we perform a fit to the most recent Supernovae Ia, Cosmic Microwave Background and Baryon Acoustic Oscillation data sets. Constraints arising from weak equivalence principle violation arguments are also discussed.

astro-ph.CO↗

arXiv2010

Gravitino dark matter in the constrained next-to-minimal supersymmetric standard model with neutralino next-to-lightest superpartner

The viability of a possible cosmological scenario is investigated. The theoretical framework is the constrained next-to-minimal supersymmetric standard model (cNMSSM), with a gravitino playing the role of the lightest supersymmetric particle (LSP) and a neutralino acting as the next-to-lightest supersymmetric particle (NLSP). All the necessary constraints from colliders and cosmology have been taken into account. For gravitino we have considered the two usual production mechanisms, namely out-of equillibrium decay from the NLSP, and scattering processes from the thermal bath. The maximum allowed reheating temperature after inflation, as well as the maximum allowed gravitino mass are determined.

hep-ph↗

arXiv2010

Bound states between dark matter particles and emission of gravitational radiation

Bound states of two weakly interactive massive particles are studied. It is assumed that the WIMPonium is formed due to the gravitational interaction, since the weak interaction can sometimes be repulsive. The lifetimes of the spontaneous emission of gravitational radiation and of the WIMPs annihilation into a pair of gravitons are computed, and are shown to be many orders of magnitude larger than the age of the universe.

gr-qc↗

arXiv2009

Big-Bang Nucleosynthesis and WIMP dark matter in modified gravity

In the present work the primordial Big-Bang Nucleosynthesis (BBN) and weakly interacting massive particle (WIMP) dark matter are discussed in a certain class of modified gravitational theories, namely $f(R) \sim R^n$ gravity. The new gravitational model is characterized by a single parameter $n$. First we determine the conditions under which the theoretical predictions for the $^{4}$He abundance are in agreement with the observations. More precisely, during BBN the physics is known and all the parameters are known. The only free parameter to be constrained is the power $n$ related to the new gravitational model. After that, for cold dark matter we use the value of $n$ determined from the BBN considerations and determine how the mass of the dark matter particle is related to the annihilation cross section in order for the cold dark matter constraint to be satisfied.

astro-ph↗