Center for Astroparticle Physics

Genève, Switzerland

Center for Astroparticle Physics

Genève, Switzerland
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Blum K.,Institute for Advanced Study | Efrati A.,Weizmann Institute of Science | Grossman Y.,Cornell University | Nir Y.,Weizmann Institute of Science | Riotto A.,Center for Astroparticle Physics
Physical Review Letters | Year: 2012

In the supersymmetric framework, prior to the electroweak phase transition, the existence of a baryon asymmetry implies the existence of a Higgsino asymmetry. We investigate whether the Higgsino could be a viable asymmetric dark matter candidate. We find that this is indeed possible. Thus, supersymmetry can provide the observed dark matter abundance and, furthermore, relate it with the baryon asymmetry, in which case the puzzle of why the baryonic and dark matter mass densities are similar would be explained. To accomplish this task, two conditions are required. First, the gauginos, squarks, and sleptons must all be very heavy, such that the only electroweak-scale superpartners are the Higgsinos. With this spectrum, supersymmetry does not solve the fine-tuning problem. Second, the temperature of the electroweak phase transition must be low, in the (1-10)GeV range. This condition requires an extension of the minimal supersymmetric standard model. © 2012 American Physical Society.

Kehagias A.,National Technical University of Athens | Kehagias A.,CERN | Riotto A.,Center for Astroparticle Physics
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2017

We investigate the recently proposed clockwork mechanism delivering light degrees of freedom with suppressed interactions and show, with various examples, that it can be efficiently implemented in inflationary scenarios to generate flat inflaton potentials and small density perturbations without fine-tunings. We also study the clockwork graviton in de Sitter and, interestingly, we find that the corresponding clockwork charge is site-dependent. As a consequence, the amount of tensor modes is generically suppressed with respect to the standard cases where the clockwork set-up is not adopted. This point can be made a virtue in resurrecting models of inflation which were supposed to be ruled out because of the excessive amount of tensor modes from inflation. © 2017 The Author(s)

Creminelli P.,Abdus Salam International Center For Theoretical Physics | Gleyzes J.,CEA Saclay Nuclear Research Center | Gleyzes J.,French National Center for Scientific Research | Gleyzes J.,University Paris - Sud | And 3 more authors.
Physical Review Letters | Year: 2014

We show that the prediction for the primordial tensor power spectrum cannot be modified at leading order in derivatives. Indeed, one can always set to unity the speed of propagation of gravitational waves during inflation by a suitable disformal transformation of the metric, while a conformal one can make the Planck mass time independent. Therefore, the tensor amplitude unambiguously fixes the energy scale of inflation. Using the effective field theory of inflation, we check that predictions are independent of the choice of frame, as expected. The first corrections to the standard prediction come from two parity violating operators with three derivatives. Also the correlator 〈γγγ〉 is standard and only receives higher derivative corrections. These results hold also in multifield models of inflation and in alternatives to inflation and make the connection between a (quasi-)scale-invariant tensor spectrum and inflation completely robust. © 2014 American Physical Society.

Zhang J.,Shanghai JiaoTong University | Ma C.-P.,University of California at Berkeley | Riotto A.,Center for Astroparticle Physics
Astrophysical Journal | Year: 2014

In the standard excursion-set model for the growth of structure, the statistical properties of halos are governed by the halo mass and are independent of the larger-scale environment in which the halos reside. Numerical simulations, however, have found the spatial distributions of halos to depend not only on their mass but also on the details of their assembly history and environment. Here we present a theoretical framework for incorporating this "assembly bias" into the excursion-set model. Our derivations are based on modifications of the path-integral approach of Maggiore & Riotto that models halo formation as a non-Markovian random-walk process. The perturbed density field is assumed to evolve stochastically with the smoothing scale and exhibits correlated walks in the presence of a density barrier. We write down conditional probabilities for multiple barrier crossings and derive from them analytic expressions for descendant and progenitor halo mass functions and halo merger rates as a function of both halo mass and the linear overdensity δ e of the larger-scale environment of the halo. Our results predict a higher halo merger rate and higher progenitor halo mass function in regions of higher overdensity, consistent with the behavior seen in N-body simulations. © 2014. The American Astronomical Society. All rights reserved..

Busoni G.,International School for Advanced Studies | de Simone A.,International School for Advanced Studies | de Simone A.,CERN | Morgante E.,Center for Astroparticle Physics | Riotto A.,Center for Astroparticle Physics
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2014

We discuss the limitations to the use of the effective field theory approach to study dark matter at the LHC. We introduce and study a few quantities, some of them independent of the ultraviolet completion of the dark matter theory, which quantify the error made when using effective operators to describe processes with very high momentum transfer. Our criteria indicate up to what cutoff energy scale, and with what precision, the effective description is valid, depending on the dark matter mass and couplings. © 2013 The Authors. Published by Elsevier B.V.

Caballero I.,University Paris Diderot | Wilms J.,Center for Astroparticle Physics
Memorie della Societa Astronomica Italiana, Supplementi - Journal of the Italian Astronomical Society, Supplement | Year: 2012

Accreting X-ray pulsars are among the most luminous objects in the X-ray sky. In highly magnetized neutron stars (B ∼ 1012 G), the flow of matter is dominated by the strong magnetic field. The general properties of accreting X-ray binaries are presented, focusing on the spectral characteristics of the systems. The use of cyclotron lines as a tool to directly measure a neutron star's magnetic field and to test the theory of accretion are discussed.We conclude with the current and future prospects for accreting X-ray binary studies. © SAIt 2012.

Kehagias A.,National Technical University of Athens | Riotto A.,Center for Astroparticle Physics
Nuclear Physics B | Year: 2013

We study the multifield inflationary models where the cosmological perturbation is sourced by light scalar fields other than the inflaton. We exploit the operator product expansion and partly the symmetries present during the de Sitter epoch to characterize the non-Gaussian four-point correlator in the squeezed limit. We point out that the contribution to it from the intrinsic non-Gaussianity of the light fields at horizon crossing can be larger than the usually studied contribution arising on superhorizon scales and it comes with a different shape. Our findings indicate that particular attention needs to be taken when studying the effects of the primordial NG on real observables, such as the clustering of dark matter halos. © 2012 Elsevier B.V.

Kehagias A.,Center for Astroparticle Physics | Kehagias A.,National Technical University of Athens | Moradinezhad Dizgah A.,Center for Astroparticle Physics | Riotto A.,Center for Astroparticle Physics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

We point out that the ability of some models of inflation, such as the Higgs inflation and the universal attractor models at large values of the coupling ξ, in reproducing the available data is due to their relation to the Starobinsky model of inflation. For large field values, where the inflationary phase takes place, all of these classes of models are indeed identical to the Starobinsky model. Nevertheless, the inflation is just an auxiliary field in the Jordan frame of the Starobinsky model, and this leads to two important consequences: first, the inflationary predictions of the Starobinsky model and its descendants are slightly different (albeit not measurably); second, the theories have different small-field behavior, leading to different ultraviolet cutoff scales. In particular, one interesting descendant of the Starobinsky model is the nonminimally coupled quadratic chaotic inflation. Although the standard quadratic chaotic inflation is ruled out by the recent Planck data, its nonminimally coupled version is in agreement with observational data and valid up to Planckian scales. © 2014 American Physical Society.

De Simone A.,International School for Advanced Studies | De Simone A.,National Institute of Nuclear Physics, Italy | Riotto A.,Center for Astroparticle Physics
Journal of Cosmology and Astroparticle Physics | Year: 2013

We propose that the Standard Model (SM) Higgs is responsible for generating the cosmological perturbations of the universe by acting as an isocurvature mode during a de Sitter inflationary stage. In view of the recent ATLAS and CMS results for the Higgs mass, this can happen if the Hubble rate during inflation is in the range (1010-1014) GeV (depending on the SM parameters). Implications for the detection of primordial tensor perturbations through the B-mode of CMB polarization via the PLANCK satellite are discussed. For example, if the Higgs mass value is confirmed to be mh = 125.5 GeV and mt,αs are at their central values, our mechanism predicts tensor perturbations too small to be detected in the near future. On the other hand, if tensor perturbations will be detected by PLANCK through the B-mode of CMB, then there is a definite relation between the Higgs and top masses, making the mechanism predictive and falsifiable. © 2013 IOP Publishing Ltd and Sissa Medialab srl.

Kehagias A.,National Technical University of Athens | Kehagias A.,Center for Astroparticle Physics | Riotto A.,Center for Astroparticle Physics
Nuclear Physics B | Year: 2014

We show that any accelerating Friedmann-Robertson-Walker (FRW) cosmology with equation of state w<-1/3 (and therefore not only a de Sitter stage with w=-1) exhibits three-dimensional conformal symmetry on future constant-time hypersurfaces if the bulk theory is invariant under bulk conformal Killing vectors. We also offer an alternative derivation of this result in terms of conformal Killing vectors and show that long wavelength comoving curvature perturbations of the perturbed FRW metric are just conformal Killing motions of the FRW background. We then extend the boundary conformal symmetry to the bulk for accelerating cosmologies. Our findings indicate that one can easily generate perturbations of scalar fields which are not only scale invariant, but also fully conformally invariant on super-Hubble scales. Measuring a scale-invariant power spectrum for the cosmological perturbation does not automatically imply that the universe went through a de Sitter stage. © 2014 The Authors.

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