arXiv subjects

Antonio Riotto

Publications and source records attributed to Antonio Riotto.

At least 127 records · Page 7Linked to original sources

arXiv2012

Testing Multi-Field Inflation with Galaxy Bias

Multi-field models of inflation predict an inequality between the amplitude tauNL of the collapsed limit of the four-point correlator of the primordial curvature perturbation and the amplitude fNL of the squeezed limit of its three-point correlator. While a convincing detection of non-Gaussianity through the squeezed limit of the three-point correlator would rule out all single-field models, a robust confirmation or disproval of the inequality between tauNL and fNL would provide crucial information about the validity of multi-field models of inflation. In this paper, we discuss to which extent future measurements of the scale-dependence of galaxy bias can test multi-field inflationary scenarios. The strong degeneracy between the effect of a non-vanishing fNL and tauNL on halo bias can be broken by considering multiple tracer populations of the same surveyed volume. If halos down to 1e13 Msun/h are resolved in a survey of volume 25(Gpc/h)^3, then testing multi-field models of inflation at the 3-σlevel would require, for instance, a detection of tauNL at the level of tauNL~1e5 given a measurement of a local bispectrum with amplitude fNL~10. However, we find that disproving multi-field models of inflation with measurements of the non-Gaussian bias only will be very challenging, unless |fNL| > 80 and one can achieve a halo mass resolution of 1e10 Msun/h.

astro-ph.CO↗

arXiv2012

Electroweak bremsstrahlung for wino-like Dark Matter annihilations

If the Dark Matter is the neutral Majorana component of a multiplet which is charged under the electroweak interactions of the Standard Model, its main annihilation channel is into W+W-, while the annihilation into light fermions is helicity suppressed. As pointed out recently, the radiation of gauge bosons from the initial state of the annihilation lifts the suppression and opens up an s-wave contribution to the cross section. We perform the full tree-level calculation of Dark Matter annihilations, including electroweak bremsstrahlung, in the context of an explicit model corresponding to the supersymmetric wino. We find that the fermion channel can become as important as the di-boson one. This result has significant implications for the predictions of the fluxes of particles originating from Dark Matter annihilations.

hep-ph↗

arXiv2012

Inflaton or Curvaton? Constraints on Bimodal Primordial Spectra from Mixed Perturbations

We consider Cosmic Microwave Background constraints on inflation models for which the primordial power spectrum is a mixture of perturbations generated by inflaton fluctuations and fluctuations in a curvaton field. If future experiments do not detect isocurvature modes or large non-Gaussianity, it will not be possible to directly distinguish inflaton and curvaton contributions. We investigate whether current and future data can instead constrain the relative contributions of the two sources. We model the spectrum with a bimodal form consisting of a sum of two independent power laws, with different spectral indices. We quantify the ability of current and upcoming data sets to constrain the difference $Δn$ in spectral indices, and relative fraction $f$ of the subdominant power spectrum at a pivot scale of $k = 0.017\ {\rm Mpc^{-1}\ h}$. Data sets selected are the WMAP 7-year data, alone and in conjunction with South Pole Telescope data, and a synthetic data set comparable to the upcoming Planck data set. We find that current data show no increase in quality of fit for a mixed inflaton/curvaton power spectrum, and a pure power-law spectrum is favored. The ability to constrain independent parameters such as the tensor/scalar ratio is not substantially affected by the additional parameters in the fit. Planck will be capable of placing significant constraints on the parameter space for a bimodal spectrum.

astro-ph.CO↗

arXiv2012

Asymmetric Higgsino Dark Matter

In the supersymmetric framework, a higgsino asymmetry exists in the universe before the electroweak phase transition. We investigate whether the higgsino is a viable asymmetric dark matter candidate. We find that this is indeed possible. The gauginos, squarks and sleptons must all be very heavy, such that the only electroweak-scale superpartners are the higgsinos. The temperature of the electroweak phase transition must be in the (1-10) GeV range.

hep-ph↗

arXiv2011

Higgs mass implications on the stability of the electroweak vacuum

We update instability and metastability bounds of the Standard Model electroweak vacuum in view of the recent ATLAS and CMS Higgs results. For a Higgs mass in the range 124--126 GeV, and for the current central values of the top mass and strong coupling constant, the Higgs potential develops an instability around $10^{11}$ GeV, with a lifetime much longer than the age of the Universe. However, taking into account theoretical and experimental errors, stability up to the Planck scale cannot be excluded. Stability at finite temperature implies an upper bound on the reheat temperature after inflation, which depends critically on the precise values of the Higgs and top masses. A Higgs mass in the range 124--126 GeV is compatible with very high values of the reheating temperature, without conflict with mechanisms of baryogenesis such as leptogenesis. We derive an upper bound on the mass of heavy right-handed neutrinos by requiring that their Yukawa couplings do not destabilize the Higgs potential.

hep-ph↗

arXiv2011

Magnetic Fields at First Order Phase Transition: A Threat to Electroweak Baryogenesis

The generation of the observed baryon asymmetry may have taken place during the electroweak phase transition, thus involving physics testable at LHC, a scenario dubbed electroweak baryogenesis. In this paper we point out that the magnetic field which is produced in the bubbles of a first order phase transition endangers the baryon asymmetry produced in the bubble walls. The reason being that the produced magnetic field couples to the sphaleron magnetic moment and lowers the sphaleron energy; this strengthens the sphaleron transitions inside the bubbles and triggers a more effective wash out of the baryon asymmetry. We apply this scenario to the Minimal Supersymmetric extension of the Standard Model (MSSM) where, in the absence of a magnetic field, successful electroweak baryogenesis requires the lightest CP-even Higgs and the right-handed stop masses to be lighter than about 127 GeV and 120 GeV, respectively. We show that even for moderate values of the magnetic field, the Higgs mass required to preserve the baryon asymmetry is below the present experimental bound. As a consequence electroweak baryogenesis within the MSSM should be confronted on the one hand to future measurements at the LHC on the Higgs and the right-handed stop masses, and on the other hand to more precise calculations of the magnetic field produced at the electroweak phase transition.

hep-ph↗

arXiv2011

Conditional Probabilities in the Excursion Set Theory. Generic Barriers and non-Gaussian Initial Conditions

The excursion set theory, where density perturbations evolve stochastically with the smoothing scale, provides a method for computing the dark matter halo mass function. The computation of the mass function is mapped into the so-called first-passage time problem in the presence of a moving barrier. The excursion set theory is also a powerful formalism to study other properties of dark matter halos such as halo bias, accretion rate, formation time, merging rate and the formation history of halos. This is achieved by computing conditional probabilities with non-trivial initial conditions, and the conditional two-barrier first-crossing rate. In this paper we use the path integral formulation of the excursion set theory to calculate analytically these conditional probabilities in the presence of a generic moving barrier, including the one describing the ellipsoidal collapse, and for both Gaussian and non-Gaussian initial conditions. While most of our analysis associated with Gaussian initial conditions assumes Markovianity (top-hat in momentum space smoothing, rather than generic filters), the non-Markovianity of the random walks induced by non-Gaussianity is consistently accounted for. We compute, for a generic barrier, the first two scale-independent halo bias parameters, the conditional mass function and the halo formation time probability, including the effects of non-Gaussianities. We also provide the expression for the two-constant-barrier first-crossing rate when non-Markovian effects are induced by a top-hat filter function in real space.

astro-ph.CO↗

arXiv2011

Excursion Set Theory for generic moving barriers and non-Gaussian initial conditions

Excursion set theory, where density perturbations evolve stochastically with the smoothing scale, provides a method for computing the mass function of cosmological structures like dark matter halos, sheets and filaments. The computation of these mass functions is mapped into the so-called first-passage time problem in the presence of a moving barrier. In this paper we use the path integral formulation of the excursion set theory developed recently to analytically solve the first-passage time problem in the presence of a generic moving barrier, in particular the barrier corresponding to ellipsoidal collapse. We perform the computation for both Gaussian and non-Gaussian initial conditions. The expression of the halo mass function for the ellipsoidal collapse barrier and with non-Gaussianity is therefore obtained in a fully consistent way and it does not require the introduction of any form factor artificially derived from the Press-Schechter formalism based on the spherical collapse and usually adopted in the literature.

astro-ph.CO↗

arXiv2011

Initial State Radiation in Majorana Dark Matter Annihilations

The cross section for a Majorana Dark Matter particle annihilating into light fermions is helicity suppressed. We show that, if the Dark Matter is the neutral Majorana component of a multiplet which is charged under the electroweak interactions of the Standard Model, the emission of gauge bosons from the initial state lifts the suppression and allows an s-wave annihilation. The resulting energy spectra of stable Standard Model particles are importantly affected. This has an impact on indirect searches for Dark Matter.

hep-ph↗

arXiv2011

On the Importance of Electroweak Corrections for Majorana Dark Matter Indirect Detection

Recent analyses have shown that the inclusion of electroweak corrections can alter significantly the energy spectra of Standard Model particles originated from dark matter annihilations. We investigate the important situation where the radiation of electroweak gauge bosons has a substantial influence: a Majorana dark matter particle annihilating into two light fermions. This process is in p-wave and hence suppressed by the small value of the relative velocity of the annihilating particles. The inclusion of electroweak radiation eludes this suppression and opens up a potentially sizeable s-wave contribution to the annihilation cross section. We study this effect in detail and explore its impact on the fluxes of stable particles resulting from the dark matter annihilations, which are relevant for dark matter indirect searches. We also discuss the effective field theory approach, pointing out that the opening of the s-wave is missed at the level of dimension-six operators and only encoded by higher orders.

hep-ph↗

arXiv2011

Evolution Equation for Non-linear Cosmological Perturbations

We present a novel approach, based entirely on the gravitational potential, for studying the evolution of non-linear cosmological matter perturbations. Starting from the perturbed Einstein equations, we integrate out the non-relativistic degrees of freedom of the cosmic fluid and obtain a single closed equation for the gravitational potential. We then verify the validity of the new equation by comparing its approximate solutions to known results in the theory of non-linear cosmological perturbations. First, we show explicitly that the perturbative solution of our equation matches the standard perturbative solutions. Next, using the mean field approximation to the equation, we show that its solution reproduces in a simple way the exponential suppression of the non-linear propagator on small scales due to the velocity dispersion. Our approach can therefore reproduce the main features of the renormalized perturbation theory and (time)-renormalization group approaches to the study of non-linear cosmological perturbations. We conclude by a preliminary discussion of the nature of the full solutions of the equation and their significance.

astro-ph.CO↗

arXiv2011

Perturbation Theory of the Cosmological Log-Density Field

The matter density field exhibits a nearly lognormal probability density distribution (PDF) after entering into the nonlinear regime. Recently, it has been shown that the shape of the power spectrum of a logarithmically transformed density field is very close to the linear density power spectrum, motivating an analytic study of it. In this paper, we develop cosmological perturbation theory for the power spectrum of this field. Our formalism is developed in the context of renormalized perturbation theory, which helps to regulate the convergence behavior of the perturbation series, and of the Taylor- series expansion we use of the logarithmic mapping. This approach allows us to handle the critical issue of density smoothing in a straightforward way. We also compare our perturbative results with simulation measurements.

astro-ph.CO↗

arXiv2011

Testing SO(10)-inspired leptogenesis with low energy neutrino experiments

We extend the results of a previous analysis of ours showing that, when both heavy and light flavour effects are taken into account, successful minimal (type I + thermal) leptogenesis with SO(10)-inspired relations is possible. Barring fine tuned choices of the parameters, these relations enforce a hierarchical RH neutrino mass spectrum that results into a final asymmetry dominantly produced by the next-to-lightest RH neutrino decays (N_2 dominated leptogenesis). We present the constraints on the whole set of low energy neutrino parameters. Allowing a small misalignment between the Dirac basis and the charged lepton basis as in the quark sector, the allowed regions enlarge and the lower bound on the reheating temperature gets relaxed to values as low as ~ 10^10 GeV. It is confirmed that for normal ordering (NO) there are two allowed ranges of values for the lightest neutrino mass: m_1 \simeq (1-5)\times 10^-3 eV and m_1\simeq (0.03-0.1) eV. For m_1\lesssim 0.01 eV the allowed region in the plane theta_13-theta_23 is approximately bounded by 44 + 3(theta_13-9) \lesssim theta_23 \lesssim 48 + 0.4 (theta_13-5), while the neutrinoless double beta decay effective neutrino mass falls in the range m_ee=(1-3)10^-3 eV for theta_13=(6 -11.5). For m_1 \gtrsim 0.01 eV, one has quite sharply m_ee\simeq m_1 and an upper bound theta_23 \lesssim 46 . These constraints will be tested by low energy neutrino experiments during next years. We also find that inverted ordering (IO), though quite strongly constrained, is not completely ruled out. In particular, we find approximately theta_23 \simeq 43 +12 log(0.2 eV/m_1), that will be fully tested by future experiments.

hep-ph↗

arXiv2011

The Kramers-Moyal Equation of the Cosmological Comoving Curvature Perturbation

Fluctuations of the comoving curvature perturbation with wavelengths larger than the horizon length are governed by a Langevin equation whose stochastic noise arise from the quantum fluctuations that are assumed to become classical at horizon crossing. The infrared part of the curvature perturbation performs a random walk under the action of the stochastic noise and, at the same time, it suffers a classical force caused by its self-interaction. By a path-interal approach and, alternatively, by the standard procedure in random walk analysis of adiabatic elimination of fast variables, we derive the corresponding Kramers-Moyal equation which describes how the probability distribution of the comoving curvature perturbation at a given spatial point evolves in time and is a generalization of the Fokker-Planck equation. This approach offers an alternative way to study the late time behaviour of the correlators of the curvature perturbation from infrared effects.

astro-ph.CO↗

arXiv2010

The Bias and Mass Function of Dark Matter Halos in Non-Markovian Extension of the Excursion Set Theory

The excursion set theory based on spherical or ellipsoidal gravitational collapse provides an elegant analytic framework for calculating the mass function and the large-scale bias of dark matter haloes. This theory assumes that the perturbed density field evolves stochastically with the smoothing scale and exhibits Markovian random walks in the presence of a density barrier. Here we derive an analytic expression for the halo bias in a new theoretical model that incorporates non-Markovian extension of the excursion set theory with a stochastic barrier. This model allows us to handle non-Markovian random walks and to calculate perturbativly these corrections to the standard Markovian predictions for the halo mass function and halo bias. Our model contains only two parameters: kappa, which parameterizes the degree of non-Markovianity and whose exact value depends on the shape of the filter function used to smooth the density field, and a, which parameterizes the degree of stochasticity of the barrier. Appropriate choices of kappa and a in our new model can lead to a closer match to both the halo mass function and halo bias in the latest N-body simulations than the standard excursion set theory.

astro-ph.CO↗

arXiv2010

Tilt and Running of Cosmological Observables in Generalized Single-Field Inflation

Employing an effective field theory approach to inflationary perturbations, we analyze in detail the effect of curvature-generated Lagrangian operators on various observables, focusing on their running with scales. At quadratic order, we solve the equation of motion at next-to-leading leading order in a generalized slow-roll approximation for a very general theory of single-field inflation. We derive the resulting power spectrum, its tilt and running. We then focus on the contribution to the primordial non-Gaussianity amplitude f_{NL} sourced by a specific interaction term. We show that the running of f_{NL} can be substantially larger than what dictated by the slow-roll parameters.

astro-ph.CO↗

arXiv2010

Strongly Scale-dependent Non-Gaussianity

We discuss models of primordial density perturbations where the non-Gaussianity is strongly scale-dependent. In particular, the non-Gaussianity may have a sharp cut-off and be very suppressed on large cosmological scales, but sizeable on small scales. This may have an impact on probes of non-Gaussianity in the large-scale structure and in the cosmic microwave background radiation anisotropies.

astro-ph.CO↗

arXiv2010

Weak Corrections are Relevant for Dark Matter Indirect Detection

The computation of the energy spectra of Standard Model particles originated from the annihilation/decay of dark matter particles is of primary importance in indirect searches of dark matter. We compute how the inclusion of electroweak corrections significantly alter such spectra when the mass M of dark matter particles is larger than the electroweak scale: soft electroweak gauge bosons are copiously radiated opening new channels in the final states which otherwise would be forbidden if such corrections are neglected. All stable particles are therefore present in the final spectrum, independently of the primary channel of dark matter annihilation/decay. Such corrections are model independent.

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