arXiv subjects

Alexey Anisimov

Publications and source records attributed to Alexey Anisimov.

14 recordsLinked to original sources

arXiv2011

Thermal production of relativistic Majorana neutrinos: Strong enhancement by multiple soft scattering

The production rate of heavy Majorana neutrinos is relevant for models of thermal leptogenesis in the early Universe. In the high temperature limit the production can proceed via the 1 <-> 2 (inverse) decays which are allowed by the thermal masses. We consider new production mechanisms which are obtained by including additional soft gauge interactions with the plasma. We show that an arbitrary number of such interactions gives leading order contributions, and we sum all of them. The rate turns out to be smooth in the region where the 1 <-> 2 processes are kinematically forbidden. At higher temperature it is enhanced by a factor 3 compared to the 1 <-> 2 rate.

hep-ph↗

arXiv2009

Cold Dark Matter from heavy Right-Handed neutrino mixing

We show that, within the see-saw mechanism, an almost decoupled RH neutrino species N_DM with mass M_DM \gtrsim 100 GeV can play the role of Dark Matter (DM). The N_DM's can be produced from non-adiabatic conversions of thermalized (source) RH neutrinos with mass M_S lower than M_DM. This is possible if a non-renormalizable operator is added to the minimal type I see-saw lagrangian. The observed DM abundance can be reproduced for M_DM δ^{1/4} \sim 10^-13 Λ_eff ξ, where Λ_eff is a very high energy new physics scale, δ\equiv (M_DM-M_S)/M_DM and ξ\lesssim 1 is a parameter determined by the RH neutrino couplings.

hep-ph↗

arXiv2008

Inflaton mass in the nuMSM inflation

We analyse the reheating in the modification of \nuMSM (Standard Model with three right handed neutrinos with masses below the electroweak scale) where the sterile neutrino providing the Dark Matter is generated in decays of the additional inflaton field. We deduce that due to rather inefficient transfer of energy from the inflaton to the Standard Model sector reheating tends to happen at very low temperature, thus providing strict bounds on the coupling between the inflaton and the Higgs particles. This in turn translates to the bound on the inflaton mass, which appears to be very light 0.1 GeV <~ m_I <~ 10 GeV, or slightly heavier then two Higgs masses 300 GeV <~ m_I <~ 1000 GeV.

hep-ph↗

arXiv2008

Viability of Dirac phase leptogenesis

We discuss the conditions for a non-vanishing Dirac phase δand mixing angle θ_{13}, sources of CP violation in neutrino oscillations, to be uniquely responsible for the observed matter-antimatter asymmetry of the universe through leptogenesis. We show that this scenario, that we call δ-leptogenesis, is viable when the degenerate limit (DL) for the heavy right-handed (RH) neutrino spectrum is considered. We derive an interesting joint condition on \sinθ_{13} and the absolute neutrino mass scale that can be tested in future neutrino oscillation experiments. In the limit of hierarchical heavy RH neutrino spectrum (HL), we strengthen the previous result that δ-leptogenesis is only very marginally allowed, even when the production from the two heavier RH neutrinos is taken into account. An improved experimental upper bound on \sinθ_{13} and (or) an account of quantum kinetic effects could completely rule out this option in the future. Therefore, δ-leptogenesis can be also regarded as a motivation for models with degenerate heavy neutrino spectrum.

hep-ph↗

arXiv2007

Majorana Dark Matter

In this letter in the framework of a simple see-saw scenario with two (or three) quasi degenerate Majorana neutrinos we propose that one of these neutrinos can be very weakly coupled, yet there is a mechanism of the generation of the abundance of such "dark" neutrino in the early universe. The mechanism of production is due to oscillations between "dark" Majorana neutrino and one of the Majorana's which has relatively large Yukawa couplings ("bright" Majorana neutrino). The transition of "bright" Majorana into a "dark" one is nonadiabatic. We point out on the similarity with the Landau-Zener transition regime. In our model one can explain the observed dark matter density, present matter-antimatter asymmetry and active neutrino data all at the same time

hep-ph↗

arXiv2005

B-Inflation

We propose a novel model of inflation based on a large class of covariant effective actions containing only the second derivatives of a scalar field. The initial conditions leading to the inflationary solutions in this model are rather generic. The graceful exit from the inflationary regime is natural once the first order derivative terms are included.

astro-ph↗

arXiv2005

The Supersymmetric Axion and Cosmology

In this lecture we review several cosmological issues associated with the axion. Axion solves the strong CP problem and is a good candidate for the dark matter. Limits, which are imposed by the value of isocurvature fluctuations fraction in the observed CMBR from WMAP data and the domain wall problem are discussed in a supersymmetric flat direction axion model.

hep-ph↗

arXiv2005

The Classical Stability Of The Ghost Condensate

We discuss classical stability of the cosmological mechanism which is responsible for the ghost condensation. We show that the simplest general covariant effective action which includes two covariant derivatives may lead to an overshoot of the condensation point into the regime, where the model becomes classically unstable. The model exhibits this behaviour for the de Sitter and matter dominated universes when the initial values taken to be in the region of validity of the low-energy effective theory. In the matter dominated case there is a finite time (which can be large enough to exceed the present age of the Universe) during which the model remains in the stable region. In inflationary regime the system does not experience an overshoot only if the Hubble parameter is larger than the characteristic energy scale of the effective field theory (EFT). This latter regime is, however, inconsistent with the EFT description at the condensation point. We discuss the limitations of the use of the truncated action and point out how one can avoid problems due to the overshoot in order to have a viable ghost condensation mechanism.

hep-ph↗

arXiv2004

Is There a Peccei-Quinn Phase Transition?

The nature of axion cosmology is usually said to depend on whether the Peccei-Quinn (PQ) symmetry breaks before or after inflation. The PQ symmetry itself is believed to be an accident, so there is not necessarily a symmetry during inflation at all. We explore these issues in some simple models, which provide examples of symmetry breaking before and after inflation, or in which there is no symmetry during inflation and no phase transition at all. One effect of these observations is to relax the constraints from isocurvature fluctuations due to the axion during inflation. We also observe new possibilities for evading the constraints due to cosmic strings and domain walls, but they seem less generic.

hep-ph↗

arXiv2003

Thermal Effects And Flat Direction Baryogenesis

In this paper we provide a detailed numerical study of the influence of thermal effects on the original picture of AD baryogenesis. These effects are found to modify the results greatly in some cases. We estimate the baryon/entropy ratio and provide numerical results on the typical behavior of the charge as a function of the strength of the potential and other parameters.

hep-ph↗

arXiv2002

Brane World Susy Breaking from String/M Theory

String and M-theory realizations of brane world supersymmetry breaking scenarios are considered in which visible sector Standard Model fields are confined on a brane, with hidden sector supersymmetry breaking isolated on a distant brane. In calculable examples with an internal manifold of any volume the Kahler potential generically contains brane--brane non-derivative contact interactions coupling the visible and hidden sectors and is not of the no-scale sequestered form. This leads to non-universal scalar masses and without additional assumptions about flavor symmetries may in general induce dangerous sflavor violation even though the Standard Model and supersymmetry branes are physically separated. Deviations from the sequestered form are dictated by bulk supersymmetry and can in most cases be understood as arising from exchange of bulk supergravity fields between branes or warping of the internal geometry. Unacceptable visible sector tree-level tachyons arise in many models but may be avoided in certain classes of compactifications. Anomaly mediated and gaugino mediated contributions to scalar masses are sub-dominant except in special circumstances such as a flat or AdS pure five--dimensional bulk geometry without bulk vector multiplets.

hep-th↗

arXiv2001

Comments on Non-Commutative Phenomenology

It is natural to ask whether non-commutative geometry plays a role in four dimensional physics. By performing explicit computations in various toy models, we show that quantum effects lead to violations of Lorentz invariance at the level of operators of dimension three or four. The resulting constraints are very stringent.

hep-ph↗

arXiv2001

Brane World Susy Breaking

In brane world models of nature, supersymmetry breaking is often isolated on a distant brane in a higher dimensional space. The form of the Kahler potential in generic string and M-theory brane world backgrounds is shown to give rise to tree-level non-universal squark and slepton masses. This results from the exchange of bulk supergravity fields and warping of the internal geometry. This is contrary to the notion that bulk locality gives rise to a sequestered no-scale form of the Kahler potential with vanishing tree-level masses and solves the supersymmetric flavor problem. As a result, a radiatively generated anomaly mediated superpartner spectrum is not a generic outcome of these theories.

hep-th↗

arXiv2001

Some Issues in Flat Direction Baryogenesis

Motivated by recent developments, we explore some issues in Affleck-Dine baryogenesis. We consider in greater detail the role of thermal effects in the production of baryon number. We find that these effects are important even for rather flat potentials, and obtain somewhat different estimates of the baryon asymmetry than those in the literature. We also consider the decay of the condensate, and possible implications of these observations for the formation of Q-balls.

hep-ph↗