arXiv subjects

A. Kartavtsev

Publications and source records attributed to A. Kartavtsev.

18 recordsLinked to original sources

arXiv2020

Accounting for the Heisenberg and Pauli principles in the kinetic approach to neutrino oscillations

While oscillations of solar neutrinos are usually studied using the single-particle quantum-mechanical approach, flavor conversions of supernovae neutrinos are typically analyzed using the kinetic equation for the matrix of densities due to the necessity of including also the scattering processes. Using the Wigner formulation of quantum mechanics we show the equivalence of the quantum-mechanical and kinetic approaches in the limit of collisionless neutrino propagation (in a background medium). Based on this observation we also argue that solutions of the kinetic equation account for the Heisenberg uncertainty principle and the related effect of wave packet separation (for single neutrinos), as well as the Pauli exclusion principle, if the initial conditions are consistent with these fundamental quantum principles. Such initial conditions can be constructed e.g. by identifying the matrix of densities with the (reduced) single-particle Wigner function computed using initial conditions for the neutrino wave function. Hence the neutrino momentum uncertainty is an integral part of the initial conditions for the matrix of densities, that may have an impact on the phenomenology of supernovae neutrinos via the effect of wave packet separation.

hep-ph↗

arXiv2017

Extragalactic photon-ALP conversion at CTA energies

Magnetic fields in extragalactic space between galaxy clusters may induce conversions between photons and axion-like particles (ALPs), thereby shielding the photons from absorption on the extragalactic background light. For TeV gamma rays, the oscillation length ($l_{\rm osc}$) of the photon-ALP system becomes inevitably of the same order as the coherence length of the magnetic field ($l$) and the length over which the field changes significantly (transition length $l_{\rm t}$) due to refraction on background photons. We derive exact statistical evolution equations for the mean and variance of the photon and ALP transfer functions in the non-adiabatic regime ($l_{\rm osc} \sim l \gg l_{\rm t}$). We also make analytical predictions for the transfer functions in the quasi-adiabatic regime ($l_{\rm osc} \ll l, l_{\rm t}$). Our results are important in light of the upcoming Cherenkov Telescope Array (CTA), and may also be applied to models with non-zero ALP masses.

astro-ph.HE↗

arXiv2015

Neutrino propagation in media: Flavor-, helicity-, and pair correlations

Neutrinos propagating in media (matter and electromagnetic fields) undergo flavor and helicity oscillations, where helicity transitions are instigated both by electromagnetic fields and matter currents. In addition, it has been shown that correlations between neutrinos and antineutrinos of opposite momentum can build up in anisotropic media. We re-derive the neutrino equations of motion in the mean-field approximation for homogeneous yet anisotropic media, confirming previous results except for a small correction in the Majorana case. Furthermore, we derive the mean-field Hamiltonian induced by neutrino electromagnetic interactions. We also provide a phenomenological discussion of pair correlations in comparison with helicity correlations.

hep-ph↗

arXiv2014

Basis invariant measure of CP-violation and renormalization

We analyze, in the context of a simple toy model, for which renormalization schemes the CP-properties of bare Lagrangian and its finite part coincide. We show that this is the case for the minimal subtraction and on-shell schemes. The CP-properties of the theory can then be characterized by CP-odd basis invariants expressed in terms of renormalized masses and couplings. For the minimal subtraction scheme we furthermore show that in CP-conserving theories the CP-odd basis invariants are zero at any scale but are not renormalization group invariant in CP-violating ones.

hep-ph↗

arXiv2014

Leptogenesis in crossing and runaway regimes

We study the impact of effective thermal masses and widths on resonant leptogenesis. We identify two distinct possibilities which we refer to as crossing and runaway regimes. In the runaway regime the mass difference grows monotonously with temperature, whereas it initially decreases in the crossing regime, such that the effective masses become equal at some temperature. Following the conventional logic the source of the asymmetry would vanish in the latter case. Using non-equilibrium quantum field theory, we analytically demonstrate that the vanishing of the difference of the effective masses does however neither imply a suppression nor a strong enhancement of the source for the lepton asymmetry. In the vicinity of the crossing point the asymmetry calculated in an (improved) Boltzmann limit develops a spurious peak, which signals the breakdown of the quasiparticle approximation. In the exact result this spurious enhancement is compensated by coherent transitions between the two mass shells. Despite the breakdown of the quasiparticle approximation off-shell contributions remain negligibly small even at the crossing point.

hep-ph↗

arXiv2013

Leptogenesis in models with keV sterile neutrino dark matter

We analyze leptogenesis in gauge extensions of the Standard Model with keV sterile neutrino dark matter. We find that both the observed dark matter abundance and the correct baryon asymmetry of the Universe can simultaneously emerge in these models. Both the dark matter abundance and the leptogenesis are controlled by the out of equilibrium decays of the same heavy right handed neutrino.

hep-ph↗

arXiv2013

Systematic approach to Delta L=1 processes in thermal leptogenesis

In this work we study the contribution to leptogenesis from Delta L=1 decay and scattering processes mediated by the Higgs with quarks in the initial and final states using the formalism of non-equilibrium quantum field theory. Starting from fundamental equations for correlators of the quantum fields we derive quantum-corrected Boltzmann and rate equations for the total lepton asymmetry improved in that they include quantum-statistical effects and medium corrections to the quasiparticle properties. To compute the collision term we take into account one- and two-loop contributions to the lepton self-energy and use the extended quasiparticle approximation for the Higgs two-point function. The resulting CP-violating and washout reaction densities are numerically compared to the conventional ones.

hep-ph↗

arXiv2012

Systematic approach to thermal leptogenesis

In this work we study thermal leptogenesis using non-equilibrium quantum field theory. Starting from fundamental equations for correlators of the quantum fields we describe the steps necessary to obtain quantum kinetic equations for quasiparticles. These can easily be compared to conventional results and overcome conceptional problems inherent in the canonical approach. Beyond CP-violating decays we include also those scattering processes which are tightly related to the decays in a consistent approximation of fourth order in the Yukawa couplings. It is demonstrated explicitly how the S-matrix elements for the scattering processes in the conventional approach are related to two- and three-loop contributions to the effective action. We derive effective decay and scattering amplitudes taking medium corrections and thermal masses into account. In this context we also investigate CP-violating Higgs decay within the same formalism. From the kinetic equations we derive rate equations for the lepton asymmetry improved in that they include quantum-statistical effects and medium corrections to the quasiparticle properties.

hep-ph↗

arXiv2011

A remark on the Koide relation for quarks

The charged lepton masses obey to high precision the so-called Koide relation. We propose a generalization of this relation to quarks. It includes up and down quarks of the three generations and is numerically reasonably close to the Koide limit.

hep-ph↗

arXiv2010

Quantum corrections to leptogenesis from the gradient expansion

Using the closed-time-path formalism we quantify gradient corrections to the kinetic equations for leptogenesis, that are neglected in the standard Boltzmann approach. In particular we show that an additional CP-violating source term arises, which is non-zero even when all species are in local thermal equilibrium. In the early universe it is proportional to the expansion rate and would vanish for static equilibrium configurations, in accordance with the Sakharov conditions. We find that for thermal leptogenesis in a standard cosmological background the additional source term is small. However, it can become the dominant source in the limit of ultra-strong washout.

hep-ph↗

arXiv2010

Medium corrections to the CP-violating parameter in leptogenesis

In two recent papers, arXiv:0909.1559 and arXiv:0911.4122, it has been demonstrated that one can obtain quantum corrected Boltzmann kinetic equations for leptogenesis using a top-down approach based on the Schwinger-Keldysh/Kadanoff-Baym formalism. These "Boltzmann-like" equations are similar to the ones obtained in the conventional bottom-up approach but differ in important details. In particular there is a discrepancy between the CP-violating parameter obtained in the first-principle derivation and in the framework of thermal field theory. Here we demonstrate that the two approaches can be reconciled if causal n-point functions are used in the thermal field theory approach. The new result for the medium correction to the CP-violating parameter is qualitatively different from the conventional one. The analogy to a toy model considered earlier enables us to write down consistent quantum corrected Boltzmann equations for thermal leptogenesis in the Standard Model (supplemented by three right-handed neutrinos) which include quantum statistical terms and medium corrected expressions for the CP-violating parameter.

hep-ph↗

arXiv2010

Systematic approach to leptogenesis in nonequilibrium QFT: self-energy contribution to the CP-violating parameter

In the baryogenesis via leptogenesis scenario the self-energy contribution to the CP-violating parameter plays a very important role. Here, we calculate it in a simple toy model of leptogenesis using the Schwinger-Keldysh/Kadanoff-Baym formalism as starting point. We show that the formalism is free of the double-counting problem typical for the canonical Boltzmann approach. Within the toy model, medium effects increase the CP-violating parameter. In contrast to results obtained earlier in the framework of thermal field theory, the medium corrections are linear in the particle number densities. In the resonant regime quantum corrections lead to modified expressions for the CP-violating parameter and for the decay width. Most notably, in the maximal resonant regime the Boltzmann picture breaks down and an analysis in the full Kadanoff-Baym formalism is required.

hep-ph↗

arXiv2010

Systematic approach to leptogenesis in nonequilibrium QFT: vertex contribution to the CP-violating parameter

The generation of a baryon asymmetry via leptogenesis is usually studied by means of classical kinetic equations whose applicability to processes in the hot and expanding early universe is questionable. The approximations implied by the state-of-the-art description can be tested in a first-principle approach based on nonequilibrium field theory techniques. Here, we apply the Schwinger-Keldysh/Kadanoff-Baym formalism to a simple toy model of leptogenesis. We find that, within the toy model, medium effects increase the vertex contribution to the CP-violating parameter. At high temperatures it is a few times larger than in vacuum and asymptotically reaches the vacuum value as the temperature decreases. Contrary to the results obtained earlier in the framework of thermal field theory, the corrections are only linear in the particle number densities. An important feature of the Kadanoff-Baym formalism is that it is free of the double-counting problem, i.e. no need for real intermediate state subtraction arises. In particular, this means that the structure of the equations automatically ensures that the asymmetry vanishes in equilibrium. These results give a first glimpse into a number of new and interesting effects that can be studied in the framework of nonequilibrium field theory.

hep-ph↗

arXiv2009

Deriving Boltzmann Equations from Kadanoff-Baym Equations in Curved Space-Time

To calculate the baryon asymmetry in the baryogenesis via leptogenesis scenario one usually uses Boltzmann equations with transition amplitudes computed in vacuum. However, the hot and dense medium and, potentially, the expansion of the universe can affect the collision terms and hence the generated asymmetry. In this paper we derive the Boltzmann equation in the curved space-time from (first-principle) Kadanoff-Baym equations. As one expects from general considerations, the derived equations are covariant generalizations of the corresponding equations in Minkowski space-time. We find that, after the necessary approximations have been performed, only the left-hand side of the Boltzmann equation depends on the space-time metric. The amplitudes in the collision term on the right--hand side are independent of the metric, which justifies earlier calculations where this has been assumed implicitly. At tree level, the matrix elements coincide with those computed in vacuum. However, the loop contributions involve additional integrals over the the distribution function.

hep-ph↗

arXiv2008

Baryogenesis via Leptogenesis in an inhomogeneous universe

We investigate the influence of primordial perturbations of the energy density and space--time metric on the generation of the lepton and baryon asymmetries. In the weak and strong washout regimes baryon isocurvature perturbations with amplitudes of the same order as those of the CMB perturbations are generated on scales of order of the respective Hubble scale. They are, however, completely washed out by baryon and photon diffusion at the later stages of the universe's evolution.

hep-ph↗

arXiv2006

Coherent pion production by neutrinos on nuclei

The main part of coherent pion production by neutrinos on nuclei is essentially determined by PCAC, provided that the leptonic momentum transferred square Q^2 remains sufficiently small. We give the formulas for the charged and neutral current cross sections, including also the small non-PCAC transverse current contributions and taking into account the effect of the μ^- mass. Our results are compared with the experimental ones and other theoretical treatments.

hep-ph↗

arXiv2006

Baryon and lepton numbers in two scenarios of leptogenesis

Baryon and lepton numbers of the Universe in leptogenesis with Dirac neutrino and leptogenesis with Majorana neutrino scenarios are considered. It is shown that despite quite different features of Dirac and Majorana fermions both scenarios yield the same relation among the initial lepton and the final baryon asymmetries. Moreover right-handed neutrinos in the leptogenesis with Dirac neutrino scenario have very little impact on the effective number of relativistic degrees of freedom, constrained by BBN. Thus the two scenarios are similar from the cosmological point of view. It is also pointed out that in thermal equilibrium the 3B+L sum is zero for left-handed fermions.

hep-ph↗

arXiv2005

Breaking of B-L in superstring inspired E6 model

In the framework of the superstring inspired E6 model, low-energy extensions of the standard model compatible with leptogenesis are considered and masses of right-handed neutrinos in two scenarios allowed by long-lived protons are discussed. The presence of two additional generations allows breaking of B-L without generating nonzero vacuum expectation values of right-handed sneutrinos of the three known generations. After the symmetry breaking, right-handed neutrinos acquire Majorana masses of order of 10^11 GeV. Within the framework of a simple discrete symmetry, assumptions made to provide a large mass of right-handed neutrinos are shown to be self-consistent. Supersymmetric structure of the theory ensures that large corrections, associated with the presence of a (super)heavy gauge field, cancel out.

hep-ph↗