Right-handed Neutrino Production Rate Research Articles

Strong cosmological constraints on the neutrino magnetic moment

A sizable magnetic moment for neutrinos would be evidence of exotic physics. In the early Universe, left-handed neutrinos with a magnetic moment would interact with electromagnetic fields in the primordial plasma, flipping their helicity and producing a population of right-handed (RH) neutrinos. In this work, we present a new calculation of the production rate of RH neutrinos in a multicomponent primordial plasma and quantify their contribution to the total energy density of relativistic species at early times, stressing the implications of the dependence on the initial time for production. We find that current cosmological data exclude values of the magnetic moment μ≳1.6×10−11μB, while future cosmological experiments will be able to probe nonthermal production of RH neutrinos, becoming competitive with stellar limits. Published by the American Physical Society 2024

Strong cosmological constraints on the neutrino magnetic moment

A sizable magnetic moment for neutrinos would be evidence of exotic physics. In the early Universe, left-handed neutrinos with a magnetic moment would interact with electromagnetic fields in the primordial plasma, flipping their helicity and producing a population of right-handed (RH) neutrinos. In this work, we present a new calculation of the production rate of RH neutrinos in a multicomponent primordial plasma and quantify their contribution to the total energy density of relativistic species at early times, stressing the implications of the dependence on the initial time for production. We find that current cosmological data exclude values of the magnetic moment μ≳1.6×10−11μB, while future cosmological experiments will be able to probe nonthermal production of RH neutrinos, becoming competitive with stellar limits. Published by the American Physical Society 2024

Status of rates and rate equations for thermal leptogenesis

In many realizations of leptogenesis, heavy right-handed neutrinos play the main role in the generation of an imbalance between matter and antimatter in the early Universe. Hence, it is relevant to address quantitatively their dynamics in a hot and dense environment by taking into account the various thermal aspects of the problem at hand. The strong washout regime offers an interesting framework to carry out calculations systematically and reduce theoretical uncertainties. Indeed, any matter–antimatter asymmetry generated when the temperature of the hot plasma [Formula: see text] exceeds the right-handed neutrino mass scale [Formula: see text] is efficiently erased, and one can focus on the temperature window [Formula: see text]. We review recent progress in the thermal field theoretic derivation of the key ingredients for the leptogenesis mechanism: the right-handed neutrino production rate, the CP asymmetry in the heavy-neutrino decays and the washout rates. The derivation of evolution equations for the heavy-neutrino and lepton-asymmetry number densities, their rigorous formulation and applicability are also discussed.

Right-handed neutrino production rate at T > 160 GeV

The production rate of right-handed neutrinos from a Standard Model plasma at a temperature above a hundred GeV has previously been evaluated up to NLO in Standard Model couplings (g ∼ 2/3) in relativistic (M ∼ πT) and non-relativistic regimes (M ≫ πT), and up to LO in an ultrarelativistic regime (M ≲ gT). The last result necessitates an all-orders resummation of the loop expansion, accounting for multiple soft scatterings of the nearly light-like particles participating in 1 ↔ 2 reactions. In this paper we suggest howthe regimes can be interpolated into a result applicable for any right-handed neutrino mass and at all temperatures above 160GeV. The results can also be used for determining the lepton number washout rate in models containing right-handed neutrinos. Numerical results are given in a tabulated form permitting for their incorporation into leptogenesis codes. We note that due to effects from soft Higgs bosons there is a narrow intermediate regime around (M ∼ g1/2T in which our interpolation is phenomenological and a more precise study would be welcome.

Thermal right-handed neutrino production rate in the relativistic regime

The production rate of right-handed neutrinos from a Standard Model plasma at a temperature above a hundred GeV is evaluated up to NLO in Standard Model couplings. The results apply in the so-called relativistic regime, referring parametrically to a mass M ~ pi T, generalizing thereby previous NLO results which only apply in the non-relativistic regime M >> pi T. The non-relativistic expansion is observed to converge for M > 15 T, but the smallness of any loop corrections allows it to be used in practice already for M > 4 T. In the latter regime any non-covariant dependence of the differential rate on the spatial momentum is shown to be mild. The loop expansion breaks down in the ultrarelativistic regime M << pi T, but after a simple mass resummation it nevertheless extrapolates reasonably well towards a result obtained previously through complete LPM resummation, apparently confirming a strong enhancement of the rate at high temperatures (which facilitates chemical equilibration). When combined with other ingredients the results may help to improve upon the accuracy of leptogenesis computations operating above the electroweak scale.

Thermal 2-loop master spectral function at finite momentum

When considering NLO corrections to thermal particle production in the "relativistic" regime, in which the invariant mass squared of the produced particle is K^2 ~ (pi T)^2, then the production rate can be expressed as a sum of a few universal "master" spectral functions. Taking the most complicated 2-loop master as an example, a general strategy for obtaining a convergent 2-dimensional integral representation is suggested. The analysis applies both to bosonic and fermionic statistics, and shows that for this master the non-relativistic approximation is only accurate for K^2 > (8 pi T)^2, whereas the zero-momentum approximation works surprisingly well. Once the simpler masters have been similarly resolved, NLO results for quantities such as the right-handed neutrino production rate from a Standard Model plasma or the dilepton production rate from a QCD plasma can be assembled for K^2 ~ (pi T)^2.

Thermal right-handed neutrino production rate in the non-relativistic regime

We consider the next-to-leading order thermal production rate of heavy right-handed neutrinos in the non-relativistic regime m_top < pi*T << M, where m_top refers to the electroweak scale. Rephrasing the problem in an OPE language and making use of different techniques than a previous analysis by Salvio et al, we confirm the general structure of their result and many of the coefficients. We also extend the analysis to the next order in the non-relativistic expansion, thereby revealing the leading non-trivial momentum dependence, as well as to NNLO in couplings, revealing the leading sensitivity to thermal resummations. Our results are expressed as a sum of simple "master" structures, which renders them a suitable starting point for determining the next-to-leading order rate also in the relativistic regime pi*T ~ M.