Leptogenesis In Models Research Articles

Fluctuation-dissipation dynamics of cosmological scalar fields

We show that dissipative effects have a significant impact on the evolution of cosmological scalar fields, leading to friction, entropy production and field fluctuations. We explicitly compute the dissipation coefficient for different scalar fields within the Standard Model and some of its most widely considered extensions, in different parametric regimes. We describe the generic consequences of fluctuation-dissipation dynamics in the post-inflationary universe, focusing in particular on friction and particle production, and analyze in detail two important effects. Firstly, we show that dissipative friction delays the process of spontaneous symmetry breaking and may even damp the the motion of a Higgs field sufficiently to induce a late period of warm inflation. Along with dissipative entropy production, this may parametrically dilute the abundance of dangerous thermal relics. Secondly, we show that dissipation can generate the observed baryon asymmetry without symmetry restoration, and we develop in detail a model of dissipative leptogenesis. We further show that this generically leads to characteristic baryon isocurvature perturbations that can be tested with CMB observations. This work provides a fundamental framework to go beyond the leading thermal equilibrium semi-classical approximation in addressing fundamental problems in modern cosmology.

Electroweak naturalness in the three-flavor type I seesaw model and implications for leptogenesis

In the Type I see-saw model, the naturalness requirement that corrections to the electroweak $\mu$ parameter not exceed 1 TeV results in a rough bound on the lightest right-handed neutrino mass, $M_{N_1}\lesssim 3\times 10^7$ GeV. In this letter we derive generic bounds applicable in any three-flavour Type I see-saw model. We find $M_{N_1}\lesssim 4\times 10^7$ GeV and $M_{N_2}\lesssim 7\times 10^7$ GeV. In the limit of one massless neutrino, there is no naturalness bound on $M_{N_3}$ in the Poincare protected decoupling limit. Our results confirm that no Type I see-saw model can explain the observed neutrino masses and baryogenesis via hierarchical ($N_1$-, $N_2$-, or $N_3$-dominated) thermal leptogenesis while remaining completely natural.

Leptogenesis in the symmetric phase of the early universe: Baryon asymmetry and hypermagnetic helicity evolution

We investigate the evolution of the baryon asymmetry of the Universe (BAU) in its symmetric phase before the electroweak phase transition (EWPT) induced by leptogenesis in the hypermagnetic field of an arbitrary structure and with a maximum hypermagnetic helicity density. The novelty of this work is that the BAU has been calculated for a continuous hypermagnetic helicity spectrum. The observed BAU $B_{obs} = 10^{-10}$ that can be in large-scale hypermagnetic fields satisfying the wave number inequality $k \leq k_{max}$ grows with increasing $k_{max}$. We will also show that the initial right-handed electron asymmetry $\xi_{eR}(\eta_0)$ used in our leptogenesis model as a free parameter cannot take too large values, $\xi_{eR}(\eta_0) = 10^{-4}$, because this leads to a negative BAU by the EWPT time. In contrast, a sufficiently small initial right-handed electron asymmetry, $\xi_{eR}(\eta_0)$, provides its further growth and the corresponding BAU growth from zero to some positive value,including the observed $B_{obs} = 10^{-10}$.

Falsifying high-scale leptogenesis at the LHC.

Measuring a nonzero value for the cross section of any lepton number violating (LNV) process would put a strong lower limit on the washout factor for the effective lepton number density in the early Universe at times close to the electroweak phase transition and thus would lead to important constraints on any high-scale model for the generation of the observed baryon asymmetry based on LNV. In particular, for leptogenesis (LG) models with masses of the right-handed neutrinos heavier than the mass scale observed at the LHC, the implied large washout factors would lead to a violation of the out-of-equilibrium condition and exponentially suppress the net lepton number produced in such LG models. We thus demonstrate that the observation of LNV processes at the LHC results in the falsification of high-scale LG models. However, no conclusions about the viability of LG models can be drawn from the nonobservation of LNV processes.

Triplet-extended scalar sector and the naturalness problem

We consider the extension of the Standard Model by a complex scalar triplet field, which occurs naturally in several models of leptogenesis and see-saw mechanism for neutrino mass generation, in the context of ameliorating the fine-tuning problem of the fundamental scalars through the Veltman Condition, i.e. by demanding the sum of the quadratically divergent corrections to vanish (or be at a reasonable level) by virtue of some possible symmetry of the underlying theory. We show that it is possible to cancel all the scalar one-loop quadratic divergences and hence obtain a viable solution for the fine-tuning problem, while satisfying the electroweak precision observables, including the $\rho$ parameter, and successfully generating the neutrino masses. The stability of the scalar potential puts important constraints on the model.

WIMPy leptogenesis with absorptive final state interactions

We consider a class of leptogenesis models in which the lepton asymmetry arises from dark matter annihilation processes which violate CP and lepton number. Importantly, a necessary one-loop contribution to the annihilation matrix element arises from absorptive final state interactions. We elucidate the relationship between this one-loop contribution and the CP-violating phase. As we show, the branching fraction for dark matter annihilation to leptons may be small in these models, while still generating the necessary asymmetry.

Natural baryogenesis from unnatural supersymmetry

We demonstrate here that the mini-split version of the Minimal Supersymmetric Standard Model (MSSM) including R-parity violating couplings naturally provides all the necessary ingredients for a novel baryogenesis mechanism. The baryogenesis is triggered by the late decay of a TeV scale bino after its thermal freezeout. A \mu-term larger than the sfermion masses is necessary for obtaining sufficient baryon asymmetry. Two example models of direct baryogenesis and leptogenesis are proposed, with viable parameter spaces presented. The cosmological conditions for the models--in particular, the requirements of a long lifetime of bino and sufficient baryon asymmetry--point towards the mini-split scale of $\sim$ 100-1000 TeV for the sfermion masses. This provides an independent motivation for mini-split SUSY, along with the constraints from flavor physics and Higgs mass measurement. We also discuss the potential multi-pronged search for the signatures of such models, including those at the Large Hadron Collider (LHC) and the low energy experiments at the intensity frontier.

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.

Seesaw mechanism with four texture zeros in the neutrino Yukawa matrix

With the recent observation of nonzero \theta_{13}, five neutrino oscillation parameters are now known. By imposing four zeros in the Yukawa coupling matrix of the type I seesaw model, the number of parameters in the neutrino mass matrix is reduced to seven, and we are able to make predictions for the lightest neutrino mass, Dirac CP phase, and neutrinoless double beta decay. Four texture zeros in the Yukawa coupling matrix is equivalent to either a single texture zero or a single cofactor zero for an off-diagonal element of the light neutrino mass matrix. We find strong similarities between single texture zero models with one mass ordering and single cofactor zero models with the opposite mass ordering. In the context of a specific class of single-flavor leptogenesis models, we find additional constraints on the parameter space.

Dirac leptogenesis with a non-anomalous U(1)′ family symmetry

We propose a model for Dirac leptogenesis based on a non-anomalous U(1)′ gauged family symmetry. The anomaly cancellation conditions are satisfied with no new chiral fermions other than the three right-handed neutrinos, giving rise to stringent constraints among the charges. Realistic masses and mixing angles are obtained for all fermions. The model predicts neutrinos of the Dirac type with naturally suppressed masses. Dirac leptogenesis is achieved through the decay of the flavon fields. The cascade decays of the vector-like heavy fermions in the Froggatt-Nielsen mechanism play a crucial role in the separation of the primodial lepton numbers. We find that a large region of parameter space of the model gives rise to a sufficient cosmological baryon number asymmetry through Dirac leptogenesis.

The role of lepton flavor symmetries in leptogenesis

AbstractThe presence of flavor symmetries in the lepton sector may have several consequences for the generation of the baryon asymmetry of the Universe via leptogenesis. We review the mechanism in general type‐I, type‐II and type‐III seesaw models. We then turn to the discussion of the cases when the asymmetry is generated in the context of seesaw models extended with flavor symmetries, before or after flavor symmetry breaking. Finally we explain how the interplay between type‐I and type‐II seesaws can (or not) lead to viable models for leptogenesis even when there is an exact mixing pattern enforced by the flavor symmetry.

Thermal production of ultrarelativistic right-handed neutrinos: complete leading-order results

The thermal production of relativistic right-handed Majorana neutrinos is of importance for models of thermal leptogenesis in the early Universe. Right-handed neutrinos can be produced both by 1↔2 decay or inverse decay and by 2 → 2 scattering processes. In a previous publication we have studied the production via 1↔2 (inverse) decay processes. There we have shown that multiple scattering mediated by soft gauge boson exchange also contributes to the production rate at leading order and gives a strong enhancement. Here we complete the leading order calculation by adding 2 → 2 scattering processes involving either electroweak gauge bosons or third-generation quarks. We find that processes with gauge interactions give the most important contributions. We also obtain a new sum rule for the Hard Thermal Loop resummed fermion propagator.

Leptogenesis at the electroweak scale

In this Letter we propose a model of leptogenesis in which the scale for the mass of the necessary heavy neutral lepton is similar to the scale of electroweak symmetry breaking.

Lepton flavor violation andθ13in minimal resonant leptogenesis

We study the impact of minimal non-supersymmetric models of resonant leptogenesis on charged lepton flavour violation and the neutrino mixing angle theta(13). Possible low-scale flavour realisations of resonant tau-, mu- and e-leptogenesis provide very distinct and predictive frameworks to explain the observed baryon asymmetry in the Universe by sphaleron conversion of an individual tau-, mu- and e-lepton-number asymmetry which gets resonantly enhanced via out-of-equilibrium decays of nearly degenerate heavy Majorana neutrinos. Based on approximate flavour symmetries, we construct viable scenarios of resonant tau-, mu- and e-leptogenesis compatible with universal right-handed neutrino masses at the GUT scale, where the required heavy-neutrino mass splittings are generated radiatively. The heavy Majorana neutrinos in such scenarios can be as light as 100 GeV and their couplings to two of the charged leptons may be large. In particular, we explicitly demonstrate the compelling role that the three heavy Majorana neutrinos play, in order to obtain successful leptogenesis and experimentally testable rates for lepton flavour violating processes, such as mu --> e gamma and mu --> e conversion in nuclei.

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.

CP violation from scatterings with gauge bosons in leptogenesis

We present an explicit computation of the CP asymmetry in scattering processes involving the heavy right-handed neutrinos of the type I seesaw framework and the Standard Model gauge bosons. Compared to CP violation in two-body decays and in scatterings with top quarks there are new contributions at one loop in the form of new type of vertex corrections as well as of box diagrams. We show that their presence implies that, unlike the CP asymmetry in scatterings with top quarks, the CP asymmetry in scatterings with gauge bosons is different from the two-body decay asymmetry even for hierarchical right-handed neutrinos. This also holds for the L-conserving CP asymmetry in scatterings with U(1)Y gauge bosons. Quantitatively however, the effects are not very different in size from those of scatterings with top quarks: the CP asymmetry per scattering is O(0.5–2)× the CP asymmetry per decay in all the relevant temperature range for leptogenesis and the generated baryon asymmetry can only be sizeably affected in leptogenesis models that are sensible to the T≳M epoch.

On fast CP violating interactions in leptogenesis

We show that when the relevant CP violating interactions in leptogenesis are fast, the different matter density asymmetries are determined at each instant by a balance condition between the amount of asymmetry being created and destroyed. This fact allows to understand in a simple way many features of leptogenesis in the strong washout regime. In particular, we find some non-trivial effects of flavour changing interactions that conserve lepton number, which are specially relevant in models for leptogenesis that rely heavily on flavour effects.

Systematic approach to leptogenesis in nonequilibrium QFT: Self-energy contribution to theCP-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.

Systematic approach to leptogenesis in nonequilibrium QFT: Vertex contribution to theCP-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.

Purely flavored leptogenesis

We study a model for leptogenesis in which the total $CP$ asymmetries in the decays and scatterings involving the $SU(2)$ singlet seesaw neutrinos ${N}_{\ensuremath{\alpha}}$ vanish (${ϵ}_{{N}_{\ensuremath{\alpha}}}=0$). Leptogenesis is possible due to nonvanishing $CP$ violating lepton flavor asymmetries, realizing a situation in which the baryon asymmetry is due exclusively to flavor effects. We study the production of a net lepton asymmetry by solving the Boltzmann equations specific to this model, and we show that successful leptogenesis can be obtained at a scale as low as the TeV. We also discuss constraints on the model parameter space arising from current experimental upper limits on lepton flavor violating decays.