Large Lepton Asymmetry Research Articles

Affleck-Dine leptogenesis scenario for resonant production of sterile neutrino dark matter

Sterile neutrino is a fascinating candidate for dark matter.In this paper, we examine the Affleck-Dine (AD) leptogenesis scenario generating a large lepton asymmetry,which can inducethe resonant production of sterile neutrino dark mattervia the Shi-Fuller (SF) mechanism.We also revisit the numerical calculation of the SF mechanism and the constraints from current X-ray and Lyman-α forest observations.We find that the AD leptogenesis scenario can explain the production of sterile neutrino dark matter by incorporating a non-topological soliton with a lepton charge called L-ball.Finally, we discussan enhancement of second-order gravitational waves at the L-ball decay and investigate the testability of our scenario with future gravitational wave observations.

Enhancement of second-order gravitational waves at Q-ball decay

The recent observation of 4He favors a large lepton asymmetry at the big bang nucleosynthesis. If Q-balls with a lepton charge decay after the electroweak phase transition, such a large lepton asymmetry can be generated without producing too large baryon asymmetry. In this scenario, Q-balls dominate the universe before the decay and induces the sharp transition from the early matter-dominated era to the radiation-dominated era. In this transition, the gravitational waves (GWs) are enhanced through a second-order effect of the scalar perturbations. We evaluate the density of the produced GWs and show that pulsar timing array observations can probe this scenario depending on the amplitude of the scalar perturbations.

Indications for a Nonzero Lepton Asymmetry from Extremely Metal-Poor Galaxies.

The recent measurement of helium-4 from the near-infrared spectroscopy of extremely metal-poor galaxies by the Subaru Survey may point to a new puzzle in the early Universe. We exploit this new helium measurement together with the percent-level determination of primordial deuterium, to assess indications for a nonvanishing lepton asymmetry during the big bang nucleosynthesis era, paying particular attention to the role of uncertainties in the nuclear reaction network. A cutting-edge Bayesian analysis focused on the role of the newly measured extremely metal-poor galaxies, jointly with information from the cosmic microwave background, suggests the existence of a nonzero lepton asymmetry at around the 2σ level, providing a hint for cosmology beyond lambda cold dark matter. We discuss conditions for a large total lepton asymmetry to be consistently realized in the early Universe.

Lepton asymmetric universe

The recent observation of 4He implies that our universe has a large lepton asymmetry.We consider the Affleck-Dine (AD) mechanism for lepton number generation.In the AD mechanism, non-topological solitons called L-balls are produced, and the generated lepton number is confined in them.The L-balls protect the generated lepton number from being converted to baryon number through the sphaleron processes.We study the formation and evolution of the L-balls and find that the universe with large lepton asymmetry suggested by the recent 4He measurement can be realized.

Cosmology Meets Functional QCD: First-Order Cosmic QCD Transition Induced by Large Lepton Asymmetries.

The lepton flavor asymmetries of the Universe are observationally almost unconstrained before the onset of neutrino oscillations. We calculate the cosmic trajectory during the cosmic QCD epoch in the presence of large lepton flavor asymmetries. By including QCD thermodynamic quantities derived from functional QCD methods in our calculation, our work reveals for the first time the possibility of a first-order cosmic QCD transition. We specify the required values of the lepton flavor asymmetries for which a first-order transition occurs for a number of different benchmark scenarios.

Big bang nucleosynthesis constraints on sterile neutrino and lepton asymmetry of the Universe

We consider the cosmological effects of sterile neutrinos with the masses of 150–450 MeV . The decay of sterile neutrinos changes the thermal history of the Universe and affects the energy density of radiation at recombination and the big bang nucleosynthesis (BBN) results. We derive severe constraints on the parameters of sterile neutrinos from the primordial abundances of helium-4 and deuterium. We also find that in a particular model the constraints can be considerably relaxed by assuming a large lepton asymmetry in the active neutrinos. In this case, the consistent parameters result in Neff ≃ 3.2–3.4 and can alleviate the Hubble tension.

Sterile neutrino dark matter via coinciding resonances

It has been proposed that two resonances could coincide in the early universe at temperatures T ∼ 0.2 … 0.5 GeV: one between two nearly degenerate GeV-scale sterile neutrinos, producing a large lepton asymmetry through freeze-out and decays; another between medium-modified active neutrinos and keV-scale sterile neutrinos, converting the lepton asymmetry into dark matter. Making use of a framework which tracks three sterile neutrinos of both helicities as well as three separate lepton asymmetries, and scanning the parameter space of the GeV-scale species, we establish the degree of fine-tuning that is needed for realizing this scenario.

Cosmological dependence of resonantly produced sterile neutrinos

The detection of a sterile neutrino could constitute the first observation of a particle that could have been produced before Big-Bang Nucleosynthesis (BBN), and could provide information about the yet untested pre-BBN era. The cosmological evolution in this era could be drastically different than typically assumed in what constitutes the standard cosmology, as happens in a variety of motivated particle models. In this work we assess the sensitivity to different pre-BBN cosmologies in which entropy is conserved of 0.01 eV to 1 MeV mass sterile neutrinos produced in the early Universe via resonant active-sterile oscillations, which requires a large lepton asymmetry. We identify mass ranges where it is possible to have two populations of the same sterile neutrino, one with a colder and one with a hotter momentum spectra, which is in principle an observable effect. Furthermore, we show the regions in mass and mixing where fully resonant production (i.e. simultaneously coherent and adiabatic) can occur. We find that in several of the cosmologies we consider, including the standard one, for a lepton asymmetry larger than ∼10−4 fully resonantly produced sterile neutrinos in the eV-mass range can evade all cosmological constraints.

Precision study of GeV-scale resonant leptogenesis

Low-scale leptogenesis is most efficient in the limit of an extreme mass degeneracy of right-handed neutrino flavours. Two variants of this situation are of particular interest: large neutrino Yukawa couplings, which boost the prospects of experimental scrutiny, and small ones, which may lead to large lepton asymmetries surviving down to T < 5 GeV. We study benchmarks of these cases within a “complete” framework which tracks both helicity states of right-handed neutrinos as well as their kinetic non-equilibrium, and includes a number of effects not accounted for previously. For two right-handed flavours with GeV-scale masses, Yukawa couplings up to |h| ∼ 0.7×10−5 are found to be viable for baryogenesis, with ΔM/M ∼ 10−8 as the optimal degeneracy. Late-time lepton asymmetries are most favourably produced with ΔM/M ∼ 10−11. We show that the system reaches a stationary state at T < 15 GeV, in which lepton asymmetries can be more than 103 times larger than the baryon asymmetry, reach flavour equilibrium, and balance against helicity asymmetries.

The cosmic QCD phase transition with dense matter and its gravitational waves from holography

Consistent with cosmological constraints, there are scenarios with the large lepton asymmetry which can lead to the finite baryochemical potential at the cosmic QCD phase transition scale. In this paper, we investigate this possibility in the holographic models. Using the holographic renormalization method, we find the first order Hawking–Page phase transition, between the Reissner–Nordström AdS black hole and thermal charged AdS space, corresponding to the de/confinement phase transition. We obtain the gravitational wave spectra generated during the evolution of bubbles for a range of the bubble wall velocity and examine the reliability of the scenarios and consequent calculations by gravitational wave experiments.

Fermion number violating effects in low scale leptogenesis

The existence of baryon asymmetry and dark matter in the Universe may be related to CP-violating reactions of three heavy neutral leptons (HNLs) with masses well below the Fermi scale. The dynamical description of the lepton asymmetry generation, which is the key ingredient of baryogenesis and of dark matter production, is quite complicated due to the presence of many different relaxation time scales and the necessity to include quantum-mechanical coherent effects in HNL oscillations. We derive kinetic equations accounting for fermion number violating effects missed so far and identify one of the domains of HNL masses that can potentially lead to large lepton asymmetry generation boosting the sterile neutrino dark matter production.

Neutrino dynamics below the electroweak crossover

We estimate the thermal masses and damping rates of active (m < eV) and sterile (M ∼ GeV) neutrinos with thermal momenta k∼ 3T at temperatures below the electroweak crossover (5 GeV < T < 160 GeV) . These quantities fix the equilibration or ``washout'' rates of Standard Model lepton number densities. Sterile neutrinos interact via direct scatterings mediated by Yukawa couplings, and via their overlap with active neutrinos. Including all leading-order reactions we find that the washout rate generally exceeds the Hubble rate for 5 GeV < T < 30 GeV . Therefore it is challenging to generate a large lepton asymmetry facilitating dark matter computations operating at T < 5 GeV, whereas the generation of a baryon asymmetry at T > 130 GeV remains an option. Our differential rates are tabulated in a form suitable for studies of specific scenarios with given neutrino Yukawa matrices.

Fermi–Boltzmann statistics of neutrinos and relativistic effective degrees of freedom in the early universe

We investigate the effect of the presence of non-pure fermionic neutrinos on the relativistic effective degrees of freedom in the early universe. The statistics of neutrinos is transformed continuously from Fermi–Dirac to Maxwell–Boltzmann statistics. We find that the relativistic degrees of freedom decreases with the deviation from pure Fermi–Dirac statistics of neutrinos if there are constant and large lepton asymmetries. Additionally, we confirm that the change of the statistics of neutrinos from Fermi–Dirac to Maxwell–Boltzmann is not sufficient to cover the excess of the effective number of neutrinos.

Sterile neutrino dark matter and low scale leptogenesis from a charged scalar.

We show that novel paths to dark matter generation and baryogenesis are open when the standard model is extended with three sterile neutrinos N_i and a charged scalar delta ^+. Specifically, we propose a new production mechanism for the dark matter particle—a multi-keV sterile neutrino, N_1—that does not depend on the active-sterile mixing angle and does not rely on a large primordial lepton asymmetry. Instead, N_1 is produced, via freeze-in, by the decays of delta ^+ while it is in equilibrium in the early Universe. In addition, we demonstrate that, thanks to the couplings between the heavier sterile neutrinos N_{2,3} and delta ^+, baryogenesis via leptogenesis can be realized close to the electroweak scale. The lepton asymmetry is generated either by N_{2,3}-decays for masses M_{2,3}gtrsim TeV, or by N_{2,3}-oscillations for M_{2,3}sim GeV. Experimental signatures of this scenario include an X-ray line from dark matter decays, and the direct production of delta ^+ at the LHC. This model thus describes a minimal, testable scenario for neutrino masses, the baryon asymmetry, and dark matter.

Thermalisation of light sterile neutrinos

We calculate the thermalisation process of light, sterile neutrinos in the early Universe for a range of mixing angles and mass–differences. We show that adding a large initial lepton asymmetry suppress thermalisation significantly. In particular, a sterile neutrino that explains the anomalies seen in ground based neutrino experiments can be made compatible with cosmology. We compute the degree of thermalisation in the (1 active + 1 sterile) scenario and give the effective number of thermalised species just before BBN begins. Resonances are taken properly taken into account, since we solve the full, momentum dependent equations, knowns as the Quantum Kinetic Equations. These proceedings are partly based on the paper [1].

Matter and antimatter in the universe

We review observational evidence for a matter–antimatter asymmetry in the early universe, which leads to the remnant matter density we observe today. We also discuss bounds on the presence of antimatter in the present-day universe, including the possibility of a large lepton asymmetry in the cosmic neutrino background. We briefly review the theoretical framework within which baryogenesis, the dynamical generation of a matter–antimatter asymmetry, can occur. As an example, we discuss a testable minimal particle physics model that simultaneously explains the baryon asymmetry of the universe, neutrino oscillations and dark matter.

First second of leptons

A poorly constrained parameter in the Standard Model of Cosmology is the lepton asymmetry . Each flavour asymmetry lf with f = e, μ, τ is the sum of the net particle density of the charged leptons nf heir corresponding neutrinos, normalized with the entropy density s. Constraints on If from BBN and CMB allow for lepton flavour asymmetries orders of magnitudes larger than the baryon asymmetry b ≃ 10−10. In this article we show how large lepton (flavour) asymmetries lf ≤ \U0001d4aa(0.1) influence the early universe, in particular the freeze out of WIMPs and the cosmic QCD transition.

Thermalisation of light sterile neutrinos in the early universe

Recent cosmological data favour additional relativistic degrees of freedom beyond the three active neutrinos and photons, often referred to as ``dark'' radiation. Light sterile neutrinos is one of the prime candidates for such additional radiation. However, constraints on sterile neutrinos based on the current cosmological data have been derived using simplified assumptions about thermalisation of νs at the Big Bang Nucleosynthesis (BBN) epoch. These assumptions are not necessarily justified and here we solve the full quantum kinetic equations in the (1 active + 1 sterile) scenario and derive the number of thermalised species just before BBN begins (T ≃ 1 MeV) for null (L = 0) and large (L = 10−2) initial lepton asymmetry and for a range of possible mass-mixing parameters. We find that the full thermalisation assumption during the BBN epoch is justified for initial small lepton asymmetry only. Partial or null thermalisation occurs when the initial lepton asymmetry is large.

WIMP abundance and lepton (flavour) asymmetry

We investigate how large lepton asymmetries affect the evolution of the early universe at times before big bang nucleosynthesis and in particular how they influence the relic density of WIMP dark matter. In comparison to the standard calculation of the relic WIMP abundance we find a decrease, depending on the lepton flavour asymmetry. We find an effect of up to 20 per cent for lepton flavour asymmetries lf = \U0001d4aa(0.1).

Large lepton asymmetry for small baryon asymmetry and warm dark matter

We propose a resonant leptogenesis scenario in a $U(1{)}_{B\ensuremath{-}L}$ gauge extension of the standard model to generate large lepton asymmetries for cosmological baryon asymmetry and dark matter. After the $B\ensuremath{-}L$ number is spontaneously broken, the inflaton can pick up a small vacuum expectation value for the mass splits of three pairs of quasidegenerately heavy Majorana neutrinos and the masses of three sterile neutrinos. With thermal mass effects of sphalerons, the observed small baryon asymmetry can be converted from large lepton asymmetries of individual flavors, although total lepton asymmetry is assumed zero. The mixing between sterile and active neutrinos is elegantly suppressed by the heavy Majorana neutrinos. Before the active neutrinos start their strong flavor conversions, the sterile neutrinos as warm dark matter can be produced by resonant active-sterile neutrino oscillations to reconcile x-ray and Lyman-$\ensuremath{\alpha}$ bounds. Small neutrino masses are naturally realized by seesaw contributions from the heavy Majorana neutrinos and the sterile neutrinos.