Abundance Of Sterile Neutrinos Research Articles

Particle Physics with the Cosmic Microwave Background with SPT-3G

The cosmic microwave background (CMB) encodes information about the content and evolution of the universe. The presence of light, weakly interacting particles impacts the expansion history of the early universe, which alters the temperature and polarization anisotropies of the CMB. In this way, current measurements of the CMB place interesting constraints on the neutrino energy density and mass, as well as on the abundance of other possible light relativistic particle species. We present the status of an on-going 1500 sq. deg. survey with the SPT-3G receiver, a new mm-wavelength camera on the 10-m diameter South Pole Telescope (SPT). The SPT-3G camera consists of 16,000 superconducting transition edge sensors, a 10x increase over the previous generation camera, which allows it to map the CMB with an unprecedented combination of sensitivity and angular resolution. We highlight projected constraints on the abundance of sterile neutrinos and the sum of the neutrino masses for the SPT-3G survey, which could help determine the neutrino mass hierarchy.

Relativistic and spectator effects in leptogenesis with heavy sterile neutrinos

For leptogenesis with heavy sterile neutrinos above the electroweak scale, asymmetries produced at early times (in the relativistic regime) are relevant, if they are protected from washout. This can occur for weak washout or when the asymmetry is partly protected by being transferred to spectator fields. We thus study the relevance of relativistic effects for leptogenesis in a minimal seesaw model with two sterile neutrinos in the strongly hierarchical limit. Starting from first principles, we derive a set of momentum-averaged fluid equations to calculate the final B − L asymmetry as a function of the washout strength and for different initial conditions at order one accuracy. For this, we take the leading fluid approximation for the relativistic CP-even and odd rates. Assuming that spectator fields remain in chemical equilibrium, we find that for weak washout, relativistic corrections lead to a sign flip and an enhancement of the asymmetry for a vanishing initial abundance of sterile neutrinos. As an example for the effect of partially equilibrated spectators, we consider bottom-Yukawa and weak-sphaleron interactions in leptogenesis driven by sterile neutrinos with masses ≳ 5 × 1012 GeV. For a vanishing initial abundance of sterile neutrinos, this can give rise to another flip and an absolute enhancement of the final asymmetry in the strong washout regime by up to two orders of magnitude relative to the cases either without spectators or with fully equilibrated ones. These effects are less pronounced for thermal initial conditions for the sterile neutrinos. The CP-violating source in the relativistic regime at early times is important as it is proportional to the product of lepton-number violating and lepton-number conserving rates, and therefore less suppressed than an extrapolation of the nonrelativistic approximations may suggest.

Viable production mechanism of keV sterile neutrino with large mixing angle

We study a model with a hidden sector coupled to keV scale sterile neutrinos. Due to nontrivial dynamics of this sector, the initially massless sterile neutrino acquires a nonzero mass at some temperature corresponding to the phase transition in the hidden sector. It shifts the onset of oscillations in plasma to later times, so that the final abundance of sterile neutrinos is strongly suppressed. We argue that in this model various cosmological and astrophysical bounds can be significantly alleviated opening new perspectives for ground-based experiments such as Troitsk ν-mass and KATRIN in the large mixing region.

Cosmologically Safe eV-Scale Sterile Neutrinos and Improved Dark Matter Structure

We show that sterile neutrinos with masses ≳1 eV, as motivated by several short baseline oscillation anomalies, can be consistent with cosmological constraints if they are charged under a hidden sector force mediated by a light boson. In this case, sterile neutrinos experience a large thermal potential that suppresses mixing between active and sterile neutrinos in the early Universe, even if vacuum mixing angles are large. Thus, the abundance of sterile neutrinos in the Universe remains very small, and their impact on big bang nucleosynthesis, cosmic microwave background, and large-scale structure formation is negligible. It is conceivable that the new gauge force also couples to dark matter, possibly ameliorating some of the small-scale structure problems associated with cold dark matter.

MeV sterile neutrinos in low reheating temperature cosmological scenarios

It is commonly assumed that the cosmological and astrophysical bounds on the mixings ofsterile with active neutrinos are much more stringent than those obtained from laboratorymeasurements. We point out that in scenarios with a very low reheating temperature MeV at the end of (the last episode of) inflation or entropy creation, the abundance ofsterile neutrinos becomes greatly suppressed with respect to that obtained within thestandard framework. Thus, in this case cosmological bounds become much lessstringent than usually assumed, allowing sterile neutrinos to be ‘visible’ in futureexperiments. Here, we concentrate on massive (mostly sterile) neutrinos with massesms>1 MeVfor TRH≤ms.

COSMOLOGY OF "VISIBLE" STERILE NEUTRINOS

We point out that in scenarios with a low reheating temperature TR << 100 MeV at the end of (the last episode of) inflation or entropy production, the abundance of sterile neutrinos becomes largely independent of their coupling to active neutrinos. Thus, cosmological bounds become less stringent than usually assumed, allowing sterile neutrinos to be "visible" in future experiments. For example, the sterile neutrino required by the LSND result does not have any cosmological problem within these scenarios.