Right-handed Neutrinos and the (Dark Sector of the) Universe

por Nick E. Mavromatos (King's College London, Physics Dept., UK)

viernes 15 feb. 2013 15:00 → 16:00 Europe/Madrid

Sala Seminarios (Dpto. de Física Teórica (UVEG))

I discuss the role of neutrinos, in particular right-handed sterile ones, in contributing to the dark sector of the Universe, especially in playing the role of dark matter. I start from a simplest extension of the standard model, proposed by Shaposhnikov and collaborators, which involves three right-handed Majorana massive neutrinos, and exploit astrophysical and cosmological constraints on the lowest of these extra states in order for it to be the dark matter candidate. This is consistent with current constraints, provided the mass of the lightest right-handed neutrino is of order of a few keV. I discuss mechanisms for which a mass of such an order can be generated. One of such mechanisms can be via non-perturbative effects, when axions (pseudoscalar fields) couple to fermions via chirality-changing Yukawa couplings with appropriately suppressed coefficients. In such theories, an effective right-handed Majorana neutrino mass can be generated at two loops by gravitational interactions that involve global anomalies related to quantum torsion, associated with a totally antisymmetric tensor field that exist in the gravitational multiplet of quantum gravity theories, such as strings, or in general loop quantum gravity models that are known to have torsion. The gravitational torsion in four space-time dimensions is equivalent to the existence of a Kalb-Ramond pesudoscalar field, which upon mixing with ordinary axions gives rise to Majorana neutrino masses as above. We estimate the magnitude of such gravitationally-induced Majorana right-handed neutrino masses and find that it is highly model dependent, ranging from multi-TeV to keV scales, the latter range being the one that makes such heavy neutrinos dark matter candidates.