Fuzzy dark matter and neutron Majorana mass from gravitational instantons

Center for Theoretical Physics, College of Physics Science and Technology, Sichuan University 610065 Chengdu, China and INFN, Sezione Roma Tor Vergata - I-00133 Rome, Italy

^(a) andrea.addazi@lngs.infn.it

Received: 28 June 2020
Accepted: 22 July 2020

Abstract

We explore the possibility that a super-light candidate for fuzzy (or wave) Dark Matter (fDM) may be a new composite boson state of fermions, rather than an axion-like Bose-Einstein condensate. We start from string-inspired massless Majorana fermions χ thought as super-modulini fields from string compactifications. We show that massless (or nearly massless) fermions can form a Bose-Einstein condensate and they can acquire a tiny mass term from quantum-gravitational non-perturbative effects such as gravitational instantons. For having a successful candidate for fDM, the so generated Majorana mass is , or so; otherwise decoherence would suppress quantum-mechanical effects in the macroscopic limit. Gravity is democratically coupled with every standard model particles; any neutral fermions acquire a Majorana mass equal to . While for neutrinos, such a mass would be completely out of any testability, a neutron Majorana mass can also be envisaged. A gravitational Majorana mass for the neutron generates baryon violating neutron-antineutron oscillations. Contrary to other competitive models, here the transition is generated without adding any new particles or interactions to the standard model: it is a genuine quantum gravity effect. Current limits on imply a Majorana mass bound and, therefore, the very same limit on the Gravi-fuzzy DM (GfDM) candidate. The next generation of experiments searching for can probe up to the , potentially excluding Gravi-fuzzy DM up to it.