A clever new paper uses the observational evidence from astronomy supporting Big Bang Nucleosynthesis to constrain the hypotheses of sterile neutrinos or non-standard neutrino interactions to almost the level reached from Earth bound experiments and other cosmology observations.
Their findings tend to reinforce the Standard Model view that there are only three flavors of neutrinos and that they have only the interactions allowed by the Standard Model (i.e. weak force interactions and neutrino oscillations according to the PMNS matrix).
In this work we investigate the impact of two phenomenological Beyond the Standard Model (BSM) scenarios concerning the role of neutrinos in the early universe: non-standard neutrino interactions (NSI) and non-unitary three-neutrino mixing.
We evaluate the impact of these frameworks on two key cosmological observables: the effective number of relativistic neutrino species (Neff), related to neutrino decoupling, and the abundances of light elements produced at Big Bang Nucleosynthesis (BBN). For the first time, neutrino CC-NSI with quarks and non-unitary three-neutrino mixing are studied in the context of BBN, and the constraints on such interactions are found to be remarkably competitive with terrestrial experiments. In the case of non-unitarity, the combination between neutrino decoupling and BBN imposes stringent constraints that can either mildly favour the existence of New Physics (NP), or reinforce the SM, depending on the choice of the experimental nuclear rates involved in the BBN calculation.
Gabriela Barenboim, Stefano Gariazzo, Alberto Sánchez-Vargas, "Big Bang Nucleosynthesis as a probe of non-standard neutrino interactions and non-unitary three-neutrino mixing" arXiv:2503.21998 (March 27, 2025).
2 comments:
On the topic of neutrino masses, I ran across a recent reddit comment
https://www.reddit.com/r/Physics/comments/1fssjjv/comment/lpnh07f/
which I thought was mildly interesting in describing the choice between Dirac-only mass and Dirac-plus-Majorana mass for neutrinos. Dirac mass only implies that lepton number is an exact global symmetry; Majorana mass implies two mass scales contributing to the net neutrino mass (i.e. seesaw).
I have been a skeptic about Dirac-only, since it requires such microscopic yukawa couplings. But I guess I find it conceivable that there is some model which explains the dark energy and microscopic yukawas for neutrinos, as part of a package deal.
I'm a fan of the possibility that the lightest neutrino mass is largely a result of its self-interactions via the weak force, and that neutrino mass more generally is largely via W boson interactions rather than Higgs boson interactions.
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