Effective neutrino number shift from keV-vacuum neutrinophilic 2HDM

TL;DR

This paper explores how a keV-vacuum neutrinophilic two-Higgs-doublet model can cause shifts in the effective neutrino number, linking high-energy CP violation with low-energy phenomena and providing testable cosmological predictions.

Contribution

It introduces a keV-vacuum induced Dirac neutrino model that impacts cosmological parameters and connects leptogenesis with observable effects in the early universe.

Findings

Current observations constrain the model.

Future measurements can test the model's predictions.

The model links high-energy CP violation to cosmological neutrino effects.

Abstract

If the Dirac neutrino masses are generated by a new scalar doublet with a vacuum at keV order, there would be rare hope to probe the framework in low-energy flavor physics, such as the lepton-flavor violating processes. However, the predicted neutrino Yukawa couplings being around the order of electronic Yukawa can realize a purely thermal Dirac leptogenesis, and thus render a more direct link between the high- and the low-energy CP violation. Despite its inert property in the low-energy flavor processes, the keV-vacuum induced Dirac neutrino model can generate a significant shift of the effective neutrino number, which can in turn be probed by the big-bang nucleosynthesis and cosmic microwave background epochs. We show that such a keV-vacuum induced Dirac neutrino model is already constrained by the current observations and will be probed with forecast sensitivity, serving therefore as a complementary avenue to test the Dirac leptogenesis.