Cosmological imprints of Dirac neutrinos in a keV-vacuum 2HDM* *Supported by the National Natural Science Foundation of China (12135006, 12075097, 12047527, 11775092), as well as by the Fundamental Research Funds for the Central Universities (CCNU20TS007, CCNU19TD012, CCNU22LJ004)

Abstract

The Dirac neutrino masses could be simply generated by a neutrinophilic scalar doublet with a vacuum being dramatically different from the electroweak one. While the case with an eV-scale vacuum has been widely explored previously, we exploit in this work the desert where the scalar vacuum is of scale. In this regime, there would be rare hope to probe the keV-vacuum neutrinophilic scalar model via the lepton-flavor-violating processes, which makes it distinguishable from the widely considered eV-scale vacuum. Although such a keV-vacuum scenario is inert in the low-energy flavor physics, we show that the baryogenesis realized via the lightest Dirac neutrino can be a natural candidate in explaining the baryon asymmetry of the Universe. Furthermore, the Dirac neutrinos with a keV-vacuum scalar can generate a shift of the effective neutrino number within the range , which can be probed by the future Simons Observatory experiments. In particular, the model with a minimal value can already be falsified by the future CMB Stage-IV and Large Scale Structure surveys, providing consequently striking exploratory avenues in the cosmological regime for such a keV-vacuum scenario.