Physics Theory Seminar with Subhajit Ghosh on Cosmological case study of a tower of neutrino states

In this talk, we discuss the cosmological effects of a tower of neutrino states (equivalently a tower of warm dark matter ) on cosmic microwave background (CMB) and large-scale structure. For concreteness, we considered the $N$-Naturalness model which is a proposed mechanism to solve the electroweak Hierarchy problem. The model predicts a tower of neutrino states, which act as warm dark matter, with increasing mass and decreasing temperature compared to the standard model neutrino. Compared to a single neutrino state, such a neutrino tower induces a more gradual suppression of the matter power spectrum. The suppression increases with the total number of states in the neutrino tower. We explore these effects quantitatively in the scalar $N$-naturalness model and show the parameter space allowed by the CMB, weak lensing, and Lyman-$\alpha$ dataset. We found that neutrinos-induced suppression of the power spectrum at the small scale puts stringent constraints on the model. We emphasize the need for a faster Boltzmann solver to study the effects of the
tower of neutrino states on smaller scales.