Constraints on the total mass and mass hierarchy of neutrinos from cosmological observations with bayesian analysis

Abstract

In the standard model neutrinos consist of three species (flavors) and are massless. However, solar neutrino problem that was solved by neutrino oscillations requires that neutrinos have non-zero masses. Another support for non-zero neutrino masses comes from kinematic experiments. Oscillation experiments give rise to a further problem regarding the order of neutrino masses, known as the neutrino mass hierarchy problem, i.e. whether the hierarchy is normal or inverted. On cosmological scales, massive neutrinos affect Cosmic Microwave Background (CMB) and Large Scale Structure (LSS) formation. The impact of massive neutrinos is imprinted in the power spectrum of CMB, and in the LSS matter power spectrum as well. Since neutrinos affect events in the universe, it is possible to constrain neutrino properties based on cosmological observations. This work is aiming at constraining neutrino total mass from cosmological observations in the two hierarchy scenarios, i.e. normal and inverted hierarchies, and also at determining which neutrino mass hierarchy is favoured by cosmological observations. To constrain neutrino total mass we employed CosmoMC and performed Bayesian analysis with MCMC algorithm, while neutrino mass hierarchy was determined based on Bayes factors, in which likelihoods of the two hierarchy scenarios were compared. In addition, neutrino individual masses were estimated by combining results of oscillation experiments. Depending on the dataset used, different total mass constraints were obtained. The tightest constraint comes from a combination of CMB + LSS + BAO + Supernova data, i.e. Σmv < 0.183 eV (normal hierarchy) and Σmv < 0.188 eV (inverted hierarchy) with 95% C.L. The Bayes factors analysis prefers normal hierarchy, although with weak evidence.