Joint constraints on the lepton asymmetry of the Universe and neutrino mass from the Wilkinson Microwave Anisotropy Probe

Abstract

We use the Wilkinson Microwave Anisotropy Probe (WMAP) data on the spectrum of cosmic microwave background anisotropies to put constraints on the present amount of lepton asymmetry L, parameterized by the dimensionless chemical potential (also called degeneracy parameter) ξ and on the effective number of relativistic particle species. We assume a flat cosmological model with three thermally distributed neutrino species having all the same mass and chemical potential, plus an additional amount of effectively massless exotic particle species. The extra energy density associated to these species is parameterized through an effective number of additional species ∆N eff others . We find that 0 < |ξ| < 1.1 and correspondingly 0 < |L| < 0.9 at 2σ, so that WMAP data alone cannot firmly rule out scenarios with a large lepton number; moreover, a small preference for this kind of scenarios is actually found. We also discuss the effect of the asymmetry on the estimation of other parameters and in particular of the neutrino mass. In the case of perfect lepton symmetry, we obtain the standard results. When the amount of asymmetry is left free, we find mν < 3.6 eV at 2σ. Finally we study how the determination of |L| is affected by the assumptions on ∆N eff others . We find that lower values of the extra energy density allow for larger values of the lepton asymmetry, effectively ruling out, at 2σ level, lepton symmetric models with ∆N eff others ≃ 0.

WMAP five-year constraints on lepton asymmetry and radiation energy density: implications for Planck

“…We obtain from our analysis a mean value of N ef f = 3.026 We get for neutrino degeneracy parameter a bound of −0.216 ≤ ξ ν ≤ 0.226 (68%CL) that represent an important improvement over the similar result obtained by using the WMAP 3-year data [45]. The CMB only is able to constrain ξ ν through its contribution to the radiation energy density during radiation domination epoch and the BBN constraints on Y p .…”

“…The delay of the epoch of matter-radiation equality shifts the LSS matter power spectrum turnover position toward larger angular scales, suppressing the power at small scales. In particular, the non-zero neutrino chemical potential leads to changes in neutrino free-streaming length and neutrino Jeans mass due to the increase of the neutrino velocity dispersion [45,46]. After WMAP3 data release there are many works aiming to constrain N ef f from cosmological observations [14,34,37,47,48,49].…”

“…The density perturbations in leptonic asymmetric cosmological models have been discussed in literature [59,60,45,46,63,64]. We applied them to modify the Boltzmann Code for Anisotropies in the Microwave Background (CAMB) [65,66,67] to compute the CMB temperature and polarization anisotropies power spectra and LSS matter density fluctuations power spectra for the case of three degenerated neutrinos/antineutrinos with the total mass m ν and degeneracy parameter ξ ν .…”

Cosmological lepton asymmetry with a nonzero mixing angleθ13

“…Some studies using CMB data can be found in the literature (see for instance [36][37][38] for limits on the degeneracy parameters using the WMAP data and [39] for the effect of the primordial helium fraction in a Planck forecast), but our paper improves on that in two directions. First, we used for our analysis the neutrino spectra in the presence of asymmetries after taking into account the effect of flavor oscillations.…”

BBN as Probe of Fundamental Physics

“…In particular, the non-zero neutrino chemical potential leads to changes in neutrino free-streaming length and neutrino Jeans mass due to the increase of the neutrino velocity dispersion (Ichiki et al, 2007;Lattanzi et al, 2005).…”