Light sterile neutrinos after BICEP-2

The recent discovery of B-modes in the polarization pattern of the Cosmic Microwave Background by the BICEP2 experiment has important implications for neutrino physics. We revisit cosmological bounds on light sterile neutrinos and show that they are compatible with all current cosmological data provided that the mass is relatively low. Using CMB data, including BICEP-2, we find an upper bound of m s < 0.85 eV (2σ Confidence Level). This bound is strengthened to 0.48 eV when HST measurements of H 0 are included. However, the inclusion of SZ cluster data from the Planck mission and weak gravi-tational measurements from the CFHTLenS project favours a non-zero sterile neutrino mass of 0.44 +0.11 −0.16 eV. Short baseline neutrino oscillations, on the other hand, indicate a new mass state around 1.2 eV. This mass is highly incompatible with cosmological data if the sterile neutrino is fully thermalised (∆χ 2 > 10). However, if the sterile neutrino only partly thermalises it can be compatible with all current data, both cosmological and terrestrial.

Physical Review D, 2013

Tantalizing cosmological and terrestrial evidence suggests the number of light neutrinos may be greater than 3, motivating a careful reexamination of cosmological bounds on extra light species. Big bang nucleosynthesis constrains the number of relativistic neutrino species present during nucleosynthesis, N BBN eff , while measurements of the cosmic microwave background (CMB) angular power spectrum constrain the effective energy density in relativistic neutrinos at the time of matterradiation equality, N CMB eff. There are a number of scenarios where new sterile neutrino species may have different contributions to ∆N BBN eff and ∆N CMB eff , for masses that may be relevant to reconciling cosmological constraints with various terrestrial claims of neutrino oscillations. We consider a scenario with two sterile neutrinos and explore whether partial thermalization of the sterile states can ease the tension between cosmological constraints on N BBN eff and terrestrial data. We then investigate the effect of a nonzero neutrino mass on their contribution to the radiation abundance, finding reductions in ∆N CMB eff of more than 5% for neutrinos with masses above 0.5 eV. While the effects we investigate here could play a role, we nevertheless find that two additional light sterile neutrinos species cannot fit all the data at the 95% confidence level.

Physical Review D, 2013

Cosmology and short baseline neutrino oscillation data both hint at the existence of light sterile neutrinos with masses in the 1 eV range. Here we perform a detailed analysis of the sterile neutrino scenario using both cosmological and SBL data. We have additionally considered the possibility that the extra neutrino degrees of freedom are not fully thermalised in the early universe. Even when analyzing only cosmological data we find a preference for the existence of massive sterile neutrinos in both (3+1) and (3+2) scenarios, and with the inclusion of SBL data the evidence is formally at the 3.3σ level in the case of a (3+1) model. Interestingly, cosmological and SBL data both point to the same mass scale of approximately 1 eV. In the (3+1) framework WMAP9+SPT provide a value of the sterile mass eigenstate m4 = (1.72 ± 0.65) eV: this result is strenghtened by adding the prior from SBL posterior to m4 = (1.27 ± 0.12) eV (m4 = (1.23 ± 0.13) eV when SDSS is also considered in the cosmological analysis). In the (3+2) scheme, two additional, non-fully thermalized, neutrinos are compatible with the whole set of cosmological and SBL data, leading to mass values of m4 = (0.95 ± 0.30) eV and m5 = (1.59 ± 0.49) eV. The inclusion of Planck data does not change our considerations about the mass scale; concerning the extra neutrino degrees of freedom, invoking a partial thermalisation the 3+1 model is still consistent with the latest data.

New Astronomy Reviews, 2006

By combining data from cosmic microwave background (CMB) experiments (including the recent WMAP third year results), large scale structure (LSS) and Lymanα forest observations, we derive upper limits on the sum of neutrino masses of Σm ν < 0.17eV at 95% c.l.. We then constrain the hypothesis of a fourth, sterile, massive neutrino. For the 3 massless + 1 massive neutrino case we bound the mass of the sterile neutrino to m s < 0.26eV at 95% c.l.. These results exclude at high significance the sterile neutrino hypothesis as an explanation of the LSND anomaly. We then generalize the analysis to account for active neutrino masses (which tightens the limit to m s < 0.23eV and the possibility that the sterile abundance is not thermal. In the latter case, the contraints in the (mass, density) plane are nontrivial. For a mass of > 1eV or < 0.05eV the cosmological energy density in sterile neutrinos is always constrained to be ω ν < 0.003 at 95% c.l.. However, for a sterile neutrino mass of ∼ 0.25eV, ω ν can be as large as 0.01.

Physical Review Letters, 2014

It has been claimed recently that massive sterile neutrinos could bring about a new concordance between observations of the cosmic microwave background (CMB), the large-scale structure (LSS) of the Universe, and local measurements of the Hubble constant, H0. We demonstrate that this apparent concordance results from combining datasets which are in significant tension, even within this extended model, possibly indicating remaining systematic biases in the measurements. We further show that this tension remains when the cosmological model is further extended to include significant tensor modes, as suggested by the recent BICEP2 results. Using the Bayesian evidence, we show that the minimal ΛCDM model is strongly favoured over its neutrino extensions by various combinations of datasets. Robust data combinations yield stringent limits of mν 0.3 eV and m eff ν,sterile 0.3 eV at 95% CL for the sum of active and sterile neutrinos, respectively.

Astroparticle Physics, 2015

The cosmological background of neutrinos thermally produced in the big bang has been definitively (albeit indirectly) detected. Measurements of the cosmic microwave background (CMB) alone have led to a constraint on the effective number of neutrino species of N eff = 3.36 ± 0.34 [1], a value 10σ away from zero and consistent with expectations. Experiments planned and underway are prepared to study this background in detail via its influence on distance-redshift relations and the growth of structure. The program for the next decade described in this document, including upcoming spectroscopic surveys eBOSS and DESI and a new Stage-IV CMB polarization experiment CMB-S4, will achieve σ( m ν ) = 16 meV and σ(N eff ) = 0.020. Such a mass measurement will produce a high significance detection of non-zero m ν , whose lower bound derived from atmospheric and solar neutrino oscillation data is about 58 meV. If neutrinos have a normal mass hierarchy, this measurement will definitively rule out the inverted neutrino mass hierarchy, shedding light on one of the most puzzling aspects of the Standard Model of particle physics -the origin of mass.

Journal of High Energy Physics, 2013

We analyze the most recent cosmological data, including Planck, taking into account the possible existence of a sterile neutrino with a mass at the eV scale indicated by short-baseline neutrino oscillations data in the 3+1 framework. We show that the contribution of local measurements of the Hubble constant induces an increase of the value of the effective number of relativistic degrees of freedom above the Standard Model value, giving an indication in favor of the existence of sterile neutrinos and their contribution to dark radiation. Furthermore, the measurements of the local galaxy cluster mass distribution favor the existence of sterile neutrinos with eV-scale masses, in agreement with short-baseline neutrino oscillations data. In this case there is no tension between cosmological and short-baseline neutrino oscillations data, but the contribution of the sterile neutrino to the effective number of relativistic degrees of freedom is likely to be smaller than one. Considering the Dodelson-Widrow and thermal models for the statistical cosmological distribution of sterile neutrinos, we found that in the Dodelson-Widrow model there is a slightly better compatibility between cosmological and short-baseline neutrino oscillations data and the required suppression of the production of sterile neutrinos in the early Universe is slightly smaller.

Physical Review Letters, 2010

Precision cosmology and big-bang nucleosynthesis mildly favor extra radiation in the universe beyond photons and ordinary neutrinos, lending support to the existence of low-mass sterile neutrinos. We use the WMAP 7-year data, small-scale CMB observations from ACBAR, BICEP and QuAD, the SDSS 7th data release, and measurement of the Hubble parameter from HST observations to derive credible regions for the assumed common mass scale ms and effective number Ns of thermally excited sterile neutrino states. Our results are compatible with the existence of one or perhaps two sterile neutrinos, as suggested by LSND and MiniBooNE, if ms is in the sub-eV range.

Physical Review D, 2011

Sterile massive neutrinos are a natural extension of the Standard Model of elementary particles. The energy density of the extra sterile massive states affects cosmological measurements in an analogous way to that of active neutrino species. We perform here an analysis of current cosmological data and derive bounds on the masses of the active and the sterile neutrino states as well as on the number of sterile states. The so-called (3+2) models with three sub-eV active massive neutrinos plus two sub-eV massive sterile species is well within the 95% CL allowed regions when considering cosmological data only. If the two extra sterile states have thermal abundances at decoupling, Big Bang Nucleosynthesis bounds compromise the viability of (3+2) models. Forecasts from future cosmological data on the active and sterile neutrino parameters are also presented. Independent measurements of the neutrino mass from tritium beta decay experiments and of the Hubble constant could shed light on sub-eV massive sterile neutrino scenarios.

2005

We derive upper limits on the sum of neutrino masses from an updated combination of data from Cosmic Microwave Background experiments and Galaxy Redshifts Surveys. The results are discussed in the context of three-flavor neutrino mixing and compared with neutrino oscillation data, with upper limits on the effective neutrino mass in Tritium beta decay from the Mainz and Troitsk experiments and with the claimed lower bound on the effective Majorana neutrino mass in neutrinoless double beta decay from the Heidelberg-Moscow experiment.