Constraining light sterile neutrino mass with the BICEP2/Keck Array 2014
  B-mode polarization data
  We explore the thermal light sterile neutrino situation from cosmological
perspective in the $\Lambda \textrm{CDM} + r_{0.05} + N_{\textrm{eff}} +
m^{\textrm{eff}}_{\textrm{s}}$ model using combinations of latest data sets
available. Here, $r_{0.05}$ is the tensor-to-scalar ratio at the pivot scale of
$k_*=0.05h$ Mpc$^{-1}$, $N_{\textrm{eff}}$ is the effective number of
relativistic species during recombination, and $m^{\textrm{eff}}_{\textrm{s}}$
is the effective mass of the sterile neutrino. Among Cosmic Microwave
Background (CMB) datasets, we use Planck 2015 temperature and low-$l$ ($l <$
30) polarization data and the latest data release on the B-mode polarization up
to and including 2014 from the BICEP2/Keck collaboration (BK14). We also use
the latest BAO data from SDSS-III BOSS DR12, MGS, and 6dFS; and a Gaussian
prior (HST) on the Hubble constant ($H_0 = 73.24 \pm 1.74$ km/sec/Mpc) from
direct measurements. We find that inclusion of BK14 data makes the constraints
on the effective mass of sterile neutrino ($m^{\textrm{eff}}_{\textrm{s}}$)
slightly stronger by preferring higher $\sigma_8$ values. The bound of
$m^{\textrm{eff}}_{\textrm{s}} <$ 0.46 eV (95\% C.L.) is found for the
combination of Planck 2015, BAO and BK14 datasets, whereas the bound is
$m^{\textrm{eff}}_{\textrm{s}} <$ 0.53 eV (95\% C.L.) without the BK14 data.
Our most aggressive bound of $m^{\textrm{eff}}_{\textrm{s}} <$ 0.28 eV (95\%
C.L.) is obtained with Planck 2015, HST and BK14. However, the HST prior also
leads to very high $N_{\textrm{eff}}$ which might be in conflict with bounds
from BBN. Our analysis indicates that fully thermalized sterile neutrinos with
mass $\sim 1$ eV are slightly more disfavoured with the inclusion of BK14 data.
It also seems to make the agreement between Planck 2015 and CFHTLenS (weak
gravitational lensing data) worse due to the higher $\sigma_8$ values (abstract
abridged).
