A Measurement of the Cosmic Microwave Background B-mode Polarization Power Spectrum at Subdegree Scales from Two Years of polarbear Data

We report an improved measurement of the cosmic microwave background B-mode polarization power spectrum with the Polarbear experiment at 150 GHz. By adding new data collected during the second season of observations (2013–2014) to re-analyzed data from the first season (2012–2013), we have reduced twofold the band-power uncertainties. The band powers are reported over angular multipoles $500⩽\ell ⩽2100$, where the dominant B-mode signal is expected to be due to the gravitational lensing of E-modes. We reject the null hypothesis of no B-mode polarization at a confidence of 3.1σ including both statistical and systematic uncertainties. We test the consistency of the measured B-modes with the Λ Cold Dark Matter (ΛCDM) framework by fitting for a single lensing amplitude parameter A _L relative to the Planck 2015 best-fit model prediction. We obtain $A_L={0.60}_{-0.24}^{+0.26}\left(stat{)}_{-0.04}^{+0.00}\right(inst)$ ± 0.14(foreground) ± 0.04(multi), where $A_L=1$ is the fiducial ΛCDM value.