We report an improved measurement of the cosmic microwave background (CMB) $B$-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 $500 leq ell leq 2100$, 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 3.1$sigma$ including both statistical and systematic uncertainties. We test the consistency of the measured $B$-modes with the $Lambda$ Cold Dark Matter ($Lambda$CDM) framework by fitting for a single lensing amplitude parameter $A_L$ relative to the Planck best-fit model prediction. We obtain $A_L = 0.60 ^{+0.26} _{-0.24} ({rm stat}) ^{+0.00} _{-0.04}({rm inst}) pm 0.14 ({rm foreground}) pm 0.04 ({rm multi})$, where $A_{L}=1$ is the fiducial $Lambda$CDM value, and the details of the reported uncertainties are explained later in the manuscript.
Download