A Measurement of the Cosmic Microwave Background $B$-Mode Polarization
  Power Spectrum at Sub-Degree Scales from 2 years of POLARBEAR Data
  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.
