Estimating the surface backgrounds in PandaX-II WIMP search data

We estimate the contribution of surface backgrounds, the radioactivities from the wall, in the search window of weakly interacting dark matter particles (WIMPs) with data outside of the window in the PandaX-II detector. In the former analysis, the surface backgrounds was suppressed to a negligible level with a strong fiducial volume (FV) cut that only includes data at the detector center. This work uses data with nominal reconstructed radius outside of the detector border, where contributions of other resources are negligible, to represent properties of the surface backgrounds except the horizontal positions. As the horizontal position reconstruction only depends on the ionization signals, the latter property of surface backgrounds is parameterized by data with the same ionization signals as the search window but different scintillation signals. With the behavior of the surface backgrounds outside the FV known, the data close to the wall can be used for the WIMP search.