Rejection of Electronic Recoils with the DMTPC Dark Matter Search

The Dark Matter Time Projection Chamber (DMTPC) collaboration is developing a low-pressure gas TPC for detecting WIMP-nucleon interactions. DMTPC detectors use optical readout with CCD cameras to search for the daily modulation of the directional signal of the dark matter wind. An analysis of several charge readout channels has been developed to obtain additional information about ionization events in the detector. In order to reach sensitivities required for the WIMP detection, the detector needs to minimize backgrounds from electron recoils. This article shows that by using the readout of charge signals in addition to CCD readout, a preliminary statistics-limited 90% C.L. upper limit on the γ and e− rejection factor of 5.6 × 10−6 is obtained for energies between 40 keVee and 200 keVee.