Towards a Depleted Argon Time Projection Chamber WIMP Search: DarkSide Prototype Analysis and Predicted Sensitivity

Abstract

Scientists have now accumulated overwhelming evidence indicating that over 80% of the mass of the universe is in the form of dark matter, neutral particles with ultra-weak couplings to ordinary matter. One compelling candidate is a Weakly Interacting Massive Particle or WIMP, with mass on the order of 100 GeV.The signal of a WIMP interaction in a detector is a low energy (typically below ~100 keV) recoiling atomic nucleus. The expected rate is at most a few interactions per year per ton of target. The most critical issue for direct WIMP searches is reducing the background in the detector below this very low rate. Argon is a promising target because measurements of the scintillation pulse shape and the scintillation-to-ionization ratio allow the reduction of gamma-induced signals, the largest source of background, by a factor of 10<super>8</super> or better. One of the major drawbacks of argon is the presence of radioactive <super>39</super>Ar, which results in a decay rate of ~1 Bq/kg in natural argon. Because <super>39</super>Ar is produced primarily in the upper atmosphere, the <super>39</super>Ar fraction can be reduced significantly by obtaining the argon from underground wells.

Our collaboration, DarkSide, is developing a series of two-phase argon time projection

chambers (TPCs) utilizing this depleted argon, along with passive shielding and active neutron and muon vetoes, for WIMP searches. I present results from a recent campaign of a 10 kg active mass prototype TPC that demonstrate the successful realization of many of the technical aspects necessary for a full-scale detector, in particular an electron-equivalent light yield of 4.5 photoelectrons per keV deposited, and a free electron lifetime in excess of 200 microseconds. Based on this successful prototype and Monte Carlo simulations, I then conclude that DarkSide-50, a 50 kg active mass TPC to be installed in LNGS, can likely acquire data background-free for three years, accumulating a fiducial exposure of ~100 kg-years

and reaching a sensitivity to the WIMP-nucleon cross section of ~3&times;10<super>45</super> cm<super>2</super>.

I also present some details on the analysis of Borexino data that resulted in the first

real-time, spectroscopic measurement of 7Be solar neutrino interactions.