Mr. Marco Rescigno (INFN/Roma)
Projects attempting the direct detection of WIMP dark matter share the common problem of eliminating sources of background or using techniques to distinguish background events from true signals. Although experiments such as DarkSide have achieved essentially background free exposures through careful choice of materials and application of efficient veto techniques, there will still be a high burden of proof to convince the greater scientific community when a discovery is claimed. A directional signature in the data would provide extremely strong evidence to distinguish a true WIMP signal from that of an isotropic background. Two-phase liquid argon time projection chambers provide an experimental apparatus which can both be scaled to the ton-scale size required to accommodate the low cross-section expected for WIMP interactions and have an anisotropy that could be exploited to evaluate the polar angles of the resulting nuclear recoils from WIMP collisions with target atoms. Two signals are acquired for each event: prompt scintillation light in the liquid argon (S1), and a second signal (S2) produced by electrons created along the recoil track which are drifted upward by a uniform electric field and extracted to the gas phase. Electrons which quickly recombine with ions contribute to the S1 signal, while those that escape contribute to S2. The efficiency of recombination and thus the ratio of ionization to scintillation light depends on the angle between the nuclear recoil and the electric field of the TPC. The ReD project at Università di Napoli Federico II will use a small novel Geiger avalanche photo-diode time projection chamber to study and calibrate the potential of polar angle sensitivity in liquid argon with mono-energetic neutrons. SiPMs will be used in place of PMTs to improve light collection efficiency and spatial resolution. The recoil energy will be assessed by detecting the scattered neutrons with liquid scintillator counters placed at angles of interest. If a directional effect exists, then recoils with the same energy but different initial polar angles relative to the electric field should show different ratios of scintillation and ionization light.