The XENON project: backgrounds and results

Experimental observations tell us that the biggest amount of mass constituting the universe does not absorb or emit electromagnetic radiation; that is the reason why it is called Dark Matter (DM). The evidences for the existence of DM are based on gravitational effects, such as anomalies in rotation curves of spiral galaxies, gravitational lensing, CMB anisotropies, etc. A consistent theoretical model of particles that includes DM has not been developed yet, thus a DM candidate is generally called Weakly Interacting Massive Particle (WIMP). There are mainly two dark matter detection techniques: indirect search, based on tagging the particles produced by the self-annihilation of WIMPs, and direct search, which consists in identifying elastic scatterings of WIMPs off target nuclei. The XENON project is a DM direct search detector, located at the Gran Sasso National Laboratories. The experiment is based on a xenon dual-phase (Liquid/Gas) Time Projection Chamber (TPC), exploiting both scintillation and ionization signals to identify and discriminate impinging WIMP particles from background. XENON100, operating up to 2016, published in 2012 the best exclusion limit, at that time, for spin-independent WIMP-nucleon elastic interactions, with a minimum cross section of 2.0×10−45cm2 for a WIMP mass of 55 GeV/c2, at 90% confidence level. Moreover, from a combined analysis over three runs, a long period of 477 live days of data has improved the sensitivity by almost a factor 2. The subsequent experiment XENON1T is the first multi-ton scale dark matter direct search detector to date, and, on May 18th 2017, the Collaboration released the results from the first science run, setting the limit for spin-independent WIMP-nucleon elastic scattering cross section, to 7.7×10−47cm2 for a 35 GeV/c2 WIMP mass at 90% confidence level. This limit is, at the moment, the best one. The work of this thesis is based both on XENON100 and XENON1T experiments. In XENON100 a study on the electromagnetic background, due to electrons or gammas scattering off xenon electrons, has been developed. The calibration data from Xe activation lines by neutron inelastic scattering, 232Th and CH3T sources have been compared with the Monte Carlo (MC) simulations. In particular, when a particle interacts with the detector, releases a certain amount of energy, which is converted into photons and electrons; the number of photons and electrons emitted per unit of energy is called light and charge yield respectively. The model used to predict the light and charge yield is Noble Element Simulation Technique (NEST) and consists of a phenomenological description of the xenon response to an ionizing radiation, through a collection of targetrelated parameters obtained by fitting up-to-date experimental results. A refined matching between MC and measured data has been achieved within a Bayesian statistical approach. For this purpose, the light and charge yield obtained by NEST have been improved by using the data from tritium calibration as input. That topic is of fundamental importance for XENON1T, where the large self-shielding power of the liquid xenon does not allow to reach the inner fiducial volume through usual calibration sources located outside the TPC. In the case of XENON1T, an active Muon Veto (MV) has been employed in order to reduce the muon-induced neutron background. It consists of a cylindrical tank, containing the TPC, filled with water and equipped with 84 photomultiplier tubes (PMTs), to tag the Cerenkov light emitted from a muon, or its daughters, crossing the water tank. I took care of the MV system since the very beginning, from the installation of the PMTs inside the water tank to its operation in the final state. Moreover, a muon analysis to tag neutron-induced muon events over the first science run of data is object of this work. 63 single scatterings off xenon nuclei inside 1 ton fiducial volume have been found in the low energy region, that is of interest for WIMP search; no one of them has been vetoed by MV, but they have been discriminated as electromagnetic background.