XENON is a direct dark matter detection project using liquid xenon as the detector medium. The goal is to detect the small charge and light signal after a dark matter particle interacts with a xenon nucleus. Its scientific reach is to be sensitive to very low WIMP-nucleon cross sections, where cosmological observations and theoretical models expect to find dark matter. The latest module, XENON100, is currently taking data and recently published the most stringent limit ever made on dark matter interactions, excluding the spin-independent cross-section above 2 x 10-45 cm2 for a WIMP mass of 55 GeV/c2 at 90% confidence level.
The XENON100 detector
The XENON100 detector is located at the Gran Sasso Underground Laboratory and it is operative since 2008. It uses the same principle of operation and many design features successfully tested in the previous prototype, XENON10. Ten times larger than its predecessor and 100 times more sensitive, it is a position-sensitive time projection chamber (TPC), with the sensitive liquid Xe volume viewed by two arrays of total 178 photomultiplier tubes (PMTs), to detect simultaneously the primary scintillation signal and the ionization signal via the proportional scintillation mechanism.
The dark matter signature
The prompt light signal (S1) is detected by photomultipliers. The ionization electrons are separated from the Xe ions and drifted upwards by a strong electric field. A second electric field extracts the charges from the liquid into the gas phase where they generate secondary scintillation light (S2) which is proportional to the charge signal and delayed to the S1 by the electron drift time. The TPC design allows the precise 3-dimensional reconstruction of the interaction vertex which can be used to reduce the background contamination by fiducial volume cuts. The ratio S2/S1 has a different value for electron recoils (background) and nuclear recoils (signal) and can be used for background discrimination.
The sensitivity on the dark matter interactions
The analysis of about 224.6 live days of data, acquired between February 2011 and March 2012 showed no evidence for dark matter: two candidate events were observed in a pre-defined signal region in which we expected 1.0±0.2 background events. A limit on the spin-independent WIMP-nucleon elastic scatterinng cross-section σ is calculated by assuming WIMPs to be distributed in an isothermal halo with a certain velocity and density. The limit depends on the unknown WIMP mass mχ and it has a minimum of σ = 2.0 x 10-45 cm2 at mχ = 55 GeV/c2.