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There is a wide range of astronomical evidence that the stars in all galaxies, including our own, are immersed and gravitationally bound in a much larger cloud of non-luminous matter, typically an order of magnitude greater in total mass. The existence of this dark matter is consistent with evidence from large-scale galaxy surveys and microwave background measurements. The conclusion from all evidence strongly supports that the majority of matter in the universe is non-baryonic. The nature of dark matter remains unknown, and the resolution of the dark matter puzzle is of fundamental importance to cosmology, astrophysics and elementary particle physics. A leading explanation, theoretically motivated by supersymmetry theory, is the existence of Weakly Interacting Massive Particles (WIMPs), formed in the early universe and subsequently clustered in association with normal matter. These could in principle be detected in terrestrial experiments by collisions with ordinary nuclei, giving observable low-energy (< 100 keV) nuclear recoils.

XENON is one such experiment, aiming at the direct detection of WIMPs with a large volume time projection chamber (TPC) filled with liquid xenon (LXe) as WIMP target and detection medium. Through the simultaneous measurement of the ionization and scintillation signals produced in LXe by radiation and through its 3D event localization capability, the XENON TPC is able to discriminate nuclear recoils produced by WIMPs (and neutrons), from electron recoils produced by gamma and beta background. The XENON concept was tested at Nevis Laboratories with several small scale prototypes, culminating with the XENON10 detector, yielding the best sensitivity for dark matter searches in 2007.

After the successful operation of this detector the collaboration realized the first 100 kg scale detector, XENON100 which started operation in 2008 and continues to take data at the Laboratori Nazionali de Gran Sasso (LNGS), Italy, one of the largest underground laboratories worldwide. With the first science run, XENON100 provided a factor 100 increase in sensitivity with respect to its predecessor, becoming the most sensitive dark matter experiment in the world. These results have challenged the dark matter interpretation of other running experiments. The second science run, 225 live-days, lowered the WIMP-nucleon cross section limit to 2x10^{-45}cm^{2} for a WIMP mass of 55 GeV.

Meanwhile, the next generation experiment, XENON1T, using for the first time a LXe detector at the multi-ton scale, has been constructed and installed in Hall B at LNGS. With a total mass of 3.5 tons and the use of new photo-sensors and materials, this new detector is actively shielded by about 700 m^{3} of pure water instrumented with photomultipliers for Cherenkov muon veto. The commissioning of the cryogenic system needed for XENON1T has been completed and the commissioning of the LXe detector has started. Figure 1 shows the XENON1T water shield and the service building which contains all the infrastructures and systems for the operation of XENON1T. Figure 2 shows a photo of the XENON1T LXe TPC during installation in the Clean Room. First data taking is expected by Spring 2016. The sensitivity of this new and largest LXe dark matter experiment is expected to be better than what achieved by XENON100 by a factor of 100. XENON1T will be able to explore cross sections down to 2x10^{-47}cm^{2} as shown in the figure. An upgrade of XENON1T - called XENONnT - with a larger detector of double the LXe mass, is foreseen by 2018, to enable another factor of 10 increase in sensitivity. The XENON1T collaboration, led by Professor Aprile of Columbia University, involves 130 researchers from 22 universities around the world.

In addition to the dark matter experiments at LNGS, R&D with smaller xenon TPCs continues to be a strong activity of Professor Aprile’s group. Several set-ups are operated at Nevis Laboratories of Columbia University with the goal to measure fundamental properties of LXe as a radiation detection medium and to improve technologies for future experiements. A good understanding of how LXe responds to radiation is essential for interpreting the results of the XENON experiments. One such detector is designed to measure nuclear and electronic recoils in Xenon (neriX). Specifically, neriX measures the amount of light and charge emitted by the LXe as a function of energy and applied electric field for different particle types. The neriX detector is optimized for very low-energy interactions, down to ~1 keV. Measurements of nuclear recoils are currently underway.

REU students accepted in the XENON group will work on the XENON1T experiment and R&D, and will be based in New York.