Currently being assembled and commisioned at the LNGS underground laboratory in Italy, the XENONnT detector will reach new levels of sensitivity in the search for the elusive dark matter particle

Array of photomultipliers with cables. Five cleanroom-suited physicists are standing behind the array
Array of photomultiplier tubes for XENONnT
Graph of excluded spin-independent cross-sections versus WIMP mass showing the XENON1T limit reaching ~3e-46 cm^2 at 30GeV plus some nT projections
The XENON1T experiment set the world's best limits on WIMPs scattering on nucleons

Astronomical evidence shows that large structures in our universe, such as galaxies and galaxy clusters are immersed in much larger halos of unseen, "dark" matter. Kinematic studies, first by Zwicky and Lundmark in the 1930s, and systematically expanded since the 1970s when Vera Rubin showed that the observed amount of ordinary, "baryonic" matter is not sufficient for starsto be gravitationally bound in galaxies. Further evidence, including large-scale galaxy surveys and microwave background measurements, strongly disfavors the possibility that dark matter could be made up of any known particle. Dark matter is central to how we believe galaxies formed and the universe expanded, and the discovery of the nature of dark matter is one of the greatest unsolved problems of physics.

A wealth of candidate particles have been proposed as dark matter. Among the most studied are weakly interacting massive particles (WIMPs). WIMPs are a feature of several theories of new physics, notably supersymmetry, and would be produced thermally in the early universe. WIMPs bound in the Milky Way would move at around 200 km/s at the Sun, and by interacting weakly with the nuclei of ordinary matter, they could deposit (≲ 100 keV/c2) of energy to the recoiling nucleus.

A recoiling nucleus will dissipate its energy as atomic motion (heat or sound), ionisation and scintillation light. The XENON range of detectors are two-phase time projection chambers (TPCs), that detect the scintillation light and the ionisation electrons deposited on a liquid xenon target. The position of the interaction can be reconstructed from the two signals, which enables self-shielding by choosing to perform the analysis in a chosen, ficucial, analysis volume, shielded by the surrounding liquid. The ratio between scintillation and ionisation energy also provides a strong discrimination between nuclear recoils and the much more numerous recoils on the electrons of xenon atoms. 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.

The upcoming experiment, XENONnT, is being constructed and installed in Hall B at LNGS. The inner, cryogenic, detector holds ~8 tonnes of liquid xenon and the TPCs, and is surrounded by a 700 m3 instrumented water tank. The TPC is instrumented with photomultiplier tubes (PMTs, one of two XENONnT arrays is shown on the upper left picture) that detect the light flashes from xenon scintillation caused by the initial recoil and by the amplification of the ionisation signal as it is accelerated at the surface of the liquid. The XENON collaboration, led by Professor Aprile of Columbia University, gathers 175 scientists from 25 institutions around the world to design and operate XENONnT.

In addition to the dark matter experiments at LNGS, Italy,, R&D with smaller xenon TPCs continues to be a strong activity of the Columbia XENON group. Set-ups are operated at the XENON laboratory at Morningside campus, with the goal to measure fundamental properties of LXe as a radiation detection medium and to improve technologies for future experiements. Good knowledge of the Xenon response as well as advanced technologies to cool and purify the liquid xenon to reach the required extreme purity are crucial to the success of the experiment.

Two REU students will be accepted in the XENON to work for the XENONnT experiment. The position may allow travel to LNGS, Italy, to work at the XENONnT detector.

For further information about XENONnT, consult the homepage of the XENON collaboration, and contact the Columbia XENON group: Prof. Aprile