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The Large Underground Xenon experiment (LUX) aims to directly detect weakly interacting massive particle (WIMP) dark matter interactions with ordinary matter on Earth. Despite the wealth of (gravitational) evidence supporting the existence of non-baryonic dark matter in the Universe, dark matter particles in our galaxy have never been directly detected in an experiment. LUX utilizes a 370 kg liquid xenon detection mass in a time-projection chamber (TPC) to identify individual particle interactions, searching for faint dark matter interactions with unprecedented sensitivity.
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The LUX experiment, which cost approximately $10 million to build, is located 1,510 m (4,950 ft) underground at the Sanford Underground Laboratory (SURF, formerly the Deep Underground Science and Engineering Laboratory, or DUSEL) in the Homestake Mine (South Dakota) in Lead, South Dakota. The detector is located in the Davis campus, former site of the Nobel Prize-winning Homestake neutrino experiment led by Raymond Davis. It is operated underground to reduce the background noise signal caused by high-energy cosmic rays at the Earth's surface.
Detector principle
The detector is isolated from background particles by a surrounding water tank and the earth above. This shielding reduces cosmic rays and radiation interacting with the xenon.
Interactions in liquid xenon generate 175 nm ultraviolet photons and electrons. The photons are immediately detected by two arrays of 61 photomultiplier tubes at the top and bottom of the detector. These prompt photons are the S1 signal. Electrons generated by the particle interactions drift upwards towards the xenon gas by an electric field. The electrons are pulled in the gas at the surface by a stronger electric field, and produce electroluminescence photons detected as the S2 signal. The S1 and subsequent S2 signal constitute a particle interaction in the liquid xenon.
The detector is a time-projection chamber (TPC), using the time between S1 and S2 signals to find the interaction depth since electrons move at constant velocity in the liquid (around 1–2 km/s, depending on the electric field). The x-y coordinate of the event is inferred from electroluminescence photons at the top array by statistical methods (Monte Carlo and maximum likelihood estimation) to a resolution under 1 cm.
Finding dark matter
WIMPs would be expected to interact exclusively with the liquid xenon nuclei, resulting in nuclear recoils that would appear very similar to neutron collisions. In order to single out WIMP interactions, neutron events must be minimized, through shielding and ultra-quiet building materials.
In order to discern WIMPs from neutrons, the number of single interactions must be compared to multiple events. Since WIMPs are expected to be so weakly interacting, most would pass through the detector unnoticed. Any WIMPs that interact will have negligible chance of repeated interaction. Neutrons, on the other hand, have a reasonably large chance of multiple collisions within the target volume, the frequency of which can be accurately predicted. Using this knowledge, if the ratio of single interactions to multiple interactions exceeds a certain value, the detection of dark matter may be reliably inferred.
Collaboration
The LUX collaboration is composed of more than 100 scientists and engineers across 18 institutions in the US and Europe. LUX is composed of the majority of the US groups that collaborated in the XENON10 experiment, most of the groups in the ZEPLIN III experiment, the majority of the US component of the ZEPLIN II experiment, and groups involved in low-background rare event searches such as Super Kamiokande, SNO, IceCube, Kamland, EXO and Double Chooz.
The LUX experiment's co-spokespersons are Richard Gaitskell from Brown University (acting as co-spokesperson since 2007) and Daniel McKinsey from Yale University (acting as co-spokesperson since 2012). Tom Shutt from Case Western Reserve University was LUX co-spokesperson between 2007-2012.
Status
Detector assembly began in late 2009. The LUX detector was commissioned overground at SURF for a six-month run. The assembled detector was transported underground from the surface laboratory in a two-day operation in the summer of 2012 and began data taking April 2013, presenting initial results Fall 2013. LUX plans to operate into 2015 and perform a blinded analysis of 300 live days. The next-generation 7-ton LUX-ZEPLIN has been approved, expected to begin in 2020.
Results
Initial unblinded data taken April to August 2013 were announced on October 30, 2013. In an 85 live-day run with 118 kg fiducial volume, LUX obtained 160 events passing the data analysis selection criteria, all consistent with electron recoil backgrounds. A profile likelihood statistical approach shows this result is consistent with the background-only hypothesis (no WIMP interactions) with a p-value of 0.35. This is the most sensitive dark matter direct detection result in the world, and rules out low-mass WIMP signal hints such as from CoGeNT and CDMS-II. These results have struck out some of the theories about WIMPs, which allows researchers to focus on fewer leads.
In the final run from October 2014 to May 2016, at four times its original design sensitivity with 368 kg of liquid xenon, LUX has seen no signs of dark matter candidate—WIMPs.