Large Underground Xenon
The Large Underground Xenon (LUX) is a 370 kg liquid xenon physics experiment that aims to directly detect interactions between Weakly Interacting Massive Particle (WIMP) dark matter and ordinary matter on Earth. Despite the wealth of evidence supporting the existence of non-baryonic dark matter in the Universe, dark matter in our galaxy has never been directly detected on Earth. The LUX experiment utilizes a large detector mass in a time-projection chamber (TPC) configuration to identify individual particle interactions in the liquid xenon volume, which will allow it to look for faint dark matter interactions with unprecedented sensitivity.
The LUX experiment is located 4,850 ft (about 1 mile) underground at the Sanford Underground Laboratory (formerly the Deep Underground Science and Engineering Laboratory, or DUSEL) in the Homestake Mine (South Dakota) in Lead, South Dakota. Underground, the detector is located in the Davis campus, the former site of the Nobel-prize winning Homestake neutrino experiment led by Raymond Davis. The LUX experiment needs to be operated underground in order to reduce signal background caused by high-energy cosmic rays at the Earth's surface.
Particle interactions in liquid xenon generate both 175 nm ultraviolet photons and electrons. The photons are immediately detected by two arrays of 61 photomultiplier tubes located at the top and bottom of the detector. These prompt photons are called the S1 signal. The electrons generated by the particle interaction are drifted upwards towards the xenon gas phase with an electric field in the liquid xenon. The electrons are extracted out of the liquid and into the gas by using a stronger electric field at the liquid xenon surface. In the gas, the electrons produce electroluminescence photons that are detected by the photomultiplier tubes as the S2 signal. The pair of an S1 and a subsequent S2 signal constitutes an individual particle interaction in the liquid xenon.
The LUX detector is a time-projection chamber (TPC) since it uses the time between the S1 and the S2 signals to find the depth of a given particle interaction. Because the electrons move with a constant velocity in the liquid xenon (around 1–2 km/s, depending on the strength of the electric field), the time delay of the S2 signal relative to the S1 signal can be converted into a physical depth (z-coordinate). The x-y coordinate of the event can be inferred by looking at the distribution of detected electroluminescence photons in the top PMT array. Since the S2 signal is generated about 5 cm below the top PMT array, the PMT nearest to the S2 electron bunch will receive the most photons. Statistical techniques (such as a Monte Carlo simulation and a maximum likelihood estimation) can be used to find the x-y location of a particle interaction from the top PMT photon hit pattern with a resolution smaller than 1 cm.
The apparatus is a cylindrical volume of liquid and gaseous xenon (hence "two-phase"). A uniform axial electric field is maintained using voltage rings, which are evenly spaced along the length of the detector. There are two arrays of photomultiplier tubes (phototubes), situated with one array at the top and the other at the bottom. The interior walls are designed to reflect as much light as possible, while trapping as few drifting electrons as possible, to maximize the detection of both.
Finding dark matter
WIMPs interact exclusively with the nuclei of the liquid xenon atoms, resulting in nuclear recoils that appear very similar to neutron collisions. In order to single out WIMP interactions, the number of neutron measurements must be reduced as much as possible. This is done through the use of proper shielding and ultra-quiet building materials.
In order to discern WIMPs from neutrons, the number of single interactions must be compared to the number of double and triple events. Since WIMPs are so weakly interacting, most of these particles will pass through the detector unnoticed. Those that do interact will have an almost non-existent chance of interacting a second time in the volume. Neutrons, on the other hand, have a reasonably large chance of having multiple collisions within the target volume. In a given volume, it is known statistically what percentage of neutrons will scatter a certain number of times. Using this knowledge, once the ratio of single interactions to multiple interactions exceeds a certain value, the detection of dark matter can be confirmed.
The LUX experimental 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 Prof. Richard Gaitskell from Brown University (acting as co-spokesperson since 2007) and Prof. Daniel McKinsey from Yale University (acting as co-spokesperson since 2012). Prof. Tom Shutt from Case Western Reserve University acted as LUX co-spokesperson between 2007-2012.
Surface commissioning and operation
The LUX detector was commissioned at a surface laboratory at SURF designed to fully emulate the underground laboratory. The detector assembly began in late 2009.
Underground transport and operation
The fully assembled detector was transported from the surface laboratory to the underground space in a two-day operation in the summer of 2012.
First LUX results
The results from an initial unblinded data run of the LUX experiment were announced on October 30, 2013. In an 85-live-day run with a 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 used by the collaboration shows that this result is consistent with the background-only hypothesis (no WIMP interactions) with a p-value of 0.35. The 85-day LUX result is the most sensitive dark matter direct detection result in the world, and rules out low-mass WIMP signal hints such as those from CoGeNT and CDMS-II Si. These results have struck out some of the theories about WIMPs, which allows researchers to focus on fewer leads.
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