Large Underground Xenon experiment

The Large Underground Xenon experiment (LUX) physics experiment aims to directly detect weakly interacting massive particle (WIMP) dark matter interactions with ordinary matter on Earth. Despite the wealth of evidence supporting the existence of non-baryonic dark matter in the Universe,^[1] dark matter in our galaxy has never been directly detected on Earth. 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.^[2]

The LUX experiment is located 4,850 ft (about 1 mile) 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 needs to be operated underground to reduce signal background caused by high-energy cosmic rays at the Earth's surface. A multi-ton upgrade is planned.

The Large Underground Xenon experiment installed 4,850 ft underground inside a 70,000 gallon water tank shield. The experiment is a 370 kg liquid xenon time projection chamber that aims to detect the faint interactions between WIMP dark matter and ordinary matter.

Detector principle

The detector is isolated from background particles by a surrounding water tank and above earth shielding that reduce 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.^[3]

Particle interactions inside the LUX detector produce photons and electrons. The photons (γ), moving at the speed of light, are quickly detected by the photomultiplier tubes. This photon signal is called S1. An electric field in the liquid xenon drifts the electrons towards the liquid surface. A much higher electric field above the liquid surface pulls the electrons out of the liquid and into the gas, where they produce electroluminescence photons (in the same way that neon sign produces light). The electroluminescence photons are detected by the photomultiplier tubes as the S2 signal. A single particle interaction in the liquid xenon can be identified by the pair of an S1 and an S2 signal.

Schematic of the Large Underground Xenon (LUX) detector. The detector consists of an inner cryostat filled with 370 kg of liquid xenon (300 kg in the inner region, called the "active volume") cooled to -100 °C. 122 photomultiplier tubes detect light generated inside the detector. The LUX detector has an outer cryostat that provides vacuum insulation. An 8-meter diameter by 6 meter high water tank shields the detector from external radiation, such as gamma rays and neutrons.

Finding dark matter

WIMPs interact exclusively with the liquid xenon nuclei, resulting in nuclear recoils that 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 so weakly interacting, most will pass through the detector unnoticed. WIMPs that interact will have negligible chance of repeated interaction. Neutrons have a reasonably large chance of multiple collisions within the target volume, which can be predicted. Using this knowledge, once the ratio of single interactions to multiple interactions exceeds a certain value, the detection of dark matter can be confirmed.

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,^[4] expecting data in 2016.

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.^[5][6] These results have struck out some of the theories about WIMPs, which allows researchers to focus on fewer leads.^[7]

References

1. Beringer, J.; et al. (2012). "2012 Review of Particle Physics" (PDF). Phys. Rev. D 86 (010001). Bibcode:2012PhRvD..86a0001B. doi:10.1103/PhysRevD.86.010001.
2. Akerib, D.; et al. (March 2013). "The Large Underground Xenon (LUX) experiment". Nuclear Instruments and Methods in Physics Research A 704: 111–126. arXiv:1211.3788. Bibcode:2013NIMPA.704..111A. doi:10.1016/j.nima.2012.11.135.
3. Akerib; et al. (May 2013). "Technical results from the surface run of the LUX dark matter experiment". Astroparticle Physics 45: 34–43. Bibcode:2013APh....45...34A. doi:10.1016/j.astropartphys.2013.02.001.
4. "Dark-matter searches get US government approval". Physics World. July 15, 2014.
5. Akerib, D. "First results from the LUX dark matter experiment at the Sanford Underground Research Facility" (PDF). Retrieved 30 October 2013.
6. Dark Matter Search Comes Up Empty Fox News, 2013 October 30
7. Dark matter experiment finds nothing, makes news The Conversation, 01 November 2013

External links

• LUX Dark Matter webpage
• Brown University article