By Christina Ignarra for the LUX collaboration
The Black Hills of South Dakota have long held a powerful lure for gold-hungry prospectors, deep in their heart. But now, physicists are looking to discover a new form of treasure down in the former Homestake gold mine, nearly a mile underground those long-fabled Black Hills. Our LUX collaboration is searching for particles called Weakly Interacting Massive Particles (WIMPs), a potential solution to the mystery of dark matter. Physicists are extremely confident now that dark matter makes up 85% of the matter in our universe, yet we have only a precious few clues to its physical makeup. The discovery of WIMPs would guide the fields of particle physics, astrophysics, and cosmology for decades to come.
LUX, or the “Large Underground Xenon” Experiment, whose central containment vessel is pictured above, is a particle detector consisting of hundreds of kilograms of xenon, located deep underground, in which we look for individual scatters from dark matter particles. The underground location is necessary in order to shield the detector from cosmic rays—charged particles which rain down on the Earth’s surface at a rate of about one particle per centimeter squared per second (at sea level). These cosmic rays would produce so much background in a surface detector that it would be impossible to sift out the much much rarer WIMP “nuggets”. However, down 4850 feet below the surface, this rate is reduced to 1-2 particles per day through the entire detector. Radiation from the rock itself is shielded by a 8 m diameter water tank, which also serves as a “veto” region to tag particles that pass through it. We are also sensitive to radiation coming from the water tank and the detector components themselves, which is shielded in part by the xenon itself due to its high density. To take advantage of that “self shielding”, it is essential that we know the location of each event.
LUX is a dual-phase liquid/gas xenon Time Projection Chamber (TPC), pictured above. Now, a TPC is not a device Dr. Who might employ, but rather one of the most sophisticated forms of modern particle detectors. When a WIMP (or other particle) interacts with one of the xenon atoms in the liquid portion of the detector, it causes the xenon nucleus to recoil. A nuclear recoil in the detector leads to a burst of scintillation light and to the ionization of the surrounding xenon atoms, leading to the liberation of “ionization electrons”. We refer to the initial burst of light as the “S1” signal. The ionization electrons subsequently drift to the top of the detector, driven by an applied electric field, where they encounter the gas-liquid interface. There, they produce a second burst of light through electroluminescence, which we call the “S2” signal. The resultant photons from each burst of light are detected by 122 Photomultiplier Tubes (PMTs), split between the top and bottom of the detector. The two signals, S1 and S2, are separated by the drift time of the electrons, from which we can infer the depth of the event. Combining this information with the geometrical patterns produced by the number of photoelectrons detected by each PMT, the three dimensional position can be reconstructed for each event. This position information is vital for discrimination between signal and background since the xenon self-shielding causes most of the background events to be concentrated near the walls of the detector. Uniform backgrounds such as the decays of radioactive elements contaminating the xenon itself, will produce electron-recoils rather than nuclear recoils in the detector. These events can be distinguished from that of nuclear recoils based on the ratios of their S1 and S2 signals.
[Plot above is from: Snowmass Community Summer Study 2013 CF1: WIMP Dark Matter Detection ]
We report our results in terms of two parameters that would describe WIMPs: their mass and their probability of interaction (which we call a cross-section). We do not know the exact values of these parameters, so we show our results in terms of excluding regions of their potential values. More sensitive experiments can probe WIMPs with less frequent interaction rates. The mass-dependent shape of the resulting limits can be explained by the effect of the mass on the detection probability. Lower mass particles will produce less of a signal in the detector due to their lower energy, making them harder to see. On the other hand, higher mass particles would have a lower flux through the detector since the observed gravitational physics of dark matter would require less of them. The figure above shows a large number of limits from current experiments (solid lines) and expected sensitivities for future experiments (dotted lines). LUX currently holds the world’s best direct dark-matter detection limit for most WIMP masses, as indicated by the shaded green region in the figure above.
LUX is currently taking a 300-day run, which will have even greater sensitivity than its previously published 2013 result of 85 days. LUX will run for several more years, at which point it will be disassembled and installation of the next generation detector, LZ, will begin in the Homestake mine. LZ combines technology and people from both the LUX and ZEPLIN detectors, and will contain 7000 kg of xenon. Our group here at SLAC is involved with R&D for LZ, improving upon the designs developed for LUX and solving the challenges associated with building a larger and more sensitive detector. These include a greater radioactive purity of xenon, particularly the removal of krypton-85 which must be reduced to sub-parts per trillion (!) quantities. LZ will explore most of the theoretically-favored WIMP parameter space. By combining the results using liquid xenon technology, sensitive to higher masses due to its heavy nucleus, with others such as germanium which is sensitive to a lower mass WIMP, we can confirm a discovery -- or lack-thereof -- of this form of dark matter within the coming years.
Above: The LUX team courageously braving the chill at their March 2014 collaboration meeting on site at the Sanford Underground Research Facility (SURF). Note the special team member at the far right, ready to jump into the analysis phase of the experiment.
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