The XENON dark matter research project, operated at the Italian Gran Sasso National Laboratory, is a deep underground detector facility featuring increasingly ambitious experiments aiming to detect hypothetical dark matter particles. The experiments aim to detect particles in the form of weakly interacting massive particles (WIMPs) by looking for rare nuclear recoil interactions in a liquid xenon target chamber. The current detector consists of a dual phase time projection chamber (TPC).
The experiment detects scintillation and ionization signals produced when external particles interact in the liquid xenon volume, to search for an excess of nuclear recoil events against known backgrounds. The detection of such a signal would provide the first direct experimental evidence for dark matter candidate particles. The collaboration is currently led by Italian professor of physics Elena Aprile from Columbia University.
The XENON experiment operates a dual phase time projection chamber (TPC), which utilizes a liquid xenon target with a gaseous phase on top. Two arrays of photomultiplier tubes (PMTs), one at the top of the detector in the gaseous phase (GXe), and one at the bottom of the liquid layer (LXe), detect scintillation and electroluminescence light produced when charged particles interact in the detector. Electric fields are applied across both the liquid and gaseous phase of the detector. The electric field in the gaseous phase has to be sufficiently large to extract electrons from the liquid phase.
Particle interactions in the liquid target produce scintillation and ionization. The prompt scintillation light produces 178 nm ultraviolet photons. This signal is detected by the PMTs, and is referred to as the S1 signal. The applied electric field prevents recombination of all the electrons produced from a charged particle interaction in the TPC. These electrons are drifted to the top of the liquid phase by the electric field. The ionization is then extracted into the gas phase by the stronger electric field in the gaseous phase. The electric field accelerates the electrons to the point that it creates a proportional scintillation signal that is also collected by the PMTs, and is referred to as the S2 signal. This technique has proved sensitive enough to detect S2 signals generated from single electrons. [1]
The detector allows for a full 3-D position determination [2] of the particle interaction. Electrons in liquid xenon have a uniform drift velocity. This allows the interaction depth of the event to be determined by measuring the time delay between the S1 and S2 signal. The position of the event in the x-y plane can be determined by looking at the number of photons seen by each of the individual PMTs. The full 3-D position allows for the fiducialization of the detector, in which a low-background region is defined in the inner volume of the TPC. This fiducial volume has a greatly reduced rate of background events as compared to regions of the detector at the edge of the TPC, due to the self-shielding properties of liquid xenon. This allows for a much higher sensitivity when searching for very rare events.
Charged particles moving through the detector are expected to either interact with the electrons of the xenon atoms producing electronic recoils, or with the nucleus, producing nuclear recoils. For a given amount of energy deposited by a particle interaction in the detector, the ratio of S2/S1 can be used as a discrimination parameter to distinguish electronic and nuclear recoil events. [3] This ratio is expected to be greater for electronic recoils than for nuclear recoils. In this way backgrounds from electronic recoils can be suppressed by more than 99%, while simultaneously retaining 50% of the nuclear recoil events.
The XENON10 experiment was installed at the underground Gran Sasso laboratory in Italy during March 2006. The underground location of the laboratory provides 3100 m of water-equivalent shielding. The detector was placed within a shield to further reduce the background rate in the TPC. XENON10 was intended as a prototype detector, to prove the efficacy of the XENON design, as well as verify the achievable threshold, background rejection power and sensitivity. The XENON10 detector contained 15 kg of liquid xenon. The sensitive volume of the TPC measures 20 cm in diameter and 15 cm in height. [4]
An analysis of 59 live days of data, taken between October 2006 and February 2007, produced no WIMP signatures. The number of events observed in the WIMP search region is statistically consistent with the expected number of events from electronic recoil backgrounds. This result excluded some of the available parameter space in minimal Supersymmetric models, by placing limits on spin independent WIMP-nucleon cross sections down to below 10×10−43 cm2 for a 30 GeV/c2 WIMP mass. [5]
Due to nearly half of natural xenon having odd spin states (129Xe has an abundance of 26% and spin-1/2; 131Xe has an abundance of 21% and spin-3/2), the XENON detectors can also be used to provide limits on spin dependent WIMP-nucleon cross sections for coupling of the dark matter candidate particle to both neutrons and protons. XENON10 set the world's most stringent restrictions on pure neutron coupling. [6]
The second phase detector, XENON100, contains 165 kg of liquid xenon, with 62 kg in the target region and the remaining xenon in an active veto. The TPC of the detector has a diameter of 30 cm and a height of 30 cm. As WIMP interactions are expected to be extremely rare events, a thorough campaign was launched during the construction and commissioning phase of XENON100 to screen all parts of the detector for radioactivity. The screening was performed using high-purity Germanium detectors. In a few cases mass spectrometry was performed on low mass plastic samples. In doing so the design goal of <10−2 events/kg/day/keV [7] was reached, realising the world's lowest background rate dark matter detector.
The detector was installed at the Gran Sasso National Laboratory in 2008 in the same shield as the XENON10 detector, and has conducted several science runs. In each science run, no dark matter signal was observed above the expected background, leading to the most stringent limit on the spin independent WIMP-nucleon cross section in 2012, with a minimum at 2.0×10−45 cm2 for a 65 GeV/c2 WIMP mass. [8] These results constrain interpretations of signals in other experiments as dark matter interactions, and rule out exotic models such as inelastic dark matter, which would resolve this discrepancy. [9] XENON100 has also provided improved limits on the spin dependent WIMP-nucleon cross section. [10] An axion result was published in 2014, [11] setting a new best axion limit.
XENON100 operated the then-lowest background experiment, for dark matter searches, with a background of 50 mDRU (1 mDRU=10−3 events/kg/day/keV). [12]
Construction of the next phase, XENON1T, started in Hall B of the Gran Sasso National Laboratory in 2014. The detector contains 3.2 tons of ultra radio-pure liquid xenon, and has a fiducial volume of about 2 tons. The detector is housed in a 10 m water tank that serves as a muon veto. The TPC is 1 m in diameter and 1 m in height.
The detector project team, called the XENON Collaboration, is composed of 135 investigators across 22 institutions from Europe, the Middle East, and the United States. [13]
The first results from XENON1T were released by the XENON collaboration on May 18, 2017, based on 34 days of data-taking between November 2016 and January 2017. While no WIMPs or dark matter candidate signals were officially detected, the team did announce a record low reduction in the background radioactivity levels being picked up by XENON1T. The exclusion limits exceeded the previous best limits set by the LUX experiment, with an exclusion of cross sections larger than 7.7×10−47 cm2 for WIMP masses of 35 GeV/c2. [14] [15] Because some signals that the detector receives might be due to neutrons, reducing the radioactivity increases the sensitivity to WIMPs. [16]
In September 2018 the XENON1T experiment published its results from 278.8 days of collected data. A new record limit for WIMP-nucleon spin-independent elastic interactions was set, with a minimum of 4.1×10−47 cm2 at a WIMP mass of 30 GeV/c2. [17]
In April 2019, based on measurements performed with the XENON1T detector, the XENON Collaboration reported in Nature the first direct observation of two-neutrino double electron capture in xenon-124 nuclei. [18] The measured half-life of this process, which is several orders of magnitude larger than the age of the Universe, demonstrates the capabilities of xenon-based detectors to search for rare events and showcases the broad physics reach of even larger next-generation experiments. This measurement represents a first step in the search for the neutrinoless double electron capture process, the detection of which would provide insight into the nature of the neutrino and allow to determine its absolute mass.
As of 2019, the XENON1T experiment has stopped data-taking to allow for construction of the next phase, XENONnT. [19] The XENON1T detector operated 2016–2018, with the detector operations ending at the end of 2018. [20]
In June 2020, the XENON1T collaboration reported an excess of electron recoils: 285 events, 53 more than the expected 232 [21] [22] with a statistical significance of 3.5σ. [23] Three explanations were considered: existence of to-date-hypothetical solar axions, a surprisingly large magnetic moment for neutrinos, and tritium contamination in the detector. Multiple other explanations were given later by others groups [24] and in 2021 an interpretation of the results not as dark matter particles but of as dark energy particles candidates called chameleons has also been discussed. [25] [26] In July 2022 a new analysis by XENONnT discarded the excess. [27] [28]
XENONnT is an upgrade of the XENON1T experiment underground at LNGS. Its systems will contain a total xenon mass of more than 8 tonnes. Apart from a larger xenon target in its time projection chamber the upgraded experiment will feature new components to further reduce or tag radiation that otherwise would constitute background to its measurements. It is designed to reach a sensitivity (in a small part of the mass-range probed) where neutrinos become a significant background. As of 2019, the upgrade was on-going and first light was expected in 2020. [19] [29]
The XENONnT detector was under construction in March 2020. Even with the problems posed by COVID-19, the project was able to finish construction and move forwards into commissioning phase by mid 2020. Full detector operations commenced in late 2020. [20] [30] In September 2021, XENONnT was taking science data for its first science run, which was ongoing at the time. [31]
On 28 July 2023 the XENONnT published the first results of its search for WIMPs, [32] excluding cross sections above at 28 GeV with 90% confidence level, [33] jointly on the same date the LZ experiment published its first results too excluding cross sections above at 36 GeV with 90% confidence level. [34]
Weakly interacting massive particles (WIMPs) are hypothetical particles that are one of the proposed candidates for dark matter.
The DAMA/NaI experiment investigated the presence of dark matter particles in the galactic halo by exploiting the model-independent annual modulation signature. Based on the Earth's orbit around the Sun and the solar system's speed with respect to the center of the galaxy, the Earth should be exposed to a higher flux of dark matter particles around June 1, when its orbital speed is added to the one of the solar system with respect to the galaxy and to a smaller one around December 2, when the two velocities are subtracted. The annual modulation signature is distinctive since the effect induced by dark matter particles must simultaneously satisfy many requirements.
The Cryogenic Dark Matter Search (CDMS) is a series of experiments designed to directly detect particle dark matter in the form of Weakly Interacting Massive Particles. Using an array of semiconductor detectors at millikelvin temperatures, CDMS has at times set the most sensitive limits on the interactions of WIMP dark matter with terrestrial materials. The first experiment, CDMS I, was run in a tunnel under the Stanford University campus. It was followed by CDMS II experiment in the Soudan Mine. The most recent experiment, SuperCDMS, was located deep underground in the Soudan Mine in northern Minnesota and collected data from 2011 through 2015. The series of experiments continues with SuperCDMS SNOLAB, an experiment located at the SNOLAB facility near Sudbury, Ontario, in Canada that started construction in 2018 and is expected to start data taking in early 2020s.
The ArDM Experiment was a particle physics experiment based on a liquid argon detector, aiming at measuring signals from WIMPs, which may constitute the Dark Matter in the universe. Elastic scattering of WIMPs from argon nuclei is measurable by observing free electrons from ionization and photons from scintillation, which are produced by the recoiling nucleus interacting with neighbouring atoms. The ionization and scintillation signals can be measured with dedicated readout techniques, which constituted a fundamental part of the detector.
DEAP is a direct dark matter search experiment which uses liquid argon as a target material. DEAP utilizes background discrimination based on the characteristic scintillation pulse-shape of argon. A first-generation detector (DEAP-1) with a 7 kg target mass was operated at Queen's University to test the performance of pulse-shape discrimination at low recoil energies in liquid argon. DEAP-1 was then moved to SNOLAB, 2 km below Earth's surface, in October 2007 and collected data into 2011.
The Large Underground Xenon experiment (LUX) aimed 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 utilized 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.
Light dark matter, in astronomy and cosmology, are dark matter weakly interacting massive particles (WIMPS) candidates with masses less than 1 GeV. These particles are heavier than warm dark matter and hot dark matter, but are lighter than the traditional forms of cold dark matter, such as Massive Compact Halo Objects (MACHOs). The Lee-Weinberg bound limits the mass of the favored dark matter candidate, WIMPs, that interact via the weak interaction to GeV. This bound arises as follows. The lower the mass of WIMPs is, the lower the annihilation cross section, which is of the order , where m is the WIMP mass and M the mass of the Z-boson. This means that low mass WIMPs, which would be abundantly produced in the early universe, freeze out much earlier and thus at a higher temperature, than higher mass WIMPs. This leads to a higher relic WIMP density. If the mass is lower than GeV the WIMP relic density would overclose the universe.
The DAMA/LIBRA experiment is a particle detector experiment designed to detect dark matter using the direct detection approach, by using a matrix of NaI(Tl) scintillation detectors to detect dark matter particles in the galactic halo. The experiment aims to find an annual modulation of the number of detection events, caused by the variation of the velocity of the detector relative to the dark matter halo as the Earth orbits the Sun. It is located underground at the Laboratori Nazionali del Gran Sasso in Italy.
EDELWEISS is a dark matter search experiment located at the Modane Underground Laboratory in France. The experiment uses cryogenic detectors, measuring both the phonon and ionization signals produced by particle interactions in germanium crystals. This technique allows nuclear recoils events to be distinguished from electron recoil events.
Manfred Lindner is a German physicist and director at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. He conducts basic research in particle and astro-particle physics.
SIMPLE is an experiment search for direct evidence of dark matter. It is located in a 61 m3 cavern at the 500 level of the Laboratoire Souterrain à Bas Bruit near Apt in southern France. The experiment is predominantly sensitive to spin-dependent interactions of weakly interacting massive particles.
The ZEPLIN-III dark matter experiment attempted to detect galactic WIMPs using a 12 kg liquid xenon target. It operated from 2006 to 2011 at the Boulby Underground Laboratory in Loftus, North Yorkshire. This was the last in a series of xenon-based experiments in the ZEPLIN programme pursued originally by the UK Dark Matter Collaboration (UKDMC). The ZEPLIN-III project was led by Imperial College London and also included the Rutherford Appleton Laboratory and the University of Edinburgh in the UK, as well as LIP-Coimbra in Portugal and ITEP-Moscow in Russia. It ruled out cross-sections for elastic scattering of WIMPs off nucleons above 3.9 × 10−8 pb from the two science runs conducted at Boulby.
The Particle and Astrophysical Xenon Detector, or PandaX, is a dark matter detection experiment at China Jinping Underground Laboratory (CJPL) in Sichuan, China. The experiment occupies the deepest underground laboratory in the world, and is among the largest of its kind.
The Cryogenic Low-Energy Astrophysics with Noble liquids (CLEAN) experiment by the DEAP/CLEAN collaboration is searching for dark matter using noble gases at the SNOLAB underground facility. CLEAN has studied neon and argon in the MicroCLEAN prototype, and running the MiniCLEAN detector to test a multi-ton design.
Elena Aprile is an Italian-American experimental particle physicist. She has been a professor of physics at Columbia University since 1986. She is the founder and spokesperson of the XENON Dark Matter Experiment. Aprile is well known for her work with noble liquid detectors and for her contributions to particle astrophysics in the search for dark matter.
The LUX-ZEPLIN (LZ) Experiment is a next-generation dark matter direct detection experiment hoping to observe weakly interacting massive particles (WIMP) scatters on nuclei. It was formed in 2012 by combining the LUX and ZEPLIN groups. It is currently a collaboration of 30 institutes in the US, UK, Portugal and South Korea. The experiment is located at about 1,500 meteres under the Sanford Underground Research Facility (SURF) in South Dakota, and is managed by the United States Department of Energy's (DOE) Lawrence Berkeley National Lab.
The CoGeNT experiment has searched for dark matter. It uses a single germanium crystal as a cryogenic detector for WIMP particles. CoGeNT has operated in the Soudan Underground Laboratory since 2009.
ANAIS is a dark matter direct detection experiment located at the Canfranc Underground Laboratory (LSC), in Spain, operated by a team of researchers of the CAPA at the University of Zaragoza.
Direct detection of dark matter is the science of attempting to directly measure dark matter collisions in Earth-based experiments. Modern astrophysical measurements, such as from the Cosmic Microwave Background, strongly indicate that 85% of the matter content of the universe is unaccounted for. Although the existence of dark matter is widely believed, what form it takes or its precise properties has never been determined. There are three main avenues of research to detect dark matter: attempts to make dark matter in accelerators, indirect detection of dark matter annihilation, and direct detection of dark matter in terrestrial labs. The founding principle of direct dark matter detection is that since dark matter is known to exist in the local universe, as the Earth, Solar System, and the Milky Way Galaxy carve out a path through the universe they must intercept dark matter, regardless of what form it takes.
Daniel S. Akerib is an American particle physicist and astrophysicist. He was elected in 2008 a fellow of the American Physical Society (APS).