Free to follow every thread. No paywall, no dead ends.
Xenon: the story on HearLore | HearLore
Xenon
In the deep silence of the Gran Sasso National Laboratory in Italy, 1400 meters beneath the Apennine Mountains, a team of physicists waits for a ghost to knock on a door. This is the XENON dark matter research project, a massive underground facility dedicated to detecting particles that have never been seen and may never be seen again. The goal is to find Weakly Interacting Massive Particles, or WIMPs, which are thought to make up the invisible scaffolding of the universe. These particles do not emit light, reflect light, or interact with normal matter in any way that is easily observable, except for the rare chance that they might collide with a xenon atom. The experiment uses a dual phase time projection chamber, a sophisticated device filled with liquid xenon that acts as both the target and the detector. When a particle strikes a xenon atom, it creates a flash of light and a trail of electrons, which are then amplified and recorded by photomultiplier tubes. The entire apparatus is designed to be so sensitive that it can detect the interaction of a single electron, yet it must also be so quiet that it ignores the constant rain of cosmic rays and background radiation that bombards the Earth. The detector is shielded by layers of water, lead, and polyethylene to ensure that only the rarest events are recorded. The collaboration, led by Italian professor of physics Elena Aprile from Columbia University, has spent over a decade refining this machine to find the first direct evidence of dark matter. The stakes are incredibly high, as a positive signal would rewrite the laws of physics and confirm the existence of a substance that constitutes about 85 percent of all matter in the universe.
The Liquid Target
The heart of the XENON experiment is a dual phase time projection chamber that relies on the unique properties of liquid xenon to distinguish between background noise and a potential dark matter signal. When a charged particle interacts with the liquid xenon, it produces two distinct signals: a prompt scintillation light known as the S1 signal, and a delayed electroluminescence signal called the S2 signal. The S1 signal consists of 178 nanometer ultraviolet photons that are detected by photomultiplier tubes located at the top and bottom of the detector. The electric field applied across the liquid and gaseous phases prevents the electrons from recombining with ions, allowing them to drift upward toward the gaseous phase. Once in the gas, a stronger electric field extracts the electrons and accelerates them to create a proportional scintillation signal, which is also collected by the photomultiplier tubes. This technique is so sensitive that it can detect S2 signals generated from single electrons, providing a level of precision that was previously unattainable. The detector allows for a full three-dimensional position determination of the particle interaction by measuring the time delay between the S1 and S2 signals and analyzing the number of photons seen by each individual photomultiplier tube. This 3D positioning enables the team to define a fiducial volume, a low-background region in the inner volume of the time projection chamber. The self-shielding properties of liquid xenon ensure that the rate of background events in this inner volume is greatly reduced compared to the edges of the detector. By using the ratio of the S2 to S1 signals, the experiment can discriminate between electronic recoils, which are caused by background radiation, and nuclear recoils, which are the expected signature of a WIMP interaction. This discrimination method suppresses backgrounds from electronic recoils by more than 99 percent while retaining 50 percent of the nuclear recoil events, making the detector one of the most powerful tools for dark matter searches.
Common questions
What is the XENON dark matter research project?
The XENON dark matter research project is a massive underground facility located 1400 meters beneath the Apennine Mountains in the Gran Sasso National Laboratory in Italy. It uses a dual phase time projection chamber filled with liquid xenon to detect Weakly Interacting Massive Particles or WIMPs that may constitute 85 percent of all matter in the universe.
Who leads the XENON collaboration and where is the experiment located?
The XENON collaboration is led by Italian professor of physics Elena Aprile from Columbia University and operates underground at the Gran Sasso National Laboratory in Italy. The facility is shielded by layers of water, lead, and polyethylene to protect the detector from cosmic rays and background radiation.
When did the XENON10 experiment begin and what were its results?
The XENON10 experiment began with installation at the Gran Sasso laboratory in March 2006 and analyzed 59 live days of data between October 2006 and February 2007. The results produced no WIMP signatures but placed limits on spin independent WIMP-nucleon cross sections down to below 10 to the minus 43 square centimeters for a WIMP mass of 100 GeV.
What happened during the XENON1T experiment in 2020 and 2022?
In June 2020 the XENON1T collaboration reported an excess of 285 electron recoils events which was 53 more than the expected 232 with a statistical significance of 3.5 sigma. A new analysis by XENONnT in July 2022 discarded the excess suggesting the anomaly was likely due to background effects rather than new physics.
When did the XENONnT experiment publish its first results and what did they show?
The XENONnT experiment published its first results of its search for WIMPs on the 28th of July 2023 excluding cross sections above 10 to the minus 47 square centimeters at 28 GeV with 90 percent confidence level. This result was published jointly on the same date the LZ experiment published its first results too excluding cross sections above 10 to the minus 47 square centimeters at 36 GeV with 90 percent confidence level.
The journey of the XENON project began in earnest with the installation of the XENON10 experiment at the Gran Sasso laboratory in March 2006. This initial detector was designed as a prototype to prove the efficacy of the XENON design and to verify the achievable threshold, background rejection power, and sensitivity. The XENON10 detector contained 15 kilograms of liquid xenon, with a sensitive volume measuring 20 centimeters in diameter and 15 centimeters in height. An analysis of 59 live days of data, taken between October 2006 and February 2007, produced no WIMP signatures, but the results were far from a failure. The number of events observed in the WIMP search region was statistically consistent with the expected number of events from electronic recoil backgrounds, which allowed the team to exclude some of the available parameter space in minimal Supersymmetric models. The experiment placed limits on spin independent WIMP-nucleon cross sections down to below 10 to the minus 43 square centimeters for a WIMP mass of 100 GeV. Due to nearly half of natural xenon having odd spin states, with 129Xe having an abundance of 26 percent and spin-1/2, and 131Xe having an abundance of 21 percent and spin-3/2, the XENON detectors could 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, establishing a benchmark for future experiments. The detector was placed within a shield consisting of an outer layer of 20 centimeters of water, a 20 centimeter layer of lead, a 20 centimeter layer of polyethylene, and on the interior a 5 centimeter copper layer to further reduce the background rate in the time projection chamber. The underground location of the laboratory provides 3100 meters of water-equivalent shielding, ensuring that the detector is protected from cosmic rays and other external interference.
Scaling the Sensitivity
The second phase of the project, XENON100, represented a significant leap in scale and sensitivity, containing 165 kilograms of liquid xenon with 62 kilograms in the target region and the remaining xenon in an active veto. The time projection chamber of the detector has a diameter of 30 centimeters and a height of 30 centimeters, and it was installed at the Gran Sasso National Laboratory in 2008 in the same shield as the XENON10 detector. 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, and in a few cases mass spectrometry was performed on low mass plastic samples. In doing so, the design goal of less than 10 to the minus 2 events per kilogram per day per kiloelectronvolt was reached, realizing the world's lowest background rate dark matter detector. The detector operated the then-lowest background experiment for dark matter searches, with a background of 50 events per tonne per year, which is equivalent to 10 to the minus 3 events per kilogram per day per kiloelectronvolt. 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 10 to the minus 45 square centimeters for a WIMP mass of 30 GeV. These results constrained interpretations of signals in other experiments as dark matter interactions and ruled out exotic models such as inelastic dark matter, which would resolve this discrepancy. XENON100 has also provided improved limits on the spin dependent WIMP-nucleon cross section, and an axion result was published in 2014, setting a new best axion limit. The top photomultiplier tube array of XENON100 contains 98 Hamamatsu R8520-06-A1 photomultiplier tubes, which are placed in concentric circles to improve the reconstruction of the radial position of observed events, while the bottom array contains 80 photomultiplier tubes which are spaced as closely as possible in order to maximize light collection efficiency.
The Ton-Scale Breakthrough
Construction of the next phase, XENON1T, started in Hall B of the Gran Sasso National Laboratory in 2014, marking a transition to the ton-scale era of dark matter detection. The detector contains 3.2 tons of ultra radio-pure liquid xenon, and has a fiducial volume of about 2 tons, housed in a 10-meter water tank that serves as a muon veto. The time projection chamber is 1 meter in diameter and 1 meter in height, and the project team, called the XENON Collaboration, is composed of 135 investigators across 22 institutions from Europe, the Middle East, and the United States. The first results from XENON1T were released by the XENON collaboration on the 18th of May 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 10 to the minus 45 square centimeters for WIMP masses of 30 GeV. In September 2018, the XENON1T experiment published its results from 278.8 days of collected data, setting a new record limit for WIMP-nucleon spin-independent elastic interactions, with a minimum of 10 to the minus 47 square centimeters at a WIMP mass of 30 GeV. 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. 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, with the detector operations ending at the end of 2018.
The Anomaly and the Answer
In June 2020, the XENON1T collaboration reported an excess of electron recoils, with 285 events observed, which was 53 more than the expected 232 with a statistical significance of 3.5 sigma. Three explanations were considered for this anomaly: the 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, 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. The question of whether dark energy had been detected by the XENON1T experiment was raised by Cambridge scientists, but in July 2022 a new analysis by XENONnT discarded the excess, suggesting that the anomaly was likely due to background effects rather than new physics. The XENON1T experiment operated from 2016 to 2018, and the data taken during this period provided crucial insights into the nature of background radiation and the sensitivity of the detector. The collaboration's ability to identify and resolve the anomaly demonstrated the robustness of their analysis methods and the importance of continuous monitoring and refinement of the detector's performance. The excess of electron recoils, while not confirming the presence of dark matter or dark energy, highlighted the challenges of distinguishing between rare signals and background noise in such a sensitive experiment. The XENON1T results also underscored the need for even larger and more sensitive detectors to push the boundaries of what is possible in dark matter research.
The Next Generation
XENONnT is an upgrade of the XENON1T experiment underground at the Gran Sasso National Laboratory, designed to 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, marking the beginning of the neutrino fog era in dark matter detection. The XENONnT detector was under construction in March 2020, and even with the problems posed by the COVID-19 pandemic, the project was able to finish construction and move forwards into commissioning phase by mid 2020. Full detector operations commenced in late 2020, and in September 2021, XENONnT was taking science data for its first science run, which was ongoing at the time. On the 28th of July 2023, the XENONnT published the first results of its search for WIMPs, excluding cross sections above 10 to the minus 47 square centimeters at 28 GeV with 90 percent confidence level, jointly on the same date the LZ experiment published its first results too excluding cross sections above 10 to the minus 47 square centimeters at 36 GeV with 90 percent confidence level. The XENONnT experiment represents the culmination of over a decade of research and development, building on the successes and lessons learned from the previous generations of XENON detectors. The collaboration continues to push the boundaries of what is possible in dark matter research, with the goal of either detecting a WIMP signal or setting even more stringent limits on the properties of these elusive particles. The XENONnT detector is a testament to the power of international collaboration and the relentless pursuit of knowledge in the face of one of the greatest mysteries in modern physics.