The external structure and tank of the XENON experiment at the INFN Gran Sasso National Laboratories (LNGS). Credit: XENON Collaboration/LNGS-INFN.
Below the Gran Sasso mountain, in central Italy, there’s a tank loaded with more than eight tonnes of xenon, a noble gas that is normally found in small traces in the atmosphere. In 2020, while the rest of the world was shutting down due to the pandemic, physicists and technicians spent months assembling and tuning the tank and the experimental apparatus that surrounds it, called XENONnT. For the next five years, they will closely observe what happens inside it, hoping to detect the feeble, distinctive signal that xenon atoms would make if a dark matter particle bumps them.
Detecting a particle of dark matter would be the next big thing to happen in physics, after the discovery of the Higgs Boson in 2012, and the first detection of a gravitational wave in 2015. In both those cases Italian researchers were heavily involved, but this time Italy is on the very short list of countries where the historic event could take place.
XENONnT, that has just started scientific work, is currently the largest and most sensitive dark matter detector in the world, with an active mass (the amount of xenon actually used for detection) of six tonnes. In addition, the National Laboratories of Gran Sasso (LNGS) is also set to host another dark matter detector based on a different technology, called DarkSide-20K and containing 50 tonnes of argon, scheduled to begin operations in 2025. And the same laboratory may one day host Darwin, a 50-tonne xenon device.
All of this puts the Italian laboratory ahead in the race towards the first dark matter detection. But the result is far from assured, because of competition with other experiments in China and the United States, ongoing negotiations on the final location of Darwin, and uncertainty on whether dark matter is really what physicists think it is.
Larger and larger
XENONnT is the scaled-up version of its predecessor XENON1T, that contained two tonnes of xenon and took data from 2016 to 2018. COVID-19 struck Italy when the new experiment was being installed at Gran Sasso delaying its commissioning by four months, explains Marco Selvi, the scientist responsible for the experiment at the National Institute of Nuclear Physics (INFN). Now the first run has started, says Selvi, and the instrument should remain operational for five years.
There are various hypothesis on what dark matter particles could be. Noble gas experiments such as XENONnT are targeted to what scientists consider the most likely option, weakly interacting massive particles (WIMPs). These particles would be heavier than the proton, and would interact with ordinary matter only via gravity and, maybe, the weak force. The appeal of WIMPs comes from the fact that they match well with supersymmetry – a theory that suggests that each known particle has a heavier counterpart that is yet to be discovered, and that is the best contender among the physical theories that try to overcome the limitations of the Standard Model.
Scientists expect that, once in a while, a WIMP could hit one of the xenon atoms in the tank, causing a burst of light and the emission of an electron that the detector in the experiment would pick up. By increasing the size of detectors, physicist try to capture candidate dark matter particles that interact less and less with the atoms in the gases, at the same time exploring a range of possible masses. With its 6 tonnes of active mass and improved technology, XENONnT will be about 10 times more sensitive than its predecessor, according to Selvi. But detecting a WIMP would be an extremely rare event anyway. The experiment’s team expect no more than one event per year at best.
XENONnT has two direct competitors based on the same technology. A 4-tonne experiment called PandaX-4t has been running for 100 days in China JinPing Underground Laboratory (CJPL) in Sichuan, and has already improved the limits previously set by XENON1T, according to its first data presented at a conference in July. At the Sanford Underground Research Facility in the US, the first scientific run of the 7-tonne LUX-ZEPLIN (LZ) experiment is expected to start before the end of 2021, according to spokesperson Hugh Lippincott, at the University of California Santa Barbara. The rationale of having several different experiments of the same sort is that “if one discovers a signal it would be nice to have an independent confirmation”, says Lippincott.
Other experiments use argon instead of xenon. Dark matter particles are expected to behave differently when bumping on different nuclei, and this would provide additional information about their features, especially their mass. Unlike the xenon community, the argon one concentrates on a single experiment, DarkSide-20k, scheduled to start at Gran Sasso in four years. “We started with different experiments, but then realized that it did not make sense to replicate the same one many times”, says Cristiano Galbiati, co-director of DarkSide-20k and a professor at Princeton University.
Gearing up for the next generation
In case dark matter is not found by the current generation of noble gases experiments, physicists are already preparing the next one.
The argon community is making plans on Argo, a detector with 300 to 400 tonnes of argon to be located at the Snolab laboratory near Sudbury, in Canada. On the xenon side, a European-led alliance has put forward a proposal for Darwin, a 50-tonne detector, and signaled intent to build it at the Gran Sasso Laboratories.
The project is currently funded by two European Research Council grants and other national grants and may be ready to start construction around 2025, according to Darwin’s spokesperson Laura Baudis, of the University of Zurich. But whether the experiment will actually end up in Italy, or its name and design, is still uncertain.
The current worldwide production of xenon does not allow more than one experiment that size. “The yearly production is about 70 tonnes, but much goes to contractors like NASA, the industry, medical applications, and so on,” says Baudis. “We are already stockpiling, in fact I just bought a few hundred kilos”. This is one of the reasons that is pushing the xenon community together. Darwin and LZ representatives signed a Memorandum of Understanding in July 2021, in which they commit to work together at the next detector, but leave the location open. “If it depends on Europe, Darwin will go to Gran Sasso”, says the laboratory’s director Ezio Previtali. But according to Lippincott, Snolab and the Sanford Underground Research Facility may also be good options.
Things could become clearer after Snowmass 2021, the planning exercise of the US particle physics community that begins this month. By the summer of 2022 it should be clear how much US physicists are committed to large dark matter experiments.
China is not part of the conversation. “Our lab is undergoing a major construction and we are targeting an experiment with about 30 tonnes of xenon,” says Jianglai Liu, spokesperson of PandaX-4t. “But it is at a very early stage”.
Wherever they end up, the next generation of detectors may be the last one. “If we don’t find dark matter particles with Darwin, it will be difficult to convince people to build an even larger detector”, says Baudis. Those devices will get too close to the so-called neutrino floor, a theoretical limit where signals from dark particles would be undistinguishable from those from neutrinos. An informal alliance of research groups from several different countries is proposing a new detector design called Cygnus, that may distinguish dark particles from neutrinos by their direction. Others have turned to smaller experiments based on solid materials (one of them, CRESST, is also at Gran Sasso), that may detect WIMPS in the lucky case that their mass is very small.
If none of this works, physicists would have to reconsider the WIMPS hypothesis altogether. Dark matter may be made by much lighter particles, like axions. Or there may not be such thing as dark matter, a conclusion that would force scientists to reconsider their understanding of gravity.
There is little disagreement among physicists that it’s worthwhile to explore every avenue. “Questions on dark matter are so important for science” Galbiati concludes. “Those are discoveries that would change physics’ history, and they don’t come for free”.
