Ettore Majorana, an Italian physicist, infamously vanished in 1938 with no sign of him. Neutrinos, one of his favorite elementary particles, might be able to perform a similar disappearing performance. Around the world, a number of new or upgraded experiments are racing to demonstrate that a very uncommon type of nuclear decay that typically yields two neutrinos might occasionally release none. Though they have gotten less funding or attention than initiatives to find dark matter, these experiments could have an equally large influence on physics. The disappearance of particles would imply that neutrinos and antineutrinos, their antimatter counterparts, are identical, a notion Majorana initially proposed1 in the 1930s. If neutrinos are Majorana particles, then these “Majorana neutrinos” may hold the key to explaining why the universe appears to have relatively little antimatter (see “Questions that could be answered if neutrinos are Majorana particles”). Additionally, they would demonstrate that neutrinos do not acquire their mass from the Higgs boson, in contrast to all other known particles of matter like quarks and electrons.

Since the search for the disappearance of neutrinos has recently intensified, physicists “have a really good shot” at finding it with the newest technology, according to Michelle Dolinski, an experimental physicist at Drexel University in Philadelphia, Pennsylvania. The current generation of experiments is an order of magnitude more sensitive than the previous generation, and newer ones planned for Japan, South Korea, Italy, Canada, and the United States will be two orders of magnitude more sensitive (see “Experiments around the world”). An experiment to detect Majorana neutrinos, which is expected to cost about $250 million, was prioritized by the US Department of Energy’s advisory council in 2015, and funding is believed to be on the way. When a proton in an unstable atomic nucleus decay into a neutron or vice versa, neutrinos or antineutrinos are produced. The electron or its antiparticle, generally known as -particles, are also ejected during this process, which is known as -decay. It was predicted in 1935 by Nobel Prize-winning physicist Maria Goeppert-Mayer2 that certain atomic nuclei can decay by simultaneously converting two of their neutrons into protons (or vice versa), ejecting two -particles. Two neutrinos or antineutrinos should also be produced by this “double decay.” The transmutation of tellurium-128 to xenon-128, which has the longest known half-life of any nuclear event at more than 1024 years, or one million billion billion years, was determined to be a rare sort of decay that Goeppert-calculation Mayer’s was accurate for.

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Wendell Furry, a scientist, noted four years after Goeppert-study Mayer’s that if Majorana was correct and neutrinos were their own antiparticles, then the two neutrinos emitted by a double decaying nucleus might occasionally annihilate each other, leading to the nucleus emitting just electrons. The goal of Majorana neutrino existence experiments is to find this neutrinoless double -decay. Take a piece of material that might go through the process and observe it for as long as you can to see if it emits two electrons carrying a specified amount of energy. The notion behind this is deceptively straightforward. However, neutrinoless radioactivity would be by far the slowest type of nuclear decay currently understood; it would be at least two orders of magnitude uncommon than common double -decay. One contender for the procedure, germanium-76, must have a half-life of more than 1.8 1026 years, according to the best result to date from the Germanium Detector Array (GERDA) at the underground Gran Sasso National Laboratories in central Italy. It is approximately 10 quadrillion times older than the universe. One of two things typically occurs when tests on rare radioactive decays boost their strength or just gather a lot of data: either they finally see the reaction they were hunting for, or they raise the bar for how long its half-life needs to be. Therefore, the possibility of limiting the half-life provides a gauge of an experiment’s sensitivity.

The half-life limit will increase to 1028 years or more with the experiments currently underway or in the planned phases aiming to be approximately 100 times more effective than GERDA. Reducing background noise, such as radioactive contaminants in or near the detectors that could provide misleading signals that appear to be electron pairs with the Majorana-neutrino signature, is a typical method for boosting sensitivity. Some teams have gone above and beyond to get rid of these. According to Dolinski, a spokesperson for the Enriched Xenon Observatory (EXO-200), a recently completed Majorana-neutrino experiment at the underground Waste Isolation Pilot Plant near Carlsbad, New Mexico, “If you pick up a little bit of dirt, it may be one part per million of radioactivity; our materials are typically one part per trillion.” It has been dubbed the “coldest cubic meter in the Universe”; another experiment at Gran Sasso, the Cryogenic Underground Observatory for Rare Events (CUORE, Italian for “heart”), maintains the core of its detector at a temperature of 0.01 kelvin to aid in signal differentiation. Using 4 tonnes of radioactively low old Roman lead that was rescued from a shipwreck, CUORE safeguards its tellurium target as well.

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The most effective experiment at decreasing background noise is GERDA; throughout its roughly ten-year working period, it saw virtually no occurrences that replicated the Majorana-neutrino signature. According to spokesperson Riccardo Brugnera, a physicist at the University of Padua in Italy, the germanium detector is crucially submerged in a tank of around 85-kelvin liquid argon, which serves three purposes. It serves as a detector to pick out radiation signals that might still sneak through to the core while also keeping the germanium cool, protecting it from external radiation, and maintaining its shielding. In order to develop the larger detector LEGEND-200, which will have a target built of 200 kilograms of germanium-76, GERDA’s team collaborated with the US-led MAJORANA partnership last year. It is currently being built at Gran Sasso and is scheduled to begin collecting data in November. The likelihood of observing a decline grows as the target’s size is scaled up. Brugnera adds, “You also need a big mass, or you’ll have to wait for centuries. By relying on the sheer size of the target, several tests attained sensitivity levels comparable to those of GERDA. In underground research facilities in Canada and Japan, physicists have repurposed enormous detectors that were initially designed to capture neutrinos. Around 750 kilograms of xenon-136 are present in the Japanese KamLAND-Zen 800, while 1,300 kilos of tellurium-130 will be present in the Canadian SNO+. Both tests detect light streaks left behind by energetic particles as they pass through a tank containing tons of mineral oil.

Dolinski’s EXO-200, which employs 200 kilograms of liquid xenon-136, is yet another innovation. Both the candidate isotope for neutrinoless decay and the medium that makes the electrons visible are the xenon atoms. The most comprehensive searches for dark matter have been made using similar xenon-based detectors calibrated to catch particles from space. EXO-200, which was developed in the early 2000s and cost less than $15 million, was “made under the surface, ignoring considerable bureaucracy,” according to Giorgio Gratta, a physicist at Stanford University in California. Gratta is hopeful that the anticipated funds from the US Department of Energy would go toward nEXO, a version that is far more ambitious and might cost up to $250 million. nEXO will contain 5 tonnes of xenon. The LEGEND-200 team, which has a plan to scale up to a “LEGEND-1000” experiment, with 1 tonne of germanium-76, is one of nEXO’s rivals for the windfall. According to physicists, the race has to include a range of huge detectors. The initial indication of a neutrinoless decay will appear as a modest data bump, and more tests will need to confirm the results. The first step would be to confirm it using a different isotope, according to Carlo Bucci, a physicist at Gran Sasso and a spokesperson for CUORE. There is no assurance that any of these experiments will eventually demonstrate that neutrinos are Majorana particles, though. Leading theoretical models suggest that they should, however the models are partially dependent on conjecture on the neutrinos’ masses. But the majority of physicists believe it will happen eventually, not if. After that, at least one of Ettore Majorana’s disappearances will be resolved.

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