The vanishing neutrinos that could upend basic physics

The SNO+ detector beneath design in Sudbury, Canada.Credit score: Volker Steger/SPL

Italian physicist Ettore Majorana notoriously disappeared in 1938 with out leaving a trace. His favorite elementary particles, neutrinos, could possibly be able of a very similar vanishing act. Many new or upgraded experiments close to the world are racing to exhibit that an extremely unusual kind of nuclear decay that generally produces two neutrinos may from time to time generate none.

These experiments have been given fewer funding or awareness than efforts to detect dim issue, but their influence throughout physics could be just as substantial. The phenomenon of disappearing particles would recommend that neutrinos and antineutrinos, their antimatter counterparts, are 1 and the exact — a likelihood that Majorana initially theorized1 in the 1930s.

These ‘Majorana neutrinos’ could be the key to knowledge why the Universe appears to comprise incredibly very little antimatter (see ‘Questions that could be answered if neutrinos are Majorana particles’). Also, they would show that not like all other recognised particles of issue, these as electrons or quarks, neutrinos really don’t get their mass from the Higgs boson.

Physicists have appeared for the disappearance of neutrinos for many years, but the look for is now ramping up substantially, which means that they “have a definitely superior shot” at detecting it with the future generation of products, says Michelle Dolinski, an experimental physicist at Drexel College in Philadelphia, Pennsylvania.

Experiments now on line or remaining built in Japan, South Korea, Italy, Canada and the United States are an order of magnitude far more sensitive than the former generation, and planned upcoming detectors would boost on that by another two orders of magnitude (see ‘Experiments close to the world’). In 2015, an advisory committee to the US Division of Electrical power recognized these types of a task as a precedence and a commitment to fund an experiment to detect Majorana neutrinos — estimated to price tag close to US$250 million — is believed to be imminent.

A extensive hold out

Neutrinos or antineutrinos are created when a proton in an unstable atomic nucleus decays into a neutron, or vice versa. This process, referred to as β-decay, also shoots out both an electron or its antiparticle — collectively identified as β-particles.

A man standing in the water tank and cryostat of the GERDA experiment.

A person standing in the drinking water tank of the LEGEND-200 experiment. The cryostat at the centre held the germanium-76 focus on chilly.Credit history: Kai Freund/GERDA collaboration

Nobel-prizewinning physicist Maria Goeppert-Mayer predicted2 in 1935 that specified atomic nuclei can decay by switching two of their neutrons into protons (or vice versa) at the moment, emitting two β-particles. This ‘double β-decay’ really should also deliver two neutrinos or antineutrinos. Goeppert-Mayer’s calculation was identified to be correct, but this variety of decay is particularly uncommon: a person circumstance3, the transmutation of tellurium-128 to xenon-128, has the longest known 50 %-existence of any nuclear reaction, at extra than 1024 decades, or 1 million billion billion decades.

4 years just after Goeppert-Mayer’s paper, physicist Wendell Furry pointed out4 that if Majorana was right and neutrinos have been their have antiparticles, the two neutrinos emitted by a doubly decaying nucleus ought to often annihilate each other, so the nucleus would emit electrons only.

Experiments screening the existence of Majorana neutrinos attempt to detect this neutrinoless double β-decay. In strategy, this is disarmingly very simple: take a chunk of product that may well undergo the procedure and observe it for as extensive as you can, to see irrespective of whether it emits two electrons carrying a unique amount of vitality.

But neutrinoless radioactivity, if it exists, would be by much the slowest form of nuclear decay recognized: at least two orders of magnitude rarer than ordinary double β-decay. The very best end result so much — from the Germanium Detector Array (GERDA) at the underground Gran Sasso Countrywide Laboratories in central Italy — has discovered5 that one particular candidate for the course of action, germanium-76, should have a 50 percent-existence of a lot more than 1.8 × 1026 several years. That is around 10 quadrillion times the age of the Universe.

When experiments on exceptional radioactive decays improve their ability or only accumulate loads of data, one particular of two issues usually comes about: both they last but not least observe the response they were searching for, or they elevate the threshold for how very long its 50 %-life has to be. The prospective to established restrictions on the 50 %-lifetime therefore gives a measure of an experiment’s sensitivity.

Experiments that are now beginning up or in the scheduling stages aim to be around 100 periods much more effective than GERDA — having the 50 %-life limit to 1028 a long time or far more.


A single common approach for increasing sensitivity is to decrease the background sounds, these types of as radioactive impurities in or all around the detectors that could give untrue alerts that seem like electron pairs with the Majorana-neutrino signature. Some teams have gone to incredible lengths to eliminate these. “If you decide up a tiny little bit of filth, it may well be a single section per million of radioactivity our supplies are typically one particular section per trillion,” says Dolinski, who is a spokesperson for the Enriched Xenon Observatory (EXO-200), a a short while ago concluded Majorana-neutrino experiment at the underground Squander Isolation Pilot Plant in close proximity to Carlsbad, New Mexico.

A different experiment at Gran Sasso — the Cryogenic Underground Observatory for Uncommon Occasions (CUORE, Italian for ‘heart’) — retains the main of its detector at a temperature of .01 kelvin to support it distinguish among many indicators it has been explained as the ‘coldest cubic metre in the Universe’. CUORE also shields its tellurium focus on working with 4 tonnes of ancient Roman guide that was recovered from a shipwreck and has significantly very low radioactivity.

Of all existing experiments, GERDA has been most prosperous at lowering history noise: during its 10 years or so running operate, it noticed basically zero activities that mimicked the Majorana-neutrino signature.

Crucially, the germanium detector is immersed in a tank of all over 85-kelvin liquid argon, which has a triple function, says spokesperson Riccardo Brugnera, a physicist at the College of Padua in Italy. It retains the germanium chilly it shields it from outside the house radiation and it acts as a detector to weed out radiation signals that might nevertheless streak as a result of to the core.

GERDA was dismantled past calendar year, as its group joined forces with a US-led collaboration identified as MAJORANA to make a bigger detector: LEGEND-200, which will have a concentrate on produced of 200 kilograms of germanium-76. Now underneath design at Gran Sasso, it is owing to start out having details in November. Scaling up the measurement of the concentrate on boosts the odds of looking at a decay. “You also will need a huge mass, normally you will have to hold out for centuries,” states Brugnera.

Other experiments reached identical sensitivities to GERDA’s by banking on the sheer size of the target. At underground laboratories in Japan and Canada, physicists have repurposed substantial detectors at first developed to catch neutrinos. Japan’s KamLAND-Zen 800 has close to 750 kilograms of xenon-136, and Canada’s SNO+ will have 1,300 kilograms of tellurium-130. Both of those experiments spot streaks of gentle made by energetic particles as they cross a tank that contains hundreds of tonnes of mineral oil.

In the operate for funding

Nonetheless yet another approach was pioneered by Dolinski’s EXO-200, which takes advantage of 200 kilograms of liquid xenon-136. The xenon functions both equally as the prospect isotope for neutrinoless decay and as the medium that reveals the electrons. Comparable xenon-primarily based detectors tuned to capture particles from room have executed the most comprehensive searches for darkish make any difference.

With a expense of much less than $15 million, EXO-200 “was constructed less than the radar, disregarding substantially bureaucracy”, suggests Giorgio Gratta, a physicist at Stanford University in California who aided to conceive it in the early 2000s. Gratta is hoping that the anticipated US Section of Power funding will go to a considerably a lot more ambitious variation named nEXO, which will have 5 tonnes of xenon and could price on the get of $250 million.

Amid nEXO’s rivals for the windfall are the LEGEND-200 crew, which has a proposal to scale up to a ‘LEGEND-1000’ experiment, with 1 tonne of germanium-76.

Physicists say it’s crucial to a have wide range of large detectors in the race. The initially hint of a neutrinoless decay will clearly show up as a small bump in the information, and other experiments will require to repeat the results. “The to start with detail that would want to materialize is that you have to confirm it with a diverse isotope,” claims CUORE spokesperson Carlo Bucci, a physicist at Gran Sasso.

However, there is no ensure that any of these experiments will show that neutrinos are Majorana particles any time quickly. Primary theoretical designs forecast that they should, but the designs are in aspect based on guesswork about the masses of neutrinos. Nonetheless, most physicists feel that it is a issue of when, not if. And then, at the very least one of the disappearing acts relating to Ettore Majorana will be solved.

Queries that could be answered if neutrinos are Majorana particles

If neutrinos are concurrently subject and antimatter, it could assistance to respond to a variety of important concerns in basic physics.

1. In which did all the antimatter go? If neutrinos are Majorana particles, it could support to describe why the Universe is made up of overwhelmingly extra make any difference than antimatter. The Massive Bang must have developed equal amounts of every single. But issue ought to have had a a bit much better likelihood of surviving reactions amongst subatomic particles in the scorching primordial soup, foremost to the current imbalance. The huge query is why. Regular double β-decay makes two electrons and two antineutrinos, so it does not adjust the harmony of particles and antiparticles. But a neutrinoless double β-decay would create only two electrons, yielding a internet maximize in the Universe’s variety of particles.

2. Where by do neutrinos get their mass — and how a great deal do they have? In the typical design of particle physics formulated in the 1970s, quarks and electrons received their masses from the Higgs boson, and neutrinos had zero mass. Then, in the 1990s, physicists identified that neutrinos do have mass — even though just how a lot is mysterious. If neutrinos are Majorana particles, they get their masses from a system other than the Higgs. Furthermore, measuring the frequency of neutrinoless decay would indirectly measure antineutrino (and consequently neutrino) masses, mainly because the additional huge the particles are, the extra probable they are to annihilate every single other.

3. Why do neutrinos generally spin the very same way? Contrary to particles such as the electron, neutrinos normally spin in just one route: their axis is always observed to align with their way of movement, and their spinning is constantly ‘left handed’, or anticlockwise. Antineutrinos have been noticed only in a suitable-handed spin. When physicists formulated the regular product, they embedded this asymmetry. But if neutrinos are Majorana particles, that means antineutrinos could just be neutrinos spinning in the reverse course.