Explained | Are neutrinos their own antiparticles?

The answer to this question has profound implications for a mystery about the universe.

February 22, 2023 12:15 pm | Updated 12:15 pm IST - Chennai

A peek inside the KamLAND detector as engineers install photomultiplier tubes.

A peek inside the KamLAND detector as engineers install photomultiplier tubes. | Photo Credit: kamland.stanford.edu

Neutrinos are the second most abundant particles in the cosmos, produced in copious amounts in the cores of stars. Because they are so ubiquitous, their properties are windows into the microscopic structure of the universe.

For example, one open question about neutrinos is whether they are their own antiparticles. If they were, physicists will have a way to explain why the universe has more matter than antimatter.

But an experiment in Japan recently reported that it failed to find “strong evidence” that this is the case, ruling out some – but not all – theories.

What are anti-particles?

Every elementary particle has an anti-particle. If the two meet, they will destroy each other in a flash of energy.

The electron’s anti-particle is the positron. Similarly, neutrinos have anti-neutrinos. However, an electron is distinguishable from a positron because they have opposite charges. Neither neutrinos nor anti-neutrinos have electric charge, nor any other properties to really differentiate between them.

One way to classify subatomic particles is as matter particles and force-carrying particles. Neutrinos are matter particles, or fermions. Fermions can be further split as Dirac fermions or Majorana fermions. Dirac fermions are not their own anti-particles, whereas Majorana fermions are.

Physicists working with the Kamioka Liquid Scintillator Antineutrino Detector (KamLAND), in the Japanese Alps, recently reported that after analysing two years’ data, they couldn’t find signs that neutrinos are Majorana fermions.

How do you search for a Majorana neutrino?

KamLAND looks for an event called neutrinoless double beta-decay (stylised as 0νββ). In normal double beta-decay, two neutrons in an atom turn into two protons by emitting two electrons and two electron anti-neutrinos (a type of anti-neutrino). In 0νββ, the anti-neutrinos aren’t emitted, which can happen only if anti-neutrinos are just different kinds of neutrinos.

“The 0νββ search is the only practical experiment to probe the Majorana nature of the neutrinos,” Itaru Shimizu, of the Research Centre for Neutrino Science, Tohoku University, and a member of the KamLAND team, told The Hindu in an email.

In a new analysis, published on January 30 in Physical Review Letters, a team including Dr. Itaru looked for signs of 0νββ in more than half a tonne of xenon-136 suspended inside a large vat of liquid. The nuclei of xenon-136 atoms are known to undergo double beta-decay.

A 3D illustration of the KamLAND detector. The spherical chamber contains a liquid called a scintillator. It emits flashes of light when particles travel through it. The orbs are photomultipliers that amplify the light for further analysis.

A 3D illustration of the KamLAND detector. The spherical chamber contains a liquid called a scintillator. It emits flashes of light when particles travel through it. The orbs are photomultipliers that amplify the light for further analysis. | Photo Credit: kamland.stanford.edu

They found that if a xenon-136 nucleus does undergo 0νββ, it happens at most once every  2.3 × 1026 years. This is one quadrillion times longer than the age of our universe.

“In the 0νββ decay model, the half-life is connected with the effective neutrino mass,” Dr. Itaru explained. This means 2.3 × 1026 years “can be converted to an effective neutrino mass of 36-156 meV.”

“The mass of the electron, the next lightest known particle, is 511 keV, so this limit is about 5,000-10,000 times lighter than that!,” Jason Detwiler, another member of the KamLAND team and an associate professor of physics at the University of Washington, Seattle, said in an email.

Even though the potential mass is so low, it matters because, during the Big Bang, the presence of neutrinos would have smoothed out the distribution of matter throughout the universe, Dr. Detwiler said.

“Similarly, in the more recent universe, neutrinos [flying through space] have slowed down considerably due to the expansion of the universe. The heavier they are, the slower they would be, and if they are too slow, they would cause galaxies to be bigger and clumpier than we observe.”

Why is the result a milestone?

En route to its finding, KamLAND also tested another idea. Neutrinos come in three types. Physicists don’t know how much they weigh nor which type is the lightest. The latter is called the neutrino mass hierarchy. We need to know it to solve some important problems in a field of study called flavour physics.

In one hypothesis, called ‘inverted mass ordering’, two neutrino types are much heavier than the third. Theories that include this feature require 0νββ to happen every 1026-1028 years. This in turn implies a certain mass range, as Dr. Itaru explained earlier.

So the new result rules out theories that predict more frequent occurrences of 0νββ as well as theories that use the ‘inverted’ hypothesis that imply neutrino masses within the excluded range. This is the first time such theories have been experimentally tested.

“The non-detection in our experiment constrains the mass scale of neutrinos, and for the first time the search sensitivity has reached the so-called ‘inverted neutrino mass ordering region’,” Dr. Itaru said. “This is a major milestone for the worldwide 0νββ community and motivates the next-generation of searches.”

Are there other experiments like KamLAND?

Dr. Detwiler is also a co-spokesperson of MAJORANA, an experiment in South Dakota that looks for 0νββ in atoms of germanium-76 and with “totally different but highly complementary experimental techniques”.

There are two ways to get better 0νββ data. KamLAND uses a large quantity of atoms that can undergo 0νββ, thus increasing the odds of spotting one event. MAJORANA works with fewer atoms but has a very low background.

“Background refers to processes that can mimic 0νββ decay, so we have to understand and quantify those very carefully,” Dr. Detwiler said. “We can only claim to have observed 0νββ decay if we see many more events than we could have expected from mimicking processes.”

MAJORANA recently concluded a demonstration run. Going ahead, it will merge with another experiment called GERDA to create LEGEND, taking forward the germanium-based search for 0νββ at higher sensitivity.

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