Identity-Switching Neutrinos Could Reveal Why We Exist At All. But Can We Find Them?

High-energy particle collisions, neutrinos. — (Image credit: Shutterstock)

For years now, an international team of researchers has hidden themselves deep beneath a mountain in central Italy, tirelessly collecting the most sensitive measurements from the coldest cubic meter in the known universe. The scientists are searching for evidence that ghostly particles called neutrinos are indistinguishable from their own antimatter counterparts. If proved, the discovery could resolve a cosmic conundrum that has plagued physicists for decades: Why does matter exist at all?

They've long known that matter has an evil twin dubbed antimatter. For every fundamental particle in the universe, there exists an antiparticle that's nearly identical to its sibling, with the same mass but opposite charge. When a particle and antiparticle meet face-to-face, they annihilate each other, creating pure energy.

"We have this apparent complete symmetry of accounting between matter and antimatter," Thomas O'Donnell, a professor of physics at Virginia Tech University, told Live Science. "Every time you make a piece of matter, you also make a balancing piece of antimatter, and every time you destroy a piece of matter, you must destroy a piece of antimatter. If this is true, you can never have more of one type than the other."

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This symmetry is at odds with our current understanding of how the universe began. According to the Big Bang Theory, when the universe expanded from an infinitesimal singularity some 13.8 billion years ago, it is believed that equal amounts of matter and antimatter came into existence. However, when astronomers look out into the cosmos today, the universe is composed almost entirely of matter with none of its evil twin in sight. More troubling, if the Big Bang Theory is correct, then we — yes, humans — shouldn't be here today.

CUORE, which means "heart" in italian, is searching for evidence that elusive subatomic particles called neutrinos are their own antiparticle, what physicists call a Majorana particle. Neutrinos, which pass like specters through most matter, are extremely hard to detect. In fact, according to NASA, trillions of neutrinos originating from the fiery nuclear furnace of our sun pass through our bodies every second.

The CUORE experiment looks for the signature of Majorana neutrinos annihilating each other in a process called neutrinoless double-beta decay. In ordinary double-beta decay, two neutrons inside the nucleus of an atom simultaneously morph into two protons, emitting a pair of electrons and antineutrinos. This nuclear event, although exceedingly rare and occurring only once every 100 quintillion years (10^20) for an individual atom, has been observed in real life.

However, if the researchers are correct and neutrinos are true Majorana particles (they are their own antiparticle), then the two antineutrinos created during the decay could annihilate each other and create a neutrinoless double-beta decay. The result? Just electrons, which are "ordinary matter." If this process proves true, it may be responsible for seeding the early universe with ordinary matter. Observing this process, however, is another story. Scientists estimate neutrinoless double-beta decay (if it exists at all), may take place just once in every 10 septillion years (10^25).

Assembled over a decade, the CUORE instrument is the coldest cubic meter in the known universe. It consists of 988 cube-shaped crystals made of tellurium dioxide, cooled to 10 milli-kelvin, or minus 460 degrees Fahrenheit (minus 273 degrees Celsius), just a hair above the coldest temperature physics will allow. To shield the experiment from interference by outside particles such as cosmic rays, the detector is encased in a thick layer of highly pure lead recovered from a 2,000-year-old Roman shipwreck.

Despite the team's technological achievements, finding the neutrinoless event has proved to be no easy task. The researchers have more than quadrupled the collected data since their initial results in 2017, representing the largest dataset ever collected by a particle detector of its kind. Their latest results, published on the preprint database arXiv, show they found no evidence of neutrinoless double-beta decay.

The little neutral ones