Deep below the mountain of Gran Sasso in central Italy, under nearly a mile of solid rock, the CUORE (Cryogenic Underground Observatory for Rare Events, and Italian for "heart") experiment is underway to help us understand one of astrophysics's great unanswered questions: why is the universe that surrounds us full of matter, when predictions suggest it should be equally split between matter and antimatter?
For every atomic particle there exists a complementary particle with equal mass but opposite charge: such is the case, for instance, with electrons and positrons, protons and antiprotons, neutrons and antineutrons. For each pair of particles, one is designated as ordinary matter and the other as antimatter (the one exception being Majorana fermions, chargeless particles – such as photons – that act as their own antiparticles).
Astrophysics tells us that the Big Bang should have produced equal amounts of matter and antimatter, but this is clearly not the case. The reason for this imbalance is a still a mystery, but may lie in the nature of the neutrino, a nearly massless subatomic particle that – just like the photon – may act as its own antiparticle. If neutrinos are indeed Majorana fermions, they may have decayed asymmetrically in the early universe and given rise to the preponderance of matter over antimatter that we see today.
This past January, a team of 150 scientists from Italy and the United States began CUORE, a five-year experiment aiming to establish whether neutrinos are indeed their own antiparticles.
CUORE seeks to do this by detecting an extraordinarily rare event known as "neutrinoless double-beta decay." Over time, two neutrons will naturally decay into two protons, two electrons, and two antineutrinos; however, if neutrinos are their own antiparticle, then very occasionally the two antineutrinos will cancel each other out in a "neutrinoless decay."
Neutrino decay can be observed in materials such as tellurium, but a neutrinoless decay is an event so rare that it occurs in a tellurium atom only once in several septillion (million billion billion) years; even then, the signature of the decay is very difficult to detect, since it consists of an energy spike of only of 2.4 MeV – less than a thousandth of a billionth of a joule.
The CUORE experiment therefore takes place as far away as possible from all interference, in a laboratory placed under nearly a mile of solid rock, and in what scientists have calculated to be "the coldest cubic meter in the universe," a refrigerator-style device that cools its interiors to only seven thousands of a degree above absolute zero. Inside the refrigerated area, 988 tellurium dioxide crystals (totaling some 100 septillion tellurium atoms) are very carefully monitored in search of the tiny temperature spike that would denote a neutrinoless decay.
Two months into the experiment, the scientists have reported they have not yet detected such an event, and as a result they concluded that the event occurs naturally at most once every 10 septillion years in a single tellurium atom.
The researchers predict they should be able to observe at least five neutrinoless decays over the next five years, in a discovery that would not only confirm that neutrinos are their own antiparticles, but also violate the Standard Model's law of conservation of lepton number.
Should the experiment not detect the desired event, the experiment's next generation, dubbed CUPID, will take its place by monitoring an even greater number of atoms; should this second experiment fail as well, one last iteration may provide a final answer to the question.
"If we don't see it within 10 to 15 years, then, unless nature chose something really weird, the neutrino is most likely not its own antiparticle," CUORE team member Lindley Winslow says. "Particle physics tells you there's not much more wiggle room for the neutrino to still be its own antiparticle, and for you not to have seen it. There's not that many places to hide."
A paper detailing the study was published this week in the journal Physical Review Letters.
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