Deep below the mountainof Gran Sasso in central Italy, under nearly a mile of solid rock,the CUORE (CryogenicUnderground 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 ofmatter, when predictions suggest it should be equally split betweenmatter and antimatter?

For every atomic particlethere exists a complementary particle with equal mass but oppositecharge: such is the case, for instance, with electrons and positrons,protons and antiprotons, neutrons and antineutrons. For each pair ofparticles, one is designated as ordinary matter and the other asantimatter (the one exception being Majorana fermions, chargeless particles – such as photons – that actas their own antiparticles).

Astrophysics tells usthat the Big Bang should have produced equal amounts of matter andantimatter, but this is clearly not the case. The reason for thisimbalance is a still a mystery, but may lie in the nature of theneutrino, a nearly massless subatomic particle that – just like thephoton – may act as its own antiparticle. If neutrinos are indeedMajorana fermions, they may have decayed asymmetrically in the earlyuniverse and given rise to the preponderance of matter overantimatter that we see today.

This past January, a team of150 scientists from Italy and the United States began CUORE, afive-year experiment aiming to establish whether neutrinos areindeed their own antiparticles.

CUORE seeks to do this bydetecting an extraordinarily rare event known as "neutrinolessdouble-beta decay." Over time, two neutrons will naturally decayinto two protons, two electrons, and two antineutrinos; however, ifneutrinos are their own antiparticle, then very occasionally the twoantineutrinos will cancel each other out in a "neutrinoless decay."

Researchers working on the cryostat CUORE collaboration

Neutrino decay can beobserved in materials such as tellurium, but a neutrinoless decay isan event so rare that it occurs in a tellurium atom only once inseveral septillion (million billion billion) years; even then, thesignature of the decay is very difficult to detect, since it consistsof an energy spike of only of 2.4 MeV – less than a thousandth of abillionth of a joule.

TheCUORE experiment therefore takes place as far away as possible from allinterference, in a laboratory placed under nearly a mile of solidrock, and in what scientists have calculated to be"the coldest cubic meter in the universe," a refrigerator-styledevice that cools its interiors to only seven thousands of a degreeabove absolute zero. Inside the refrigerated area, 988 telluriumdioxide crystals (totaling some 100 septillion tellurium atoms) arevery carefully monitored in search of the tiny temperature spike thatwould denote a neutrinoless decay.

Two months into theexperiment, the scientists have reported they have not yet detectedsuch an event, and as a result they concluded that the event occursnaturally at most once every 10 septillion years in a singletellurium atom.

The researchers predictthey should be able to observe at least five neutrinoless decays overthe next five years, in a discovery that would not only confirm thatneutrinos are their own antiparticles, but also violate the StandardModel's law of conservation of lepton number.

Shouldthe experiment not detect the desired event, the experiment's nextgeneration, dubbed CUPID, will take its place by monitoring an evengreater number of atoms; should this second experiment fail as well,one last iteration may provide a final answer to the question.

"Ifwe don't see it within 10 to 15 years, then, unless nature chosesomething really weird, the neutrino is most likely not its ownantiparticle," CUORE team member Lindley Winslow says. "Particlephysics tells you there's not much more wiggle room for theneutrino to still be its own antiparticle, and for you not to haveseen it. There's not that many places to hide."

Apaper detailing the study was published this week in the journalPhysical Review Letters.

Source: MIT