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 neutrinos 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.
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
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.
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.
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.”
paper detailing the study was published this week in the journal
Physical Review Letters.