A physics The experiment found no evidence that the elusive particles called Majorana neutrinos exist, according to a new study published in nature today, but that doesn’t mean it won’t lead to new discoveries.
The Cryogenic Underground Laboratory for Rare Events (CUORE) collaboration has released its latest findings on the search for a controversial property of neutrinos – particles so light and rare that they interact with other matter that they are often referred to as ghost particles. This property, if present in the fundamental particles, could explain why the universe has matter left over from the Big Bang at all.
Scientists still don’t know some of the basic properties of neutrinos, such as Frank Avignone, a particle astrophysicist at the University of South Carolina who has been part of the CUORE project from the start The opposite.
Here is the background – CUORE consists of 988 detectors, each consisting of a 2-inch-wide cube of tellurium dioxide crystal to which is attached an incredibly sensitive thermometer, built into towers and placed deep underground in Italy’s Gran Sasso massif. The cubes are kept at an extremely low temperature and operate at 11 to 15 millikelvins (thousandths of a degree above absolute zero). The cubes are sensitive enough to see the small temperature changes caused by radioactive decay.
Ettore Fiorini of the University of Milan came up with the idea of the network of tiny detectors. “I once told Fiorini he was crazy, and it turns out I was,” says Avingone. Back then, Fiorini was experimenting with detectors the size of sugar cubes, while Avignone weighed several pounds.
One of the great mysteries the collaboration aims to unveil is the balance of matter and antimatter in the universe. During the Big Bang, as many particles as antiparticles were created. But as the universe began to cool, the particles and antiparticles began to combine and annihilate each other, says Avignone. So why didn’t matter and antimatter completely destroy each other or leave both types? After all, “we don’t see many anti-protons … no matter where we look,” he says.
What they have done – The most popular explanation for how this happened is called leptogenesis and requires neutrinos to be Majorana particles, which essentially means they are their own antiparticle, and if two collided they would annihilate themselves – destroying each other and just energy leave behind.
Observing some kind of radioactive decay closely is “the only practical way” to find out if the neutrino is a Majorana particle, says Avignone. In normal beta decay, a neutron decays into a proton, an electron, and a neutrino. In double beta decay, this process happens twice, “in a chain,” he says.
If they are Majorana particles, the two neutrinos produced in rapid succession in this double beta decay could recombine and disappear, leaving only energy. Otherwise both neutrinos would survive.
Neutrinos interact with matter so rarely that it’s nearly impossible to record two of them, but in these crystal detectors, scientists can also study these radioactive decays based on the total energy the sensor picks up during a decay. This amount of energy changes depending on whether neutrinos are escaping.
What’s new – The team chose the tellurium-130 isotope for their detector cubes because it naturally undergoes double beta decay, converting first iodine and then xenon as it emits electrons and neutrinos.
When the neutrinos disappear, the decay would produce an energy spike with a certain frequency. This data would provide evidence that the neutrinos are Majorana particles and help determine the mass of the neutrino.
However, when neutrinos are emitted, they affect the energy release d — turning it into a broader “continuum” of frequencies that the sensor can pick up, as opposed to one frequency, Avignone says.
In the team’s newly released results, they don’t see any evidence of the energy spike that would suggest neutrinos are Majorana particles, but that doesn’t mean the spikes don’t exist – the team just showed that the spike doesn’t exist can be sensitivity of your instrument. For this reason, the collaboration is already planning an upgrade called CUPID, which would use a different crystal to detect two signals – a heat signal and a light signal from radioactive decay, which could be combined to get even better sensitivity.
CUPID’s concept received early approval from the Department of Energy, but it still has a long way to go before it can be realized, Avignone says. In the meantime, CUORE will continue to work and collect evidence to look for signs of Majorana neutrinos.
Why it matters – Not finding the particle does not mean that the experiment failed. The measurements helped narrow down the properties of the neutrino – and indirectly gave a better indication of its mass, he says.
It is important for cosmology to set the limits on the possible mass of the particle – the understanding of the overall structure and origin of the universe. That’s very valuable data, says Avignone, “even if we never see it.”
In a presentation he once called the project “the search for neutrino properties hiding under an Italian mountain”.
People sometimes ask him why the experiment takes place in caves under an Italian mountain. He replies that the layer of solid rock shields the ever-present cosmic rays that would disrupt the experiment, Avignone says. He also says: “The wine is good there.”