How do #neutrinos get their #mass?
Issued by Symmetry
06/09/20Jessica Romeo
Neutrinos don’t seem to get their mass in the same way as other particles in the Standard Model.
In 1998, researchers made a discovery that challenged their understanding of particle physics and vaulted an unassuming particle into the spotlight for decades to come.
The Standard Model, the theoretical framework that should explain ordinary matter and its interactions, predicted that particles called neutrinos had no mass. In experiments, neutrinos appeared to move at the speed of light, something only a massless particle can accomplish.
But then, physicists at the Super-Kamiokande Observatory in Japan collected the first evidence that neutrinos had a mass that was tiny but not zero.
“This was the first observed phenomenon that we didn’t know how to explain,” says André de Gouvêa, a theoretical physicist and professor at Northwestern University. “Our model failed. That means that there’s some ingredient missing.”
So now we know: Neutrinos aren’t massless, they’re just incredibly light—a million times lighter than the next lightest particle, the electron. Trillions of neutrinos pass harmlessly through your body each second, and in fact, rarely interact with any matter at all.
“Because they’re so weakly interacting, we don’t know as much about neutrinos as other Standard Model particles,” says Jessica Turner, a postdoc studying neutrino phenomenology at the US Department of Energy’s Fermi National Accelerator Laboratory. “We know that they’re there. We’ve got many experiments that detect their interactions, but we know relatively little about them.”
We do know some things. We know neutrinos come in three flavors. And they don’t stick to just one of those flavors; they oscillate from flavor to flavor as they move through space. This feat is only possible because they have non-zero mass.
But where does that mass come from?
Neutrinos are a type of fundamental particle known as a fermion. All other fermions, such as leptons and quarks, gain their mass through their interactions with the Higgs boson. But neutrinos don’t seem to follow that trend.
Physicists have proposed hundreds of theories for how neutrinos might get their mass, and everyone has their favorite. Maybe there’s another source of mass that we do not know about. Maybe the neutrino masses are the interplay of the Higgs boson and this new source of mass. For many, the thrill comes from trying to narrow it down.
“If we’re lucky, the question becomes: Which is the right answer?” says de Gouvêa. “What is the way that nature chooses to give neutrinos a mass? We still don’t know the answer to that.”
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