Neutrinos are veritable ghosts of subatomic particles — marvelously abundant, yet so elusive, so intangible, they almost might as well not exist at all.
In fact, trillions of them just passed harmlessly through your body, and you never felt a thing.
They pass through whole planets, whole suns, whole galaxies the same way.
Particle physicists have been working to find and then unlock the mysteries of neutrinos ever since they were first theorized back in the 1930s. More recently, they’ve turned to the study of antineutrinos, their antimatter equivalent.
Now, a physicist at the College of William and Mary in Williamsburg is among a collaboration of some 240 scientists and engineers from around the world who announced Monday that they’ve moved the needle in the understanding of neutrinos and antineutrinos — for the first time, they found strong evidence of muon antineutrinos oscillating into electron antineutrinos.
This is important, scientists say, because more precise measurements and comparisons of shape-shifting neutrinos and antineutrinos will help resolve some of the mysteries of the universe.
“The Standard Model (of physics) explains how particles interact, and neutrinos and antineutrinos together are one of the most abundant particles in the universe,” said Patricia Vahle, William and Mary’s Mansfield associate professor of physics, in a phone call Monday. “So, really understanding how neutrinos work is crucial to understanding how the universe works on a subatomic level.”
The experiment, called NOvA, has been running for several years at Fermilab, a national laboratory outside Chicago. Vahle is one of NOvA’s two spokespersons.
A neutrino is forged in a number of ways — by stars, by nuclear reactor decay or by reactions between cosmic radiation and the Earth’s atmosphere.
It rarely interacts with other particles, has no electric charge and comes in three kinds, or “flavors” — muon, electron and tau.
It was initially thought that neutrinos had no mass. Then, in 2015, two physicists won Nobel Prizes for their experiments proving that they do.
They accomplished this by demonstrating that neutrinos can oscillate, or change identities — essentially, morph from one flavor to another. This is something a mass-less particle can’t do.
While it’s understood that antineutrinos can also oscillate, the shift into electron antineutrinos over long distances had not been definitely observed.
So NOvA researchers decided to alter their experiment to see if they could capture the transition.
“We flipped the polarity,” Vahle said. “We essentially run the current backwards through our focusing element (so) we can focus opposite charges of particles, which essentially gives us the antineutrino beam instead of a neutrino beam.”
Starting in February of last year, the scientists used Fermilab’s powerful accelerators to shoot an intense beam of mostly muon antineutrinos into a particle detector at the lab, then through the ground to a larger 14,000-ton detector 500 miles away in northern Minnesota.
The entire interstate trip takes less than three milliseconds.
And NOvA scientists found that the far-detector in Minnesota picked up far more electron antineutrinos than the near-detector at Fermi, providing what scientists call “strong evidence” of oscillation.
“The number of oscillation events we see is far greater than we would expect if antineutrinos didn’t oscillate from muon type to electron,” said Peter Shanahan of Fermilab in a statement Monday announcing the results. Shanahan is co-spokesperson for NOvA.
Going forward, that oscillation can help NOvA scientists solve their experiment’s primary puzzle: the mass hierarchy or mass ordering of neutrinos.
“Neutrino oscillation is connected to the fact that neutrinos have mass, but we actually don’t even know what the neutrino mass is,” said Vahle.
But by measuring the differences in different neutrino states, she said, they can establish whether the particle’s three flavors consist of two light neutrinos and one heavy or vice-versa.
“The best way of figuring this out is by comparing this oscillation probability between neutrinos and antineutrinos,” Vahle said.
There’s also an even bigger puzzle tied to mass ordering, she said: whether or not neutrinos violate a certain symmetry of nature in which the same laws of physics apply to a particle and its antiparticle.
Cosmologists came up with the so-called charge parity (CP) violation theory to explain why the universe has more matter than antimatter.
“In order to explain why our universe is made up of matter and we don’t see antimatter lying around anywhere in the universe, some processes have to violate CP symmetry,” Vahle said.
“We’re hoping that neutrinos will help us figure out exactly what went on in the early universe to explain this difference in the matter versus antimatter abundance in the universe.”
Their findings so far are taken from NOvA’s first data set, she said, and there are more years of research to go.
“It’s early days,” Vahle said, “but NOvA is already giving us new insights.”
NOvA is a collaboration of researchers from nearly 50 institutions in seven countries.
Fermilab is a national laboratory for particle physics and accelerator research under the U.S. Department of Energy Office of Science.