Neutrinos, as we already know, defy the Standard Model. The question is, by how much do you mean by that?
The Standard Model of particle physics, which has been tested and verified with increasing precision, is a wonderfully elegant approach of describing the links between particles and their interactions.
But physicists recognize that it isn’t the whole story: it doesn’t resolve some perplexing concerns, such as the identity of the unseen dark matter that makes up the vast majority of the universe’s mass.
As a result, one area of particular interest in the hunt for physics beyond the Standard Model remains neutrinos.
Neutrinos exist in three flavors in the Standard Model: electron neutrinos, muon neutrinos, and tau neutrinos. This is similar to the Standard Model’s other matter particles, which can be classified into three types. However, several investigations have hinted at the existence of a novel form of neutrino that does not fit well into this basic image.
According to Kate Scholberg, a neutrino physicist at Duke University, “there’s muttering noisiness behind the scenes that maybe there’s something more out there.” “It may be nothing or something incredibly exciting,” says the narrator.
The results of the Liquid Scintillator Neutrino Detector and the MiniBooNE experiment show that there is an extra neutrino that doesn’t line up with the Standard Model’s generations of particles. It would be “sterile,” which means it wouldn’t interact with any Standard Model particles directly. It’s possible that it’s a type of dark matter.
Whether more neutrino flavors exist or not, neutrinos have already demonstrated that they exist outside the realm of regular physics in various ways.
Neutrinos should have no mass, according to the Standard Model. They aren’t, though; their masses are unusually small, and they don’t appear to fit in with the masses of the other fundamental particles.
A change in the theory could possibly account for this fact. It could also have far-reaching consequences for our knowledge of the universe.
Scholberg, who has been engaged in numerous neutrino tests over the years, adds, “It’s a picture we’ve gotten used to, so it doesn’t seem very strange anymore.” “However, it was not included in the initial Standard Model.”
Changing the flavor profile
Scientists can use the fact that neutrinos have mass to test if particular sorts of sterile neutrinos exist in the first place.
Before scientists were certain that neutrinos possessed mass, they understood that even a small quantity of mass would cause the particles to “oscillate,” or switch flavors. Observing neutrino oscillations in action explained why prior tests only identified approximately a third of the expected quantity of neutrinos from the sun.
Neutrino oscillations were found roughly 20 years ago, and several investigations since then have verified the unexpected results. The behavior of neutrinos emitted from the sun and created in the Earth’s atmosphere was studied in some experiments (SuperK and SNO). Other investigations looked at neutrinos produced by a nuclear reactor (Daya Bay, Double Chooz, and RENO) or a particle accelerator (MINOS, NOvA, Super-K, and T2K), measuring neutrinos shortly after they were born and then calculating how many of each flavor showed up in a detector some distance away.
During the 1990s, the US Department of Energy’s Los Alamos National Laboratory used the Liquid Scintillator Neutrino Detector to seek for neutrino oscillations before they had been completely confirmed by experiment, monitoring the appearance of electron neutrinos in a muon neutrino beam.
Theorist Joachim Kopp of Johannes Gutenberg University in Mainz states, “[LSND] saw a relatively big amount of electron-flavored neutrinos, far more than you’d expect.” “That’s their signal, which has been around from the beginning of time and for which no one has a credible explanation.”
Subsequent tests yielded a mixed bag of results. The Fermi National Accelerator Laboratory’s MiniBooNE experiment, which was developed in part to explain the LSND anomaly, also discovered an excess of electron neutrinos.
“On their own, LSND and MiniBooNE might be properly fit within the premise that there is a fourth neutrino flavor in nature,” says Kopp, whose research compares the results of different neutrino studies. “The issue is that LSND and MiniBooNE aren’t separated.”
Other experiments, including as MINOS and IceCube, have released data that are hard to reconcile with LSND’s sterile neutrinos. The statistics didn’t add up when Kopp and his colleagues sought for evidence of a fourth neutrino flavor. “It’s obviously not implausible that a more intricate scenario with more neutrinos may suit the data,” Kopp says, adding that this does not rule out sterile neutrinos.
The tooth fairy neutrino
Neutrinos with mass require additional ingredients that are not present in the Standard Model. Extra neutrinos with a massive mass—far greater than anything that could be created in a particle collider—would be one of the simplest additions. The mass of these particles would be the same as conventional neutrinos, but they would not participate in oscillations. These extra neutrinos come with exciting bonus predictions in some theories, such as lower-mass sterile neutrinos.
According to Kevork Abazajian of the University of California, Irvine, “the most fundamental neutrino mass process provides you a sterile neutrino for free.” “It’s the most basic beyond-the-Standard Model [explanation] for both neutrino mass and dark matter in some ways.”
The LSND and MiniBooNE results could be explained by those extra sterile neutrinos participating in neutrino oscillations.
The issue, as Abazajian argues, is that sterile neutrinos of the right mass to explain the LSND anomaly are incompatible with a number of cosmological findings. This includes the large-scale structure of the cosmos, which is the observed arrangement of galaxies. To make everything work, some other ideas must be rethought, which might be a deal breaker for those particles.
“In a manner, you have to have numerous tooth fairies,” he explains. “These sterile neutrinos would have to be combined with something else to be consistent with large-scale structure.”
Abazajian, like Kopp, isn’t ruling out sterile neutrinos just yet. He argues, “I wouldn’t make inferences based purely on cosmology.” “It needs to be solved in the laboratory, not merely from cosmology,” says the author.
Thankfully, numerous forthcoming investigations, including Fermilab’s Short-Baseline Neutrino program, will use three detectors to explore the LSND/MiniBooNE anomaly: MicroBooNE, ICARUS, and the Short-Baseline Near Detector. Others are searching for sterile neutrinos in different detectors. As a result, we should be able to determine if a fourth neutrino flavor is required to explain oscillation data in the future years.
“In neutrino physics, we’re living in fascinating times,” adds Kopp. “I’d be ecstatic if these abnormalities were confirmed, and the good news is that there’s an opportunity to put them to the test.”
Other oscillation experiments, meanwhile, will continue to try to figure out what gave neutrinos their mass in the first place—one of the first signs of physics beyond the Standard Model. The issue remains: how far will these strange particles carry us beyond known physics, and what new riddles will they require us to solve?