This season preview for neutrino-watchers will offer you an overview of what to expect from neutrino research in the future years.
In the realm of neutrinos, small particles that constantly run through us unnoticed, there’s a lot to look forward to.
According to SLAC National Accelerator Laboratory theorist Alexander Friedland, the area of neutrino study 20 years ago would be unrecognizable compared to what it is now. He says, “The developments have been really astonishing.” “It’s come a long way.”
Neutrinos revealed a flaw in the Standard Model of particle physics, scientists’ best explanation of the fundamental particles and forces that make up everything, twenty years ago, in 1998. Neutrinos should have no mass, according to the Standard Model. They did, however, according to the Super-Kamiokande and then the Sudbury Neutrino Observatory experiments. They were already known to come in three forms, but the fact that they had mass meant that they could shift from one type to the next while they flew at nearly the speed of light.
Many questions about these small particles remain unanswered: Do neutrinos really come in four varieties, as some experiments suggest? What do neutrinos’ masses mean? Is it true that neutrinos have their own antiparticles? What can neutrinos teach us about the Standard Model, astrophysics, and the origins of the universe?
According to Lindley Winslow, an MIT physicist, “our current neutrino experiments have all reached a sort of midway point.” She explains, “We’re refueling and looking at maps and thinking out our next steps into this completely undiscovered country.” “Now is a good time to congratulate ourselves on getting to this point before making the big push into the unknown.”
With Neutrino 2018, the XXVIII International Conference on Neutrino Physics and Astrophysics, just around the horizon, we asked some neutrino experts for their short thoughts on the lineup of experiments heading into this season, as well as their predictions for future successes in the field. This is what they had to say about it.
In search of undiscovered flavors
As they travel through space, neutrinos are known to fluctuate between three flavors: electron, muon, and tau. However, in 1995, scientists at Los Alamos National Laboratory working on the Liquid Scintillator Neutrino Detector, or LSND, discovered hints that there might be an extra flavor lurking in the shadows. It was dubbed a “sterile neutrino,” a neutrino flavor that did not interact with other neutrinos.
Neutrinos outnumber electrons, protons, and neutrons in today’s cosmos by a factor of ten billion, according to University of Michigan physicist Joshua Spitz.
“Given this, it’s simple to understand how the existence of a fourth form of neutrino, as well as its mixing with the other three, would have had a huge impact on the universe’s evolution. The arrival of a new sort of neutrino could have an impact on large scale structure, galaxy formation, dark matter, cosmic microwave background observables, and the generation and abundance of heavy metals, among other things.”
Physicists have been devising tests to track down this secret flavor in the years since the LSND anomaly. At Fermi National Accelerator Laboratory, the MiniBooNE experiment began gathering data related to this in 2002.
So far, the results have revealed an excess of MiniBooNE events consistent with the LSND signal, although it’s unclear how this fits into a sterile neutrino model. Richard Van de Water and Rex Tayloe, MiniBooNE’s co-spokespeople, expect to present updated results at Neutrino 2018 that will include substantial new information.
“The findings will add to our understanding of the LSND and MiniBooNE excesses, particularly the question of whether the two data sets are consistent, indicating whether new physics such as sterile neutrinos or other more intricate theories are at work,” Van de Water says.
Furthermore, new, more sensitive experiments are only getting started. The experiment that will succeed MiniBooNE is named MicroBooNE, and it is scheduled to publish its first physics results next year. MicroBooNE will eventually be joined at Fermilab by ICARUS and SBND, establishing the Short-Baseline Neutrino Program, a trio of detectors.
A number of radioactive-source and reactor-based experiments, including PROSPECT, STEREO, DANSS, CHANDLER, and SOLID, are also working and aim to catch the postulated sterile neutrino sometime in the near future.
Taking on the problem of bulk ordering
We also know that there are three different neutrino masses, just as there are at least three different flavors of neutrinos. But it’s still a mystery how these massive states are arranged. Neutrino mass states can be organized in one of two ways: normal or inverted. Despite the fact that numerous signals point to normal ordering, the final decision is still being made.
Knowing whether neutrinos have a regular or inverted mass ordering can help scientists examine different universe models, such as one in which the four forces of nature combine at high energies to form one.
Unlike the short-baseline experiments looking for sterile neutrinos, investigations looking into mass ordering are designed to last a long time. The T2K experiment, which is hosted by KEK accelerator laboratory and monitors a beam of neutrinos traveling more than 180 miles across Japan, and the NOvA experiment, which is hosted by Fermilab and studies a beam that originates about 500 miles from the detector in the United States, are the two major long-baseline experiments in operation. Fermilab has upgraded its accelerators, and an upgrade this summer will improve the sensitivity of the T2K experiment’s detector. The inquiry also includes reactor-based studies, such as the Daya Bay Reactor Neutrino Experiment in China.
Many of the specialists interviewed for this story, including Northwestern’s André de Gouvêa and SLAC’s Friedland, say they’re looking forward to a deluge of NOvA and T2K outcomes in the next years that could bring us closer than ever to figuring out mass ordering.
According to Michigan’s Spitz, telescope-based investigations of large-scale structure are rapidly developing sensitivity to determining the sum of neutrino masses by examining its impact on the gravitational clumping of matter in the early cosmos. Scientists may be able to uncover the neutrino mass ordering by combining this with previous findings.
“Seeing agreement between NOvA, T2K, and telescopic observations of this neutrino property will be really extraordinary,” he adds, adding that “seeing disagreement would even be more interesting.” When we can start linking the features of the neutrino to the origin of the cosmos, this will really be ‘astroparticle physics.’”
Other experiments are attempting to determine the combined mass of the three neutrinos kinds. KATRIN, a German neutrino experiment with a 200-ton spectrometer at its heart, has recently begun collecting data. The experiment will check for extremely minor deviations in the energy of the electrons ejected during the decay of the radioactive isotope tritium, which will reveal the neutrino’s absolute mass.
According to Alexander Himmel, a physicist at Fermilab, “the absolute neutrino mass is one of those things that oscillation experiments can’t see at all.” “With KATRIN, we’re just getting started with data collection. It’s a very demanding experiment that’s taken a long time to get up and running, so we’re looking forward to obtaining really nice measurements from them over the next few years, which I think will be extremely exciting.”
Project 8, another experiment aimed at determining the neutrino’s absolute mass, will likewise employ tritium, but will measure the energy of individual electrons by measuring the frequency of their spiraling motion in a magnetic field, rather than measuring the energy of individual electrons. Physicists plan to scale up the process in the future, despite the fact that Project 8’s goal is to demonstrate the technology.
When such fouls occur, the whistle is blown
Most particles in our universe have antiparticles, which have the same charge as their companions but are charged in the opposite direction.
Scientists believe that there should have been an equal amount of matter and antimatter in the universe during the Big Bang. When matter and antimatter collide, however, they destroy each other. This match should have ended in a stalemate, with matter and antimatter canceling each other out and leaving just energy behind.
Despite this, matter triumphed, as you might expect given the matter-filled world we live in. Scientists are still baffled as to why this is. Charge-parity violations come into play in this situation.
For a long time, scientists thought there had to be some type of symmetry between particle behavior and that of their antimatter counterparts, known as CP symmetry. This indicates that if antineutrinos were to take the place of neutrinos, the cosmos would regard them the same way. However, if this symmetry is disrupted in any way, it could explain how matter gained the upper hand.
Long-baseline experiments like NOvA and T2K, together with reactor-based experiments like Daya Bay, have been tracking neutrinos and antineutrinos oscillations to see if they are fundamentally different. This would show that CP is broken, providing a plausible explanation for why matter triumphed in the universe’s creation.
One of the major neutrino announcements due shortly, according to Friedland, is the release of antineutrino run data from the NOvA experiment, which, when combined with T2K, would either enhance current signs of CP violation or send teams of scientists running in different directions.
Kendall Mahn, a physicist at Michigan State University, says, “We’re finding signs that something intriguing is happening between neutrinos and antineutrinos.” “We’re gathering additional information to determine if this will develop into something truly interesting or if it will fizzle out. It just goes to prove that we’re on the cutting edge of something.”
Lepton number violation is another hypothetical symmetry-breaking that may have played a role in shaping the cosmos as we know it. If neutrinos were their own antiparticles, this would happen. Scientists are putting this theory to the test by hunting for a phenomenon called neutrinoless double-beta decay, in which neutrinos behave as their own opposites and cancel each other out.
Experiments including CUORE, Majorana Demonstrator, GERDA, and NEXT are on the attack, with all of them publishing fresh data recently. By the end of the year, KamLAND-Zen 800 results should be available.
“Just turning the detector on was a triumph in and of itself,” says MIT’s Winslow of CUORE. “Now we have the difficult task of keeping it operating for five years and achieving the ultimate sensitivity where we believe we should be able to notice something,” says the researcher.
The fitness test based on the Standard Model
Scientists aren’t just doing neutrino experiments; they’re also developing Standard Model tests. Last summer, physicists at Oak Ridge National Laboratory’s COHERENT experiment were able to quantify for the first time a phenomena predicted by the Standard Model that had been searched for four decades but had gone undetected. Coherent elastic neutrino-nucleus scattering is a phenomenon that occurs in supernovae explosions as well.
A neutrino impacting the nucleus of an atom in coherent elastic neutrino-nucleus scattering does not simply hit one component of it—a proton or a neutron—but kicks the entire nucleus.
Kate Scholberg, a physicist at Duke, compares it to hitting a bowling ball with a ping pong ball. “Neutrinos almost seldom contact, but because this cross-section is so huge, the chance of a collision is 100 times higher than for a conventional neutrino interaction. The issue is that when you strike a bowling ball with a ping pong ball, it’s difficult to get the bowling ball to roll very quickly, and there’s a very low-energy recoil [that’s difficult to notice].”
COHERENT, the world’s smallest neutrino detector, will continue to publish data in the coming months, searching for this phenomenon in different nuclei and eventually leading to larger detectors capable of searching for additional oscillation effects.
According to Janet Conrad, an MIT physicist, taking multiple approaches is crucial in moving neutrino research forward. IceCube, a large South Pole neutrino observatory made up of a cubic kilometer of Antarctic ice, is another equipment she’s excited to use for precision measurements that will test the Standard Model.
“IceCube is a unique detector that has produced excellent dark matter discoveries and a really fascinating sterile neutrino limit,” she says, “but I think many people don’t appreciate how fantastic a beyond-Standard-Model search detector IceCube is.” And it’s only getting better as we learn more about the detectors. Within the particle physics world, IceCube is the unsung hero who has yet to be noticed.”
The unpredictable factor
When a large star bursts, its fast, unimpeded neutrinos are the first messengers it sends throughout the galaxy. Because these neutrinos are escaping from the star’s collapsing core, they hold information about the early phases of supernovae that cannot be obtained any other way.
Experiments will continue to evolve and improve over the next 10 to 15 years. The Deep Underground Neutrino Experiment (DUNE), hosted by Fermilab, will send neutrinos racing over 800 miles across the United States in order to better comprehend their oscillations and maybe answer some of our present issues firmly.
Each question scientists answer is linked to others, and each point earned moves physicists closer to breakthroughs that could transform our understanding of the universe, from its tiniest particles to its biggest scale astrophysical occurrences.
“Every day I come to work, we take a small step forward toward a new understanding,” Mahn says. “There is more out there, and we are getting closer.”