Reactor data hint at existence of fourth neutrino

In tunnels deep inside a granite mountain at Daya Bay, a nuclear reactor facility some 55 kilometers from Hong Kong, sensitive detectors are hinting at the existence of a new form of neutrino, one of nature’s most ghostly and abundant elementary particles.

Neutrinos, electrically neutral particles that sense only gravity and the weak nuclear force, interact so feebly with matter that 100 trillion zip unimpeded through your body every second. They come in three known types: electron, muon and tau. The Daya Bay results suggest the possibility that a fourth, even more ghostly type of neutrino exists — one more than physicists’ standard theory allows.

Dubbed the sterile neutrino, this phantom particle would carry no charge of any kind and would be impervious to all forces other than gravity. Only when shedding its invisibility cloak by transforming into an electron, muon or tau neutrino could the sterile neutrino be detected. Definitive evidence “would open up a whole new avenue of research,” says particle physicist Stephen Parke of the Fermi National Accelerator Laboratory in Batavia, Ill.

Possible evidence for the sterile particle comes from a mismatch between theory and experiment. If a nuclear reactor produces a beam of just one type of neutrino, theory predicts that some should change their identity as they travel to a far-off detector (SN Online: 10/6/15). Analyzing more than 300,000 electron antineutrinos (the antimatter counterpart of the electron neutrino) collected from the Daya Bay nuclear reactors during 217 days of operation, researchers found 6 percent fewer of the particles than predicted by the standard particle physics model. Particle physicist Kam-Biu Luk of the University of California, Berkeley and the Lawrence Berkeley National Laboratory and colleagues report the findings in the Feb. 12 Physical Review Letters.

One explanation for the deficit is that some of the electron antineutrinos have transformed into an undetectable, lightweight sterile neutrino, about one-millionth the mass of an electron, says Luk. Other nuclear reactor studies, including an experiment at the Bugey reactor in Saint-Vulbas, France, have seen similar electron antineutrino deficits, he notes. Studies with muon antineutrino beams at some particle accelerators have seen an excess of electron antineutrinos, which might be attributed to a different kind of sleight-of-hand by the unseen sterile neutrinos.

The Daya Bay result provides the most precise measure yet of the energies of electron antineutrinos at a nuclear reactor. Even so, the statistical significance of the deficit is not high enough to rate the finding a discovery. The result is a “three-sigma” finding, meaning that there’s about a 0.3 percent probability that such a paucity of electron antineutrinos would have occurred if no sterile neutrino exists. Physicists generally want a discrepancy to have a significance of five-sigma, or a 0.00003 percent chance of being a fluke, before they will label it a discovery.

Besides the hint of sterile neutrinos, the Daya Bay results reveal a second strange feature — an excess of electron antineutrinos (compared with theoretical predictions) at an energy of around 5 million electron volts. That could be a sign of completely new physics awaiting discovery (or simply that scientists don’t have a detailed enough grasp of the output of nuclear reactors). A revised understanding of that feature might even do away with the need for a lightweight sterile neutrino to explain the overall deficit in electron antineutrinos.

But if definitive evidence for a light sterile neutrino is eventually found, it “would turn the theory community on its head,” says Parke, and could have a bigger impact than the discovery of the Higgs boson, the Nobel-winning finding that explains why elementary particles have mass.

“Finding a sterile neutrino is extremely important because it would be the first discovery of a particle which cannot be accommodated in the framework of the so-called standard model,” says particle physicist Carlo Giunti of the University of Turin in Italy.

One of the earliest experiments that suggested the presence of sterile neutrinos was the Liquid Scintillator Neutrino Detector, which operated at the Los Alamos National Laboratory in New Mexico from 1993 to 1998. The LSND found that muon antineutrinos beamed into 167 tons of mineral oil had morphed into electron antineutrinos in a way that seemed to require a fourth type of neutrino to exist. A follow-up experiment at Fermilab, called MiniBooNE, ran from 2002 to 2012, with equivocal results. Another Fermilab experiment, MicroBooNE, began operation last October. MicroBooNE is the first of three liquid argon detectors, spaced at different distances near neutrino sources at Fermilab, that will track with unprecedented precision the transformation of neutrinos from one type to another.

Located 470 meters from Fermilab’s Booster Neutrino Beamline, MicroBooNE is the middle of the trio, to be joined in 2018 by ICARUS, the farthest detector, at a distance of about 600 meters from the beamline, and the Short-Baseline Near Detector, placed just 100 meters from the source. First results from the trio are expected in 2021, says experimental particle physicist Peter Wilson of Fermilab.

The detectors will also serve as a prototype for the Deep Underground Neutrino Experiment, a large-scale experiment that will send Fermilab-generated neutrinos on a 1,300-kilometer journey to the Sanford Underground Research Facility near Lead, S.D.

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In the meantime, the Daya Bay collaboration has teamed up with another Fermilab experiment, the Main Injector Neutrino Oscillation Search, to continue to seek signs of the sterile neutrinos. Although data from accelerator and reactor experiments do not yet paint a consistent picture, “we will soon know better whether a light sterile neutrino is waiting for us to unveil,” says Luk.

If a light sterile neutrino exists, it might have siblings about 1,000 times heavier. These particles could contribute to the as-yet-unidentified dark matter, the invisible gravitational glue that keeps galaxies from flying apart and shapes the large-scale structure of the universe. Fingerprints of this particle will be sought with an experiment called KATRIN, which examines the radioactive decay of tritium, a heavy isotope of hydrogen, at the Karlsruhe Institute of Technology in Germany.  

Sterile neutrinos that are even more massive, more than a trillion times heavier than the electron, could explain an ever bigger cosmic mystery — the mismatch between the amounts of matter and antimatter in the cosmos. Possessing an energy at least a million times greater than can be produced at the Large Hadron Collider, the world’s most powerful particle accelerator, a superheavy sterile neutrino in the early universe would have made a smidgen more matter than antimatter. Over time, the tiny imbalance, reproduced in countless nuclear reactions, would have generated the matter-dominated universe seen today (SN: 1/26/13, p. 18).

“For cosmology, the [lightweight] sterile neutrino that we are talking about cannot solve the problem of the matter-antimatter asymmetry, but it is likely that the sterile neutrino is connected with other new particles that can solve the problem,” says Giunti.

Scientists see another, more practical, benefit for studying neutrinos. By recording the antineutrino output of nuclear reactors, detectors can discern the relative amounts of plutonium and uranium, the raw materials for making nuclear weapons. Gram for gram, fissioned plutonium and uranium have distinctive fingerprints in both the energy and rate of antineutrinos they produce, says physicist Adam Bernstein of the Lawrence Livermore National Laboratory in California. Closeup monitoring of reactors, from a distance of 10 to 500 meters, has already been demonstrated; detectors capable of monitoring weapons activity from several hundred kilometers away is possible but will require additional research and funding, Bernstein says.

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Editor’s note: This story was updated February 26, 2016, to correct the location of Fermilab’s planned ICARUS detector.