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Physicists Intensify Hunt for the Sterile Neutrino

Photomultiplier tubes line the walls of the Daya Bay neutrino detector. | Roy Kaltschmidt, Lawrence Berkeley National Laboratory

Scientists have learned a great deal about the ghostly neutrino in recent years, including Nobel Prize-winning work on the particle's ability to shape-shift or oscillate from one neutrino variety to another as it zips through the cosmos. But the neutrino continues to surprise physicists, who regard it as a bit of a misfit whose behavior cannot be fully encompassed within the so-called Standard Model of nature's particles and forces.

New results from a neutrino experiment in China are the latest indication that all is not well with theoretical models of neutrino behavior. Kam-Biu Luk, a co-spokesperson for the International Daya Bay Collaboration, told a news briefing at the 2016 AAAS Annual Meeting about discrepancies between experiment and nuclear theory that have been revealed at a set of nuclear reactors 30 miles northeast of Hong Kong.

Neutrinos, fundamental subatomic particles without an electric charge, are produced by the decay of radioactive elements. They are produced in explosions of stars and the nuclear fusion processes of the sun, and pass through each of us harmlessly by the trillions each second. They also can be created in nuclear reactors and particle accelerators. Researchers look at all neutrino sources to try to better understand them.

The Daya Bay experiment looks specifically at electron antineutrinos, one of three known species of neutrino. Over the course of 217 days at the reactor complex, the experiment collected more than 300,000 antineutrinos. That allowed the researchers to make the most precise measurement to date of the energy spectrum of the electron antineutrino — how many of the particles are being produced at particular energies.

The data, just published in the journal Physical Review Letters, found two discrepancies with theoretical predictions: a 10% excess of antineutrinos at an energy level of around five million electron volts and, intriguingly, 6% fewer antineutrinos overall than predicted.

The deficit in overall antineutrino flux — dubbed the "Reactor Antineutrino Anomaly" — could simply be due to problems with the theoretical models, scientists say, but the missing neutrinos also could be the result of an oscillation to a new kind of neutrino, the so-called sterile neutrino. The unexpected disagreement between observation and prediction strongly suggests that current calculations need some refinement, according to Luk.

(From left) Kam-Biu Luk, Peter Wilson, and Olga Mena Requejo at the 2016 AAAS Annual Meeting. | Ashley Gilleland / AAAS

The sterile neutrino, if it exists, would have no charge of any kind — including the "color" charge associated with the strong nuclear force of the Standard Model. There also could be several varieties. Particle physicists are looking for a version that would have a mass one million times smaller than the electron's. Cosmologists and astrophysicists are on the hunt for a variety predicted to be somewhat heavier — with a mass only 100 times smaller than the electron.

Should the new neutrinos be found, said Olga Mena Requejo, a cosmologist at the University of Valencia, it could provide clues about the relationship between matter and antimatter in the early universe. Theorists also have suggested that sterile neutrinos could be a component of the dark matter that seems to pervade the cosmos and exert a pull on regular matter.

Several new experiments using neutrinos from radioactive sources, nuclear reactors, or particle accelerators will be taking data over the next several years in an effort to determine if there are sterile neutrinos. Peter Wilson of the Fermi National Accelerator Laboratory (Fermilab), near Chicago, described an ambitious international effort to construct a chain of three particle detectors at Fermilab to look for evidence of the neutrino. The first of the three detectors, known as MicroBooNE, began receiving neutrinos from a particle beam last year. The second and largest detector known as ICARUS, is currently being refurbished at CERN, the European Organization for Nuclear Research, and will be shipped to Fermilab for installation in 2017. The third detector, the Short Baseline Near Detector, is now being designed and constructed by a collaboration of Swiss, British, and U.S. universities working with CERN and Fermilab, and will be installed in a new building in 2018.

Wilson noted that CERN will be participating in experiments outside of Europe for the first time, a fact that speaks to "the blossoming of neutrino programs in the United States." He said that if all goes well, the collaboration at Fermilab should have enough data by 2021 to "really tell whether there are sterile neutrinos in the region" that particle physicists are interested in.


Earl Lane