Evidence that neutrinos oscillate in flavor typically takes the form of a deficit. An experiment tuned to detect, say, muon neutrinos sees fewer of them than expected because some might have transformed into tau neutrinos. In the case of neutrinos produced inside the Sun or by cosmic rays in Earth’s atmosphere, the detected deficits are consistent with the three familiar flavors—electron, muon, and tau.
However, neutrinos that are produced in accelerators or reactors and travel short distances to a detector appear to oscillate differently. Theory has a ready explanation for the anomalies: Neutrinos can also transform into one or more additional flavors—sterile neutrinos—which interact with matter only through the gravitational force.
In 2000 the Super-Kamiokande experiment in Japan reported that flavor oscillations in 20 GeV atmospheric neutrinos can be accounted for by invoking the three usual flavors and no more. (See Physics Today, January 2001, page 16.) Now the IceCube experiment at the South Pole has looked at atmospheric neutrinos in the energy range from 320 GeV to 20 TeV. At such high energies, nonsterile neutrinos and nonsterile antineutrinos pass through Earth at measurably different rates. If those neutrinos have the option of transforming into sterile flavors, then the angular distribution of muon neutrinos and antineutrinos that emerge from Earth’s surface should bear a telltale signature.
Credit: IceCube/NSF
The IceCube Neutrino Observatory consists of 5160 optical detectors buried under Antarctic ice at vertical separations of 17 meters and horizontal separations of 125 meters. (The accompanying photo shows one of the detectors being installed.) After analyzing 20 145 high-energy events gathered for almost a year in 2011–12, the IceCube team found no evidence of sterile neutrinos.
Even though these latest bounds from IceCube rule out much of the sterile neutrino parameter space favored by the reactor and accelerator experiments, sterile neutrinos cannot be ruled out yet. Detecting neutrinos, not to mention inferring the presence of sterile neutrinos, remains beset by systematic effects that are subtle and elusive.
Several current and future experiments are searching for sterile neutrinos. Earlier this year, the Daya Bay Reactor Neutrino Experiment in Guangdong, China, reported a puzzling excess of antineutrinos between 5 MeV and 7 MeV that could conceivably be attributed to sterile neutrinos. (See Physics Today, May 2016, page 16.) At Fermilab, the MicroBooNE experiment is taking data, while two new experiments, ICARUS and the Short-Baseline Near Detector, are under construction.
Whether sterile neutrinos exist is an important question to settle: One of the most appealing models to account for matter’s preponderance over antimatter, the seesaw mechanism, depends on them. (M. G. Aartsen et al., Phys. Rev. Lett., 117, 071801, 2016.)
