For 86 years after they were proposed by Wolfgang Pauli, and 60 years after they were discovered by Clyde Cowan and Frederick Reines with an experiment aptly named Project Poltergeist, neutrinos have remained the most elusive ghosts of particle physics.
One of the great scientific discoveries of the last century, and one of the first indications found of physics beyond the Standard Model, is that neutrinos oscillate. That is, if a neutrino starts off as one flavor, after traveling some distance, it is possible it will be detected as a different flavor.
Most experiments measuring neutrino oscillation have seen results consistent with the premise that there are only three of these neutrino flavors – electron, muon and tau. However, results from a few experiments, such as LSND at Los Alamos National Laboratory and Fermilab’s MiniBooNE, can be explained by neutrino oscillations only if we include one or more new neutrino flavors different from the three familiar ones.
Neutrinos interact through the weak force, and precision measurements with the Large Electron-Positron collider (LEP, the LHC predecessor at CERN) showed that the weak force interaction can couple only with three neutrino flavors. That means that any extra neutrino flavors must not participate in these weak interactions, and so we call these extra hypothetical particles sterile neutrinos. Because they have virtually no interactions with matter, these sterile neutrinos are even harder to detect than the neutrinos we know. They are truly a ghost particle’s ghost!
There are two weak interaction channels for neutrinos: charged-current and neutral-current interactions. Charged-current interactions involve a neutrino, a W boson, and a charged lepton (electron, muon or tau). These interactions are sensitive to the familiar “active” neutrino flavors and hence to oscillations between those flavors. Neutral-current interactions involve a neutrino and a Z boson, without the emerging charged lepton, so they are blind to the active neutrino flavor and hence blind to oscillations between those flavors.
Measuring neutral-current neutrino interactions lets one measure the overall number of neutrinos of any active flavor, regardless of whether they have changed flavor while traveling. Neutral-current neutrino interactions therefore offer a prime avenue for determining whether oscillations between the three active neutrinos and sterile neutrinos are occurring. If so, those oscillations would lead to a partial deficit in the total number of active neutrino interactions, and we would see that telltale signature in our measurement of the number of neutral-current interactions. This deficit is something we can look for with a neutrino experiment like NOvA.
NOvA is a two-detector experiment using an incredibly intense source of neutrinos produced by the Main Injector particle accelerator at Fermilab. NOvA has a small detector on site at Fermilab, one that we use to measure the rate of neutral-current neutrino interactions happening before any oscillations take place. It also has a second detector located 810 kilometers away that measures the neutrinos after they may have oscillated into sterile flavors.
The powerful source of neutrinos it measures means that NOvA samples a very high rate of neutrino interactions among the global landscape of neutrino experiments and also observes a large number of neutral-current neutrino events.
The NOvA detectors have been collecting data since summer 2014 and, so far, NOvA has analyzed 16 percent of the total data it expects to collect. At the recent Neutrino 2016 conference, NOvA released the first results for its search for neutral-current oscillations. It predicted to see 84 of these neutral-current neutrino events and observed 95, as you can see in the first plot above. This of course means that no deficit of events was observed, showing no evidence for oscillations into sterile neutrinos. Instead, we can use these data to place constraints on the parameters that would govern oscillations into sterile neutrinos. The limits NOvA sets on how strongly a new sterile neutrino flavor could mix with the three active neutrino flavors are shown in the second plot.
NOvA has collected only a small fraction of data from its design goal, but it is already making competitive measurements within the global environment of sterile neutrino oscillations. It will be very exciting to see how NOvA improves the world’s understanding of sterile neutrino oscillations in the coming years.
Gavin Davies of Indiana University will present these new NOvA results at Fermilab during the Joint Experimental-Theoretical Seminar on Friday, July 29.
Adam Aurisano is a postdoc at the University of Cincinnati. Gareth Kafka is a graduate student at Harvard University.