Neutrino physicists have had a rich and storied relationship with the little neutral ones. First suggested by Wolfgang Pauli as a solution to the problem of missing energy in radioactive decay these light neutral particles have always proven to be as frustrating as they are fascinating. Pauli himself famously said, “I have done a terrible thing, I have postulated a particle that cannot be detected.”
But detect it physicists did, and we found it to be even stranger than we first expected. Perhaps most fascinating is the fact that neutrinos change among the seemingly distinct types as they travel. Physicists around the world and at Fermilab have made much progress in understanding these neutrino oscillations, but key questions remain unanswered. Does the ordering of neutrino masses match our intuition based on what we know of other families of particles, or is it inverted? Do neutrinos oscillate the same as antineutrinos? These questions are themselves compelling and tie in to grander theories. For example, leptogenesis seeks to explain why our universe has far more matter than antimatter.
Many experiments have worked to answer these questions. At Fermilab the NuMI muon neutrino beam enables a program of study of neutrino oscillations. Over the long journey from Fermilab to northern Minnesota, these neutrinos change type. The MINOS experiment has already used this beam to study the disappearance of muon neutrinos. The NOvA experiment is now providing another key piece of the puzzle by studying the appearance of electron neutrinos.
In many ways the entire NOvA experiment was optimized to see electron neutrino appearance. The detector has a high resolution and is instrumented with specialized photodetectors such that it can resolve the key signatures of an electron neutrino interaction. Excellent timing systems allow us to disentangle neutrino beam events from cosmic activity. The NuMI beam is operating at its highest-ever power to provide as many neutrinos as possible to the experiment, and the detector is off the main axis of the NuMI beam so it sees neutrinos at the perfect energy.
The first measurement of electron neutrino appearance by NOvA has also required a complex analysis of our data, using sophisticated image processing algorithms trained on large sets of simulated data to pull out a pure sample of electron neutrino candidates and data-driven studies using beam and cosmic events at our near and far detectors. Four graduate students will earn their doctorates with their work on this result, and more have made significant contributions.
The first appearance result, presented at Thursday’s Joint Experimental-Theoretical Seminar, shows six events selected with our primary analysis and 11 with our secondary analysis, with an expected background of approximately one in each case. This observation proves conclusively that the NOvA experiment can measure electron neutrino appearance and confirms oscillations at greater than 3 sigma with our primary analysis or 5 sigma with our secondary analysis. While this first result represents one-twelfth of the final exposure, it has already reached excellent agreement with measurements from existing experiments such as MINOS and T2K.
NOvA has shown that it will be able to contribute significantly to the world’s knowledge of neutrino oscillations in the coming decade. It also represents a start of another exciting road as we set out to make the best possible use of world-class detectors and a world-class beam to provide leading discoveries using electron neutrino appearance.
Alexander Radovic is a scientist at the College of William and Mary.