Search for sterile neutrinos in MINOS and MINOS+

MINOS far detector as seen in 2012. Photo: Reidar Hahn

The MINOS+ collaboration at the Department of Energy’s Fermilab has published a paper in Physical Review Letters about their latest results: new constraints on the existence of sterile neutrinos. The collaboration has exploited new high-statistics data and a new analysis regime to set more stringent boundaries on the possibility of sterile neutrinos mixing with muon neutrinos. They have significantly improved on their previous results published in 2016. With close to 40 publications that have garnered more than 6,000 citations, MINOS has been at the forefront of studying neutrino oscillations physics since its first data-taking days in 2005.

The experiment uses two iron-scintillator sampling-and-tracking calorimetric particle detectors: The near detector is placed 1.04 kilometers from the neutrino source at Fermilab, and the far detector is placed 735 kilometers away in Minnesota. The MINOS experiment collected data using a low-energy neutrino beam from May 1, 2005, to April 29, 2012, and MINOS+ collected data with a medium-energy neutrino beam from Sept. 4, 2013 to June 29, 2016.

The detectors have accumulated high-statistics samples of muon neutrino interactions. Using a Fermilab neutrino beam composed of almost 100 percent muon neutrinos, they measured the disappearance of muon neutrinos as the particles arrived at the far detector. The collaboration used these data to obtain some of the most precise to-date measurements of standard three-neutrino mixings. These data also restrict phenomena beyond the Standard Model, including the hypothetical light sterile neutrinos.

The analysis has simultaneously employed the energy spectra of charged-current (W boson exchange) and neutral-current (Z boson exchange) interactions between the neutrinos and the atoms inside the detector.

Using a neutrino oscillation model that assumed the existence of the three known kinds of neutrinos plus a fourth type of neutrino referred to as a single sterile neutrino, the MINOS+ collaboration found no evidence of sterile neutrinos. Instead, the collaboration was able to set rigorous limits on the mixing parameter sin2θ24 for the mass splitting Δm241 > 10−4 eV2.

The results significantly increase the tension with results obtained by experiments conducted with single detectors studying electron neutrino appearance in a muon neutrino beam. The LSND and MiniBooNE techniques and limited statistics present challenges that are now being tackled by the MicroBooNE experiment at Fermilab, designed specifically for this task.

Scientists from 33 institutions in five countries — the United States, UK, Brazil, Poland and Greece — are members of the MINOS+ collaboration. More information can be found on the MINOS+ website.

This work is supported by the U.S. Department of Energy Office of Science.