Does matter matter for neutrino flavor?

By combining its neutrino and antineutrino data sets, MINOS has constrained the non-standard interaction parameter εμτ, finding that the results are consistent with εμτ=0, shown by the gray line. The angle θ and the parameter Δm2 relate to the relative masses of the neutrinos and to how quantum mechanically “mixed” the flavors are.

The NuMI (Neutrinos at the Main Injector) beam is generated here at Fermilab and points toward the Soudan Underground Laboratory in Soudan, Minn. The MINOS collaboration detects this beam of neutrinos in its journey twice: once at Fermilab right after it is generated and once at Soudan Lab after the neutrinos have traveled 450 miles through the Earth’s crust. At its generation, the beam is made up of muon-flavored neutrinos (neutrinos come in three flavors: electron, muon, and tau). After traveling such a long distance, some of the neutrinos change flavor, primarily into tau neutrinos and a few into electron neutrinos. This phenomenon of flavor change is called neutrino oscillation. By counting the number (and measuring the energy) of muon neutrinos before and after travel, MINOS can measure parameters that govern neutrino oscillations.

The presence of matter in the neutrino path may also have an impact on flavor change. If it does, the flavor count after travel would be altered. Some of these interactions are expected from the tiny number of oscillation-generated electron neutrinos, but extra interactions of muon or tau neutrinos with the Earth are non-standard and are thus called non-standard interactions, or NSI for short. (The Earth is made up of regular matter—electrons, protons and neutrons—and not of matter in muon or tau flavors.)

NSI affects neutrinos and antineutrinos in opposite ways, whereas most of the standard oscillations are expected to be the same for both. Thanks to NuMI’s ability to generate separate neutrino and antineutrino beams and to the magnetized detectors’ ability to discriminate neutrino and antineutrino interactions, MINOS can search for non-standard interactions and measure the NSI parameter called εμτ. In this new analysis, MINOS combined its neutrino and antineutrino data collected over five years and performed the first direct search for NSI. The results showed that NSI made no significant contribution to flavor change, in agreement with previous hints.

Because of its magnetized detectors, MINOS remains the most suitable experiment to further investigate NSI. Starting this spring, MINOS+ will collect data in a complementary energy regime. This will allow for a more precise determination of the impact of NSI in neutrino flavor change.

—Zeynep Isvan, Brookhaven National Laboratory