First, the cast of characters: Neutrinos, the quarries of the MINERvA experiment, are subtle particles. They come in three types, two of which play a prominent role in this story: the electron neutrino and the muon neutrino. Neutrinos' behavior could change our understanding of physics and the universe. And they are hard to capture. So scientists do the next best thing: study the particles resulting from neutrinos' interactions in a detector. Unfortunately, the interactions aren't always clear signatures. In a situation of innocent deception, electron neutrinos and muon neutrinos can be mistaken. To tamp down on cases of mistaken identity, MINERvA measured the frequency of the signature of a muon neutrino interaction—the emergence of a particle called a muon and a second called a neutral pion. This signature can be easily mistaken for the signature left by an electron neutrino.
This measurement lets scientists better identify muon neutrinos masquerading as electron neutrinos. Measuring neutral-pion production at MINERvA also helps scientists understand the relationship between a neutrino's energy and the resulting particles in the detector. One of the biggest problems in oscillation experiments (which look at how neutrinos change from one type into another) occurs when a neutrino of one energy masquerades as a neutrino of another energy. This happens frequently. Why? Two reasons. Some of the particles get stuck inside the nucleus of the detector material, or they transform into other particles. MINERvA's measurement of neutral pion production is sensitive to these effects and helps theorists improve their models of the nucleus.
Scientists are working to resolve a case of mistaken identity in the world of physics. They want to disambiguate two types of neutrinos. Oscillation experiments look for electron neutrinos (which leave electrons in their wake) that were originally muon neutrinos (which produce mostly muons). If they see an electron, they know their muon neutrino has oscillated into an electron neutrino. If they see a muon, they conclude the muon neutrino did not oscillate by the time it reached them.
Neutrino interactions—including those of electron neutrinos or muon neutrinos—can also produce electrically neutral pions. Neutral pions sometimes disguise themselves as electrons—the telltale sign that an electron neutrino has passed through—in a particle detector. So when scientists think they have identified electrons, they might actually be looking at pions.
Why does this happen? Pions decay into two photons, particles of light. Consider the case in which one photon escapes detection, and the other, as often happens, looks a lot like an electron. This can trick the experiment into counting the arrival of two photons as an electron sighting—therefore identifying a muon neutrino as an electron neutrino. So it seems like the neutrino has oscillated when, really, it has not.
To avoid such misidentification, MINERvA scientists measured how often a neutrino interaction results in a muon and a neutral pion.
Fermi National Accelerator Laboratory
Fermi National Accelerator Laboratory
This study was done by members of the MINERvA collaboration using the resources of the Fermi National Accelerator Laboratory (Fermilab), a Department of Energy (DOE), Office of Science, High Energy Physics user facility. Fermilab is managed by Fermi Research Alliance, LLC. These resources included support for the MINERvA construction project; support for construction also was granted by the National Science Foundation (NSF) and by the University of Rochester. Support for participating scientists was provided by NSF and DOE (USA); by CAPES and CNPq (Brazil); by CoNaCyT (Mexico); by Proyecto Basal FB 0821, CONICYT PIA ACT1413, and Fondecyt 3170845 and 11130133 (Chile); by CONCYTEC, DGI-PUCP, and IDI/IGI-UNI (Peru); and by the Latin American Center for Physics.
O. Altinok, et al., "Measurement of ?? charged-current single ?0 production on hydrocarbon in the few-GeV region using MINERvA." Physical Review D 96, 072003 (2017). [DOI: 10.1103/PhysRevD.96.072003]
Fermilab news at work article: Neutral-pion production at MINERvA, or how to cope with cases of mistaken identity