20 years later: Neutrino-induced coherent pions are back to Fermilab

Display of an event captured in the ArgoNeuT detector. The track on the top corresponds to a muon, the one below it is a charged pion. These particles are produced by the interaction of a muon neutrino with an argon atom in the detector.

Display of an event captured in the ArgoNeuT detector. The track on the top corresponds to a muon, the one below it is a charged pion. These particles are produced by the interaction of a muon neutrino with an argon atom in the detector.

The neutrino is known for how rarely it interacts with matter. But when it does, the interaction can take place numerous ways, and some interaction types happen more often than others. The ArgoNeuT experiment recently looked at one of the more rare cases — one that comes to only about 1 percent of all the possible ways a neutrino can interact. As one might expect, its infrequency poses a great challenge in our efforts to measure it.

This month, the ArgoNeuT collaboration released a new measurement of this rare interaction, called charged-current coherent pion production induced by neutrinos on nuclei. In this process, a neutrino interacts with a nucleus as a whole, producing a muon and a pion without breaking the nucleus apart or leaving it in an excited state. Seen in the detector, the events look like the one shown above, where two very forward-going tracks leave the interaction point.

Historically, there have been only a handful of experiments that observed coherent pion production. Back in 1993, the FNAL E632 experiment, conducted using a 15-foot bubble chamber, measured interactions of this type at a neutrino energy of 70 to 90 GeV. In more recent years, the K2K and SciBooNE experiments also attempted to measure this cross section at a much lower energy (1 to 2 GeV) but found no sign of it in the charged-current channel. The null results motivated renewed interest by the theoretical community, who modified the favored models of the time and proposed new ones.

The primary authors of this result are Tingjun Yang of Fermilab, left, and Edward Santos of Imperial College London.

The primary authors of this result are Tingjun Yang of Fermilab, left, and Edward Santos of Imperial College London.

These days, Fermilab’s ArgoNeuT and MINERvA collaborations are in hot pursuit of these interactions, measuring them using the low-energy NuMI beam. The ArgoNeuT collaboration has measured the likelihoods of charged-current pion production, reporting the interactions with neutrinos and antineutrinos at the mean energies of 3.6 GeV and 10 GeV, respectively. These measured probabilities, the results of a five-month run of antineutrino-enhanced NuMI beam, are in good agreement with theoretical predictions and are attracting much interest within the neutrino community.

This is the first time that scientists measured the process in a liquid-argon detector and using an automated reconstruction. Researchers also once again demonstrated the potential of the liquid-argon technique for the measurement of neutrino interactions. Key pieces of this success were ArgoNeuT’s capabilities for precisely measuring the particles ejected from a neutrino interacting with an argon nucleus.

Although ArgoNeuT’s small detector size limits the precision of this measurement, the techniques developed during this analysis will be used by future, larger experiments, such as MicroBooNE and LAr1-ND, to gain new insights into coherent pion production.