In February, the MINERvA experiment at Fermilab reported its findings of what happens when a neutrino produces a pion (a particle made of a quark and an antiquark) by interacting with a proton or neutron inside the nucleus. In today’s wine and cheese seminar, MINERvA will release its measurement of what happens when a neutrino or antineutrino produces a pion outside a nucleus by interacting with the nucleus as a whole but leaving the nucleus intact. Neutrino physicists refer to this reaction as coherent pion production.
A neutrino interaction with a nucleus is like the break shot at the beginning of a billiards game where the cue ball is shot into a tightly packed group of target balls to break up the group. If coherent pion production were to happen in billiards, the target balls would remain tightly packed after being struck and an additional ball (the pion) would emerge from the collision.
Coherent pion production can be a background to neutrino oscillation experiments that measure how neutrinos change from one type of neutrino to another as they travel through space. Predictions for coherent pion production disagree in how much background the reaction should produce in oscillation experiments. In addition, recent experiments that looked for coherent pion production at neutrino energies important to oscillation experiments came up empty — until now, that is.
MINERvA has measured coherent pion production on carbon atoms where the interaction changes the neutrino (or antineutrino) into a muon (a heavier cousin of the electron). MINERvA searches for coherent pion production using its defining characteristic — that the interaction does not breakup the nucleus.
MINERvA can see whether or not breakup of the nucleus occurs in two ways. First, it can detect the particles ejected from the nucleus when it is broken up and can require that only a muon and a pion are detected at the interaction point. Second, MINERvA can measure the momentum transferred to the nucleus by measuring the muon and pion momentum and can require it be consistent with not breaking the nucleus apart.
These two signatures together greatly reduce the background and allow MINERvA to measure, for the first time, the details of coherent pion production to understand how it produces background for oscillation experiments.