Who let the pions out (of the nucleus)?

This plot shows what a neutral pion looks like in the MINERvA detector when produced with a muon. Colors correspond to energy deposited in each triangular scintillator bar.

This plot shows what a neutral pion looks like in the MINERvA detector when produced with a muon. Colors correspond to energy deposited in each triangular scintillator bar.

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Neutrinos are odd particles: They rarely interact in matter and can change character back and forth over time in a process called oscillation. When neutrinos do interact with matter, however, they do so in ways that are similar to how other high-energy particles produced by Fermilab accelerators interact: by making still more particles. So even though neutrinos themselves contain no quarks, they are still able to produce pions, quark-antiquark pairs that can be either charged or neutral. At today’s Joint Experimental-Theoretical Physics Seminar, MINERvA will release its new result on how neutral pions are produced in a beam of antineutrinos from Fermilab’s NuMI beamline.

A previous MINERvA result described how charged, rather than neutral, pions are made from neutrinos. At least “on paper,” that result is similar to today’s new result. Both of these interactions are predicted to happen and even to have similar probabilities.

However, they leave very different footprints in detectors and so present different challenges. In fact the neutral pion’s footprint is a worry for oscillation experiments because it can look like something it’s not. So oscillation experiments need good measurements of how many neutral pions are made in neutrino and antineutrino beams.

Measuring both charged- and neutral-pion production at similar neutrino energies also helps us better understand the nucleus with which a neutrino interacts, since the two different kinds of pions see the nucleus differently as they exit it. Before the research that led to today’s result, though, only a few dozen neutral pion-antineutrino events have ever been seen in a single experiment.

This plot shows the cross section (likelihood per proton or neutron) of a neutral pion and an antimuon being made from an antineutrino as a function of the pion momentum. The two different models represent turning on and off the effects of the nucleus where the neutrino interacted. The effects of the nucleus were clearly "turned on" in the data. The inner error bars are statistical and the outer error bars are the total uncertainties.

This plot shows the cross section (likelihood per proton or neutron) of a neutral pion and an antimuon being made from an antineutrino as a function of the pion momentum. The two different models represent turning on and off the effects of the nucleus where the neutrino interacted. The effects of the nucleus were clearly “turned on” in the data. The inner error bars are statistical and the outer error bars are the total uncertainties.

Trung Le of Rutgers University will present MINERvA's latest results at today's wine and cheese seminar at 4 p.m. in One West.

Trung Le of Rutgers University will present MINERvA’s latest results at today’s wine and cheese seminar at 4 p.m. in One West.

Neutral pions are harder to see than charged pions because they decay very rapidly and must be detected through their decay products — two neutral photons, which interact on average about a foot away from where the neutral pion decayed in the first place. For today’s result, the neutral pion is produced at the same time as a muon, which is a heavier version of an electron.

This new measurement adds more than 400 new events to the world’s collection for this novel interaction and tells us much more about how neutrinos and pions are both affected by the nucleus.

There has been a lot of interest in pion production because the best theories are unable to describe previous MiniBooNE measurements of charged pions. Although the best calculation was also unable to reproduce the MINERvA charged-pion data, it failed in a different way, extending the controversy. Experimenters don’t stop, though. They just keep trying to find another way to measure what’s happening inside the nucleus until they understand it. Now MINERvA’s new result, which sees better agreement between the best calculation and the prediction (see figure below), paints a new picture of the nucleus.