Neutrinos are notoriously antisocial, nearly always slipping past atoms of matter without so much as a “how do you do.” But new research indicates that on the rare occasion a neutrino and an atomic nucleus do make contact, the interaction is surprisingly involved.
By training a beam of neutrinos on a plastic target, researchers at the MINERvA experiment at Fermi National Accelerator Laboratory in Batavia, Ill., have found that when a neutrino collides with an atom it often knocks free not just one proton or neutron, but two. Some of the particles within the atomic nuclei, it appears, are pairing up rather than moving about independently, only to be sprung free in twos when a neutrino strikes. The results will have implications for precision neutrino measurements, which often rely on carefully reconstructing the physics of rare collisions between neutrinos and atoms.
MINERvA, an apparatus about the size of a bread truck, is parked in the path of a Fermilab neutrino beam, nearly all of which passes cleanly through the detector and into another neutrino experiment called MINOS. MINERvA’s detector contains a variety of different materials, including layers of lead and iron fronting the hydrocarbon plastic material of the inner detector. “It’s chewy on the inside and crunchy on the outside,” says Deborah Harris, a Fermilab physicist and co-spokesperson for the MINERvA collaboration. “One of the goals is to measure the neutrino interaction on several different nuclei.” Now the experiment has produced its first physics results, an analysis of neutrino interactions with carbon nuclei in the plastic portion of the detector.
In two new studies that will appear in the journal Physical Review Letters, the MINERvA collaboration reports on several months of experimental operation in 2010 and 2011. The analyses focus on so-called quasi-elastic scattering, which in the simplest case involves a neutrino colliding with a neutron in one of the carbon atoms. The interaction of those two electrically neutral particles yields two oppositely charged particles, a positively charged proton and a negatively charged muon, which scatter outward like billiard balls struck by a cue ball. “It spits out a proton, and leaves the rest of the nucleus basically undisturbed,” Harris says. “Some fraction of the time, it looks like more than just one proton comes out.”
The appearance of an extra proton alongside a neutron-turned-proton indicates that neutrinos tend to strike particle pairs. “Twenty-five percent of the time, with some uncertainties, protons are traveling around with neutrons,” Harris says. The physicists observed a similar trend in analogous reactions involving antineutrinos—the particles’ antimatter counterpart. “Let’s say that the carbon nucleus was really just six pairs of protons and neutrons” rather than a dozen independent particles, Harris explains, “so whenever you hit a proton you’re also hitting a neutron. That’s kind of an extreme view of what might be going on in the nucleus.”
Neutrinos and antineutrinos come in three flavors—electron, muon and tau—each of them associated with a charged elementary particle of the same name. But as a neutrino zips through space at nearly the speed of light, it oscillates between the three possible flavors, a phenomenon that several experiments around the world are currently investigating. The tendency toward nuclear pairings documented at MINERvA could inform the analysis of those neutrino-oscillation experiments. “It is not accounted for in the standard kinds of simulations of how neutrinos interact in all these oscillation experiments,” Harris says. “In order to predict what the neutrino energy was coming in, you have to make some assumptions about what was going on in the nucleus.”
Adds physicist John Arrington of Argonne National Laboratory, who did not contribute to the new research: “You really have to understand those reaction mechanisms to know what’s going on” in experiments where neutrinos scatter off of atomic nuclei. “That really just wasn’t possible with the types of neutrino-scattering experiments that have been done before.”