The hammer of neutrino interactions

A “hammer” neutrino interaction in ArgoNeuT with back-to-back protons (p1 and p2) and one forward-going muon (μ).

Physicists from the ArgoNeuT collaboration may be seeing some interesting new ways in which a neutrino leaves its signature. ArgoNeuT uses a detector called a liquid-argon time projection chamber, which was operated in the NuMI beamline, to collect thousands of neutrino interactions on argon. LArTPCs are very well-suited to the task of observing neutrino interactions due to their very high-quality imaging capabilities. This same detector technology will soon be used on a much larger scale by MicroBooNE, and eventually by LBNE, to further understand the behavior of neutrinos.

A single argon atom floating around in a LArTPC detector contains 40 nucleons (18 protons and 22 neutrons) within its nucleus. This nucleus presents a complicated environment for the neutrino to interact within. In the simplest interaction that can occur, an incoming neutrino knocks into a neutron in the nucleus, producing an outgoing proton and muon. Based on experiments that scatter electrons off nuclear targets, there is evidence that some nucleons can partner up, existing as a duo. Neutrinos may end up interacting with both, and not just one, of the members of the duo. This twist on the neutrino interaction can have measurable consequences that need to be accounted for in order to properly interpret experimental results and compare them against theoretical predictions.

One suggested signature of a neutrino interacting with a nucleon pair, as opposed to a lone nucleon, is two back-to-back protons emerging from the nucleus as opposed to the single proton described before. Visually this signature gives the appearance of a hammer, with the muon forming the handle and the protons forming the head. Among ArgoNeuT’s data sample, several events are observed that have this telltale hammer signature, which could be the smoking gun for correlated nucleons playing a role in neutrino interactions. The back-to-back protons can be very low in energy, making them hard to detect, which is where the strengths of the LArTPC technique come into play. The excellent resolution and calorimetry of the LArTPC detector allows both protons to be identified cleanly, and the back-to-back signature is easily discerned.

ArgoNeuT’s studies of these nuclear effects will help inform phenomenologists who create the tools vital to modern particle physics analysis. Theoretical models of the nuclear environment are complicated, so providing experimental data can help the model builders. These studies will also help future LArTPC experimenters know what to expect when they send the intrepid neutrino into the heart of an argon atom.

—Mitch Soderberg