Catching (anti)neutrinos in a liquid-argon detector

The angular distribution with respect to the incoming (anti)neutrino beam of the outgoing muon in the charged-current interaction. GENIE and NUWRO are simulated-neutrino-event generators.

The ArgoNeuT experiment has measured the first antineutrino differential cross sections in a liquid-argon time projection chamber, or LArTPC.

Neutrinos share some properties with quarks, the particles that constitute the familiar proton and neutron. Like quarks, neutrinos come in three types. Also like quarks, neutrinos can oscillate, or change from one of their three types into another.

That’s about where the resemblance ends. Neutrino masses are many orders of magnitude smaller than the quark masses. Although they oscillate among themselves, they do so with entirely different parameters than do quarks. And while every quark has an antiquark partner, neutrinos may or may not be their own antiparticles.

Before we stampede to the grand unified theory that explains all these bizarre observations, we need to do the most pedestrian thing: We must carefully characterize neutrino interactions in our liquid-argon detectors.

When a neutrino or antineutrino interacts with a liquid-argon nucleus, it produces secondary particles, such as a muon.

Some crucial properties of these antineutrino interactions can be characterized by measuring the properties of the outgoing muon. These are called charged-current inclusive interactions. The ArgoNeuT detector — the first LArTPC to record neutrino data in the United States — offers a very high-resolution view into the nature of these particle interactions.

The 170-liter detector took data from 2009-10 in the NuMI beam. The MINOS near detector, located behind ArgoNeuT, measures the charge and momentum of muons from ArgoNeuT’s (anti)neutrino interactions.

During ArgoNeuT’s data collection, a short one-month run in neutrino beam mode was followed by a longer run in antineutrino beam mode. ArgoNeuT released the results of the charged-current cross section in neutrino beam mode two years ago. New results from the antineutrino beam mode — which contains both neutrino and antineutrino sources — show agreement with simulations and no surprises in the distributions of the angle and momentum of the outgoing muon.

There is little data on the rates of these interactions in argon and even less on specific properties of the outgoing particles that arise from them. The LArTPC technology will be used in future long- and short-baseline experiments at Fermilab, and the measurements reported here lay the foundation for automated reconstruction techniques using the LArSoft software package. Such techniques will be important to these programs. ArgoNeuT’s neutrinos are at roughly the same energy that will be studied in future long-baseline neutrino experiments, so this result can already help shape preparations for similar analyses on those experiments.

LArTPCs, with measurements like this one, are rewarding the investment placed in them by the U.S. high-energy physics community.

Eric Church, Yale University, and Tingjun Yang, Fermilab

Eric Church (left) of Yale University and Tingjun Yang of Fermilab conducted this analysis.