What actually happens when a neutrino hits something?

The ArgoNeuT liquid argon time projection chamber’s (LArTPC) neutrino cross section measurements are depicted here in terms of the muon momentum (top) and angle (bottom).

The neutrino almost never interacts with matter. “The Earth is just a silly ball / To them, through which they simply pass,” wrote John Updike in 1960. But how often do neutrinos actually hit the “silly ball”? And, what exactly happens when a neutrino does interact? A number of Fermilab-based experiments, including the ArgoNeuT liquid argon time projection chamber (LArTPC) detector, are seeking answers to these questions. A LArTPC provides a richly detailed image of a neutrino event by collecting the ionization tracks of particles created in the interaction.

ArgoNeuT ran in the NuMI beamline at Fermilab from September 2009 to February 2010. The experiment, located just upstream of the MINOS near detector, collected the first thousands of neutrino and anti-neutrino events ever with a LArTPC in a low energy neutrino beam. ArgoNeuT found that only about two out of every 1,000,000,000,000 neutrinos passing through the 90 cm long detector interacted to produce a muon, the electron’s heavier cousin. That’s about as rare as being dealt a royal flush with the first five cards—twice in a row!

In addition to this total cross section measurement, ArgoNeuT measured the angle and momentum of the outgoing muon in such events. The results are consistent with the world’s data and theoretical expectations. Along with fundamental importance in particle physics, neutrino cross section measurements such as these contain essential information for those experiments seeking to probe neutrino oscillation. The outgoing particle kinematic information is useful for forming a complete picture of how neutrinos interact and provides much needed data for the proper simulation and modeling of neutrino events.

LArTPC technology is one of the rabbits in the race to measure CP violation in neutrinos. CP violation is a fundamental difference between matter and anti-matter that could help to explain why there exists so much matter and so little anti-matter in the universe today.

ArgoNeuT’s measurements are among the first physics results obtained with a LArTPC and go a long way in demonstrating the technology’s viability for a future ultra-large detector and the pursuit of this physics goal. As Updike once wrote, “Rabbit, run.”

*ArgoNeuT thanks the MINOS collaboration for providing their data for use in this analysis.

Learn more

—Joshua Spitz, ArgoNeuT