The Parno Group

Experimental neutrino physics at Carnegie Mellon University

Coherent Elastic Neutrino-Nucleus Scattering

Neutrinos are fundamental particles with no electrical charge, no color charge, and a very small (but nonzero) mass. They interact with other particles via the nuclear weak force, which is aptly named, so it's difficult to detect them directly; in fact, it took about a quarter of a century to detect them for the first time after their existence was predicted!

Coherent elastic neutrino-nucleus scattering (CEvNS), first proposed about forty years ago but never yet detected, is actually relatively common for a neutrino interaction with a cross section around 10-39 cm2 for low-energy neutrinos on moderately sized nuclei. (Particle physicists measure the probability of a reaction occurring using the analogy of targets: a larger target, analogous to a larger cross-sectional area, is easier to hit.) In this process, a neutrino scatters from a nucleus by exchanging an electrically neutral Z boson. The scattering process is coherent: the neutrino interacts with the nucleus as a whole, rather than with individual nucleons or groups of nucleons. And it's elastic: a neutrino and a nucleus go in, and the same neutrino and the same nucleus come out, conserving kinetic energy. The problem is that even the lightest nucleus is literally hundreds of millions of times more massive than a neutrino. Even when the neutrino is carrying a fair amount of energy (50 MeV is a typical upper bound for nuclei of medium mass), the recoil imparted to the nucleus is quite small, hundreds of eV or a few keV. That's not easy to detect.

Luckily for CEvNs, though, a large community of physicists has spent decades designing and refining detectors to look for dark matter (specifically, WIMPs, or weakly interacting massive particles) via small nuclear recoils. In fact, the next generation of WIMP detectors will be so sensitive that they'll see a CEvNS background from solar neutrinos! So the time is right to make a dedicated measurement of CEvNS: detect the process, validate the models used in dark matter detection and supernova dynamics, and refine the measurement to look for the signature of new physics beyond the standard model. The COHERENT collaboration is preparing to make such a measurement at Oak Ridge National Laboratory, using the Spallation Neutron Source as a serendipitous source of pulsed neutrinos with the right energy range.

COHERENT has made the first discoveries of CEvNS in two different detectors -- cesium iodide (paper in Science) and liquid argon (paper in Physical Review Letters). We are continuing to make these measurements more accurate -- by analyzing more data and by better understanding our systematics -- and we are working to measure CEvNS in additional target types. Our group is especially involved in understanding neutron backgrounds using the MARS detector, and in understanding the rate of neutrinos coming out of the Spallation Neutron Source. We've come about as far on the second goal as simulations can take us. Working with our colleagues in COHERENT, we are now working to build a two-module heavy-water detector that will measure this neutrino flux directly. You can see a recent preprint paper for details.