A New Mexico State University physics professor and graduate student are among collaborators on a one-of-a-kind physics experiment at the Department of Energy’s Oak Ridge National Laboratory, using the world’s smallest neutrino detector.
The result is the first measurement of coherent scattering of low-energy neutrinos off nuclei.
The COHERENT experiment, performed at ORNL’s Spallation Neutron Source for more than a year and published in the journal Science this month, provides compelling evidence for a neutrino interaction process predicted by theorists 43 years ago, but never seen.
NMSU assistant physics professor Robert Cooper and NMSU physics graduate student Hector Moreno in the College of Arts and Sciences are among the authors of the paper and participants in COHERENT, a collaboration of 80 researchers from 19 institutions and four nations.
“This first measurement has brought together a diverse team of scientists from across this country and abroad,” Cooper said. “As a member of COHERENT, NMSU students are literally at the cutting edge of physics research. Students like Hector Moreno are in a position to discover new things and make an impact on the future direction of neutrino research.”
Moreno, who is from Colombia, has been working with the experiment at ORNL in Tennessee since March as part of his study. While part of another collaborative research effort in Illinois, he met with researchers who encouraged him to apply for the physics Ph.D. program at NMSU.
“Neutrinos are a very interesting area of research right now and this is a terrific discovery,” Moreno said. “For me it has been an amazing experience to be at the national lab in the research environment. It has allowed me to work closely with the particle physics experimental area. Dr. Cooper has played an important role for me in encouraging me and guiding me through this research.”
NMSU has been part of the COHERENT collaboration since fall 2015. “We have played active roles in the measurement of background neutron fluxes with the SciBath detector, and with the liquid argon detector,” Cooper said. SciBath is a prototype neutral particle detector offering high-precision reconstruction of neutrino and neutron events.
Typically, neutrinos, electrically neutral particles that interact only weakly with matter, interact with individual protons or neutrons inside a nucleus. But in “coherent” scattering, an approaching neutrino “sees” the entire weak charge of the nucleus as a whole and interacts with all of it.
“The energy of the SNS neutrinos is almost perfectly tuned for this experiment ¬– large enough to create a detectable signal, but small enough to take advantage of the coherence condition,” said ORNL physicist Jason Newby, technical coordinator and one of 11 ORNL participants in COHERENT. “The only smoking gun of the interaction is a small amount of energy imparted to a single nucleus.”
That signal is as tough to spot as a bowling ball’s tiny recoil after a pingpong ball hits it.
“It is crucial that we finally measured this process,” Cooper said. “It is well-predicted, and now after the first measurement, a series of future measurements can utilize the Coherent Elastic Neutrino Nucleus Scattering process to explore a wide variety of new physics: non-standard model sterile neutrinos, nuclear structure via nuclear form factors, the structure of the weak interaction, supernova processes, and many other topics.”
The calculable fingerprint of neutrino – nucleus interactions predicted by the Standard Model and seen by COHERENT is not just interesting to theorists. In nature, it also dominates neutrino dynamics during neutron star formation and supernovae explosions.
“When a massive star collapses and then explodes, the neutrinos dump vast energy into the stellar envelope,” said physicist Kate Scholberg of Duke University, COHERENT’s spokesperson. “Understanding the process feeds into understanding of how these dramatic events occur.”
Coherent elastic scattering is also relevant for detecting the enormous neutrino burst from a supernova. “When such an event occurs in the Milky Way, neutrinos of all flavors will bump into nuclei, and sensitive dark matter detectors may observe a burst of tiny recoils,” she said.
“COHERENT’s data will help with interpretation of measurements of neutrino properties by experiments worldwide,” Scholberg said.