The neutrino is a confounding little particle that is believed to have played a major role in the evolution of our Universe. They also possess very little mass, have no charge, and interact with other particles only through the weak nuclear force and gravity. As such, finding evidence of their interactions is extremely difficult and requires advanced facilities that are shielded to prevent interference.
One such facility is the Oak Ridge National Laboratory (ORNL) where an international team of researchers are conducting the COHERENT particle physics experiment. Recently, researchers at COHERENT achieved a major breakthrough when they found the first evidence of a new kind of neutrino interaction, which effectively demonstrates a process known as coherent elastic neutrino-nuclear scattering (CEvNS).
The research was conducted by the COHERENT collaboration – which includes 80 researchers from 19 institutions in 4 nations – using the Spallation Neutron Source (SNS) at the ORNL. As they indicate in a study (which recently appeared in the Physical Review Letters), their research consists of observing a process known as Coherent Elastic Neutrino-nucleus Scattering (CEvNS, pronounced “sevens”).
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According to The Standard Model of particle physics, neutrinos are leptons, an elementary particle similar to an electron – but with no electric charge and very little mass (if any). They are created through radioactive decay, stellar fusion, supernovae, and as a byproduct of the Big Bang. For this reason, they are believed to be the most abundant particle in the Universe.
The CEvNS process comes down to neutrinos colliding with the much-larger nucleus of an element, which results in a tiny transfer of energy and causes the nucleus to recoil almost imperceptibly. The process was first predicted in 1973 but evaded detection because of the tiny amounts of energy and motion involved. This changed in 2017 when members of the COHERENT collaboration used the SNS to measure the CEvNS process for the first time.
In this instance, they observed neutrinos generated by the SNS as they interacted with heavier cesium and iodine nuclei. This time around, the team observed neutrinos as they collided with even smaller argon nuclei, which caused even tinier levels of recoil. As Duke University physicist Kate Scholberg, a spokesperson and organizer of science and technology goals for COHERENT, explained in an ORNL press release:
“The Standard Model of Particle Physics predicts coherent elastic scattering of neutrinos off nuclei. Seeing the neutrino interaction with argon, the lightest nucleus for which it has been measured, confirms the earlier observation from heavier nuclei. Measuring the process precisely establishes constraints on alternative theoretical models.”
In addition to being the smallest neutrino detector in the world, the SNS accelerator-based system is also the world’s brightest source of pulsed neutron beams. This consists of protons that are fired at atoms of mercury, a process which smashes them apart to produce massive amounts of neutrons and neutrinos as a by-product.
These neutrinos are used in a dedicated neutrino laboratory beneath the SNS, an experiment known as “Neutrino Alley” developed by the COHERENT team. This alley is outfitted with highly-sensitive CENNS-10 detectors, which rely on cesium iodide scintillator crystals to detect tiny light signals produced by subatomic interactions. The SNS was further augmented in 2017 with the addition of sodium iodide detectors.
Yuri Efremenko, a physicist at the University of Tennessee, oversaw the most recent addition to the experiment, which consisted of liquid argon detectors. As Efremenko explained, this made the experiment even more sensitive, to the point that it could provide data on even tinier collisions:
“Argon provides a ‘door’ of sorts. The CEvNS process is like a building that we know should exist. The first measurement on cesium and iodine was one door that let us in to explore the building. We’ve now opened this other argon door. Seeing something unexpected would be like opening the door and seeing fantastic treasures.”
The COHERENT team gathered 18-months of data from the SNS, the analysis of which revealed 159 CEvNS events – which is consistent with the Standard Model prediction. In the future, the collaboration team hopes to scale their experiment so they can observe 25 times as many CEvNS events per year. In the process, they hope to obtain detailed spectra that might reveal the signatures of new physics.
To further their goal of observing CEvNS on a variety of nuclei, the team plans to install an even bigger 10-ton (9 metric ton) liquid argon detector at the SNS’s Second Target Station. There are also plans for adding a 16-kg (~35 lbs) detector based on germanium nuclei (bigger than argon but smaller than cesium and iodine) to Neutrino Alley next year.
In the meantime, the data from this latest analysis will help researchers around the world interpret their neutrino measurements and investigate the possibility of new physics. These results also have practical applications in the laboratory and the field. For example, the method employed by the COHERENT team could be used by particle physicists to measure the distributions of neutrons inside nuclei.
Meanwhile, astrophysicists could use it to determine the density of neutron stars, providing another “door” for investigating physics under the most extreme and exotic of domains. Said physicist Jason Newby, ORNL’s lead for the COHERENT experiment:
“We’re looking for ways to break the Standard Model. We love the Standard Model; it’s been really successful. But there are things it just doesn’t explain. We suspect that in these small places where the model might break down, answers to big questions about the nature of the universe, antimatter and dark matter, for instance, could lie in wait.”