The First Observation of Coherent Elastic Neutrino Nucleus Scattering
Based on recently published research by KAIST researchers, coherent elastic neutrino nucleus scattering (CEvNS) has been detected for the first time.
The predicted cross-section of the CEvNS provides new opportunities to study neutrino properties and leads to a miniaturization of detector size with potential applications to emerge in the future.
Professor Jongee Yoo (Department of Physics) collaboration with a network of scientitsts called “COHERENT,” observed this form of coherent elastic scattering for the first time.
CEvNS was proposed but had never been observed since its first prediction appeared in the scientific literature in 1974.
The condition for coherent interactions between neutrinos and nuclei requires a sufficiently small momentum transfer from a neutrino to an interacting nucleon in a nucleus in order for the nuclear wave function to change minimally.
Therefore, the wavelength of the scattered neutrinos should be much larger than the size of the nucleus, which limits the upper energy of the neutrino beam to 50 MeV.
The sole signature of CEvNS in an experiment is nucleus recoiling with 10s of keV kinetic energy.
Consequently, technical difficulties of developing a sufficiently large, low-energy threshold, and low-background detector have historically prevented the discovery of CEvNS.
The CEvNS cross-section, which is precisely known in the Standard Model (SM), is considerably larger than other neutrino interaction channels at the MeV energy scale.
Therefore, CEvNS is a very powerful tool for future neutrino experiments.
The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) generates the most intense pulsed neutron beams in the world, produced by the interactions of accelerator-driven about a giga-electron-volt (GeV) energy of protons striking a mercury target.
This beam creates a significant yield of neutrinos as well, generated when pions, themselves a byproduct of proton interactions in the target, decay at rest.
The resulting low neutrino energies are favorable for CEνNS.
The scientific collaboration COHERENT identified a location in a basement corridor which benefits from continuous shielding against beam-related neutrons; this location reduces cosmic ray-induced backgrounds, while sustaining an instantaneous neutrino flux very high.
The COHERENT collaboration observed this process at a 6.7σ confidence level, using a low-background, 14.6-kilogram CsI scintillator exposed to the neutrino emissions from the SNS at ORNL.
The discovery places tighter bounds on exotic, beyond-the-SM interactions involving neutrinos.
There are ongoing discussions of various applications of the CEvNS process, such as remote monitoring of nuclear reactors, non-proliferation purposes, new opportunities of studying neutrino properties, research into dark matter, and supernova processes.
* Professor Yoo has focused on measuring the CEvNS process since 2010. He was leading a detector development until he moved to KAIST in 2016; his detector is currently installed and running in the SNS site at ORNL.
This study was published in Science as a cover article (15 September 2017)