A team of physicists with the COHERENT collaboration is the first to detect and characterize coherent elastic scattering of low-energy neutrinos off atomic nuclei.
Neutrinos, miniscule subatomic particles that rarely interact with matter, are often described as ‘ghost-like.’
Chargeless and nearly without mass, trillions of neutrinos pass through our bodies every second, but we have no way feeling them.
In 1974, Fermilab physicist Daniel Freedman predicted a novel way for neutrinos to interact with matter.
More than four decades later, the COHERENT collaboration built the world’s smallest neutrino detector to observe the elusive interaction — called coherent elastic scattering — for the first time.
“Larger neutrino detectors are usually better for spotting these particles because they simply have more matter for the neutrinos to interact with, making it that much more likely that they will be able to catch at least one of the rare events,” said Duke Professor Kate Scholberg, spokesperson for the COHERENT collaboration.
“But detecting coherent elastic scattering is a little different from detecting other types of neutrino interactions.”
“Scattering interactions occur much more often, but are also much lower in magnitude, than some of the other behaviors. As a result, smaller but extremely sensitive detectors can be more effective.”
When a neutrino bumps into the nucleus of an atom, it creates a tiny, barely measurable recoil.
Making a detector out of heavy elements such as iodine, cesium or xenon dramatically increases the probability for this mode of neutrino interaction, compared to other processes.
But there’s a trade-off, since the tiny nuclear recoils that result become more difficult to detect as the nucleus grows heavier.
To detect that bit of tiny recoil, the scientists figured out that a cesium iodide crystal doped with sodium was the perfect material.
The detector built by the team measures about 13 inches long and 4 inches wide, and weighs only 32 pounds (14.5 kg). In comparison, the world’s most famous neutrino observatories are equipped with thousands of tons of detector material.
Physicists are excited about the discovery because careful measurements of coherent elastic scattering could be a powerful tool for testing the limits of the Standard Model, physicists’ best guess at an overarching mathematical description of the Universe.
“This is useful not just for studying the fundamental properties of neutrinos themselves, but also using their interactions to constrain our knowledge of nuclear physics, the Standard Model and possible extensions beyond the Standard Model,” said team member Grayson Rich, a graduate student at the University of North Carolina.
“This discovery is exciting in and of itself, but it really is just step zero. But it’s a pretty big step zero.”
“COHERENT’s data will help with interpretation of measurements of neutrino properties by experiments worldwide,” Professor Scholberg said.
“We may also be able to use coherent scattering to better understand the structure of the nucleus.”
The team’s findings were published online August 3, 2017 in the journal Science.
_____
D. Akimov et al (COHERENT Collaboration). Observation of coherent elastic neutrino-nucleus scattering. Science, published online 3, 2017; doi: 10.1126/science.aao0990