The world's smallest neutrino detector has just discovered something quite big. Researchers have observed an atom ricochet from the tiny impact of a neutrino for the first time. Such an occurrence was first predicted in theory 43 years ago, but had so far eluded scientists.
The results of the experiment, conducted at the Oak Ridge National Laboratory (ORNL), are published in Science. It involves something called coherent scattering, where the neutrinos interact with the whole atom, imparting a little bit of energy to the nucleus. Previously, they've only been seen interacting with individual protons or neutrons in the nuclei of atoms.
"The one-of-a-kind particle physics experiment at Oak Ridge National Laboratory was the first to measure coherent scattering of low-energy neutrinos off nuclei," co-author Jason Newby, technical coordinator at the ORNL, said in a statement.
Typically, neutrino detectors are huge and look for neutrinos emitted from far away sources. This includes some natural, like the Sun, and some artificial, like the T2K experiment, where neutrinos are sent over 295 kilometers (183 miles) to a detector the size of a building. The more distant these detectors are, the more easily the properties of neutrinos can be measured.
However, this experiment instead uses a detector that weighs just 14.5 kilograms (32 pounds), and the neutrinos emitted are a byproduct of an instrument called the Spallation Neutron Source (SNS) just 20 meters (65 feet) from the target. Although it’s a modest setup, the detector is 100 times more likely to detect neutrinos compared to the observatories that study solar neutrinos. This is because there are a lot of neutrinos coming from the SNS, and they're pretty close.
"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," Newby said. "The only smoking gun of the interaction is a small amount of energy imparted to a single nucleus."
The measured observation is consistent with predictions from the Standard Model, which is a testament to how good the theoretical foundation of particle physics is. However, this result also has consequences in astronomy. Supernova explosions release a huge amount of neutrinos, which interact with the gas released by the dying star.
"Understanding the process feeds into understanding of how these dramatic events occur," said physicist Kate Scholberg of Duke University, the spokesperson for the COHERENT collaboration that runs the experiment. "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."
Researchers will now conduct the experiment with different type of detectors to improve our understanding of the phenomenon and learn more about the structure of the nucleus.