February 13, 2025 by Bob Yirka , Phys.org
Collected at: https://phys.org/news/2025-02-size-neutrino-physicists-considerably-larger.html
An international team of physicists has successfully measured the size of a certain type of neutrino to a certain degree. In their paper published in the journal Nature, the group describes experiments they conducted that involved measuring the radioactive decay of the element beryllium.
Neutrinos are subatomic particles with a mass very close to zero. They also have a half-integral spin and rarely react with normal matter. To date, three kinds of neutrinos have been identified, each by association with an electron, muon or tau particle. Physicists have become more interested in neutrinos over the past several years because it is thought better understanding them may lead to a better understanding of why there is more matter than antimatter in the known universe.
One of the first questions that needs to be answered about neutrinos is their size. This is important because it allows building the right size and shape of neutrino detectors. Currently, they are very large, which allows for what is believed to be their largest possible theoretical size—several meters—though it is believed they are smaller. In this new effort, the research team conducted experiments with beryllium to measure the size of an electron-associated neutrino.
The experiment consisted of measuring radioactive decay in beryllium, which decayed into lithium. As it does so, an electron in a single atom combines with a proton, producing a neutron, resulting in the creation of a lithium atom. As that happens, energy is released, pushing the atom in one direction and the neutrino produced in the other. By starting the process in a particle accelerator and placing extremely sensitive neutrino detectors along the sides, they were able to measure the momentum of the lithium atoms and use that to calculate the size of the neutrino.
The experiments showed that the neutrinos were at least 6.2 picometers, making them hundreds of times bigger than a typical atomic nucleus. Such a size increase is possible, the researchers note, because of the nature of neutrinos; rather than consisting of physical particles, they are fuzzy-type waves that move due to their vibrations. Their size is measured by marking the boundaries of their wave packet, which is the part of their wave that vibrates the most strongly.
More information: Joseph Smolsky et al, Direct experimental constraints on the spatial extent of a neutrino wavepacket, Nature (2025). DOI: 10.1038/s41586-024-08479-6
Journal information: Nature
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