Scientists from an international experimental group claim to have found a new way to analyze the components within the nucleus of atoms, utilizing a novel approach involving mysterious “ghost particles” renowned for their rare interactions with matter. Scientists from the University of Rochester’s Department of Physics and the MINERvA neutrino experiment accomplished what had previously been thought impossible: they used a beam of neutrinos to study the internal structure of protons. Their findings are published in the scientific journal Nature. It was in the 1950s that scientists at Stanford University first used electron beams from an accelerator to determine the size of protons. New research out of Rochester, though, took a similar tack but used neutrino beams instead of electrons.

Because of how little they interact with matter, neutrinos are often referred to as “ghost particles,” despite the fact that they have no electrical charge and barely enough mass to be detected. Despite being some of the most common particles in the cosmos, neutrinos have a reputation for being elusive due to their peculiar features. According to Tejin Cai, a postdoctoral research associate at York University and a Ph.D. candidate with the University of Rochester’s Neutrino Group, the team discovered a novel approach to analyzing the structure of protons through their investigation of neutrinos in conjunction with the MINERvA experiment. Scattering weakly interacting neutrinos “gives the chance to quantify both vector and axial vector form factors of the nucleon,” Cai and colleagues write in a recent publication.

To assess the size and form of the protons that make up atomic nuclei, neutrinos were used. “We weren’t sure at first if it would work,” Cai said in a statement. “It’s like using a phantom ruler to make a measurement,” Cai, who also authored the new Nature research, explains. Physicist Kevin McFarland, who is also the Dr. Steven Chu Professor of Physics at Rochester, referred to the new method as a “very indirect means of assessing anything.” The team’s method, however, “enables us to relate the structure of an object—in this case a proton—to how many deflections we witness at different angles,” he says. Although the Rochester group is quick to point out that their new neutrino-based technique does not provide any clearer imagery of proton structure than previous efforts relying on electron beams, the benefit of their new methodology is that it gives physicists a rare opportunity to gauge interactions that take place between the “ghost particles” and protons.

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There are “two essential elements” in the interaction between neutrinos and protons (or neutrons), as McFarland explained to The Debrief through email. The proton’s “size” was first measured in the 1950s, and subsequent studies involving the interaction of protons and electrons have resulted in increasingly precise measurements. “One is exactly the information that you can acquire by scattering electrons from protons,” the author writes. According to what McFarland told The Debrief, the second component is impossible to quantify in the same way since it depends on something called the axial vector form factor. McaFarland said that prior inferences of this form factor relied on computations “either to correct for a measurement being done on a bound nucleon, to tie some other process to the axial vector form factor, or to merely compute it from fundamental principles without any data at all.”

We scatter neutrinos off of protons to get the first direct measurement of this effect,” McFarland explains. It was previously only possible to deduce such data indirectly, using theoretical models in conjunction with other measures. The novel method was developed by Cai, McFarland, and their colleagues in the hopes that it will allow for future research to be conducted in which the effects of neutrino scattering on protons can be disentangled from those of neutrino scattering at the atomic level. “the tools developed for this analysis and the result presented are substantial advancements in our capabilities to understand the nucleon structure in the weak sector,” the authors write in their paper’s conclusion. They also note that the tools will aid in constraining neutrino interaction models in both ongoing and future neutrino oscillation experiments. The group now intends to utilize the method to disentangle the effects of neutrino scattering on protons from those of neutrino scattering on atomic nuclei, which include bonded groupings of protons and neutrons.

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