Scientists working with an international experimental group claim to have discovered, for the first time, a revolutionary approach to investigating the constituents of atoms’ nuclei. This approach involves the use of enigmatic “ghost particles,” which are distinguished by the rarity of their interactions with matter.
Scientists at the University of Rochester, working with the MINERvA neutrino experiment, have successfully studied the structure of protons using a neutrino beam, an accomplishment that was previously thought to be unachievable. Their findings are published in the journal Nature.
Scientists at Stanford University were able to estimate the size of protons as early as the 1950s by employing electron beams generated by an accelerator. The Rochester team’s most recent study, however, concentrated on a comparable strategy—albeit one that used neutrino beams rather than electrons.
Due to their negligible interactions with atoms, neutrinos—which have no electrical charge and nearly no observable mass—have earned the moniker “ghost particles.” Although neutrinos are among the most plentiful particles known to exist in the cosmos, they are also notorious for being challenging to detect due to their peculiar features.
According to Tejin Cai, a Ph.D. student with the University of Rochester’s Neutrino Group and postdoctoral research associate at York University, the research team discovered a novel technique for analyzing the structure of protons as a result of their investigation of neutrinos in connection with the MINERvA experiment.
In a recent publication, Cai and his colleagues write, “Scattering weakly interacting neutrinos allows measuring both vector and axial vector form factors of the nucleon, providing an additional, complementary probe of their structure.”
“We weren’t sure at first if it would work, but in the end, we found we could measure the size and shape of the protons that make up the nuclei of atoms using neutrinos,” Cai said in a statement.
The lead author of the recently published research in Nature, Cai, compares it to measuring with a “ghost ruler.”
The new technique is a “very indirect way of measuring something,” according to Rochester University’s Dr. Steven Chu Professor of Physics Kevin McFarland. However according to him, the method used by the researchers “allows us to relate the structure of an object—in this case, a proton—to how many deflections we see in different angles.”
The Rochester team is quick to point out that while their novel approach using neutrinos does not provide a clearer image of the proton structure than previous methods utilizing electron beams, it does give physicists a rare chance to measure interactions that take place between the protons and the “ghost particles.”
“There are two main ingredients in the interaction between neutrinos and protons (or neutrons),” McFarland emailed The Debrief. “One is precisely the information that can be obtained by scattering electrons from protons, which is how the first measurements of the proton’s “size” were made starting in the 1950s and have been improved upon ever since with increasingly superior proton-electron interactions in experiments.”
The second ingredient, which involves the axial vector form factor, cannot be assessed in the same way, according to McFarland, who spoke with The Debrief.
According to McFarland, “previous inferences of this form factor relied on calculations in one way or another—either to relate an axial vector form factor to another process, correct for a measurement being made on a bound nucleon, or just calculate it from first principles without any data at all.”
By scattering neutrinos against protons, we can measure it directly for the first time, according to McFarland.
Before, the only way to deduce this kind of information was indirectly, by using theoretical models in conjunction with other measurements. The new method, according to Cai, McFarland, and the team, should make it easier to conduct future research in which the effects of neutrino scattering on protons can be distinguished from related atomic-level processes.
The tools developed for this analysis and the result presented are significant advances in our understanding of the nucleon structure in the weak sector, and they also aid in improving the constraints on neutrino interaction models for both current and upcoming neutrino oscillation experiments, as the authors conclude in their paper. The team’s current goal is to apply the method to distinguish between the impacts of neutrino scattering on protons and bound groupings of protons and neutrons that occur during neutrino scattering on atomic nuclei.
“It can now be used to predict other processes, such as the outcome of neutrinos scattering from neutrons, or possibly even neutrinos or antineutrinos scattering against the bound protons and neutrons within an atom’s nucleus,” McFarland told The Debrief. “This is because our measurement is unambiguously a measurement of this other description of the proton’s shape that the ‘axial vector form factor’ provides.”
“At least now, data on those effects cannot be complicated by the possibility that what we actually have wrong is the part that comes from the scattering on free protons or neutrons,” adds McFarland. “The latter requires models of how those protons and neutrons are bound to work.”
According to a recent statement from McFarland, “We will be better able to conduct our future studies of neutrino characteristics by using our new measurement to increase our understanding of these nuclear effects.”
The axial vector form factor was measured using antineutrino-proton scattering, and the results were reported in Nature on February 1, 2023.
