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While neutrinos are common in the universe, they are notoriously challenging to detect and study due to their lack of electrical charge and negligible mass. Because of how rarely they interact with atoms, they are commonly referred to as “ghost particles.” However, their sheer number means they play a significant part in helping researchers solve some of the universe’s most perplexing mysteries. Scientists from the international cooperation MINERvA have utilized a beam of neutrinos at the Fermi National Accelerator Laboratory, or Fermilab, to study the structure of protons for the first time. The study, conducted by academics from the University of Rochester, was published in Nature. Researchers in the MINERvA experiment were not initially interested in studying protons because of the project’s focus on neutrinos. However, their achievement, which was previously deemed unachievable, has given scientists a new perspective on the minute components of an atom’s nucleus.

Tejin Cai, the paper’s first author, explains that the idea for the study of protons came to him while he was studying neutrinos as part of the MINERvA experiment. This work was done while Cai was a PhD student of Kevin McFarland, the Dr. Steven Chu Professor in Physics at Rochester and a prominent member of the University’s Neutrino Group. Cai is now a postdoctoral research associate at York University. Neutrinos were used to determine the size and form of protons within atomic nuclei, a finding that at first seemed unlikely. In other words, it’s like trying to measure with a phantom ruler. Scientists have a hard difficulty getting accurate readings when trying to measure individual atoms or their nuclei’ protons and neutrons. Instead, they use a stream of highly energetic particles to bombard atoms, piecing together a picture of the shape and structure of the atom’s constituent parts in the process. The researchers then record the distance and angle at which the particles rebound from the atom’s constituent parts.

Consider the scenario of hurling marbles against a box. If you knew the angle at which the marbles were hitting the box, you could use this information to calculate its dimensions and thus locate the box even while it was out of sight. McFarland notes that while this is a “indirect” method of measurement, it does allow them to connect the structure of an object (a proton) to the number of deflections observed at various angles. The linear accelerator at Stanford University was the first place where the size of protons was measured, in the 1950s. The novel method proposed by Cai, McFarland, and colleagues uses neutrino beams rather than beams of accelerated electrons. McFarland says that while the new method does not yield a clearer picture than the old one, it may provide scientists with new insights into the interactions between neutrinos and protons, insights that are currently only inferred through theoretical calculations or a combination of theory and other measurements.

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To illustrate the differences between the old and new methods, McFarland uses the analogy of observing a flower first in regular, visible light and then again under UV. While it’s the same flower, different lighting conditions reveal new details, as McFarland explains. Even if our image isn’t any clearer thanks to the neutrino measurement, we get a new perspective on things. Atomic nuclei are bonded collections of protons and neutrons, and they hope to utilize this technique to disentangle the effects of neutrino scattering on protons from those on atomic nuclei. In contrast to prior methods, which relied on theoretical calculations, “our result directly quantifies that scattering,” as Cai puts it. Further, McFarland explains, “Our new measurement will help us better understand these nuclear effects, paving the way for more precise studies of neutrino characteristics in the future.” Created when atomic nuclei either collide or disintegrate, neutrinos have no mass and cannot be directly observed. Neutrinos are a consequence of nuclear fusion in the sun and come from this source in vast quantities.

To give just one example, if you stand in direct sunshine, millions of neutrinos will flow through your body every second, doing no harm. Neutrinos are more common in the cosmos than electrons, but scientists have a more difficult time experimentally harnessing them in huge numbers because neutrinos pass through stuff like ghosts, whereas electrons interact with matter more frequently. Cai claims that out of the billions of neutrinos that pass through your body every second, just one or two will interact with you over the course of a year. Getting enough protons to study and figuring out how to get enough neutrinos through that large assembly of protons presents a significant technological hurdle for our studies. The scientists got around this issue in part by utilizing a neutrino detector with a dual-target made of hydrogen and carbon atoms. In most proton measurement investigations, just hydrogen atoms are used.

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Hydrogen is the most common element in the universe, and its atoms are the simplest, with just one proton and one electron. However, there wouldn’t be enough neutrinos to cause significant atomic interaction in a target of pure hydrogen. McFarland explains that they are using a “chemical trick” to make the hydrogen molecules into sub-atomic particle detectors by binding the hydrogen up into hydrocarbon molecules. In its investigations, the MINERvA team utilized Fermilab’s high-power, high-energy particle accelerator. The neutrino accelerator is the world’s most powerful source of high-energy neutrinos. The neutrino beam was directed towards the detector made of hydrogen and carbon atoms, and the team logged data for nearly nine years. The data from the hydrogen atoms has to be separated from the “noise” produced by the carbon atoms. Cai explains that because hydrogen and carbon are chemically bound together, the detector may observe interactions on both elements simultaneously.

When I removed the effects of the carbon interactions, I was able to view hydrogen on its own, and this revelation changed the way I approached my research. Subtracting the massive background from neutrino scattering on protons in the carbon nucleus was a significant aspect of our work. According to Deborah Harris of York University, who is also a co-spokesperson for MINERvA, “When we suggested MINERvA, we never believed we’d be able to obtain measurements from the hydrogen in the detector. For this to be possible, the detector had to function exceptionally well, scientists had to come up with novel ways of analyzing the data, and the accelerator at Fermilab had to be “run” for several years. When McFarland first considered the possibility, he, too, assumed neutrinos would make it nearly impossible to get an accurate reading of the proton signal. I believed it would be very difficult to do this analysis when Tejin and our colleague Arie Bodek (the George E. Pake Professor of Physics at Rochester) originally suggested it,” McFarland adds.

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The old vision of protons, however, has been well investigated, so we tried this strategy to get a new view—and it worked. According to Cai, the success of the study project would not have been possible without the combined knowledge and efforts of the experts at MINERvA. There is a need for “creativity and collaboration” in data analysis, he says, as evidenced by the findings and the innovative methods created. “Although many of the analysis’s components already existed, bringing them together in the appropriate way made a real impact; and this cannot be done without professionals with varied technical backgrounds pooling their knowledge to make the experiment a success.” Additionally useful for anticipating neutrino interactions for other studies measuring neutrino properties, this study sheds light on the ubiquitous stuff that makes up the cosmos.

McFarland and his team are involved in a number of neutrino experiments, such as the Deep Underground Neutrino Experiment (DUNE), the Imaging Cosmic and Rare Underground Signals (ICARUS) neutrino detector, and the T2K neutrino investigations. To determine which neutrinos are heavier than others and whether there are distinctions between neutrinos and their anti-matter counterparts, “we need extensive knowledge about protons,” Cai explains. Our findings get us closer to answering some of the most fundamental problems in neutrino physics, which is the ultimate goal of many current and future large-scale scientific endeavors.

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