Scientists have discovered a new way to investigate the structure of protons using neutrinos, known as ‘ghost particles’

TORONTO, Feb. 1, 2023 – Scientists are that much closer to understanding protons today after using a novel technique involving a high-energy neutrino beam to precisely measure their size, which could change how these kinds of experiments are done and answer many more questions, say researchers from 91��ɫ.

“We need detailed information about protons to answer questions like which neutrinos have more mass than others and whether or not there are differences between neutrinos and their anti-matter partners,” says Tejin Cai, 91��ɫ postdoctoral researcher and lead author of the published today in Nature. “Our work is one step forward in answering the fundamental questions about neutrino physics that are the goal of these big science projects in the near future.”

The research involved a series of experiments with neutrinos, often referred to as “ghost particles,” over nearly a decade. It was part of the international , which studies neutrinos at ).

“While we were studying neutrinos as part of the MINERvA experiment, I realized a technique I was using might be applied to investigate protons,” says Cai, who did the research, involving an international team of scientists, while completing his PhD in the lab of , the Dr. Steven Chu Professor in Physics and the Acting Vice Provost for Academic Affairs at the University of Rochester.

They found that the proton radius as seen by neutrinos is 0.73 femtometres – a quadrillionth of one metre.

“When we proposed MINERvA, we never thought we’d be able to extract measurements from the hydrogen in the detector,” says Professor Deborah Harris, a particle physicist in 91��ɫ’s Faculty of Science, a senior scientist at Fermilab and a co-spokesperson at MINERvA. “Making this work required great performance from the detector, creative analysis from scientists, and years of running the most intense high-energy neutrino beam on the planet.”

That’s the novel part of this experiment. The use of a beam of neutrinos to investigate the structure of protons was once thought impossible. The MINERvA group used a high-power, high-energy particle accelerator, which produces the strongest source of high-energy neutrinos on the planet. This new technique offers scientists a new way of looking at the small components of an atom’s nucleus.

Although neutrinos are one of the most abundant particles in the universe, they are notoriously difficult to detect and study as they don’t have an electrical charge and nearly zero mass. They are often referred to as “ghost particles” because they rarely interact with atoms, but they play a large role helping scientists answer fundamental questions about the universe.

Atoms, and the protons and neutrons that make up an atom’s nucleus, are so small that researchers have a difficult time measuring them directly. Instead, they build a picture of the shape and structure of an atom’s components by bombarding atoms with a beam of high-energy particles. They then measure how far and at what angles the particles bounce off the atom’s components.

For example, if marbles were thrown at a box, they would bounce off it at certain angles, enabling someone to determine where the box was, its size and shape – even if the box was not visible.

“This is a very indirect way of measuring something, but it allows us to relate the structure of an object – in this case, a proton – to how many deflections we see in different angles,” says McFarland.

A new technique

Specifically, the researchers are hoping to use the technique to separate the effects related to neutrino scattering on protons from the effects related to neutrino scattering on atomic nuclei, which are bound collections of protons and neutrons.  

“Our previous methods for predicting neutrino scattering from protons all used theoretical calculations, but this result directly measures that scattering,” says Cai.

McFarland adds, “By using our new measurement to improve our understanding of these nuclear effects, we will better be able to carry out future measurements of neutrino properties.”

What is a neutrino?

Neutrinos are created when atomic nuclei either come together or break apart. The sun is a large source of neutrinos, which are a byproduct of the sun’s nuclear fusion. If you stand in the sunlight, for example, trillions of neutrinos will harmlessly pass through your body every second.

Even though neutrinos are more abundant in the universe than electrons, it is harder for scientists to experimentally harness them in large numbers; neutrinos pass through matter like ghosts, while electrons interact with matter far more frequently.

“Over the course of a year, on average, there would only be interactions between one or two neutrinos out of the trillions that go through your body every second,” says Cai of 91��ɫ’s Faculty of Science. “There’s a huge technical challenge in our experiments in that we have to get enough protons to look at, and we have to figure out how to get enough neutrinos through that big assembly of protons.”

A chemical trick

The researchers solved this problem in part by using a detector containing a target of both hydrogen and carbon atoms. A target of pure hydrogen wouldn’t be sufficiently dense for enough neutrinos to interact with the atoms.

“We’re performing a ‘chemical trick’, so to speak, by binding the hydrogen up into hydrocarbon molecules that make it able to detect sub-atomic particles,” McFarland says.

To isolate only the information from the hydrogen atoms, the researchers then had to subtract the background “noise” from the carbon atoms.

“The hydrogen and carbon are chemically bonded together, so the detector sees interactions on both at once,” Cai says. “I realized that a technique I was using to study interactions on carbon could also be used to see hydrogen all by itself once you subtract the carbon interactions. A big part of our job was subtracting the very large background from neutrinos scattering on the protons in the carbon nucleus.” 

The collective expertise of MINERvA’s scientists and the collaboration within the group was essential in accomplishing the research, says Cai.

“The result of the analysis and the new techniques developed highlight the importance of being creative and collaborative in understanding data. While a lot of the components for the analysis already exist, putting them together in the right way really made a difference, and this cannot be done without experts with different technical backgrounds sharing their knowledge to make the experiment a success.”

The paper, , is published today in the journal Nature.

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91��ɫ research confirms protons are smaller than expected

��TORONTO, September 5, 2019 – 91��ɫ researchers have made a precise measurement of the size of the proton – a crucial step towards solving a mystery that has preoccupied scientists around the world for the past decade.

Scientists thought they knew the size of the proton, but that changed in 2010 when a team of physicists measured the proton-radius value to be four percent smaller than expected, which confused the scientific community. Since then, the world’s physicists have been scrambling to resolve the proton-radius puzzle – the inconsistency between these two proton-radius values. This puzzle is an important unsolved problem in fundamental physics today.

Now, a study published in the journal finds a new measurement for the size of the proton at 0.833 femtometres, which is just under one trillionth of a millimetre. This measurement is approximately five percent smaller than the previously-accepted radius value from before 2010.

The study, led by researchers in 91��ɫ’s Faculty of Science, presents a new electron-based measurement of how far the proton’s positive charge extends, and it confirms the 2010 finding that the proton is smaller than previously believed.

“The level of precision required to determine the proton size made this the most difficult measurement our laboratory has ever attempted,” said Distinguished Research Professor , Department of Physics & Astronomy, who led the study.

“After eight years of working on this experiment, we are pleased to record such a high-precision measurement that helps to solve the elusive proton-radius puzzle,” said Hessels.

The quest to resolve the proton-radius puzzle has far-reaching consequences for the understanding of the laws of physics, such as the theory of quantum electrodynamics, which describes how light and matter interact.

Hessels, who is an internationally-recognized physicist and expert in atomic physics, says three previous studies were pivotal in attempting to resolve the discrepancy between electron-based and muon-based determinations of the proton size.

The 2010 study was the first to use muonic hydrogen to determine the proton size, compared to prior experiments that used regular hydrogen. At the time, scientists studied an exotic atom in which the electron is replaced by a muon, the electron’s heavier cousin. While a 2017 study using hydrogen agreed with the 2010 muon-based determination of the proton charge radius, a 2018 experiment, also using hydrogen, supported the pre-2010 value.

Hessels and his team of scientists spent eight years focused on resolving the proton-radius puzzle and understanding why the proton radius took on a different value when measured with muons, rather than electrons.

The 91��ɫ team studied atomic hydrogen to understand the deviant value obtained from muonic hydrogen. They conducted a high-precision measurement using the frequency-offset separated oscillatory fields (FOSOF) technique, which they developed for this measurement. This technique is a modification of the separated oscillatory fields technique that has been around for almost 70 years and won Norman F. Ramsey a Nobel Prize. Their measurement used a fast beam of hydrogen atoms created by passing protons through a molecular hydrogen gas target. The method allowed them to make an electron-based measurement of the proton radius that is directly analogous to the muon-based measurement from the 2010 study. Their result agrees with the smaller value found in the 2010 study.

Hessels’ team consisted of graduate students Nikita Bezginov and Travis Valdez, Physics & Astronomy Professor , postdoctoral research assistant Alain Marsman, and former postdoctoral Fellow Amar Vutha, now assistant professor of physics at the University of Toronto.

Funding for the study was provided by the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation, the Ontario Research Fund, the United States National Institute for Standards and Technology, the Canada Research Chair program, Compute Canada, Compute Ontario and 91��ɫ.

��91��ɫ champions new ways of thinking that drive teaching and research excellence. Our students receive the education they need to create big ideas that make an impact on the world. Meaningful and sometimes unexpected careers result from cross-disciplinary programming, innovative course design and diverse experiential learning opportunities. 91��ɫ students and graduates push limits, achieve goals and find solutions to the world’s most pressing social challenges, empowered by a strong community that opens minds. 91��ɫ U is an internationally recognized research university – our 11 faculties and 25 research centres have partnerships with 200+ leading universities worldwide. Located in Toronto, 91��ɫ is the third largest university in Canada, with a strong community of 53,000 students, 7,000 faculty and administrative staff, and more than 300,000 alumni.

91��ɫ U's fully bilingual Glendon Campus is home to Southern Ontario's Centre of Excellence for French Language and Bilingual Postsecondary Education.

Media Contact: Vanessa Thompson, 91��ɫ Media Relations, 416-736-2100 ext. 22097,��vthomps@yorku.ca

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