TORONTO, Nov. 13, 2020 – Precision matters. 91��ɫ Distinguished Research Professor Eric Hessels, who has conducted the most precise measurement to date of the fine structure of helium and of the hydrogen n=2 Lamb shift to come closest to solving the proton-size puzzle, deals in precision every day. It is his work in the precision measurement field that has earned him the .

It is the first time in the association’s history, since the award began in 1956, that it has gone to a 91��ɫ Department of Physics and Astronomy professor. The annual medal awards researchers for their distinguished service to physics over an extended time or a recent outstanding achievement. Hessels fits both categories.

“We could not be more delighted or more proud that Dr. Hessels will receive this prestigious honour for his lifetime of dedication to research in physics,” said Faculty of Science Dean Rui Wang. “Dr. Hessels’ work has far-reaching consequences for our understanding of the laws of physics and��may help answer some of the unresolved fundamental��questions of the Universe. His impact at 91��ɫ and within our faculty has been enormous, as is the imprint his research will leave on the discipline of physics.”

91��ɫ Distinguished Research Professor Eric Hessels receives the 2020 CAP Medal for Lifetime Achievement in Physics. Photo by Paola Scattolon

Hessels, renowned for his work in atomic, molecular and optical physics, is a master at developing novel and sophisticated laser and microwave experiments to ascertain the properties of atoms and nuclei, such as the dual charge exchange method and the frequency offset separated oscillatory fields (FOSOF) technique. The first method made antihydrogen atoms cold enough to trap and study, while the FOSOF technique was used to measure the radius of a proton.

“Professor Hessels specializes in precision measurement to test fundamental theories of physics. His work is important in advancing our understanding of nature and how it works,” says Patrick Hall, chair of the Department of Physics and Astronomy. "His research group provides invaluable opportunities for graduate and undergraduate students and postdoctoral fellows at 91��ɫ to be involved in cutting-edge science."

His most recent research involves measuring the electron electric dipole moment using polar molecules frozen into a cryogenic argon solid, which could help explain why there is an abundance of matter, but very little anti-matter in our Universe. He is leading a collaboration of 19 physicists and chemists working on this measurement.

“I am very honoured to receive this award,” says Hessels. “I would like to acknowledge the hard work and intelligence of my students and collaborators, as the work being honoured here would not have been possible without them.”

A professor at 91��ɫ since 1992, Hessels has won several prestigious awards over the years, including The John Charles Polanyi Prize, the Herzberg Medal, and the U.S. National Institute of Standards, Technology Precision Measurements Award and two Canada Research Chairs.

Hessels will deliver a plenary medal talk in the coming months, which will be posted to the website.

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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:

Sandra McLean, 91��ɫ Media Relations, 416-272-6317, sandramc@yorku.ca

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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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