Proton Radius Measurement

Utilizing the first new methodology in half a century for measuring the dimension of the proton through electron scattering, the PRad collaboration has produced a brand new worth for the proton’s radius in an experiment carried out at the Division of Vitality’s Thomas Jefferson Nationwide Accelerator Facility. Credit score: DOE’s Jefferson Lab

Physicists get nearer to fixing the proton radius puzzle with distinctive new measurement of the cost radius of the proton. New measurement yields smaller proton radius.

Utilizing the first new methodology in half a century for measuring the dimension of the proton through electron scattering, the PRad collaboration has produced a brand new worth for the proton’s radius in an experiment carried out at the Division of Vitality’s Thomas Jefferson Nationwide Accelerator Facility.

The consequence, revealed as we speak (November 6, 2019) in the journal Nature, is certainly one of the most exact measured from electron-scattering experiments. The brand new worth for the proton radius that was obtained is 0.831 fm, which is smaller than the earlier electron-scattering worth of 0.88 fm and is in settlement with latest muonic atomic spectroscopy outcomes.

“We are happy that years of hard work of our collaboration is coming to an end with a good result that will help critically toward solution of the so-called proton radius puzzle,” says Ashot Gasparian, a professor at North Carolina A&T State College and the experiment’s spokesperson.

All seen matter in the universe is constructed on a cloud of three quarks certain along with sturdy pressure vitality. The ever-present proton, which sits at the coronary heart of each atom, has been the topic of quite a few research and experiments aimed toward revealing its secrets and techniques. But, an surprising consequence from an experiment to measure the dimension of this cloud, when it comes to its root-mean-square cost radius, has united atomic and nuclear physicists in a flurry of exercise to re-examine this fundamental amount of the proton.

Prior to 2010, the most exact measurements of the proton’s radius got here from two totally different experimental strategies. In electron-scattering experiments, electrons are shot at the protons, and the proton’s cost radius is decided by the change in path of the electrons after they bounce off, or scatter from, the proton. In atomic spectroscopy measurements, the transitions between vitality ranges by electrons are noticed (in the type of photons which can be given off by the electrons) as they orbit a small nucleus. Nuclei which have usually been noticed embody hydrogen (with one proton) or deuterium (with a proton and a neutron). These two totally different strategies yielded a radius of about 0.88 femtometers.

In 2010, atomic physicists introduced outcomes from a brand new methodology. They measured the transition between vitality ranges of electrons in orbit round lab-made hydrogen atoms that changed an orbiting electron with a muon, which orbits a lot nearer to the proton and is extra delicate to the proton’s cost radius. This consequence yielded a price that was 4% smaller than earlier than, at about 0.84 femtometers.

In 2012, a collaboration of scientists led by Gasparian got here collectively at Jefferson Lab to revamp electron-scattering strategies in hopes of manufacturing a novel and extra exact measurement of the proton’s cost radius. The PRad experiment was given precedence scheduling as certainly one of the first experiments to take information and full its run following an improve of the Steady Electron Beam Accelerator Facility, a DOE Person Facility for nuclear physics analysis. The experiment took electron-scattering information in Jefferson Lab’s Experimental Corridor B in 2016.

“When we started this experiment, people were searching for answers. But to make another electron-proton scattering experiment, many skeptics didn’t believe that we could do anything new,” says Gasparian. “If you want to come up with something new, you have to come up with some new tools, some new method. And we did that — we did an experiment which is completely different from other electron-scattering experiments.”

The collaboration instituted three new methods to enhance the precision of the new measurement. The primary was implementation of a brand new sort of windowless goal system, which was funded by a Nationwide Science Basis Main Analysis Instrumentation grant and was largely developed, fabricated and operated by Jefferson Lab’s Goal group.

The windowless goal flowed refrigerated hydrogen fuel instantly into the stream of CEBAF’s 1.1 and a pair of.2 GeV accelerated electrons and allowed scattered electrons to transfer practically unimpeded into the detectors.

“When we say windowless, we are saying that the tube is open to the vacuum of the accelerator. Which seems like a window – but in electron-scattering, a window is a metal cover on the end of the tube, and those have been removed,” says Dipangkar Dutta, an experiment co-spokesperson and a professor at Mississippi State College.

“So this is the first time that people actually put a gas-flow target onto the beamline at Jefferson Lab,” says Haiyan Gao, an experiment co-spokesperson and Henry Newson professor at Duke College. “The vacuum was good, so that we could have electron beam going through our target to do the experiment, and we actually have a hole in the entrance foil and another in the exit foil. Essentially, the beam just passed through directly to the hydrogen gas, not seeing any window.”

The subsequent main distinction was the use of a calorimeter relatively than the historically used magnetic spectrometer to detect scattered electrons ensuing from the incoming electrons putting the hydrogen’s protons or electrons. The repurposed hybrid calorimeter HyCal measured the energies and positions of the scattered electrons, whereas a newly constructed fuel electron multiplier, the GEM detector, additionally detected the electrons’ positions with even larger accuracy.

The info from each detectors was then in contrast in actual time, which allowed the nuclear physicists to classify every occasion as an electron-electron scattering or an electron-proton scattering. This new methodology of classifying the occasions allowed the nuclear physicists to normalize their electron-proton scattering information to electron-electron scattering information, vastly decreasing experimental uncertainties and rising precision.

The final main enchancment was placement of those detectors extraordinarily shut in angular distance from the place the electron beam struck the hydrogen goal. The collaboration was in a position to get that distance down to lower than one diploma.

“In electron scattering, in order to extract the radius, we have to go to as small a scattering angle as possible,” says Dutta. “To get the proton radius, you need to extrapolate to zero angle, which you cannot access in an experiment. So, the closer to zero you can get, the better.”

“The region that we explored is at such a forward angle and at such small four-momentum transfer squared that it has never been reached before in electron-proton scattering,” provides Mahbub Khandaker, an experiment co-spokesperson and a professor at Idaho State College.

The collaborators say the result’s distinctive, as a result of it used a brand new method through electron-scattering to decide the proton cost radius. Now, they’re trying ahead to evaluating the consequence to new spectroscopic determinations of the proton radius and upcoming electron- and muon-scattering measurements which can be being carried out worldwide.

Additional, this consequence additionally sheds new mild on conjecture of a brand new pressure of nature that was proposed when the proton radius puzzle first surfaced.

“When the initial proton radius puzzle came out in 2010, there was hope in the community that maybe we have found a fifth force of nature, that this force acts differently between electrons and muons,” says Dutta. “But the PRad experiment seems to shut the door on that possibility.”

They are saying the subsequent step is to contemplate conducting additional investigations utilizing this new experimental methodology to obtain even larger precision measurements on this and associated matters, comparable to the radius of the deuteron, the nucleus of deuterium.

“There is a very good chance we can improve our measurements by a factor of two or maybe even more,” Gao says.

Reference: “A small proton charge radius from an electron–proton scattering experiment” by W. Xiong, A. Gasparian, H. Gao, D. Dutta, M. Khandaker, N. Liyanage, E. Pasyuk, C. Peng, X. Bai, L. Ye, Okay. Gnanvo, C. Gu, M. Levillain, X. Yan, D. W. Higinbotham, M. Meziane, Z. Ye, Okay. Adhikari, B. Aljawrneh, H. Bhatt, D. Bhetuwal, J. Brock, V. Burkert, C. Carlin, A. Deur, D. Di, J. Dunne, P. Ekanayaka, L. El-Fassi, B. Emmich, L. Gan, O. Glamazdin, M. L. Kabir, A. Karki, C. Keith, S. Kowalski, V. Lagerquist, I. Larin, T. Liu, A. Liyanage, J. Maxwell, D. Meekins, S. J. Nazeer, V. Nelyubin, H. Nguyen, R. Pedroni, C. Perdrisat, J. Pierce, V. Punjabi, M. Shabestari, A. Shahinyan, R. Silwal, S. Stepanyan, A. Subedi, V. V. Tarasov, N. Ton, Y. Zhang and Z. W. Zhao, 6 November 2019, Nature.
DOI: 10.1038/s41586-019-1721-2


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