Scientists measure precise proton radius to help resolve decade-old puzzle

Scientists measure precise proton radius to help resolve decade-old puzzle
Distinguished research professor Eric Hessels in his physics lab at York University. Credit: York University

York University 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 to be published in the journal Science 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 York University's Faculty of Science, presents a new electron-based measurement of how far the proton's 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 Eric Hessels, 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 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 York University 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 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.


Explore further

New measurement with deuterium nucleus confirms proton radius puzzle is real

More information: "A measurement of the atomic hydrogen Lamb shift and the proton charge radius" Science (2019). science.sciencemag.org/cgi/doi … 1126/science.aau7807
Journal information: Science

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Sep 05, 2019
Then, the quark radius must be less than one-third of 0.833 fm.

Sep 06, 2019
Then, the quark radius must be less than one-third of 0.833 fm.
As we conventionally pack three spheres in a circumscribing sphere, slightly less than half, assuming a least-energy configuration of quarks. But I'm wondering if such dimensions as 'radii', at this quantum/Heisenbergian realm of scales may have a more abstract meaning than what we would apply to fitting model ships in bottles, much as atomic 'spin', in its ups and downs; sideways and otherways, 0, 1, -1, etc. means something quite other than the rotating round-and-round-stuff's angular trajectory comprising a child's top.

Sep 06, 2019
That will give the defense boys a frequency in terms of wavelength needed to disintegrate protons and create intranuclear bombs of unimaginable ferocity this solar system has not witnessed since the destruction of the planet of which the asteroid belt is made.

Sep 06, 2019
Then, the quark radius must be less than one-third of 0.833 fm.
This is trivially true, but doesn't tell much either about the orbits of quarks within the proton or their interactions (and the self-interactions) of the gluons which makes most of the mass of the proton.

Sep 06, 2019
I calculated, if three spheres of equal radii fit in a larger sphere, then the radii of small spheres will be 0.464 times that of the circumscribing sphere. So, I was incorrect when I said less than one-third.

Sep 06, 2019
I calculated, if three spheres of equal radii fit in a larger sphere, then the radii of small spheres will be 0.464 times that of the circumscribing sphere. So, I was incorrect when I said less than one-third.

1. Who says they're equal?
2. What if they integrate(overlap)?

Sep 06, 2019
This Proton Radius Mystery: by Dr Hamish Johnston

The proton radius puzzle
has been reinforced
by a precise measurement of an atomic transition in muonic deuterium
which suggests the radius of the deuterium nucleus is much smaller than expected
This latest result tallies with a similar experiment on muonic hydrogen
which found that the radius of the proton is also smaller than expected
The discrepancy could mean
that theory describing how muons and electrons interact with the proton is incorrect
or that there is an error in how the radius is calculated
Another possibility is that the Rydberg constant
which defines the energy scale for atomic transitions in hydrogen
requires a slight correction
https://physicswo...p-short/

As this proton radius puzzle deepen
using electrons
scattering
spectroscopy
and muons
give differing set of values for this protons radius

This Proton appears to Shrink and Grow

Sep 06, 2019
Now to figure why / how an 'electron' result should differ from a 'muonic' result, and if any QCD parameters need an update...

Sep 06, 2019
Now to figure why / how an 'electron' result should differ from a 'muonic' result, and if any QCD parameters need an update...
no Nik.

-Before 2010 proton "r" > 0.833fm measured with electron
-In 2010 more accurate proton "r" ~ 0.833fm measured with muon
-Now high accuracy measurement with electron "r" = 0.833fm

Measurement with electron confirms measurement with muon.

Sep 06, 2019
This is certainly a meritorious experimental achievement; however, I would be more impressed if the charge distribution of the proton would have been defined. To talk about the radius leads to a somewhat naïve spherical representation of the proton structure, i.e. roughly seen as a rigid ball. Its structural charge distribution may be quite complex (similarly to the atomic orbital structure), and its presumed radius would just express a rough average of its structural distribution. Furthermore, could it not be that the proton structural charge distribution would suffer a distortion according to experimental conditions? So, the different values of the proton radius obtained should be "taken with a grain of salt" since this fact may express the little we know about the structural charge distribution of the proton.

Sep 06, 2019
To talk about the radius leads to a somewhat naïve spherical representation of the proton structure, i.e. roughly seen as a rigid ball. Its structural charge distribution may be quite complex (similarly to the atomic orbital structure), and its presumed radius would just express a rough average of its structural distribution. Furthermore, could it not be that the proton structural charge distribution would suffer a distortion according to experimental conditions? So, the different values of the proton radius obtained should be "taken with a grain of salt" since this fact may express the little we know about the structural charge distribution of the proton.

Perhaps, more to your favour, would be to say - sphere...no wait...domain.. of influence. Now, what is the radius of that?

Sep 06, 2019
This isn't really the size of the proton. It is the size of the charge radius.

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