Scientists thought they knew the approximate size of a proton until a 2010 experiment came back with a very different number, creating the proton-radius puzzle. » Subscribe to Seeker!
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After almost a decade of reexamination, scientists think they’ve solved what’s known as the proton-radius puzzle.
First some basics: The proton itself is not a smooth billiard-ball, but more like a cloud of quarks held together by gluons. The quarks a proton is made up of give it its positive charge, and the threshold of that positive charge can be thought of as the proton’s size.
There are two main approaches scientists have used to find the radius of a proton’s boundary.
The first involves firing electrons at atoms, like hydrogen, which in its simplest form is a single proton nucleus with one orbiting electron. Then, based on how the electrons bounce off the nucleus, scientists can determine where the proton’s positive charge begins to fade.
The second method involves measuring how much energy it takes to excite an atom’s electron from one state to the next, and like method one, hydrogen is typically the atom of choice.
The other method measures how much energy it takes to excite an atom’s electron from one state to the next, and again, hydrogen is often the atom of choice.
Over the years, these two methods came more or less to the agreement that a proton’s radius was about 0.8768 femtometers, and all was well until about a decade ago when someone artificially swapped out hydrogen’s electron with a muon.
Now a muon is like an electron in every way, except it is 207 times more massive.
That’s right 207 times.
And that added weight means that the muon spends more time inside the proton, making its switch to a higher energy state millions of times more sensitive to the proton’s size than the electron in regular hydrogen. By measuring the proton using muonic hydrogen, they came back with a result 4% smaller than the previously accepted size, a difference that’s not insignificant.
So, were their previous measurements off? Or was this a hint at something more tantalizing? Did the discrepancy reveal some unknown physics or new elementary particles?
Find out more about a proton’s size and what we know so far, on this episode of Elements.
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New Subatomic Particles Are Shifting the Standard Model of Physics
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Proton-size puzzle deepens
"The proton might truly be smaller than was thought. Experiments on an exotic form of hydrogen first found1 a puzzling discrepancy with the accepted size in 2010. Now, evidence from a German and Russian team points to a smaller value for the size of the proton with ordinary hydrogen, too."
Scientists measure precise proton radius to help resolve decade-old puzzle
"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."
Physicists Finally Nail the Proton’s Size, and Hope Dies
"The new result implies that earlier attempts to measure the proton’s radius in electronic hydrogen tended to overshoot the true value. It’s unclear why this would be so."
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