New measurement with deuterium nucleus confirms proton radius puzzle is real

August 12, 2016 by Bob Yirka, Phys.org report
The quark structure of the proton. There are two up quarks in it and one down quark. The strong force is mediated by gluons (wavey). The strong force has three types of charges, the so-called red, green and the blue. Note that the choice of green for the down quark is arbitrary; the "color charge" is thought of as circulating among the three quarks. Credit: Arpad Horvath/Wikipedia

A large team made up of researchers from across the globe has repeated experiments conducted several years ago that showed a different radius for a proton when it was orbited by a muon as opposed to an electron—a finding dubbed the proton radius puzzle—using a deuterium nucleus this time and has found the same puzzle. In their paper published in the journal Science, the team describes the experiments they conducted, what they found and offer a few possible ideas to help dispel the notion that the puzzle indicates that there may be some problems with the Standard Model.

Scientists have been able to calculate the radius of a (0.88 ± 0.01 femtometers) for some time using the charge of the electron that orbits around it and doing so has helped confirm theories regarding the Standard Model. But, in trying to improve the accuracy of the measurement by using a negatively charged muon (which orbits closer to the proton), researchers at the Max Planck Institute back in 2010 found a different radius—one that was 7 deviations from what was considered the official value. This proton radius puzzle has had physicists scratching their heads ever since because it suggests there is an error in the Standard Model somewhere. Over the past six years various researchers have offered theories to solve the puzzle, most of which have involved ways to preserve the Standard Model, but to date, the puzzle still remains.

In this latest effort the researchers sought to gain more insight into the problem by adding another piece to the puzzle, a neutron, i.e. by using a deuterium nucleus. Their thinking was that the presence of the neutron would change the way that electrons and muons perceived the proton charge. They report that they found that the measurement they made of the radius of the proton was still different from that found with just an electron and proton, by approximately 7.5 sigma.

The results by the team offer no new explanations for the measurement discrepancies—it remains a puzzle, but they do offer some possible avenues for further investigation, e.g. ways to improve measurements and forcing muons to interact with the protons to see if there might be any evidence of an unknown force at work.

Explore further: Proton radius puzzle may be solved by quantum gravity

More information: R. Pohl et al, Laser spectroscopy of muonic deuterium, Science (2016). DOI: 10.1126/science.aaf2468

Abstract
The deuteron is the simplest compound nucleus, composed of one proton and one neutron. Deuteron properties such as the root-mean-square charge radius rd and the polarizability serve as important benchmarks for understanding the nuclear forces and structure. Muonic deuterium μd is the exotic atom formed by a deuteron and a negative muon μ. We measured three 2S-2P transitions in μd and obtain rd = 2.12562(78) fm, which is 2.7 times more accurate but 7.5σ smaller than the CODATA-2010 value rd = 2.1424(21) fm. The μd value is also 3.5σ smaller than the rd value from electronic deuterium spectroscopy. The smaller rd, when combined with the electronic isotope shift, yields a "small" proton radius rp, similar to the one from muonic hydrogen, amplifying the proton radius puzzle.

Press release

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shavera
5 / 5 (10) Aug 12, 2016
Questions like these are always tough. QCD is notoriously hard to calculate, so maybe the effect is just some quirk in the approximations we use. So maybe it doesn't tell us "new" physics, but refines what we have. OTOH, it could be something more fundamental about the standard model, some kind of coupling or broken symmetry, and that could tell us where the standard model needs to go next.

It would be interesting if it has something to do with the fact that a muon is very nearly the mass of a pion, and maybe there's some kind of enhancement process there (though by no means am I saying this is the case, or even if it is plausible, it just strikes me as an interesting coincidence).
daqman
1.8 / 5 (8) Aug 12, 2016
There isn't enough info in either the article or the paper to indicate why this such a puzzle.

In the case of a hydrogen atom the electron is bound to a proton. The electron has a mass 1/2000 of the mass of the proton. Using a simple model where the electron is in an orbit around the proton the center of the orbit and the center of the proton are the same.

Now replace the electron with a muon. The muon is about 200 electron masses, about 10% of the proton mass. This makes the radius of the orbit smaller and the proton and muon orbit the center of mass of the muon-proton system which is not the center of the proton. This would be fine if the proton was a solid object but it is a system of three quarks and the gluons that bind them. I would expect a combination of the close proximity of the charged muon and the rotation of the proton about the center of mass of the system to distort it's shape.
ReGuess
5 / 5 (8) Aug 12, 2016
The 1st sentence of the 2nd paragraph states that "Scientists have been able to calculate the radius of a proton ... for some time using the charge of the proton that orbits around it...." It should be corrected to read "the charge of the electron that orbits around it"
nkalanaga
5 / 5 (8) Aug 12, 2016
They changed it. Thank you.
shavera
4.9 / 5 (8) Aug 12, 2016
daqman: the problem is that you can't use "the simple model" where an electron orbits classically. It's all quantum mechanics at that level. In a way, where you expect it to be most often is 'inside' the nucleus itself; though the nucleus is just such a very very small volume that such a comparison only makes sense when you compare it to all the other similarly small volumes within an atom.

But following from your idea, the muon would be even *more* likely to reside within the proton's volume because of its increased mass.

That being said, I'm sure the theorists have already accounted for that changed charge distribution when they speak of what they expect the proton's size to be. What this 'problem' is is that the proton's volume still doesn't match that expectation. (Also the EM force is way way weaker than the strong force, and so maybe it doesn't really affect the size at all)
TheKnowItAll
5 / 5 (2) Aug 12, 2016
Anyone with access to the original paper? I'm just curious to know if they tried the same measurements with an extra muon.
tinitus
Aug 12, 2016
This comment has been removed by a moderator.
Phys1
4.6 / 5 (9) Aug 12, 2016
QED corrections, many of them are chosen arbitrarily

That seems unlikely. Which ones?
the experimental errors are quite large BTW

0.04%? Do you see that as large ? Do you find 780 zeptometer large ?
That is a first.
You know what I think?
I think you don't know the first thing about it.
torbjorn_b_g_larsson
4.6 / 5 (10) Aug 12, 2016
The head for the old and new experiment suspects the measurement of the Rydberg constant, thought to be one of the most precise ones, are off. [ https://www.quant...-puzzle/ ] Another suspect could be the Lamb shift calculation.

@daqman: The electron cloud orbitals (or "orbits" in your non-QM model) are decided by EM forces of the particle fields, the masses are negligible and even more mass differences.

There are other sorts of atom radius by the way, but this is the EM one.

@shavera: To nitpick, it is only the low energy S orbital where the electron (or muon) visit the nucleus much. The other orbitals are outside, not that it hinders the electron/muon field ripples (particles) being found inside at very low probability.

@TKIA: http://science.sc...6300/669 (from the Quanta article),
Reg Mundy
1.6 / 5 (10) Aug 13, 2016
Over the past six years various researchers have offered theories to solve the puzzle, most of which have involved ways to preserve the Standard Model, but to date, the puzzle still remains.

Is the Standard Model worth preserving? There are increasing reasons why it needs a complete rethink.
Phys1
3.8 / 5 (10) Aug 13, 2016
@RM
There are increasing reasons to discard your posts.
tinitus
Aug 13, 2016
This comment has been removed by a moderator.
torbjorn_b_g_larsson
4.6 / 5 (11) Aug 13, 2016
There are increasing reasons why it needs a complete rethink
Standard Model is modern version of epicycles - its fitted to data with many arbitrary constants, so it's not so easily to disprove it.


No, it is very obviously not epicycles. It is self contained, houses the 5 fundamental forces (with the new Higgs boson = interaction), and has 3 generations in both charged and neutral (neutrino) leptons - the same number of neutrino generations predicted by Standard Cosmology. [Planck legacy archive.]

The Standard Model of particles has a lot of non-arbitrary constants that are fitted, and it is unknown why so far.

By the way, the old claim of "epicycles" is philosophical, and rejected by nature: nature insists in being simple. (Say, having just 18 particle fields in Core Theory. [ Low energy quantum physics version of semi-classical physics = Standard Model + graviton; http://www.prepos...-shirts/ ]
torbjorn_b_g_larsson
5 / 5 (7) Aug 13, 2016
Sorry, I meant 18 quantum fields obviously, not the confusing "particle fields".
Captain Stumpy
4.4 / 5 (8) Aug 13, 2016
No, it is very obviously not epicycles
@torbjorn_b_g_larsson
just in case you didn't know: you're talking to zephir's latest incarnation/sock
reason, logic and evidence will not be heard, read or considered because it directly contradicts her religious belief in aether/DA etc ... she's only here to proselytize and seek converts

just sayin'

.

.

Is the Standard Model worth preserving? There are increasing reasons why it needs a complete rethink
@regMORON
1- the standard model is based upon empirical evidence and validated physics

2- your own model is based on your personal delusional belief and no evidence (you can't even post valid references or evidence to support your claims)

3- the SM may need adjustment, but all Theories in science likely will need this because we don't have all the info

if anything requires a complete rethink it is your book and pseudoscience beliefs which are proven chronic lies
Captain Stumpy
Aug 13, 2016
This comment has been removed by a moderator.
Reg Mundy
Aug 13, 2016
This comment has been removed by a moderator.
eachus
1.6 / 5 (7) Aug 13, 2016
Let me take a leap off the deep end and see where it winds up. One way magnetic monopoles could be hidden is if trying to isolate a monopole required more energy than creating a new monopole pair. Hmmm. The color force has three flavors, but that is just a mapping problem. Say red is North and anti-red is South, yellow, blue and their antiparticles become mixes of North and South. Think if the cube root of minus one as the mixing angle.

What do we gain from mapping quarks to magnetic monopoles? It should explain this effect. (Which becomes a test for the theory.) The quantum of magnetic charge will be large compared to the quantum of electric charge, but we already knew that the color force is stronger than the electric part of the electromagnetic force. The fine structure constant then implies a quark has a magnetic charge about 137 times the charge of an electron.

Don't quarks mass much more than this model requires? Not really. Gluons get involved.
Phys1
Aug 13, 2016
This comment has been removed by a moderator.
Captain Stumpy
Aug 13, 2016
This comment has been removed by a moderator.
Hyperfuzzy
1 / 5 (6) Aug 15, 2016
Given that a proton and an electron may converge and within the nucleus charge will always comply to charge, i.e. the opposite charges will be closer than the repulsive charges; one does not need the standard model or strong and weak forces, since these will be supplied by charge. Geometry, then will define how an atom is confined. Therefore, it is rejectable that the standard model defines anything, as it is unnecessary. It may also be shown, however unnecessary, that the standard model is even sufficient. To continue as a theory is actually the mystery.

Given only two diametrical spherical fields can be shown as necessary and sufficient to define atoms and space. One should not overestimate to zero the effects of all charge within every space.
Hyperfuzzy
1 / 5 (6) Aug 15, 2016
Magnetic mono-poles does not define magnetism as defined by a current loop or the time variation of the E field in 3 dimensions. Hence, a magnetic mono pole is an impossibility. This is known and empirical physics. A study of EM theory versus particle physics is the correct physics, not this.
Hyperfuzzy
1 / 5 (6) Aug 15, 2016
Given that a proton and an electron may converge and within the nucleus charge will always comply to charge, i.e. the opposite charges will be closer than the repulsive charges; one does not need the standard model or strong and weak forces, since these will be supplied by charge. Geometry, then will define how an atom is confined. Therefore, it is rejectable that the standard model defines anything, as it is unnecessary. It may also be shown, however unnecessary, that the standard model is even sufficient. To continue as a theory is actually the mystery.

Given only two diametrical spherical fields can be shown as necessary and sufficient to define atoms and space. One should not overestimate to zero the effects of all charge within every space.

I meant the SM is unnecessary and not sufficient.
ThunderDolts
Aug 16, 2016
This comment has been removed by a moderator.
RNP
not rated yet Aug 18, 2016
@Hyperfuzzy

A bunch of unmitigated rubbish! Scientifically inspired word salad. No basis in any form of reality.

There! I got that off my chest.

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