Researchers orbit a muon around an atom, confirm physics is broken

Although tiny, a proton takes up a finite amount of space, enough to fit three quarks, a host of virtual particles, and their associated gluons. The size of a proton’s radius is determined by these particles and their interactions, and so is fundamentally tied in to theories like the Standard Model and quantum chromodynamics.

We can measure the radius because the proton’s charge is spread across it, which influences the orbit of any electrons that might be circling it. Measurements with electrons produce a value that’s easily in agreement with existing theories. But a few years back, researchers put a heavier version of the electron, called a muon, in orbit around a proton. This formed an exotic, heavier version of the hydrogen atom. And here, measuring the proton’s radius produced an entirely different value—something that shouldn’t have happened.

This “proton radius puzzle” suggests there may be something fundamentally wrong with our physics models. And the researchers who discovered it have now moved on to put a muon in orbit around deuterium, a heavier isotope of hydrogen. They confirm that the problem still exists, and there’s no way of solving it with existing theories.

> But a few years back, researchers put a heavier version of the electron, called a muon, in orbit around a proton. This formed an exotic, heavier version of the hydrogen atom.

Yep, I remember that.

> And here, measuring the proton’s radius produced an entirely different value—something that shouldn’t have happened.

I didn’t notice that. What sigma? Was it confirmed by an independent laboratory?

> This “proton radius puzzle” suggests there may be something fundamentally wrong with our physics models.

It would. The only similar thing I can think of is the “anomalous magnetic moment of the muon”, and that’s in borderline agreement with existing physics models.

> the researchers who discovered it have now moved on to put a muon in orbit around deuterium, a heavier isotope of hydrogen. They confirm that the problem still exists, and there’s no way of solving it with existing theories.

We need to take it seriously then, and look up the details of the experiment and the order of magnitude of the disagreement.

> And here, measuring the proton’s radius produced an entirely different value—something that shouldn’t have happened.

This isn’t mentioned in wikipedia. Why? It wouldn’t be new information. All Wikipedia has is:
“Since a muon is more massive than an electron, the Bohr orbits are closer to the nucleus in a muonic atom than in an ordinary atom, and corrections due to quantum electrodynamics are more important. Study of muonic atoms’ energy levels as well as transition rates from excited states to the ground state therefore provide experimental tests of quantum electrodynamics.”

The radius of the proton is quantum chromodynamics not quantum electrodynamics. So why doesn’t Wikipedia mention it?

mollwollfumble said:

> And here, measuring the proton’s radius produced an entirely different value—something that shouldn’t have happened.

I didn’t notice that. What sigma? Was it confirmed by an independent laboratory?

> the researchers who discovered it have now moved on to put a muon in orbit around deuterium, a heavier isotope of hydrogen. They confirm that the problem still exists, and there’s no way of solving it with existing theories.

We need to take it seriously then, and look up the details of the experiment and the order of magnitude of the disagreement.

Went to 7.

Does this suggest that quarks can change flavors, as neutrinos do, dependent upon the lepton field environment?

Going back to the first measurements of a muonic atom, Oct 2010, we have

> For the transition, they needed look no further than a Lamb shift. A Lamb shift occurs when an electron moves between the 2s and 2p energy levels in an atom. The difference in binding energy between the two is very small, and leaves little room for external effects to muck up the measurements. The researchers found that the muonic hydrogen needed to be shot with a laser with a frequency of 50 terahertz in order to transition up to the 2p state. When they plugged this measurement into a quantum electrodynamics equation that relates proton radius to binding energies, they found the needed energy indicated a proton radius of 0.841 femtometers—four percent smaller and five standard deviations off the currently accepted radius of 0.876 femtometers.

From way back, even with the normal hydrogen atom the Lamb shift has been infamous. Explaining the precise measured value defeated more than one Nobel prizewinning physicist (including Dirac) before Hans Bethe (I think) got it right. Its precise value required the mathematics of the renormalization of mass (an electron changes mass due to the cloud of virtual particles in the vacuum it sits in). The Lamb shift currently provides a measurement of the fine-structure constant α to better than one part in a million.

More on the Lamb shift at https://en.wikipedia.org/wiki/Lamb_shift

Now, where does the radius of the proton appear in all this mathematics?

Possibly in this way.

“This result diverges when no limits about the integral (at both large and small frequencies). As mentioned above, this method is expected to be valid only when ν > πc/a0, or equivalently k > π/a0. Therefore, we can choose the upper and lower limit of the integral and these limits make the result converge.” Here a0 is the Bohr radius https://en.wikipedia.org/wiki/Bohr_radius
“The Bohr radius (a0) is a physical constant, approximately equal to the most probable distance between the proton and electron in a hydrogen atom in its ground state. In the simplest atom, hydrogen, a single electron orbits the nucleus and its smallest possible orbit, with lowest energy, has an orbital radius almost equal to the Bohr radius. (It is not exactly the Bohr radius due to the reduced mass effect. They differ by about 0.1%.) The Bohr radius including the effect of reduced mass in the hydrogen atom depends on the Compton wavelength of the proton, effectively the proton’s size.

Could it be, I wonder, that what we’re seeing here is a change in the mean distance between the proton and orbiting particle (electron or muon) due to the induced movement of the proton (or quarks within the proton)? A muon, because of its larger mass, orbits close and so the induced motion of the proton is larger than that due to the electron. This would change the Bohr radius of the hydrogen atom slightly and hence (this is only a wild guess, remember) the effective radius of the proton. It’s a thought, but if this was the case then that would also change our interpretation of the Lamb shift for the normal hydrogen atom, which would totally put the wind up the whole of QED. ie. They broke physics.

Unless their experiment is wrong.

So, let’s go back to the original muon hydrogen paper from 2010.
http://www.nature.com/nature/journal/v466/n7303/full/nature09250.html

“The size of the proton” by Pohl et al. in Nature.

Summary:
> The root-mean-square charge radius, r_p, has been determined with an accuracy of 2% (at best) by electron–proton scattering experiments. To 1%, this value is based mainly on precision spectroscopy of atomic hydrogen. Here we use pulsed laser spectroscopy to measure a muonic Lamb shift. Our result implies that either the Rydberg constant has to be shifted, or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient. The muon is about 200 times heavier than the electron. The atomic Bohr radius is correspondingly about 200 times smaller. S states are shifted because the muon’s wavefunction at the location of the proton is non-zero. In contrast, P states are not significantly shifted (Note: remember your S,P,D etc orbitals, the Lamb shift is the energy difference between the S and P orbitals).

> The Lamb shift in muonic hydrogen is the sum of radiative, recoil, and proton structure contributions

OK, so in the paper they’ve already considered and eliminated both of my wild guesses above, the induced motion of the proton and the induced motion of the quarks. In fact, it turns out that ONLY these are related to the radius of the proton. So my wild guess above was the opposite of the solution, it was the problem. The induced motion of the proton is proportional to the radius of the proton squared and the induced motion of the quarks is proportional to the radius of the proton cubed.

I don’t see any fundamental flaw in the experiment or results.

Short of having an expert in fundamental physics look at these results, I have to agree that they broke physics. The most reasonable explanation I can see is that their assumed constant attached to “the radius of the proton cubed” is wrong. That would be an error not in QED but on QCD, which is easier to stomach.

>Could it be, I wonder, that what we’re seeing here is a change in the mean distance between the proton and orbiting particle (electron or muon) due to the induced movement of the proton (or quarks within the proton)?

Seems reasonable.

Was up there pruning this

when I was told to bugger off and find my own spot.

and even long after I’d given up ‘cause it was a bit dark to be hanging out of trees with sharp stuff in hand, it was hanging out of the tree reminding me to bugger off.

Let’s see what the experts say.

http://arxiv.org/pdf/1008.3861.pdf

“QED is not endangered by the proton’s size” by Rujula

Rujula’s paper has no quibbles with the experiments by Pohl et al., or with the results, but only with the interpretation. He claims that QED is correct, and what is wrong is the “dipole form factor” for the hydrogen atom. This agrees with what I said (ie. I hoped) at the end of my last post. ie. that the constants in the equation describing the effect of the proton movement and quark movement on the Lamb shift are wrong. This equation in full, with r_p the radius of the proton is:
Lamb shift = 209.9779 – 5.2262 r_p^2 + 0.0347 r_p^3

Rujula replaces this with
Lamb shift = 209.9779 – 5.2262 r_p^2 + 0.00913 r_p^3

Another independent paper is “The size of the proton: Closing in on the radius puzzle” by Lorenz et al.
http://link.springer.com/article/10.1140/epja/i2012-12151-1

> We analyze the recent electron-proton scattering data from Mainz using a dispersive framework that respects the constraints from analyticity and unitarity on the nucleon structure. We also perform a continued fraction analysis of these data. We find a small electric proton charge radius, r_Ep = 0.84−0.01+0.01 fm, consistent with the recent determination from muonic hydrogen measurements and earlier dispersive analyses. We also extract the proton magnetic radius, r_Mp = 0.86−0.03+0.02 fm, consistent with earlier determinations based on dispersion relations.

So Lorenz’s paper gives two different sizes for the proton (typical Lorenz, in-joke).

> The proton charge radius is a fundamental quantity
of physics. It is truly remarkable that, despite decade long
experimental and theoretical efforts, its precise value
is not yet determined. The recent controversy about the
size of the proton was triggered by the precision measurement
of the Lamb shift in muonic hydrogen that led to
a “small” charge radius, r_pE = 0.84184 fm. This
result came as a big surprise as it was in stark contrast
to the commonly accepted “large” CODATA value
of 0.8768 fm, based on the measurements of the
Lamb shift in electronic hydrogen and the analysis of
electron-proton scattering data. The large value was further
strengthened by the high-precision electron-proton
scattering measurements at MAMI-C. The analysis
of these data including two-photon corrections led to
r_pE = 0.876 fm. These authors also found a magnetic
radius of the proton that came out much smaller than
commonly accepted values, r_pM = 0.803 fm.
On the theoretical side, a precise ab initio calculation
based on lattice QCD is not yet available due to various
conceptual problems to be overcome, but the most
recent and sophisticated QCD calculation led to
values consistent with the ones from muonic hydrogen,
r_pE = 0.844 fm . The value for the proton’s magnetic
radius was r_pM = 0.854 fm.

etc. The paper re-analyses the MAMI-C data to get different results.

Can of worms.

Summary of summary.

Pohl’s calculation of the size of the proton using the Lamb shift of atoms with muons instead of electrons is in conflict with accepted values in CODATA. Accepted values in CODATA have come from electron-proton scattering measurements and the Lamb shift of the hydrogen atom.

Rujula claims that Pohl has misinterpreted QCD by giving too large a coefficient to the influence of individual quarks within the proton on the energy levels of the atom. To prove it, he recalculates this coefficient from first principles.

Lorenz claims that Pohl is right and that the accepted value in CODATA is wrong. To prove it he re-analyses electron-proton scattering measurements to get a radius for the proton that is the same as Pohl’s.

Stay tuned for the next thrilling installment in 6 or so years.

Would all physics experiments so far conducted have occurred naturally either in the past, present or future (as the conditions of the universe continually change) or have we created something so artificial in nature that its highly unlikely to ever occur naturally.

Cymek said:

Would all physics experiments so far conducted have occurred naturally either in the past, present or future (as the conditions of the universe continually change) or have we created something so artificial in nature that its highly unlikely to ever occur naturally.

This is a very important discovery at this moment IMO. With Juno about to take data on Jupiter this type of observation will be essential for assessing metallic hydrogen and other high pressure elemental interactions.