QED Proton Size: 4% Smaller Than Thought?

[my_wan]

http://www.nature.com/news/2010/100707/full/news.2010.337.html

Apparently the proton is about 4% smaller than thought, which is somewhat challenging for QED. It could be new effects or experimental error of some sort. Any such guessing is speculative at this time, but what would you think?

 

[alxm]

(New crack? I didn't know there was an old one ;) )

QED has successfully calculated the Lamb shifts for hydrogen and helium to a high degree of accuracy, so it would appear QED 'works'. And has for some time - it's not very new stuff, calculating the Lamb shift of hydrogen - but muonic hydrogen, on the other hand, is relatively new stuff. And there's room for mistakes when going from hydrogen to muonic hydrogen, since the relative masses are very different, and the Lamb shift energy includes mass-dependent recoil corrections.

So that's my offhand guess - errors in the recoil corrections.

 

[my_wan]

Yeah well it was just sensationalism
I suppose the vacuum catastrophe might sort of qualify for one

 

[alxm]

Hmm, looking deeper into this, I'm not so sure - it seems the recoil corrections are overall too small to be able to account for the discrepancy. Of course, if a guy like me could find a correction that could possibly account for this, it should be immediately obvious to the people working on this thing, and we might not be having this discussion ..

The opinion I'd most like to hear is that of http://www.fuw.edu.pl/~krp/" , since he's the one who's done most of the calculations used for the muonic Lamb shift.

Me, I'm just a chemical physicist, so as far as I'm concerned the nucleus is a point charge, and the Lamb shift is just a rounding error!

 

[my_wan]

...and the Lamb shift is just a rounding error!

:rofl:
It's not something I'm seeing any obvious way to correct either. My first thought was binding energy, but... still seems the large mass difference highly likely has something to do with it. I'm stuck on this one without even any really reasonable guesses.

 

[Dadface]

If the size of the proton is actually smaller does this imply that other quantities such as proton mass are different?

 

[Dickfore]

QED does not deal with protons.

/thread

 

[alxm]

If the size of the proton is actually smaller does this imply that other quantities such as proton mass are different?

Not likely. Basically they're talking about the charge radius here. This is measured using scattering (typically), and this number is then used calculating the Lamb shift. The mass is also used, but this is determined independently, and better. (For instance, you have to use it to get the unshifted values of hydrogen to begin with, it's several orders of magnitude larger).

QED does not deal with protons.

What on Earth do you mean by that?

Lamb-shift calculations, which are done using QED include a whole host of corrections which are dependent on the properties of the nucleus/proton. (charge radius, mass, polarizability, etc) Everyone agrees it's not likely to be a fundamental failure of QED here, but it's at least a distant possibility.

 

[Dickfore]

What on Earth do you mean by that?

Lamb-shift calculations, which are done using QED include a whole host of corrections which are dependent on the properties of the nucleus/proton. (charge radius, mass, polarizability, etc)

Look up form factors. The Lamb shift calculation does not use QED, but effective field theory where the proton is a "black box" with some phenomenological parameters associated to it.

 

[Dadface]

Thanks alxm.My feeling is that it's early days yet and we can't rule out the possibility of other surprises cropping up.Interesting stuff.

 

[alxm]

Look up form factors. The Lamb shift calculation does not use QED, but effective field theory where the proton is a "black box" with some phenomenological parameters associated to it.

I know about the form factors (in fact I thought it was the experimental determination of them we were talking about), I don't know why you think that that means it doesn't use QED. What would that imply?

All I can say that your definition of 'using QED' must be different from everyone else's, because it certainly seems everyone else thinks that QED corrections are used, including the people who wrote the paper in question, and the people who did the Lamb shift calculations that they refer to, and the textbooks, and Wikipedia.

 

[humanino]

I know about the form factors (in fact I thought it was the experimental determination of them we were talking about), I don't know why you think that that means it doesn't use QED. What would that imply?

All I can say that your definition of 'using QED' must be different from everyone else's, because it certainly seems everyone else thinks that QED corrections are used, including the people who wrote the paper in question, and the people who did the Lamb shift calculations that they refer to, and the textbooks, and Wikipedia.

I would even argue that QCD has very little theoretical relevance in this context : not only most of the Lamb shift corrections are just QED, but in addition form factors which could in principle be calculated from QCD in fact are not manageable (non-perturbative), and are just experimentally measured using QED probes (DIS) !

 

[my_wan]

Size can take on different meanings depending on the context. It's not necessarily a simple geometric volume in space. Would you call a charge radius a size? It really makes no difference what ontological notions you attach to size in this case, it's about 4% smaller than what we can account for.

 

[Dadface]

Is it true that the Rydberg constant is used to calculate the size of the proton?

 

[humanino]

Size can take on different meanings depending on the context. It's not necessarily a simple geometric volume in space. Would you call a charge radius a size? It really makes no difference what ontological notions you attach to size in this case, it's about 4% smaller than what we can account for.

The charge radius is defined consistently in electron scattering, ordinary hydrogen Lamb shift and muonic hydrogen Lamb shift. The definition chosen is the round-mean-squared of the charge 1-D distribution as a function of radius.

We already expect from models that the mass radius (by which I mean : the RMS of the mass 1-D distribution; also other radii, such as from angular momentum density or force density) are different BTW.

 

[my_wan]

The root-mean-square charge radius is, and that goes into defining the Rydberg constant.

 

[Dickfore]

A qoute from the introduction to the paper:

Here we use pulsed laser spectroscopy to measure a muonic Lamb shift of 49,881.88(76)GHz.On the basis of present calculations^11–15 of fine and hyperfine splittings and QED terms, we find rp50.84184(67) fm, which differs by 5.0 standard deviations from the CODATA value³ of 0.8768(69) fm. Our result implies that either the Rydberg constant has to be shifted by 2110 kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.

The references used here are:

• 3. Mohr, P. J., Taylor, B. N. & Newell, D. B. CODATA recommended values of the
  fundamental physical constants: 2006. Rev. Mod. Phys. 80, 633–730 (2008)
  
• 11. Pachucki, K. Theory of the Lamb shift in muonic hydrogen. Phys. Rev. A 53,
  2092–2100 (1996)
  
• 12. Pachucki, K. Proton structure effects in muonic hydrogen. Phys. Rev. A 60,
  3593–3598 (1999)
  
• 13. Borie, E. Lamb shift in muonic hydrogen. Phys. Rev. A 71, 032508 (2005)
  
• 14. Martynenko, A. P. 2S Hyperfine splitting of muonic hydrogen. Phys. Rev. A 71,
  022506 (2005)
  
• 15. Martynenko, A. P. Fine and hyperfine structure of P-wave levels in muonic
  hydrogen. Phys. At. Nucl. 71, 125–135 (2008)

I see three (of the many) authors:

Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France

Eric-Olivier Le Bigot, Paul Indelicato,


Institut für Teilchenphysik, ETH Zürich, 8093 Zürich, Switzerland

Franz Kottmann

who 'did work on QED theory', according to the paper.

 

[my_wan]

The charge radius is defined consistently in electron scattering, ordinary hydrogen Lamb shift and muonic hydrogen Lamb shift. The definition chosen is the round-mean-squared of the charge 1-D distribution as a function of radius.

We already expect from models that the mass radius (by which I mean : the RMS of the mass 1-D distribution; also other radii, such as from angular momentum density or force density) are different BTW.

Yes, but by how much and at what variance from the muon data?

 

[my_wan]

Here's something that may help:
http://engineering.library.cornell.edu/node/4510

The size of the proton

Nature 466, 213 (2010). doi:10.1038/nature09250

Authors: Randolf Pohl, Aldo Antognini, François Nez, Fernando D. Amaro, François Biraben, João M. R. Cardoso, Daniel S. Covita, Andreas Dax, Satish Dhawan, Luis M. P. Fernandes, Adolf Giesen, Thomas Graf, Theodor W. Hänsch, Paul Indelicato, Lucile Julien, Cheng-Yang Kao, Paul Knowles, Eric-Olivier Le Bigot, Yi-Wei Liu, José A. M. Lopes, Livia Ludhova, Cristina M. B. Monteiro, Françoise Mulhauser, Tobias Nebel, Paul Rabinowitz, Joaquim M. F. dos Santos, Lukas A. Schaller, Karsten Schuhmann, Catherine Schwob, David Taqqu, João F. C. A. Veloso & Franz Kottmann
The proton is the primary building block of the visible Universe, but many of its properties—such as its charge radius and its anomalous magnetic moment—are not well understood. The root-mean-square charge radius, rp, has been determined with an accuracy of 2 per cent (at best) by electron–proton scattering experiments. The present most accurate value of rp (with an uncertainty of 1 per cent) is given by the CODATA compilation of physical constants. This value is based mainly on precision spectroscopy of atomic hydrogen and calculations of bound-state quantum electrodynamics (QED; refs 8, 9). The accuracy of rp as deduced from electron–proton scattering limits the testing of bound-state QED in atomic hydrogen as well as the determination of the Rydberg constant (currently the most accurately measured fundamental physical constant). An attractive means to improve the accuracy in the measurement of rp is provided by muonic hydrogen (a proton orbited by a negative muon); its much smaller Bohr radius compared to ordinary atomic hydrogen causes enhancement of effects related to the finite size of the proton. In particular, the Lamb shift (the energy difference between the 2S1/2 and 2P1/2 states) is affected by as much as 2 per cent. Here we use pulsed laser spectroscopy to measure a muonic Lamb shift of 49,881.88(76) GHz. On the basis of present calculations of fine and hyperfine splittings and QED terms, we find rp = 0.84184(67) fm, which differs by 5.0 standard deviations from the CODATA value of 0.8768(69) fm. Our result implies that either the Rydberg constant has to be shifted by −110 kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.

Link: http://feeds.nature.com/~r/nature/rss/current/~3/6zg1oLLn4QI/nature09250
Author: Randolf Pohl

 

[Dadface]

A qoute from the introduction to the paper:



The references used here are:

• 3. Mohr, P. J., Taylor, B. N. & Newell, D. B. CODATA recommended values of the
  fundamental physical constants: 2006. Rev. Mod. Phys. 80, 633–730 (2008)
  
• 11. Pachucki, K. Theory of the Lamb shift in muonic hydrogen. Phys. Rev. A 53,
  2092–2100 (1996)
  
• 12. Pachucki, K. Proton structure effects in muonic hydrogen. Phys. Rev. A 60,
  3593–3598 (1999)
  
• 13. Borie, E. Lamb shift in muonic hydrogen. Phys. Rev. A 71, 032508 (2005)
  
• 14. Martynenko, A. P. 2S Hyperfine splitting of muonic hydrogen. Phys. Rev. A 71,
  022506 (2005)
  
• 15. Martynenko, A. P. Fine and hyperfine structure of P-wave levels in muonic
  hydrogen. Phys. At. Nucl. 71, 125–135 (2008)

I see three (of the many) authors:

Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France

Eric-Olivier Le Bigot, Paul Indelicato,


Institut für Teilchenphysik, ETH Zürich, 8093 Zürich, Switzerland

Franz Kottmann

who 'did work on QED theory', according to the paper.

Here's something that may help:
http://engineering.library.cornell.edu/node/4510

Wow the implications here are really interesting.:tongue2:

 

[ViewsofMars]

http://www.nature.com/news/2010/100707/full/news.2010.337.html

Apparently the proton is about 4% smaller than thought, which is somewhat challenging for QED. It could be new effects or experimental error of some sort. Any such guessing is speculative at this time, but what would you think?

Thank you my_wan. Well, I followed up by also searching for further information that pertains to the Nature "News" article. I am fond of Dr. Randolf Pohl of Max Planck Institute of Quantum Optics. Max Planck Institute provides added insight: PRESS RELEASE - Garching, 08.07.2010, How small is the proton?
http://www.mpq.mpg.de/cms/mpq/en/news/press/pdf/2010/PR_10_07_08.pdf[/URL]

Here are two consecutive quotes from the press release from Max Planck Institute, though I encourage people to read the entire press release. I have highlighted in red what I thought was important to remember.

[QUOTE]Finally the experiment has been realized in a joined effort in which each team brought in its own expertise in the fields of accelerator physics, atomic physics, laser technologies and detectors. First measurements in 2002, 2003 and 2007 were not encouraging. Although the experiment seemed to work as planned, there was no sign of the expected resonance. “First we thought our lasers were not good enough, so we rebuilt large parts of the laser system using the new disk laser technology developed at the University of Stuttgart. But then it turned out that we had simply looked at the wrong place: apparently the theoretical prediction of the transition frequency was wrong”, explains Dr. Aldo Antognini from PSI.

The breakthrough happened in the summer of 2009. After three months of set up time and three weeks of data taking – day and night – the scientists could unambiguously observe the signal on the evening of July 5, 2009. After a long and careful analysis of this signal, the deduced value of the proton radius is ten times more precise, 0.84184 femtometers (1 femtometer = 0.000 000 000 000 001 meter), but in strong disagreement with the international accepted value (0.8768 femtometers) deduced from previous experiments. The scientists are still discussing the possible reasons for the discrepancy. Everything is under scrutiny now: Previous high-precision measurements, complicated calculations, and maybe at some point even the world’s most precise and best-tested fundamental theory itself: quantum electrodynamics. “[COLOR="Red"]However, before we question the validity of QED theorists have to check their calculations”, says Dr. Pohl. A hint, which interpretation could be correct, may come from the new project, planned for 2012. Then, the researchers want to perform the same kind of experiment with muonic helium. The required muon source and lasers, says Randolf Pohl, are already available. .[/COLOR] Meyer-Streng(MPQ)/Piwnicki(PSI)[/QUOTE]

It appears to me to be an ongoing work-in-progress. I'm excited! The new project planned for 2012 should give us further information. Wow! I'll be looking forward to tune-in on this topic in 2 more years. :biggrin:

 

[alxm]

Is it true that the Rydberg constant is used to calculate the size of the proton?

The Rydberg constant is naturally necessary for determining the absolute values of the the electronic levels, but the "classical" Lamb shift, being the relative shift of the 2S and 2P levels, is independent of the Rydberg constant; it cancels out.

But the accuracy of the Rydberg constant is limited by the accuracy of these lower (1S, 2S, 2P) levels, which in turn is limited by the Lamb shift. As I read it, they're saying that if their value of the Lamb shift is correct, and the charge radius calculated from it is correct, then the Rydberg constant will be changed by an improbable amount.

 

[Dadface]

The Rydberg constant is naturally necessary for determining the absolute values of the the electronic levels, but the "classical" Lamb shift, being the relative shift of the 2S and 2P levels, is independent of the Rydberg constant; it cancels out.

But the accuracy of the Rydberg constant is limited by the accuracy of these lower (1S, 2S, 2P) levels, which in turn is limited by the Lamb shift. As I read it, they're saying that if their value of the Lamb shift is correct, and the charge radius calculated from it is correct, then the Rydberg constant will be changed by an improbable amount.

Changed by an improbable amount but does improbable mean zero?

Can someone please confirm,clarify or otherwise the following.As far as the whole atom is concerned the most probable radius is the Bohr radius(for ground state) and muonic hydrogen has a much smaller radius than ordinary hydrogen(for the non reduced mass equation the radius being inversely proportional to the lepton mass)

 

[alxm]

Changed by an improbable amount but does improbable mean zero?

No, their calculated value differed by "−110 kHz/c (4.9 standard deviations)".
It's a very large deviation, and given that there are lamb-shift independent means of determining it, I suspect they simply mean it's impossible. So they're basically doing the old "either the calculations are wrong or <something impossible> is true" argument. Of course the theorists will inevitably raise the additional possibility of experimental error.

Can someone please confirm,clarify or otherwise the following.As far as the whole atom is concerned the most probable radius is the Bohr radius(for ground state)

In this context (extremely accurate hydrogen calcs) I would have to say no. These days, the Bohr radius is defined as the non-relativistic value for a fictional hydrogen with infinite nuclear mass. It does not take into account reduced-mass, or relativistic effects, or indeed the Lamb shift. So it's not exactly the most probable radius, and the ground-state energy is not exactly 0.5 a.u.

muonic hydrogen has a much smaller radius than ordinary hydrogen(for the non reduced mass equation the radius being inversely proportional to the lepton mass)

Entirely correct, for the ordinary clamped-nucleus Schrödinger equation a change in electron mass is just a linear scaling of coordinates. So the muon 'orbits' about 200x closer to the nucleus, hence nuclear structure effects have a correspondingly much, much larger effect. Which is why this experiment was done.

 

[Dadface]

No, their calculated value differed by "−110 kHz/c (4.9 standard deviations)".
It's a very large deviation, and given that there are lamb-shift independent means of determining it, I suspect they simply mean it's impossible. So they're basically doing the old "either the calculations are wrong or <something impossible> is true" argument. Of course the theorists will inevitably raise the additional possibility of experimental error.



In this context (extremely accurate hydrogen calcs) I would have to say no. These days, the Bohr radius is defined as the non-relativistic value for a fictional hydrogen with infinite nuclear mass. It does not take into account reduced-mass, or relativistic effects, or indeed the Lamb shift. So it's not exactly the most probable radius, and the ground-state energy is not exactly 0.5 a.u.




Entirely correct, for the ordinary clamped-nucleus Schrödinger equation a change in electron mass is just a linear scaling of coordinates. So the muon 'orbits' about 200x closer to the nucleus, hence nuclear structure effects have a correspondingly much, much larger effect. Which is why this experiment was done.

Great stuuff alxm,thanks a lot.I need to look more into the Bohr radius thing.

 

[ViewsofMars]

I've located the actual LETTER from Nature. I should remind people that Nature is an internationally peer-reviewed journal. It is one of the very finest journals known by professionals. Scientists love it!

Nature 466, 213-216 (8 July 2010) | doi:10.1038/nature09250; Received 22 March 2010; Accepted 1 June 2010

The size of the proton

Randolf Pohl1, Aldo Antognini1, François Nez2, Fernando D. Amaro3, François Biraben2, João M. R. Cardoso3, Daniel S. Covita3,4, Andreas Dax5, Satish Dhawan5, Luis M. P. Fernandes3, Adolf Giesen6,13, Thomas Graf6, Theodor W. Hänsch1, Paul Indelicato2, Lucile Julien2, Cheng-Yang Kao7, Paul Knowles8, Eric-Olivier Le Bigot2, Yi-Wei Liu7, José A. M. Lopes3, Livia Ludhova8, Cristina M. B. Monteiro3, Françoise Mulhauser8,13, Tobias Nebel1, Paul Rabinowitz9, Joaquim M. F. dos Santos3, Lukas A. Schaller8, Karsten Schuhmann10, Catherine Schwob2, David Taqqu11, João F. C. A. Veloso4 & Franz Kottmann12

1.Max-Planck-Institut für Quantenoptik, 85748 Garching, Germany
2.Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France
3.Departamento de Física, Universidade de Coimbra, 3004-516 Coimbra, Portugal
4.I3N, Departamento de Física, Universidade de Aveiro, 3810-193 Aveiro, Portugal
5.Physics Department, Yale University, New Haven, Connecticut 06520-8121, USA
6.Institut für Strahlwerkzeuge, Universität Stuttgart, 70569 Stuttgart, Germany
7.Physics Department, National Tsing Hua University, Hsinchu 300, Taiwan
8.Département de Physique, Université de Fribourg, 1700 Fribourg, Switzerland
9.Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009, USA
10.Dausinger & Giesen GmbH, Rotebühlstr. 87, 70178 Stuttgart, Germany
11.Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland
12.Institut für Teilchenphysik, ETH Zürich, 8093 Zürich, Switzerland
13.Present addresses: Deutsches Zentrum für Luft- und Raumfahrt e.V. in der Helmholtz-Gemeinschaft, 70569 Stuttgart, Germany (A.G.); International Atomic Energy Agency, A-1400 Vienna, Austria (F.M.).
Correspondence to: Randolf Pohl1 Email: randolf.pohl@mpq.mpg.de


Abstract
The proton is the primary building block of the visible Universe, but many of its properties—such as its charge radius and its anomalous magnetic moment—are not well understood. The root-mean-square charge radius, rp, has been determined with an accuracy of 2 per cent (at best) by electron–proton scattering experiments1, 2. The present most accurate value of rp (with an uncertainty of 1 per cent) is given by the CODATA compilation of physical constants3. This value is based mainly on precision spectroscopy of atomic hydrogen4, 5, 6, 7 and calculations of bound-state quantum electrodynamics (QED; refs 8, 9). The accuracy of rp as deduced from electron–proton scattering limits the testing of bound-state QED in atomic hydrogen as well as the determination of the Rydberg constant (currently the most accurately measured fundamental physical constant3). An attractive means to improve the accuracy in the measurement of rp is provided by muonic hydrogen (a proton orbited by a negative muon); its much smaller Bohr radius compared to ordinary atomic hydrogen causes enhancement of effects related to the finite size of the proton. In particular, the Lamb shift10 (the energy difference between the 2S1/2 and 2P1/2 states) is affected by as much as 2 per cent. Here we use pulsed laser spectroscopy to measure a muonic Lamb shift of 49,881.88(76) GHz. On the basis of present calculations11, 12, 13, 14, 15 of fine and hyperfine splittings and QED terms, we find rp = 0.84184(67) fm, which differs by 5.0 standard deviations from the CODATA value3 of 0.8768(69) fm. Our result implies that either the Rydberg constant has to be shifted by −110 kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.

1.Max-Planck-Institut für Quantenoptik, 85748 Garching, Germany
2.Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France
3.Departamento de Física, Universidade de Coimbra, 3004-516 Coimbra, Portugal
4.I3N, Departamento de Física, Universidade de Aveiro, 3810-193 Aveiro, Portugal
5.Physics Department, Yale University, New Haven, Connecticut 06520-8121, USA
6.Institut für Strahlwerkzeuge, Universität Stuttgart, 70569 Stuttgart, Germany
7.Physics Department, National Tsing Hua University, Hsinchu 300, Taiwan
8.Département de Physique, Université de Fribourg, 1700 Fribourg, Switzerland
9.Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009, USA
10.Dausinger & Giesen GmbH, Rotebühlstr. 87, 70178 Stuttgart, Germany
11.Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland
12.Institut für Teilchenphysik, ETH Zürich, 8093 Zürich, Switzerland
13.Present addresses: Deutsches Zentrum für Luft- und Raumfahrt e.V. in der Helmholtz-Gemeinschaft, 70569 Stuttgart, Germany (A.G.); International Atomic Energy Agency, A-1400 Vienna, Austria (F.M.).
Correspondence to: Randolf Pohl1
http://www.nature.com/nature/journal/v466/n7303/full/nature09250.html

No, their calculated value differed by "−110 kHz/c (4.9 standard deviations)".
It's a very large deviation, and given that there are lamb-shift independent means of determining it, I suspect they simply mean it's impossible. So they're basically doing the old "either the calculations are wrong or <something impossible> is true" argument. Of course the theorists will inevitably raise the additional possibility of experimental error.

Well, alxm, there are Theory physicists aka physicists' theorists. Also, your remark seems to me that you might be knocking down all those organizations (1- 13) that are affliated with the document above with your statement. I hope I am mistaken. I happen to like what Paul Scherrer Institute has provided me:

The size of the proton

Research Department Neutrons and Muons (NUM).

Here, a technically challenging spectroscopic experiment is described: the measurement of the muonic Lamb shift. The results lead to a new determination of the charge radius of the proton. The new value is 5.0 standard deviations smaller than the previous world average, a large discrepancy that remains unexplained. Possible implications of the new finding are that the value of the Rydberg constant will need to be revised, or that the validity of quantum electrodynamics theory is called into question.
Citation: R. Pohl, A. Antognini, F. Nez, F.D. Amaro, F. Biraben, J.M.R. Cardoso, D.S. Covita, A. Dax, S. Dhawan, L.M.P. Fernandes, A. Giesen, T. Graf, T.W. Hänsch, P. Indelicato, L. Julien, C.Y. Kao, P. Knowles, E.O. Le Bigot, Y.W. Liu, J.A.M. Lopes, L. Ludhova, C.M.B. Monteiro, F. Mulhauser, T. Nebel, P. Rabinowitz, J.M.F. dos Santos, L.A. Schaller, K. Schuhmann, C. Schwob, D. Taqqu, J.F.C.A. Veloso, F. Kottmann, Nature 466, 213 (2010)
http://www.psi.ch/science/num-highlights

Paul Scherrer Institute's Condensed Matter Research with Neutrons and Muons (NUM) has very impressive publications focusing on recent scientific highlights. Awesome!
http://num.web.psi.ch/highlights.html

I should mention SCIENCE doesn't exist without research.
They are partners that go hand in hand. That is one of the beautiful things about Science that I love. The journey never ends. There is so much to explore. I love it!

 

[Andy Resnick]

http://www.nature.com/news/2010/100707/full/news.2010.337.html

Apparently the proton is about 4% smaller than thought, which is somewhat challenging for QED. It could be new effects or experimental error of some sort. Any such guessing is speculative at this time, but what would you think?

Ha! experiment once again befuddles theory :)

 

[jal]

Proton is smaller than we thought

I found the following ...
http://physicsworld.com/cws/article/news/43128
Proton is smaller than we thought
Jul 7, 2010
Now an international team led by Randolf Pohl at the Max Planck Institute for Quantum Optics in Garching, Germany has measured the Lamb shift in muonic hydrogen for the first time and found the proton radius to be 0.8418*fm with uncertainty 0.0007*fm

Until recently the best estimate of the proton radius was 0.877*femtometres with an uncertainty of 0.007*fm

According to Jeff Flowers of the UK's National Physical Laboratory there are three possible explanations for the discrepancy. The most likely is that QED is correct, but has been misapplied in what he describes as a "very difficult calculation". Alternatively there is a problem with the experiment – but Flowers, who was not involved in the measurement, believes that Pohl's team has done an excellent job. The least likely – but most exciting explanation – according to Flowers is that there is something wrong with QED.

While QED rests on a weak mathematical foundation, it has been extremely successful in predicting the outcome of experiments. "Changing QED would be big philosophical change for physicists", says Flowers.
The result has already caused a flurry of experimental and theoretical activity, with some physicists carefully redoing Lamb shift calculations and others trying to improve electron-based measurements of the proton radius.
Meanwhile, Pohl's team will repeat its experiment and do a new series of measurements on muonic helium to measure the radius of the helium nucleus.
The research is described in Nature.

However, the explanation and discussion at

http://scienceblogs.com/principles/2010/07/protons_even_smaller_than_we_t.php
by Chad Orzel is even more informative.

jal

 

[Orion1]

quark spins appear to account for only 20-30% of the neutron and proton spins (Jaffe 1995, Hellemans 1996). The current best two values for the proton's radius disagree: 0.805 ± 0.011 and 0.862 ± 0.012 femtometer (Stein 1995). Weingarten (1993) calculated the mass of the proton to within 6% using "lattice" quantum chromodynamics.

The charge radius is measured mainly through elastic electron-proton scattering and is 0.870 fm.

The internationally accepted value of the proton's charge radius is 0.8768 femtometers. This value is based on measurements involving a proton and an electron.

However since July 5, 2009 an international research team has been able to make measurements involving a proton and a negatively charged muon. After a long and careful analysis of those measurements the team concluded that the root-mean-square charge radius of a proton is "0.84184(67) fm, which differs by 5.0 standard deviations from the CODATA value of 0.8768(69) fm

Measured proton charge radii:
0.805 ± 0.011 fm
0.8418467 fm ± 0.0007 fm
0.862 ± 0.012 fm
0.870 ± ? fm
0.876869 ± 0.0069 fm
0.877 ± ? fm
0.88015 ± ? fm

Our result implies that either the Rydberg constant has to be shifted by −110 kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.

What is the QED Penning trap upper limit for the proton particle radius?

http://scienceworld.wolfram.com/physics/Proton.html"[/URL]
[URL]http://en.wikipedia.org/wiki/Nucleon#Proton"[/URL]
[URL]http://en.wikipedia.org/wiki/Proton#Charge_Radius"[/URL]
[PLAIN]http://en.wikipedia.org/wiki/Penning_trap"[/URL]
[PLAIN]http://en.wikipedia.org/wiki/Lattice_QCD"[/URL]
http://www.nature.com/nature/journal/v466/n7303/abs/nature09250.html"
[URL]http://www.vniim.ru/sgk/psas/book/muonic.htm"[/URL]

 

[alxm]

I've located the actual LETTER from Nature. I should remind people that Nature is an internationally peer-reviewed journal. It is one of the very finest journals known by professionals. Scientists love it!

This was linked to in the first post.

Well, alxm, there are Theory physicists aka physicists' theorists.

That sentence doesn't make any sense.

Also, your remark seems to me that you might be knocking down all those organizations (1- 13) that are affliated with the document above with your statement.

In what way would I be knocking these organizations? Besides, you don't speak for them. Have you even read the article? Do you even know something about the topic? I suspect not. You're just dumping a bunch of stuff anybody could find on google in a few seconds.

 

[ZapperZ]

Two different threads on the same topic have been merged.

Zz.

 

[petergreat]

Forgive me for asking a naive question. Is it possible to determine the proton size using muon-proton scattering in an accelerator? Would this method be sufficiently accurate to shed light on this problem?

 

[sanman]

Proton Smaller Than Previously Thought?

There have been a spate of articles about the size of the proton being measured as smaller than previously believed:

http://arstechnica.com/science/news/2010/07/-we-may-have-been.ars

What is the significance of this finding? What impact does it have on the broader picture?

This sounds like some basic things in physics are going to have to be revised, no matter what.

Does this affect things like nuclear cross-section values for nuclear energy applications?

 

[jtbell]

Threads merged. Sanman, check out the previous posts. I left your post intact because it has a reference to another article on the subject.

 

[sanman]

I found the following ...
http://physicsworld.com/cws/article/news/43128
Proton is smaller than we thought
Jul 7, 2010

...

Meanwhile, Pohl's team will repeat its experiment and do a new series of measurements on muonic helium to measure the radius of the helium nucleus.

What about experimenting with μ+ and the anti-proton?

Shall we hope for similar results there? Or could you see a CP-violation on top of everything?