Physicists confirm surprisingly small proton radius

Jan 24, 2013

International team of physicists confirms surprisingly small proton radius with laser spectroscopy of exotic hydrogen. The initial results puzzled the world three years ago: the size of the proton (to be precise, its charge radius), measured in exotic hydrogen, in which the electron orbiting the nucleus is replaced by a negatively charged muon, yielded a value significantly smaller than the one from previous investigations of regular hydrogen or electron-proton-scattering. A new measurement by the same team confirms the value of the electric charge radius and makes it possible for the first time to determine the magnetic radius of the proton via laser spectroscopy of muonic hydrogen.

The experiments were carried out at the Paul Scherrer Institut (PSI) (Villigen, Switzerland) which is the only research institute in the world providing the necessary amount of muons. The included the Max Planck Institute of (MPQ) in Garching near Munich, the Swiss Federal Institute of Technology ETH Zurich, the University of Fribourg, the Institut für Strahlwerkzeuge (IFSW) of the Universität Stuttgart, and Dausinger & Giesen GmbH, Stuttgart. The new results fuel the debate as to whether the discrepancies observed can be explained by standard physics, for example an incomplete understanding of the systematic errors that are inherent to all measurements, or whether they are due to new physics.

The hydrogen atom has played a key role in the investigation of the fundamental laws of physics. Hydrogen consists of a single positively charged orbited by a negatively charged electron, a model whose success in explaining spectroscopy data dates back to its proposal by Bohr in 1913. The energy levels of this simplest of atoms can be predicted with excellent precision from the theory of quantum electrodynamics. However, the calculations have to take into account that – in contrast to the point-like electron – the proton is an extended object with a finite size, made of three quarks bound by so-call 'gluons'. Therefore, the electric charge as well as the magnetism of the proton is distributed over a certain volume. The extended nature of the proton causes a shift of the energy levels in hydrogen. Hence the electric and the magnetic charge radii can be deduced from a measurement of the level shifts.

In 2010, the first results on the spectroscopic determination of the shift of the so-called 2S energy level in muonic hydrogen were published. The exotic atoms were generated by bombarding a target of regular hydrogen with muons from an accelerator at PSI. Muons behave a lot like electrons, except for their mass: muons are 200 times heavier than electrons. The atomic orbit of the muon is therefore much closer to the proton than the electron's orbit in a regular hydrogen atom. This results in a much larger sensitivity of the muon's energy level to the proton size and hence to a stronger shift of the energy levels. Measuring the level shifts is very technologically demanding: muonic hydrogen is very short-lived (muons decay after about two millionths of a second), so the light pulses for the excitation of the resonance have to be fired onto the hydrogen target only nanoseconds after the detection of a muon. The new disk laser technology developed by the Institut für Strahlwerkzeuge (IFSW) of the Universität Stuttgart was an important element to fulfil this requirement. The lasers necessary for exciting the resonance were developed by the Max-Planck-Institute of Quantum Optics in cooperation with the Laboratoire Kastler Brossel (Paris).

In the experiment described in the newly published Science article, the energy shift was determined for another transition. This leads to a new measurement of the electric charge radius of the proton. Its value of 0.84087(39) femtometres (1 fm = 0.000 000 000 000 001 metre) is in good agreement with the one published in 2010, but 1.7 times as precise. The discrepancy with existing radius measurements made in regular hydrogen or by electron-proton-scattering, the so-called proton size puzzle, has thus been reaffirmed.

In addition, the new measurement allows a determination of the magnetic radius of the proton for the first time by of muonic hydrogen. This results in a value of 0.87(6) femtometres, in agreement with all previous measurements. Though the precision is, at present, of the same order as in other experiments, laser spectroscopy of muonic hydrogen has the potential of achieving a much better accuracy in the determination of the magnetic proton radius in the future.

around the world are actively seeking a solution to the proton size puzzle. Previous measurements in regular and by electron-proton-scattering are being reanalyzed and even repeated. Theorists of various disciplines suggested ways to explain the discrepancy. Very interesting proposals explain the discrepancies by physics beyond the standard model. Other explanations suggest a proton structure of higher complexity than assumed today which only reveals itself under the influence of the heavy muon. New measurements are needed to check on these possibilities. Muon-proton-scattering experiments are being developed at PSI, new precision measurements at the electron accelerator in Mainz are being considered, and the PSI team plans to measure, for the first time ever, laser spectroscopy of the muonic helium atom in the course of this year.

The required modifications of the current laser system are being investigated in the frame of the project "Thin-disk laser for muonic atoms spectroscopy" which (financed by the Swiss National Science Foundation (SNSF) and the Deutsche Forschungsgemeinschaft (DFG)) is carried out at the ETH Zürich (Prof. Dr. Klaus Kirch, Dr. Aldo Antognini) and at the IFSW (Prof. Dr. Thomas Graf, Dr. Andreas Voß). The Project "Muonic Helium" is also generously supported by the European Research Council (ERC) by an ERC Starting Grant held by Dr. Randolf Pohl from the MPQ in Garching.

Explore further: X-ray powder diffraction beamline at NSLS-II takes first beam and first data

More information: Proton Structure from the Measurement of 2S-2P Transition Frequencies of Muonic Hydrogen, Science 25 January 2013: Vol. 339 no. 6118 pp. 417-420 DOI: 10.1126/science.1230016

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User comments : 23

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Shootist
2.3 / 5 (21) Jan 24, 2013
Replacing electrons with muons?

Yes. We call "closing the barn door". It makes everything so much easier.

lengould100
2.3 / 5 (4) Jan 24, 2013
It sort of puzzles me how a proton-muon atom can be compared physically to a normal atom of hydrogen, and the spectral frequency of emitted photons compared. Obviously I expect the investigators have dealt with the question, but isn't the absorbtion or emission spectrum of an atom determined by the various full or half wavelength orbit pathlengths (sic) possible of the outer orbital electron(s) of an atom before it is entirely ionized off? And normal hydrogen has only 4 possible orbit changes for an electron, from 420 nm to about 600 nm. So my problem is, with the muon being 20x heavier than the electron, it's wavelength must be much longer than the electron, yet it apparently orbits much closer to the proton (eg. much shorter orbits). ?? I'd like to see the emission or absorbtion lines they are detecting, or at least a statement of the wavelengths.
douglaskostyk
4.3 / 5 (7) Jan 24, 2013
It sort of puzzles me how a proton-muon atom can be compared physically to a normal atom of hydrogen, and the spectral frequency of emitted photons compared. Obviously I expect the investigators have dealt with the question, but isn't the absorbtion or emission spectrum of an atom determined by the various full or half wavelength orbit pathlengths (sic) possible of the outer orbital electron(s) of an atom before it is entirely ionized off? And normal hydrogen has only 4 possible orbit changes for an electron, from 420 nm to about 600 nm. So my problem is, with the muon being 20x heavier than the electron, it's wavelength must be much longer than the electron, yet it apparently orbits much closer to the proton (eg. much shorter orbits). ?? I'd like to see the emission or absorbtion lines they are detecting, or at least a statement of the wavelengths.


λ = h / p ..... smaller wavelength for larger mass
ValeriaT
2 / 5 (11) Jan 24, 2013
Why physicists should be so impressed by this result (if we neglect the technical part of experiment, which is indeed brilliant) - when they know already, every particle is surrounded by less or more dense coat of virtual quarks (virtual quark-anti-quark pairs)? The similar effect is responsible for Casimir force at different energy density scale.

This difference would be the effect of compactified extradimensions near proton (the same situation could be observed near black holes, which should appear the smaller, the more we approach to them). I hope, these guys considered the fact, muon affects the location of center of mass of proton more, then the electron during its motion around proton - it would be a trivial mistake to forget the classical physics here. I.e. the physics, which served for spectroscopic discovery of deuterium in 1932.
ValeriaT
1.8 / 5 (10) Jan 24, 2013
Note that Riemann manifolds do shrink in gravity field - which allows to have larger object inside of smaller one - well, theoretically...
Whydening Gyre
1.7 / 5 (13) Jan 25, 2013
Very concise description of the researchers process. However, I saw no reference to the neutron half of the nucleus. It seems to me that a measurement of that might provide some potentially interesting correlations.
Len,
Great questions and corresponding data...
I'll bet Dougs response was a forehead smacker, tho...:-)
Whydening Gyre
1.4 / 5 (9) Jan 25, 2013
Wait, another question - did the muon's additional mass merely confirm the measurement of the protons "size" or did IT cause the proton charge (Field) to shrink?
lengould100
3 / 5 (2) Jan 25, 2013
I'll bet Dougs response was a forehead smacker, tho...:-)
Yeah, OUCH! LOL. Sorry guys (m/f), its been too long for me since I've been doing this, I need to learn to reference my stuff better.
Noumenon
2.8 / 5 (19) Jan 25, 2013
Very concise description of the researchers process. However, I saw no reference to the neutron half of the nucleus. It seems to me that a measurement of that might provide some potentially interesting correlations.

There's no neutron in regular hydrogen.

did the muon's additional mass merely confirm the measurement of the protons "size" or did IT cause the proton charge (Field) to shrink?


The presence of the muon can't change the proton's field, but can make the effects of it more observationally 'sensitive' via energy levels, as I gather from the article,... it allowed confirmation of a discrepancy in "field size" measured with regular electron-proton hydrogen.
omerbashich
Jan 25, 2013
This comment has been removed by a moderator.
Whydening Gyre
1.4 / 5 (9) Jan 25, 2013
There's no neutron in regular hydrogen.

Thanks Nou, I was not aware of that. I just assumed all atoms required one.

And omar... Ease up on the caffeine, eh?
dschlink
2.3 / 5 (3) Jan 25, 2013
Hydrogen with a neutron in it is called Deuterium (D or 2H), the nucleus is called a deuteron. One with two neutrons is tridium (which is unstable).
Shabs42
4.1 / 5 (9) Jan 25, 2013
You can use them ALL your fake accounts to give me as many stars as you can! Thank you oh thank you oh...!! not...

In the meantime, in real life: "How Wineland & Haroche Stole My Discovery (and got 2012 PHYSICS NOBEL PRIZE for it...)":
http://sites.goog...ci#Nobel


So you don't care what we think about your comments, but you care what we think about your website? I'm going to continue flagging your comments as spam until you're banned or learn to stick to commenting on the article.
Q-Star
2.8 / 5 (13) Jan 25, 2013
In the meantime, in real life: "How Wineland & Haroche Stole My Discovery (and got 2012 PHYSICS NOBEL PRIZE for it...)"


How did Wineland and Haroche steal your discovery?

Did you publish before them?

Do you have collaborators that can confirm that? (I mean real people who have a track record.)

Maybe you could tell us about the grant that was funding your stolen work? (I mean an actual grant awarded by some body with a track record of funding such work.)

Why did you not complain to the Nobel Committee prior to the actual awarding of the Prize? Surely you must have been aware that Wineland and Haroche had been nominated and were contenders?

And most urgently,,,, why would a Crown Prince and renown scientist such as yourself, need to use a cobbled together free web page from the "Internet-Gurus-R-Us" provided by Google for the moron community. It reminds me of the Facebook "United-Vortex-Theory" page of AntonKole.

By the By: Are you related to a guy named Zephyr?
rah
2.3 / 5 (12) Jan 25, 2013
Oh look, a Wineland and Haroche type of "research"! They can't mint enough medals for them all ingeniahs westernahs...
God knows what hungered Chinese grad student they stole this discovery from... Scum like Edison, Marconi, Bell... Today from Asians and eastern Europeans, as yesterday from Tesla, Meucci and countless others...

You can use them ALL your fake accounts to give me as many stars as you can! Thank you oh thank you oh...!! not...

In the meantime, in real life: "How Wineland & Haroche Stole My Discovery (and got 2012 PHYSICS NOBEL PRIZE for it...)":
http://sites.goog...ci#Nobel


I want you to call Dr. Shmuel Mandelbaum and make an appointment to see him. He is a really excellent psychiatrist in Chicago.
ValeriaT
2.1 / 5 (11) Jan 25, 2013
: Are you related to a guy named Zephyr?
Negative. I don't think, the aether model will be stolen soon with some nobelist.
Q-Star
2.8 / 5 (11) Jan 25, 2013
: Are you related to a guy named Zephyr?
Negative. I don't think, the aether model will be stolen soon with some nobelist.


Well, I do quite agree with ya. I don't the aether model will be stolen by anyone, Nobel winner or otherwise.
wavettore
1.5 / 5 (8) Jan 26, 2013
3 to close the circle.
In spite of two findings from Einstein and Planck, one to show the equation between atoms and energy and the other to discover the constant between waves' frequencies and energy, traditional science has never closed the circle.
Still today, this science does not recognize the transformation from waves to atoms.
The repercussions of Wavevolution are huge and overturn many common beliefs but, the bureaucracy of traditional science prevents the recognition of any event unless certain criteria are first met.
The problem exists precisely in the compilation of these "laws" or criteria introduced by a few scientists on behalf of all science and from the limits imposed that makes it impossible to recognize the concept of simultaneity.
One new Progressive Science includes the transformation from waves to atoms and has new understanding of the relation between Space and Time.
Q-Star
3 / 5 (12) Jan 26, 2013
In spite of two findings from Einstein and Planck, one to show the equation between atoms and energy and the other to discover the constant between waves' frequencies and energy, traditional science has never closed the circle.


Wavevolution eh? At least you're not calling yourself a prince,, yet.

The repercussions of Wavevolution are huge and overturn many common beliefs but, the bureaucracy of traditional science prevents the recognition of any event unless certain criteria are first met.


Oh, I see, another unsung hero of science who is being conspired against by "the bureaucracy". I bet you also have a ToE that answers ALL the unanswered questions, only it is being repressed, censored, stolen or some such Mephistophelean evil.

Oh goody, another earth shattering truth. We can add it to the collection,,, Aether Waves, Plasma Cosmology, Unified Vortex Theory, Unified Absolute Relativity Theory, and my vary favorite, Leprechauns-Riding-Pink-Unicorns-Theory.
ValeriaT
1.5 / 5 (8) Jan 26, 2013
The repercussions of wavevolution are huge and overturn many common beliefs but, the bureaucracy of traditional science prevents the recognition
It has nothing to do with the above subject anyway. This problem is quantitative, it consists of nearly two dozens of QED corrections already and only quantitative theory could help with it.
packrat
2.4 / 5 (8) Jan 27, 2013
Total layman and probably dumb question here. If a small planet can make a star wobble enough that we can find the planet light years away couldn't the much closer and heavier muon compared to an electron make the proton wobble more than normal and make it look like it's bigger to the instruments measuring it? It just seems like an apt comparison to me.
antialias_physorg
3.5 / 5 (8) Jan 27, 2013
Mass (gravity) isn't the dominant force at these scales. Electromagnetism is.
ValeriaT
1.5 / 5 (8) Jan 27, 2013
couldn't the much closer and heavier muon compared to an electron make the proton wobble more than normal and make it look like it's bigger to the instruments measuring it
It would make it more lightweight/smaller instead and it was discussed above already (ValeriaT Jan 24, 2013).
swordsman
2.1 / 5 (7) Jan 28, 2013
I agree with antialias. You are presumably measuring the size of a dynamic electromagnetic field, in which case a logical choice would be the size of a half wavelength. It is no difference from the time of Planck, who analyzed atoms as electronic circuits. Rediscovery is a wonderful thing.

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