9 ms·

Hydrogen made with muons reveals proton size conundrum

123 points by iProject 14y ago

richardjordan 14y ago
This is tremendously interesting. In particular their suggestion that one solution to the conundrum is the existence of additional very light force carriers. Perhaps a more up to date particle physicist than I can chip in bu such particles must presumably be outside of the Standard Model (as iirc from my particle physics, which is admittedly getting on for 20 years ago, there ain't any more slots left). I know there are some interesting new theories which are very widely disputed and which have a lot of other problems, such as the Exceptionally Simple Theory, which predict a whole host of additional particles - but there are reasons such things are on the fringe right now. With all the progress in the last couple of decades, and all these new results pointing to new Physics in all kinds of areas, it's a fascinating time for the field - and makes one wonder where we'd be if the entire faculty hadn't dedicated decades down the String theory rabbit hole.
- pdonis 14y ago
  such particles must presumably be outside of the Standard Model Yes, they are. With the detection of the Higgs we have now detected experimentally all of the Standard Model particles. Any new "very light force carriers" would definitely be outside of the Standard Model.
  - aidenn0 14y ago
    Are gravitons predicted by the Standard Model?
    - pdonis 14y ago
      Gravitons are not included in the Standard Model, which doesn't cover gravity at all, and is not intended to. As far as whether gravitons could be the "very light force carriers", I don't see how that would be possible; gravity is way too weak a force to make that much of a difference in measurements on hydrogen atoms, even muonic ones.
- incision 14y ago
  >...decades down the String theory rabbit hole. Where does string theory stand these days? As a fascinated layman, strings always reminded me quite a bit of epicycles and I've had an impression that string theory had been falling out of favor for some time now.
  - michaels0620 14y ago
    From what I understand (as a fellow fascinated layman) is that String Theory is still the dominant StandardModel++ theory in academia but there are a few who are being more vocal about its validity. Peter Woit is a notable skeptic of the viability of String Theory and has a blog which contains links to a number of interesting physics blogs. http://www.math.columbia.edu/~woit/wordpress/ http://www.math.columbia.edu/~woit/wordpress/
    - InclinedPlane 14y ago
      The dominant StandardModel++ theory is the standard model + super symmetry. String Theory is still in its formative years and best described as a collection of ideas than as a proper theory.
  - Steuard 14y ago
    The "popular fad" aspect of string theory has certainly lost much of its luster, no question, particularly as far as the general public is concerned. (The early excitement about the theory in the 1980's was apparently rather over the top.) The fact that folks like Peter Woit and Lee Smolin were able to publish popular books about their distaste for string theory reflects that (and those books in turn did a lot to establish the "string theory is overrated" counter-meme). But within actual physics departments, as far as I've been able to tell string theory holds much the same position that is has for many years: it's the direction that most people interested in pursuing a "theory of everything" seem to find most promising. (Plenty of physicists have other interests, of course, even within particle physics: the phenomenology involved in interpreting LHC data and predicting new phenomena to look for at that scale is a big deal these days, and string theory really isn't relevant to those questions.) As far as I can tell from the inside, that's primarily a genuine process of ongoing scientific judgement rather than pure groupthink. As a string theorist, I can see to some degree why you'd say it reminds you of epicycles: I think most of us would agree that there's got to be some deeper underlying truth that would make the structures of string theory seem more unified and elegant. The difference between that and epicycles, at least to a string theorist, is that we're optimistic that string theory as we know it is a valid, well-defined approximation to the underlying theory, whatever form that may take. Epicycles, on the other hand, were (as I understand it) sort of a hack, without any intrinsic connection to the true heliocentric model.
drucken 14y ago
The article makes this seem like a relatively straightfoward experiment concept and implementation plus high justification. Why have measurements of this sort not been made already? Is it because of the difficulty of forming muonic hydrogen or making measurements before the decay?
- ChuckMcM 14y ago
  The experimental apparatus is pretty specialized.
- nagrom 14y ago
  None of it is easy. I haven't read this particular paper, but I have worked on similar experiments. First, you have to create muons and then isolate them in the particle beam and this costs quite some money (typically fire accelerated protons at a target and allow the pions and other decay products to be dumped in some dumb absorber that the muons pass through). Then you have to capture the muons into orbits around protons that come from hydrogen that needs to be disassociated and ionised. Then you have to pulse the hydrogen with a laser and excite the muon into a new energy state and accurately measure the energy of the photons that come off in the decay. You almost certainly need subnanosecond accuracy in your timing at this point. Of course, you can never be absolutely certain what you're measuring in any particular event so you sum over millions of events and average. And you have to simulate the whole thing (typically using a C++ framework called GEANT running on massive processor farms analysed with a horror-show of an analysis toolkit called ROOT) to see what could go wrong and determine the possible sources of error and uncertainty in your measurement. To get the money to do this, you have to apply for government grants that may take up to a year to get approved. And no government wants to fund it so you need an 'in' with the lab, because they'll be the ones that sponsor your application for it to have a chance of success, and you'll need to know people from a whole load of other countries to collaborate with and prove your bona fides. And the people writing all these grant applications and doing most of the middle management are distracted by teaching literally hundreds of students every week during term time, so they can't even work full-time on it. There are other experimental ways of doing this (e.g. measure correlated form factors and parton distribution functions from elastic scattering and extrapolate to zero momentum transfer), but they're all harder. They'll probably be done within the next 30 years. Probably. This particular effort is likely the work of 30-50 PhDs working for five or more years (at 50% of the salary they'd earn working for commercial companies), discounting any of the commercially available technology like electronics developed by CAEN or detector components developed by the likes of ElJen or Hamamatsu. And I would be surprised if they didn't mint another 10-15 PhDs during the effort. And I haven't covered the highly trained technicians and electronic engineers that are usually needed to build a lot of the custom experimental equipment.
ChuckMcM 14y ago
I find the physics fascinating, but that muonic hydrogen stuff, that has marketing gold all over it. SmartWater, now made with Muonic Hydrogen, get it before it decays! I expect its going to take several days to digest the paper though, it is quite dense.
- SideburnsOfDoom 14y ago
  > get it before it decays A muon has a mean lifetime of 2.2 microseconds (2.2 * 10^−6 seconds). Hurry, if you blink you'll miss it two hundred thousand times over.
  - sliverstorm 14y ago
    You don't have to tell your customers that. Or maybe you can combine it with homeopathy while you're at it. The regular water will remember the properties of the muon water.
  - deleted 14y ago
    [deleted]
- CoffeeDregs 14y ago
  >SmartWater, now made with Muonic Hydrogen, get it before it decays! Wait. Isn't hydrogen a chemical? I'm pretty sure I've heard that chemicals are bad for me... ;)
  - andrewflnr 14y ago
    Make sure your water isn't contaminated with dihydrogen monoxide.
    - davrosthedalek 14y ago
      A dangerous toxin found in almost all tumor tissue. Did you know that DHMO is found in virtual every school restroom? Act now -> http://www.dhmo.org/ http://www.dhmo.org/
shardling 14y ago
This has been a pretty big deal for a while now -- its a huge, unexplained effect that ever theoretical physicist in a remotely related field will have poked at. (And the expected result is actually pretty easy to calculate, as far as anything in QED is.) A lot of folk were betting on some sort of experimental error (just as ended up happening with the superluminal neutrinos), so if they've managed to replicate the result with more in depth experiments, that's kind of important.
habosa 14y ago
I'm no physics expert but I did find the article very interesting. Can someone who knows more than I do possibly explain, simply, what may be going on here and what its implications would be?
- davrosthedalek 14y ago
  In my PhD, I measured the proton form factor and, from that, calculated the radius. So let me explain my view :) There are essentially three ways to measure the radius: 1) Electron-proton scattering. This gives you the form factors (related to the charge distribution by the Fourier transform), and from that you can calculate the proton radius. 2) Measurements of the electron energy levels of "normal" Hydrogen 3) Measurements of the muon-proton energy levels. Re 1) The Mainz proton form factor experiment is to date the most precise proton radius determination from electron scattering and is compatible with the larger value. Our results are along the line of earlier measurements using similar techniques. The first measurements of this kind where done in the 50's and 60's (but produced quite a wrong radius). Re 2) These measurements are very hard, the proton size effect is very small. Nevertheless, the results are of similar precision as those from 1) and give a compatible radius Re 3) In muonic hydrogen, the muon is "much closer" to the proton and therefore the proton size has a much larger influence. Because of this, the method produces by far the best precision. The 7 standard deviations are calculated with the precision of 1) and 2), the error of 3) is negligble! So, what can go wrong? In my opinion, the muon experiment is very clean, so I don't believe in an experimental error. The fact that 1) and 2) agree make it unlikely that they are wrong, as they are complete separate methods. However, it is possible. It could be that we are missing an important part in our understanding of the radiative corrections, i.e. the theory needed to calculate the levels. This could mean a simple error in one of the calculations, but most if not all of them have been checked by different groups. It could also mean a flaw in one of the solving techniques. Or maybe something which was overlooked. It is also possible that there is another particle at work here. A possible candidate is a dark photon. This solution has some benefits, especially since such a new particle might also solve the muon g-2 puzzle. But it is not easy to construct a theory of such a particle without violating other experiments. A lot of fine tuning. It could also be that the muon just behaves differently from the electron. That would shatter a rather basic and widely accepted believe. I attended a very interesting workshop recently which focused on this puzzle. Unfortunately, we didn't find a solution. However, there are a lot of experiments in the pipeline which might clarify the situation: - There are several experiments to measure the proton radius using eletron scattering, with specialized instruments and new methods. -Muse is an experiment which will scatter a combined electron and muon beam from protons. This will test certain aspects of the radiative corrections and allow a direct comparison of the exctracted radii. -There will be measurements of other muonic atoms. All in all, this is a very interesting topic right now, with the added benefit that it brings together the often separate communities of nuclear and atomic physics. Nice tidbit: Our result and the first muonic result was presented at the same conference... and both speakers didn't know what the other would say. Other nice tidbit: The first try at the muonic measurement didn't work out. When they got it working, they scanned the energy region around the suspected radius value. But they didn't find a resonance. After improving the experiment and double checking everything, they still didn't find anything, until they started looking further away, when suddenly, the resonance appeared. They just didn't scan wide enough the first time!
goodcanadian 14y ago
Obligatory: http://xkcd.com/955/ http://xkcd.com/955/ Humour aside, I love these sorts of things. Even if there is no new physics involved, if it turns out to be some sort of error in the experiment, we are bound to learn something.
- gus_massa 14y ago
  I read the preprint in the arxive, but I'm not an expert in this area. This doesn't look like the usual mixture of crackpotology and misinterpretation. It's better skip this article and to keep your $200 for the next one.
deleted 14y ago
[deleted]
TerraHertz 14y ago
Annnnd.... once again when we want to read the actual paper (paid for with public funds) we hit a damned paywall. http://www.sciencemag.org/content/339/6118/417.full http://www.sciencemag.org/content/339/6118/417.full Can anyone suggest a free source?
- gus_massa 14y ago
  Once again, every preprint (in some areas) is in the arxiv: http://arxiv.org/pdf/1208.2637.pdf http://arxiv.org/pdf/1208.2637.pdf
  - TerraHertz 14y ago
    I wasn't sure that was the same paper, due to the different title and somewhat different list of authors. Even though the subject matter is the same.
    - gus_massa 14y ago
      My bad. Same team, different (recent) paper. Edit: There is a discussion about this in reddit: http://www.reddit.com/r/Physics/comments/177wfm/hydrogen_made_with_muons_reveals_proton_size/ http://www.reddit.com/r/Physics/comments/177wfm/hydrogen_mad...
TerraHertz 14y ago
So how is the mystery of observed isotopic decay rate variations (with Earth/Moon/Sun spin/orbit period correlations) going these days? That's now two solid results that indicate things we thought were constants, are not. What's the bet they are related?

123 points by iProject 14y ago

This is tremendously interesting. In particular their suggestion that one solution to the conundrum is the existence of additional very light force carriers. Perhaps a more up to date particle physicist than I can chip in bu such particles must presumably be outside of the Standard Model (as iirc from my particle physics, which is admittedly getting on for 20 years ago, there ain't any more slots left). I know there are some interesting new theories which are very widely disputed and which have a lot of other problems, such as the Exceptionally Simple Theory, which predict a whole host of additional particles - but there are reasons such things are on the fringe right now. With all the progress in the last couple of decades, and all these new results pointing to new Physics in all kinds of areas, it's a fascinating time for the field - and makes one wonder where we'd be if the entire faculty hadn't dedicated decades down the String theory rabbit hole.

pdonis 14y ago
such particles must presumably be outside of the Standard Model Yes, they are. With the detection of the Higgs we have now detected experimentally all of the Standard Model particles. Any new "very light force carriers" would definitely be outside of the Standard Model.
- aidenn0 14y ago
  Are gravitons predicted by the Standard Model?
  - pdonis 14y ago
    Gravitons are not included in the Standard Model, which doesn't cover gravity at all, and is not intended to. As far as whether gravitons could be the "very light force carriers", I don't see how that would be possible; gravity is way too weak a force to make that much of a difference in measurements on hydrogen atoms, even muonic ones.
incision 14y ago
>...decades down the String theory rabbit hole. Where does string theory stand these days? As a fascinated layman, strings always reminded me quite a bit of epicycles and I've had an impression that string theory had been falling out of favor for some time now.
- michaels0620 14y ago
  From what I understand (as a fellow fascinated layman) is that String Theory is still the dominant StandardModel++ theory in academia but there are a few who are being more vocal about its validity. Peter Woit is a notable skeptic of the viability of String Theory and has a blog which contains links to a number of interesting physics blogs. http://www.math.columbia.edu/~woit/wordpress/ http://www.math.columbia.edu/~woit/wordpress/
  - InclinedPlane 14y ago
    The dominant StandardModel++ theory is the standard model + super symmetry. String Theory is still in its formative years and best described as a collection of ideas than as a proper theory.
- Steuard 14y ago
  The "popular fad" aspect of string theory has certainly lost much of its luster, no question, particularly as far as the general public is concerned. (The early excitement about the theory in the 1980's was apparently rather over the top.) The fact that folks like Peter Woit and Lee Smolin were able to publish popular books about their distaste for string theory reflects that (and those books in turn did a lot to establish the "string theory is overrated" counter-meme). But within actual physics departments, as far as I've been able to tell string theory holds much the same position that is has for many years: it's the direction that most people interested in pursuing a "theory of everything" seem to find most promising. (Plenty of physicists have other interests, of course, even within particle physics: the phenomenology involved in interpreting LHC data and predicting new phenomena to look for at that scale is a big deal these days, and string theory really isn't relevant to those questions.) As far as I can tell from the inside, that's primarily a genuine process of ongoing scientific judgement rather than pure groupthink. As a string theorist, I can see to some degree why you'd say it reminds you of epicycles: I think most of us would agree that there's got to be some deeper underlying truth that would make the structures of string theory seem more unified and elegant. The difference between that and epicycles, at least to a string theorist, is that we're optimistic that string theory as we know it is a valid, well-defined approximation to the underlying theory, whatever form that may take. Epicycles, on the other hand, were (as I understand it) sort of a hack, without any intrinsic connection to the true heliocentric model.

drucken 14y ago

The article makes this seem like a relatively straightfoward experiment concept and implementation plus high justification. Why have measurements of this sort not been made already? Is it because of the difficulty of forming muonic hydrogen or making measurements before the decay?

ChuckMcM 14y ago
The experimental apparatus is pretty specialized.
nagrom 14y ago
None of it is easy. I haven't read this particular paper, but I have worked on similar experiments. First, you have to create muons and then isolate them in the particle beam and this costs quite some money (typically fire accelerated protons at a target and allow the pions and other decay products to be dumped in some dumb absorber that the muons pass through). Then you have to capture the muons into orbits around protons that come from hydrogen that needs to be disassociated and ionised. Then you have to pulse the hydrogen with a laser and excite the muon into a new energy state and accurately measure the energy of the photons that come off in the decay. You almost certainly need subnanosecond accuracy in your timing at this point. Of course, you can never be absolutely certain what you're measuring in any particular event so you sum over millions of events and average. And you have to simulate the whole thing (typically using a C++ framework called GEANT running on massive processor farms analysed with a horror-show of an analysis toolkit called ROOT) to see what could go wrong and determine the possible sources of error and uncertainty in your measurement. To get the money to do this, you have to apply for government grants that may take up to a year to get approved. And no government wants to fund it so you need an 'in' with the lab, because they'll be the ones that sponsor your application for it to have a chance of success, and you'll need to know people from a whole load of other countries to collaborate with and prove your bona fides. And the people writing all these grant applications and doing most of the middle management are distracted by teaching literally hundreds of students every week during term time, so they can't even work full-time on it. There are other experimental ways of doing this (e.g. measure correlated form factors and parton distribution functions from elastic scattering and extrapolate to zero momentum transfer), but they're all harder. They'll probably be done within the next 30 years. Probably. This particular effort is likely the work of 30-50 PhDs working for five or more years (at 50% of the salary they'd earn working for commercial companies), discounting any of the commercially available technology like electronics developed by CAEN or detector components developed by the likes of ElJen or Hamamatsu. And I would be surprised if they didn't mint another 10-15 PhDs during the effort. And I haven't covered the highly trained technicians and electronic engineers that are usually needed to build a lot of the custom experimental equipment.

ChuckMcM 14y ago

I find the physics fascinating, but that muonic hydrogen stuff, that has marketing gold all over it. SmartWater, now made with Muonic Hydrogen, get it before it decays! I expect its going to take several days to digest the paper though, it is quite dense.

SideburnsOfDoom 14y ago
> get it before it decays A muon has a mean lifetime of 2.2 microseconds (2.2 * 10^−6 seconds). Hurry, if you blink you'll miss it two hundred thousand times over.
- sliverstorm 14y ago
  You don't have to tell your customers that. Or maybe you can combine it with homeopathy while you're at it. The regular water will remember the properties of the muon water.
- deleted 14y ago
  [deleted]
CoffeeDregs 14y ago
>SmartWater, now made with Muonic Hydrogen, get it before it decays! Wait. Isn't hydrogen a chemical? I'm pretty sure I've heard that chemicals are bad for me... ;)
- andrewflnr 14y ago
  Make sure your water isn't contaminated with dihydrogen monoxide.
  - davrosthedalek 14y ago
    A dangerous toxin found in almost all tumor tissue. Did you know that DHMO is found in virtual every school restroom? Act now -> http://www.dhmo.org/ http://www.dhmo.org/

shardling 14y ago

This has been a pretty big deal for a while now -- its a huge, unexplained effect that ever theoretical physicist in a remotely related field will have poked at. (And the expected result is actually pretty easy to calculate, as far as anything in QED is.) A lot of folk were betting on some sort of experimental error (just as ended up happening with the superluminal neutrinos), so if they've managed to replicate the result with more in depth experiments, that's kind of important.

habosa 14y ago

I'm no physics expert but I did find the article very interesting. Can someone who knows more than I do possibly explain, simply, what may be going on here and what its implications would be?

davrosthedalek 14y ago
In my PhD, I measured the proton form factor and, from that, calculated the radius. So let me explain my view :) There are essentially three ways to measure the radius: 1) Electron-proton scattering. This gives you the form factors (related to the charge distribution by the Fourier transform), and from that you can calculate the proton radius. 2) Measurements of the electron energy levels of "normal" Hydrogen 3) Measurements of the muon-proton energy levels. Re 1) The Mainz proton form factor experiment is to date the most precise proton radius determination from electron scattering and is compatible with the larger value. Our results are along the line of earlier measurements using similar techniques. The first measurements of this kind where done in the 50's and 60's (but produced quite a wrong radius). Re 2) These measurements are very hard, the proton size effect is very small. Nevertheless, the results are of similar precision as those from 1) and give a compatible radius Re 3) In muonic hydrogen, the muon is "much closer" to the proton and therefore the proton size has a much larger influence. Because of this, the method produces by far the best precision. The 7 standard deviations are calculated with the precision of 1) and 2), the error of 3) is negligble! So, what can go wrong? In my opinion, the muon experiment is very clean, so I don't believe in an experimental error. The fact that 1) and 2) agree make it unlikely that they are wrong, as they are complete separate methods. However, it is possible. It could be that we are missing an important part in our understanding of the radiative corrections, i.e. the theory needed to calculate the levels. This could mean a simple error in one of the calculations, but most if not all of them have been checked by different groups. It could also mean a flaw in one of the solving techniques. Or maybe something which was overlooked. It is also possible that there is another particle at work here. A possible candidate is a dark photon. This solution has some benefits, especially since such a new particle might also solve the muon g-2 puzzle. But it is not easy to construct a theory of such a particle without violating other experiments. A lot of fine tuning. It could also be that the muon just behaves differently from the electron. That would shatter a rather basic and widely accepted believe. I attended a very interesting workshop recently which focused on this puzzle. Unfortunately, we didn't find a solution. However, there are a lot of experiments in the pipeline which might clarify the situation: - There are several experiments to measure the proton radius using eletron scattering, with specialized instruments and new methods. -Muse is an experiment which will scatter a combined electron and muon beam from protons. This will test certain aspects of the radiative corrections and allow a direct comparison of the exctracted radii. -There will be measurements of other muonic atoms. All in all, this is a very interesting topic right now, with the added benefit that it brings together the often separate communities of nuclear and atomic physics. Nice tidbit: Our result and the first muonic result was presented at the same conference... and both speakers didn't know what the other would say. Other nice tidbit: The first try at the muonic measurement didn't work out. When they got it working, they scanned the energy region around the suspected radius value. But they didn't find a resonance. After improving the experiment and double checking everything, they still didn't find anything, until they started looking further away, when suddenly, the resonance appeared. They just didn't scan wide enough the first time!

goodcanadian 14y ago

Obligatory: http://xkcd.com/955/ http://xkcd.com/955/ Humour aside, I love these sorts of things. Even if there is no new physics involved, if it turns out to be some sort of error in the experiment, we are bound to learn something.

gus_massa 14y ago
I read the preprint in the arxive, but I'm not an expert in this area. This doesn't look like the usual mixture of crackpotology and misinterpretation. It's better skip this article and to keep your $200 for the next one.

deleted 14y ago

[deleted]

TerraHertz 14y ago

Annnnd.... once again when we want to read the actual paper (paid for with public funds) we hit a damned paywall. http://www.sciencemag.org/content/339/6118/417.full http://www.sciencemag.org/content/339/6118/417.full Can anyone suggest a free source?

gus_massa 14y ago
Once again, every preprint (in some areas) is in the arxiv: http://arxiv.org/pdf/1208.2637.pdf http://arxiv.org/pdf/1208.2637.pdf
- TerraHertz 14y ago
  I wasn't sure that was the same paper, due to the different title and somewhat different list of authors. Even though the subject matter is the same.
  - gus_massa 14y ago
    My bad. Same team, different (recent) paper. Edit: There is a discussion about this in reddit: http://www.reddit.com/r/Physics/comments/177wfm/hydrogen_made_with_muons_reveals_proton_size/ http://www.reddit.com/r/Physics/comments/177wfm/hydrogen_mad...

TerraHertz 14y ago

So how is the mystery of observed isotopic decay rate variations (with Earth/Moon/Sun spin/orbit period correlations) going these days? That's now two solid results that indicate things we thought were constants, are not. What's the bet they are related?