Proton mass finally calculated

[Martin]

Not just the proton mass, but several hadrons---proton, neutron...etc.

Took a year of supercomputer time. I'll get the link

http://www.newscientist.com/article/dn16095-its-confirmed-matter-is-merely-vacuum-fluctuations.html

 

"It's confirmed, matter is merely vacuum fluctuations."

 

For simplicity, I'll focus on the case of the proton. The point is that the three quarks that make up a proton have themselves very little mass. Most of the proton's mass is somehow the result of the interaction of those three component quarks.

 

We could always experimentally measure the proton mass, but what these people did is calculate what it theoretically ought to be (by modeling the interactions) and thank heaven they got a theoretical answer which agreed with empirical measurement.

http://www.durr.itp.unibe.ch/

 

http://www.sciencemag.org/cgi/content/abstract/sci;322/5905/1224

Ab Initio Determination of Light Hadron Masses

S. Dürr,1 Z. Fodor,1,2,3 J. Frison,4 C. Hoelbling,2,3,4 R. Hoffmann,2 S. D. Katz,2,3 S. Krieg,2 T. Kurth,2 L. Lellouch,4 T. Lippert,2,5 K. K. Szabo,2 G. Vulvert4

 

"More than 99% of the mass of the visible universe is made up of protons and neutrons. Both particles are much heavier than their quark and gluon constituents, and the Standard Model of particle physics should explain this difference. We present a full ab initio calculation of the masses of protons, neutrons, and other light hadrons, using lattice quantum chromodynamics. Pion masses down to 190 mega–electron volts are used to extrapolate to the physical point, with lattice sizes of approximately four times the inverse pion mass. Three lattice spacings are used for a continuum extrapolation. Our results completely agree with experimental observations and represent a quantitative confirmation of this aspect of the Standard Model with fully controlled uncertainties."

 

1 John von Neumann–Institut für Computing, Deutsches Elektronen-Synchrotron Zeuthen, D-15738 Zeuthen and Forschungszentrum Jülich, D-52425 Jülich, Germany.

2 Bergische Universität Wuppertal, Gaussstrasse 20, D-42119 Wuppertal, Germany.

3 Institute for Theoretical Physics, Eötvös University, H-1117 Budapest, Hungary.

4 Centre de Physique Théorique (UMR 6207 du CNRS et des Universités d'Aix-Marseille I, d'Aix-Marseille II et du Sud Toulon-Var, affiliée à la FRUMAM), Case 907, Campus de Luminy, F-13288, Marseille Cedex 9, France.

5 Jülich Supercomputing Centre, FZ Jülich, D-52425 Jülich, Germany.

 

http://www.sciencemag.org/cgi/content/summary/sci;322/5905/1198

The Weight of the World Is Quantum Chromodynamics

Andreas S. Kronfeld

 

"Ab initio calculations of the proton and neutron masses have now been achieved, a milestone in a 30-year effort of theoretical and computational physics."

Theoretical Physics Group, Fermi National Accelerator Laboratory, Batavia, IL 60510, USA.

 

In a sense, this corroborates the vision of matter presented in Frank Wilczek's book The Lightness of Being.

 

The general idea of matter consisting of vacuum fluctuations, or condensing out of empty space, reminded me of a passage that starts around page 91.

 

Technically (it's not the quite the same as determining hadron masses!) it's about the formation of a chiral symmetry-breaking condensate. Technically sigma mesons which he calls Q Qbar pairs. The passage begins with a kind of thought experiment, or at least an interesting "What-If" gambit.

 

What if you could completely clean out a patch of space? Then he presents the idea that the empty space would have an explosive potential to actually liberate energy by bringing Q-Qbar pairs into existence. Because the quark masses are small and so the energy cost of realizing them can be LESS than the binding energy released.

 

He illustrates this on page 93 by referring to experiments at the Brookhaven RHIC (relativistic heavy ion collider). He says that a collision of two gold nuclei creates a fireball with enough energy to clear out a small region, hot enough to evaporate the condensate---and then we get to witness the aftermath, as the region cools back down and the Q-Qbar condensate forms in it again.

Edited November 21, 2008 by Martin

[ajb]

This a lattice gauge theory calculation?

[Martin]

Yes!

 

These lattice calculations take enormous amounts of computer time. Wilczek describes one of the supercomputer installations working on calculating proton mass, at Brookhaven where he visited. It's kind of amusing. He walks into one of the rooms with banks of computers all trying to figure out the mass of a single proton.

 

Essentially his comment is: "here we have 10^30 protons and neutrons hard at work, and they will work for months (10^7 seconds) in order to figure out what a single proton knows inside of 10^-24 second."

 

Namely how much it weighs.

I've paraphrased the sense, this is not an exact quote.

Edited November 21, 2008 by Martin

[Gilded]

I was kind of confused when I was reading some more simplified pop science reports of this last night (with main focus on "E=mc^2 finally corroborated!"), but now it makes a bit more sense. Nifty stuff!

[Mr Skeptic]

Cool stuff! Did they actually calculate the proton mass to 9 significant figures and have it match up with reality? I couldn't read the article as I don't have a subscription.

[Royston]

How odd, not only did I read the New Scientist article earlier, when looking for a different article concerning a recent thread, but I was actually reading about the fluctuations in a vacuum as per 'The Lightness of Being' this morning, on the way to work (Gluons and the Grid)...I stopped on the sub-chapter 'Grid Weighs' (incredibly good book BTW)

 

The more I read about QCD, I've only covered the absolute basics so far, the more fascinating it becomes. I was ignorant in thinking that QCD solely dealt with the strong nuclear force, but it seems far more reaching than that.

[Riogho]

Ha! You beat yourself to this post on the other forum

[Severian]

It is so annoying when papers are published in Science rather than a more respectable journal, because I can never read them! Does anyone know what lattice action they used? The staggered action?

[Daecon]

For simplicity, I'll focus on the case of the proton. The point is that the three quarks that make up a proton have themselves very little mass. Most of the proton's mass is SOMEHOW the result of the interaction of those three component quarks.

I don't like that word.

[BenTheMan]

It is so annoying when papers are published in Science rather than a more respectable journal, because I can never read them! Does anyone know what lattice action they used? The staggered action?

 

Yeah I don't know why they published in AAAS. I read the paper, and I didn't see anything about a proton mass, and I can't recall all of the lattice terminology---I do recall that they're using the quenched approximation, which seems a bit odd. In this approximation, you turn off the quark-anti-quark virtual pairs (I seem to recall), but the New Scientist article makes a big deal about the fact that this is what they were calculating.

[Severian]

I do recall that they're using the quenched approximation, which seems a bit odd.

 

Are you sure? Hardly any serious calculations are done with the quenched approximation. The linked article mentions the calculation of the [math]B_c[/math] mass, and makes it sound like this is the next step (since it is for lighter quarks). That calculation (whose author sits in the office just across the hall from me) was not quenched and used a staggered action.

 

I find it hard to believe they would get 2% accuracy with a quenched approximation.

[Mr Skeptic]

Cool stuff! Did they actually calculate the proton mass to 9 significant figures and have it match up with reality? I couldn't read the article as I don't have a subscription.

 

"figure within 2% of the value measured by experiments."

 

Or, maybe they just got vaguely in the neighborhood and called it a success.

[BenTheMan]

Are you sure?

 

No---I skimmed through the article the other day, and I seem to recall that they were working in that approximation, but I could be wrong. Like I said, it is confusing because they make some claim in the New Scientist article about vacuum fluctuations, which is what the quenched calculation does away with.

[Severian]

The quenched approximation neglects the creation of quark-antiquark pairs, so you could still have gluonic vacuum fluctuations (since the gluon is colored).

[BenTheMan]

Absolutely. The New Scientist article says something like "Until recently, lattice QCD calculations concentrated on the virtual gluons, and ignored another important component of the vacuum: pairs of virtual quarks and antiquarks."...not that I trust whatever journalist who wrote this knows what they're talking about.

 

But this doesn't seem to mesh with the "quenched" approximation.

 

I probably looked too quickly at the article

[Martin]

Are you sure? Hardly any serious calculations are done with the quenched approximation...

I find it hard to believe they would get 2% accuracy with a quenched approximation.

The quenched approximation neglects the creation of quark-antiquark pairs, so you could still have gluonic vacuum fluctuations (since the gluon is colored).

 

I looked at the article. They make explicit up front that they aren't using the quenched approximation, and that they do include the creation of quark-antiquark pairs. This is pointed out somewhere in the first couple of paragraphs, as I recall. The quenched approximation is mentioned only as a method which earlier calculations used over the course of some 20 years.

 

I don't imagine anyone who read serious coverage was really in doubt about this but wanted to settle it just in case some confusion remained.

Edited November 24, 2008 by Martin

[BenTheMan]

Yeah sorry---

 

They say "calculations have been preformed using the quenched calculation..." which I took to mean that THEY used that approximation.

 

Sverian, PM me and I'll send you a copy of the article.

[Yuri Danoyan]

http://www.nature.com/nature/journal/v456/n7221/pdf/456449a.pdf

Particle physics: Mass by numbers p449

A highly precise calculation of the masses of strongly interacting particles, based on fundamental theory, is testament to the age-old verity that physical reality embodies simple mathematical laws.

Frank Wilczek

 

a milestone paper, Dürr et al.1 report a first-principles calculation of the masses of strongly interacting particles (hadrons, such as the proton), starting from the basic equations for their constituent particles (quarks and gluons), and including carefully documented estimates of all sources of error. Their results, published in Science, highlight a remarkable correspondence between the ideal mathematics of symmetry and the observed reality of the physical world.

Edited November 26, 2008 by Yuri Danoyan
multiple post merged