In quantum chromodynamics, the modern theory of the nuclear force, most of the mass of the proton and the neutron is explained by special relativity. The mass of the proton is about eighty times greater than the sum of the rest masses of the quarksthat make it up, while the gluons have zero rest mass. The extra energy of thequarks and gluons in a region within a proton, as compared to the energy of the quarks and gluons in the QCD vacuum, accounts for over 98% of the mass.
The internal dynamics of the proton are complicated, because they are determined by the quarks exchanging gluons, and interacting with various vacuum condensates.Lattice QCD provides a way of calculating the mass of the proton directly from the theory to any accuracy, in principle. The most recent calculations[7][8] claim that the mass is determined to better than 4% accuracy, arguably accurate to 1% (see Figure S5 in Dürr et al.[8]). These claims are still controversial, because the calculations cannot yet be done with quarks as light as they are in the real world. This means that the predictions are found by a process of extrapolation, which can introduce systematic errors.[9] It is hard to tell whether these errors are controlled properly, because the quantities that are compared to experiment are the masses of the hadrons, which are known in advance.
These recent calculations are performed by massive supercomputers, and, as noted by Boffi and Pasquini: “a detailed description of the nucleon structure is still missing because ... long-distance behavior requires a nonperturbative and/or numerical treatment..."[10] More conceptual approaches to the structure of the proton are: thetopological soliton approach originally due to Tony Skyrme and the more accurateAdS/QCD approach which extends it to include a string theory of gluons, various QCD inspired models like the bag model and the constituent quark model, which were popular in the 1980s, and the SVZ sum rules which allow for rough approximate mass calculations. These methods don't have the same accuracy as the more brute force lattice QCD methods, at least not yet.
8 comments:
http://marcofrasca.wordpress.com/2011/06/13/back-from-paris
“We do not understand proton” - plots show systematic deviation from standard Montecarlo simulations of events signaling that their low-energy models are failing. This is rather understandable in view of the fact that low-energy QCD is currently forefront research with a lot to be clarified also for experimental data...
They showed a systematic deviation of their Montecarlo simulations from experimental data. This means for us, working in this area, that their modeling of low-energy QCD is bad and their possible estimation of the background unsure. There is no way currently to get an exact evaluation of the proton scattering section. I am somewhat surprised by this as so far, as I have always pointed out in this blog, at least the structure of the gluon propagator at low energies should be known exactly from the lattice. So, modeling the proton in such Montecarlo models should be a mitigated issue. This does not seem to be so and these different communities do not seem to talk each other at all.
Is that why D0 said in arxiv 11096.1457 that for multijet background calculations (particularly including the electron channel in the studies of the CDF Wjj bump) a “data-driven method” was used because “estimation of this background from Monte Carlo simulations is not reliable” ?
Fermilab, sea quarks: It is often said that a proton is made of three quarks: two of the same type, called up quarks, and one of a different type called a down quark. But that's not the whole story. In the space between these three stable quarks there is a boiling soup of quark–antiquark pairs. That is, a quark and an antimatter quark spontaneously come into existence, drift a while, and then recombine, destroying one another. This happens all the time...
Counting W+ and W¯ bosons yields new insight into the dynamic structure of protons, which is too complicated to compute from first principles with current techniques. It also informs predictions of new physics: The rate at which hypothetical particles would be produced depends on the density of quark-antiquark pairs, for the same reason that W bosons do.
http://tgd.wippiespace.com/public_html/paddark/shrinkingproton.pdf
If electrons are changed to muons.
http://www.galaxyzooforum.org/index.php?topic=277969.0 The nature of mass.
Hi Ulla. I'm just starting my study of this stuff. QCD always amazed me, how do we know this stuff? Anyway, Frank Wilczek has a very pleasant writing style, and goes into this stuff in some detail in "The Lightness of Being." Have you read it? Cool stuff.
In any event, a knowledge of Fourier Analysis, specifically Fourier Transforms, seems to be in order in order to understand. I studied that stuff in Calc V in college, sometime around when Edison was inventing the light bulb, so I have to review, and so I can't comment until such time as said review is complete. Thanks for posting.
QCD is badly understood. There are some perturberations with the weaker forces, but how? So CKM isn't in order either?
I cannot claim I understand these, long from that. That's why I use citations. But that something isn't what is told us I can see.
Have you read "The Lightness of Being" by Frank Wilczek ?
No.
Hi Steven,
I always hated Gell-Mann’s quarks and more preferred George Zweig’s aces. That is the way I have it figured if QM has god to play dice he might also enjoy poker. Now picture god imagining a proton as a texas hold’em hand with after the flop receiving one down ace and two up you have one hell of a hand;-)
Best,
Phil
Can you solve problems in QCD, Phil? Like the Lagrangian? It's only tensors and spinors, with one free parameter, the coupling constant g.
I can't, but I want to. Anyone know the fastest way to learn? Wikipedia has all the passion and emotion and thus interest to me as a Mr. Spock. i'd rather watch paint dry or grass grow, frankly. There has to be a better way.
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