Evidence for 3 quarks in protons

[hkyriazi]

I'm curious about the nature of the evidence for three quarks -- two up and one down -- in the proton. I assume the data is derived from high energy scattering experiments, and looking at the angles and momenta of the scattered particles. But I don't see how, from such experiments, one can say there are two ups and one down, rather than their simply existing in a ratio of 2:1. In other words, might there be six quarks rather than three (four up, two down)? Is the determination of three based more on theoretical arguments, rather than experimental data?

 

[Sideways]

Yours is a non-trivial question, indeed.

Early scattering experiments at SLAC and CERN did reveal that there was a substructure of protons; these constituents were called "partons". Many physicists over the decades that followed asked the exact same question you did, whether or not these "partons" were indeed the same thing as the "quarks" of Gell-Mann's Eightfold Way.

This question has been largely resolved due to ever-more accurate scattering experiments, but the answer is not that simple. It turns out that, depending on the scattering energy, you "see" not only the three quarks, but the products of gluons and quark-antiquark pairs exchanged between the quarks.

That's all I have time to say for now, but a basic answer is "yes", experiment corroborates the basic 3-quark model (though things are a bit more tricky...)

 

[Meir Achuz]

The original argument for three quarks in baryons was the fact that this produced the stable spin 1/2 baryon octet and the lowest spin 3/2 baryon decuplet resonances. More quarks would lead to many more unobserved states.

 

[humanino]

You may want to check [thread=276180]this discussion[/thread] for instance, or Jerome I. Friedman - Nobel Lecture

 

[hkyriazi]

Thanks, Sideways, Clem, and Humanino. Sideways' response was most suited to my level of (in)expertise, but I got something out of them all. (Clem's comment that six quarks would lead to more states than observed was enlightening.) I'll check out the "parton" ideas, and Gell-Mann's Eightfold Way. (The Nobel Lecture was over my head.)

 

[humanino]

It should be emphasized that Clem correctly said "the original idea" for SU(3) color. One can study the possible representations of the groups that are compatible with the observed symmetry patterns in the spectrum, and come up with SU(3) flavor for u, d and s. Then, from here, one has to postulate a SU(3) color group for the observed states like $\Delta^{++}$. But this still calls confirmation.

The Nobel Lecture was over my head.

The physical argument is given in page 734

[...]the $F^{3}$ structure function, which uniquely occurs in the general expression for the inelastic neutrino and antineutrino nucleon cross sections as a consequence of parity non-conservation in the weak interaction

allows us to distinguish between quarks and antiquarks. By the same token, the Adler sum rule gives you the difference between the net number of u and d quarks.

See also Quarks, partons and Quantum Chromodynamics par Jiˇr´ı Ch´yla

 

[hkyriazi]

Thanks, Humanino. The last link, to the 210-page book, is quite good (taking a novice from the ground up). And, actually, I've been scanning the Friedman lecture, and it gives me a feel, at least, for what's involved (and it's pretty involved!). But I have to tell you, I know next to nothing about all these groups and symmetries. My level of understanding is to imagine things banging together, rebounding, breaking up and perhaps interconverting to various extents. I'll be happy if I can grasp what is meant by the various proton and higher mass resonances (first paragraph of Friedman's essay).