Quarks and gluons in hadrons and nuclei Page: 18 of 31
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The valence picture above implicitly assumed that uv(x) = 2dv(x) for all x.
However, unpolarized data show this to be untrue in that it would require that
In practice, this ratio drops as x + 1, suggesting that the u(x + 1) d(x + 1),
a phenomenon which follows from spin dependence via single gluon exchange.
Chromomagnetic hyperfine energy shifts split the a-N masses and elevate u(x + 1)
over d(x + 1). They also cause u (x + 1) to dominate over u+(x + 1), which the
consequence that Ap'n(x + 1) + 1. Thus, a qualitative expectation for Ap emerged:
Ap(x + 0) + 0; Ap(x = 1/3) = 1/3 A Ap(x + 1) + 1.
These predictions turned out to be remarkably well verified and even agree with
the latest EMC data.
Recently Close and Thomas28 showed that, within the framework of the MIT bag
model, one could relate the x-dependent distortion of the valence distributions to
the measured chromomagnetic energy shift in the a-N masses. All of this suggests
that the valence quark polarizations measured in polarized deep inelastic scat-
tering are similar to the polarizations of the constituent quarks manifested in
low-energy spectroscopy. This is an important constraint on model builders. The
memory of the constituent quark spins is not lost as one proceeds to the deep in-
elastic: the valence quarks are highly polarized.
If, as is being claimed, the quarks and antiquarks contribute (within errors)
nothing to the net spin polarization of the proton, then we must conclude that
something is canceling the contribution of the valence quarks. Candidates include
orbital angular momentum polarized gluons or a negatively polarized sea.
We already noted that in the constituent limit it is over naive to ignore
orbital angular momentum. The presence of polarized gluons may be probed by
studying the polarization dependence of direct photon production or spin depen-
dence of heavy flavor production; a polarized sea may affect the inclusive produc-
tion of hadrons33,34 and fast K-(su) production may be a tag for scattering from
Dziembowski et al.35 have studied the relation between constituent quarks and
partons. They view the constituent quarks as being a conglomerate of partons-
quarks, antiquarks, and gluons, thus
qi ,(xQ2) = G f x v/N gi/vv . Q2),
where the X are helicity labels. The constituent quark distributions Gv/N(y)
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Close, F. E. Quarks and gluons in hadrons and nuclei, article, December 1, 1989; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1058782/m1/18/: accessed March 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.