Quarks and gluons in hadrons and nuclei Page: 22 of 31
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gA F,D Aq(P) Data
np F + 0 Au - ad 1.26 0.005
Ap F +3 D 3(2u-Ad-As) 0.72 0.02
A F - D 3(eu+Ad-2es) 0.25 0.05
En F - D Ad - As -0.33 0.02
Thus F/D = (Au-As)/(Au+As-2.d). Extracting the individual-contributions involves
a correlated fit. The EMC values, corrected for F/D, become &u = 0.80 0.06, &d
= -0.45 0.06, and as = -0.15 0.06. One possibility is that es = 0, so that s
= -a/2n AG. In this case, we obtain for
Au u - as=0.95 0.06 (53)
Ad =_ d - s = -0.30 0.06 (54)
It is interesting to note that these values are consistent with those extracted
from the magnetic moments (Eq. (4)) viz
Auv = 1, Adv = -0.25
The proton helicity is given by
= Z Aq+ (AG+ L) (55)
Hence AG = 3.5 and LZ = -3.35 at Q2 = 10 GeV2. As one devalues to lower Q2,
d/dQ2(AG+LZ) = 0, and the individual contributions fall. It is an open question
whether the "passive" Lz in the constituent model (i.e., the dilution of SZ due to
relativistic spinors) at low Q2 provides a consistent picture between constituent
spin polarization and "parton" polarization.
Ellis et a1.31 suggest that the modification to eq(x) be driven by evolution
fi(x) = Aq(x) - jf yy AG(y)a(x/y)
where a(Z) is the cross section for Y*g + qq. If so, then g(x+0) < 0, the cross-
over from positive to negative moving to smaller x values as Q2 increases.
It is tantalizing that such a picture may already be manifested at low Q2 in
the resonance region. It is well known that the prominent D13(1520) and F15(1690)
resonances are excited dominantly in 03/2 when Q2 = 0, but in 01/2 for Q2 * 0.
The change in helicity structure,38 or change in sign of gi(Q2), occurs at Q2 =
0.4 GeV2 for D13 and Q2 = 0.7 GeV2 for F15. It is amusing that these correspond
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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/m1/22/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.