Local quark-hadron duality in structure functions Page: 3 of 6
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3. Neutron and Proton Structure Functions Below Threshold
Knowledge of structure functions at large x is vital for several reasons, not least of
which is that it allows one to test mechanisms for the breaking of spin-flavor symmetry in
the nucleon. There are a number of predictions for the z - 1 behavior of the neutron to
proton F2 structure functions, ranging from 2/3 in the SU(6) symmetric quark model, to
1/4 in broken SU(6) through d quark suppression, to 3/7 in broken SU(6) via helicity flip
suppression (see Refs. [6,7] and references therein). Although it is well established that the
large-x F2/F2 data deviate from the SU(6) expectation, they are at present inconclusive
about the precise x -> 1 limit because of large nuclear corrections in the extraction of F2
from deuterium cross sections beyond x ~i 0.6 [6].
0.8
-0.6 2/3
0.4 --- ~-- - 3/7
20
Q (GeV )
Figure 1. Neutron to proton ratio for F (dashed), F2 (solid) and gi (dot-dashed) structure
functions in the limit x -+ 1.
The ratios of the neutron to proton F, F2 and gi structure functions are shown in Fig. 1
as a function of Q2, using the parameterisation of the global form factor data in Ref.[8].
While the F2 ratio varies quite rapidly at low Q2, beyond Q2 ~ 3 GeV2 it remains almost
Q2 independent, approaching the asymptotic value (dGM/dQ2)/(dGM/dQ2). Because the
F1 /FP ratio depends only on Gm, it remains flat over nearly the entire range qf Q2. At
asymptotic Q2 the model predictions for F1(x -* 1) coincide with those for F2; at finite
Q2 the difference between F and F2 can be used to predict the x -+ 1 behavior of the
longitudinal structure function, or the R = 0L/crT ratio.
The pattern of SU(6) breaking for the spin-dependent structure function ratio g'/gP
essentially follows that for F2/F2, namely 1/4 in the d quark suppression and 3/7 in
the helicity flip suppression scenarios [6,7]. However, the gi structure function ratio
approaches the asymptotic limit somewhat more slowly than F or F2, which may indicate
a more important role played by higher twists in spin-dependent structure functions than
in spin-averaged.
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Melnitchouk, W. Local quark-hadron duality in structure functions, article, February 1, 2000; Newport News, Virginia. (https://digital.library.unt.edu/ark:/67531/metadc712080/m1/3/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.