Quarks and gluons in hadrons and nuclei Page: 26 of 31
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The latter should be unity and is within errors when one combines data from
two energies, 140 GeV and 286 GeV incident n beams. (However, if one restricts
attention to the lower energy sample, the situation is more messy and the pion
distributions do not seem to factorize. Why this should be is unclear to me, but
bear it in mind as an empirical observation for later reference.)
So the message is: do not be misled by a = Aa, a = 1 for Drell-Yan pair pro-
duction on nuclei. While this may be approximately true for the total rate, there
can be (and are) non-trivial effects in x and pt2. A depletion at large x may be
compensated by an enhancement as x + 0 for example. The kinematic conditions of
experiments may emphasize different regions of x for the beam and target. The
extent to which A-dependent effects will arise depends on qA (x) and gA (x), about
which we know almost nothing. Rescaling and pion models imply that both q(x,Q2)
and g(x,Q2) have non-trivial A dependence; moreover, shadowing effects will modify
them as x + 0. Empirical information is only now beginning to emerge.
4.3. Antiquarks in Nuclei
The Drell-Yan process with incident nucleons can probe q in the target if
suitable kinematics are chosen, e.g., xl = 0.7 and x2 x1. An investigation of
this in various models has been made by Bickerstaffe et al.48 and by Berger et
al.46 As an example, in Fig. 1, I show the predictions for qA/qN(x) in iron in
three models compared with information gleaned from CDHS.49 The dramatic rise in
the Berger-Coester model at x > 0.3 is due to their prediction that q leak out to
moderate x values in nuclei. However, it is illusory to some degree as both
qA and qN are vanishingly small; even so, experiment E772 may be able54 to test
this. Independent of specific models, it is an interesting question whether q
leak to "large" x in nuclei as this will have a bearing on the Q2 shape of Drell-
Yan pairs in nuclei which may differ from the 4 production (produced by gluons)
and potentially provide a background to the plasma signal sought in heavy-ion
collisions. Indeed, there are hints of a change in the 4/Drell-Yan ratio in com-
paring nucleon data with that from oxygen-uranium (NA38 collaboration at CERN,55
but close examination suggests that the shape of the Drell-Yan continuum has
changed. This implies that "conventional" A dependence of q(x) may be signifi-
cant, and that this is not a first hint of plasma formation.56
Recently WA25 and WA59 in collaboration have studied the EMC effect using v
and v interactions in neon and deuterium. The x,y distributions allow separation
of quark and antiquark distributions, and there is some indication that the sea
decreases in going from deuterium to neon. Depending upon the model assumed for
QL/UT, the fractions of sea in neon is 7 6% or 18 10% less than in deuterium.
Presumably, these data are being dominated by nuclear shadowing, like the
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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/m1/26/: accessed March 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.