Quarks and gluons in hadrons and nuclei Page: 24 of 31
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4. PARTON DISTRIBUTIONS IN NUCLEI: QUAGMA OR QUAGMIRE?
In this lecture I review the emerging information on the way quark, anti-
quark, and gluon distributions are modified in nuclei relative to free nucleons.
I place particular emphasis on Drell-Yan and production on nuclei and caution
against premature use of these as signals for quagma in heavy-ion collisions.
If we are to identify the formation of quark-gluon plasma in heavy-ion colli-
sions by changes in the production rates for relative to Drell-Yan lepton pairs,
then it is important that we first understand the "intrinsic" changes in parton
distributions in nuclei relative to free nucleons. So, I will review our emerging
knowledge on how quark, antiquark, and gluon distributions are modified in nuclei
relative to free nucleons, and briefly summarize the emerging theoretical concen-
4.1. Partons in Nuclei
The best known nuclear distortion is that of the EMC effect which reveals a
modification of the valence quark distributions in nuclei relative to those in
All experiments now show broad agreement.39,40 ThE rise in FA/FN at x > 0.7
is due to Fermi motion causing the structure function FA to leak out to x > 1;
dramatic as this appears, it occurs where FAN = 0, and is, in fact, a very minor
contributor to the overall phenomenon. Indeed, overall, the effect is a subtle
10% affair, and we don't need to rewrite the nuclear physics textbooks. As x + C,
we are beginning to see evidence for shadowing, a subject on which theory is now
also starting to develop.41
4.2. Quarks in Nuclei
In the "intermediate" region 0.2 < x < 0.6 the ratio falls below unity as
(valence) quarks '.se momentum due to nuclear binding. The A dependence was suc-
cessfully predicted in advance of data"2 and is rather well understood. It was
first predicted in the context of the resealing analysis and subsequently verified
by experiments at SLAC.39 However, the A-dependence is more general than the re-
scaling model43 and will arise in any model in which (i) the EMC effect is fitted
or predicted for one A value, say iron, (ii) the physics underwriting the effect
has a finite range in coordinate space, and (iii) the effect is expressible as a
functional of the nuclear density operator, p(r). If gk(r1'...rkxQ2) is an A-
dependent function expressing the change in FA arising from the overlap of k-
nucleons, pA (r) is the mean nuclear density, then
6Fk=2(xQ 2) = j d3rl d3r2 aA lA(r21 2 l 2 '
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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/m1/24/: accessed March 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.