Quarks and gluons in hadrons and nuclei Page: 3 of 31
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Nuclear Physics deals with the collective properties of the many-body nucleus
consisting of nucleons. Hadron physics deals with the structure and interaction
dynamics of those hadrons. Particle physics uses these parties as tools with
which to elucidate deeper truths, seeking the origins of matter, the nature of
mass, and the several other parameters which presently have to be invoked ad hoc
(the weak mixing angles, fermion masses, etc.).
With tongue in cheek, one might contrast the two extremes. In extreme high-
energy particle physics, there is infinite theorizing but almost no data; in the
nuclear structure field, we have copious data but no truly fundamentally useful
theory. In hadron physics we have much data and the hope of confronting them with
the fundamental QCD theory of interacting quarks and gluons. This is stimulating
but also difficult. A major problem is confinement of colored quarks and gluons
within the nucleons - this is simulated on computers where space-time is described
as a discrete lattice, but its origins analytically are still rather poorly
This confinement phenomenon is also the catch-22 of "deriving" nuclear
physics from QCD. As Bob Jaffe once remarked, "Looking for evidence of quarks in
nuclei is like looking for the mafia in Sicily: everyone knows they are there,
but it's hard to find the evidence." The quarks are confined in nucleons, and so
any successful description of the nuclear structure must reduce to that of quarks
clustered inside individual nucleons. Thus we have first to understand the proton
and neutron - hence the interest in hadron physics. There is the interesting
possibility that the interactions and overlaps among closely packed nucleons in
large or dense nuclei, or in high-energy collisions of heavy ions, may disturb the
distributions of quarks - their spatial or momentum distributions - relative to
their behavior when confined in isolated free nucleons. There are indeed hints of
such behavior, e.g., the "EMC effect" where quarks in iron have a slightly differ-
ent momentum distribution relative to that in the deuteron. Their mean momenta
are reduced in iron and other heavy nuclei suggesting greater spatial freedom. Is
this "liberation" a "cold" precursor to "hot" deconfinement? Are nucleons in
nuclei physically "enlarged", or is this a manifestation of quarks exchanged be-
tween nucleons, tying the nucleus together and having more spatial mobility than
when in a single free nucleon? Future experiments may help to answer these ques-
tions - questions raised, in large degree, by the underlying quark theory.
I would like now to draw some analogies between QED (electrical charges,
atoms, and molecules) on the one hand, and on the other, QCD (color, hadrons, and
nuclei). The similarity is such that one could rewrite Bjorken and Drell's QED
text by inserting a traceless 3x3 matrix (a of SU(3)) at the fermion gauge boson
vertices and let QED become QCD (with a replaced by as = 1/10). However, the
gluons themselves have color and so mutually interact via the color forces (con-
trast the photon of QED which transmits but does not directly "feel" 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/m1/3/: accessed March 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.