OPPORTUNITIES IN NUCLEAR SCIENCE A Long-Range Plan for ...

THE SCIENCE • PROTONS AND NEUTRONS: STRUCTURE AND INTERACTIONS
tion coming from gluons or perhaps from the orbital
motion of the quarks? This is one of the most important
open questions in hadronic physics.
The coming years afford an outstanding opportunity to
tackle this question with three different experimental
approaches, which are described in the following paragraphs:
• Direct measurements of gluon polarization
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0.8
0.6
0.4
0.2
Tesla-N
HERMES
COMPASS
RHIC
• Determination of the flavor structure of the quark
polarization
• Transversity measurements
∆G/G
0
– 0.2
Experiments planned at CERN, SLAC, and RHIC aim
to determine the gluon contribution to the proton’s spin. At
CERN the COMPASS experiment will employ beams of
longitudinally polarized muons interacting with longitudinally
polarized proton targets to probe gluon polarization.
SLAC will study the photon-gluon fusion process with real
photons. And at RHIC, where the proton beams can be
polarized, quarks whose polarization is already known will
be used to analyze the degree to which gluons are polarized
within the proton via the quark-gluon Compton process.
A comparison of the projected results is shown in Figure
2.6. When complete, these experiments will extend knowledge
of the gluon polarization to momentum fractions as
small as x gluon ~ 0.01. Measurements of gluon polarization
at still-lower momentum fractions (which are necessary to
ensure that the total gluon contribution is precisely measured)
will require a new collider of polarized electrons and
polarized ions.
To obtain a full understanding of this problem, it will
also be necessary to know how the spin is shared among the
different quark and antiquark flavors over a wide range of x.
Initial information about these flavor structures is available
from semi-inclusive hard-scattering measurements, in
which final-state hadrons are detected in coincidence with
the scattered lepton. The recent HERMES run, with its
ring-imaging Cerenkov detector that cleanly identifies the
final-state hadrons, promises a more precise determination
of the flavor structure of the quark and antiquark polarization.
Additional information on flavor dependence will be
forthcoming from RHIC, including measurements of the
Drell-Yan process, production of electroweak bosons (W ± ),
and measurements of parity-violating asymmetries in collisions
between longitudinally polarized proton beams.
– 0.4
10 – 2 10 – 1
Further insights into the enigma of the nucleon’s spin will
come from measurements of the so-called transversity structure
function, which encodes the quark and antiquark polarizations
in a transversely polarized proton. Comparing the
transversity and net helicity distributions will expose one
aspect of the degree to which relativistic effects are important
in the quark structure of the nucleon. Preliminary results
from the HERMES collaboration are intriguing, and further
measurements are planned at HERMES, RHIC, and e + e –
colliders.
x gluon
Figure 2.6. Future data on gluon polarization. Comparison of the
anticipated precision and x range for data on gluon polarization
from planned experiments (TESLA-N is still under consideration).
Three different models of gluon polarization, all consistent with the
existing polarized deep inelastic scattering data, are also shown.
QCD in the Confinement Regime
In the early 1970s, the spectrum of mesons and baryons
led to the proposal that the quarks inside these particles are
effectively tied together by strings. Today, the string theories
that emerged from this idea are being examined as candidates
for the ultimate theory of nature. While the strong
interactions are described by QCD, which is not fundamentally
a string theory, numerical simulations of QCD (lattice
QCD) have demonstrated that this early conjecture was
essentially correct: In chromodynamics, a stringlike chro-
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