The Gerasimov-Drell-Hearn (GDH) sum rule relates the difference in total photo-absorption cross sections for left- and right-handed polarized photons to the square of the nucleon's anomalous magnetic moment. This fundamental sum rule arises because any particle with a non-zero anomalous magnetic moment must have internal structure and an excitation spectrum. The energy scale at which the sum rule is saturated gives an indication of the scale beyond which nucleonic excitations become spin-independent.
This experiment will measure the spin-dependent total cross section for circularly polarized photons absorbed on longitudinally polarized protons and neutrons in the photon energy range 5 <k <40 GeV. This will be the first measurement of the difference between left- and right-handed polarized photoproduction above the nucleon resonance region. Measurements of the magnitude and energy-dependence of polarized photoabsorption at high energy will provide the baseline for understanding of soft Regge physics, essential to the interpretation of data taken with virtual photons.
The photon beam will be produced using collimated coherent bremsstrahlung from a diamond crystal. The polarized targets will use polarized ammonia and deuterated ammonia to provide polarized protons and deuterons. Calorimetric detectors will be used to identify hadronic interactions and reject electromagnetic backgrounds.
Griffioen is co-spokesperson (long with D. Crabb of U. Virginia and P. Bosted of U. Massachusetts) of this experiment which will measure the A-dependence of the J/psi and psi' photoproduction cross sections using an unpolarized coherent bremsstrahlung beam (15, 25 and 35 GeV) incident on various nuclear targets.
These first-ever measurements of the A-dependence of psi' photoproduction will test the claim that the psi'-nucleon cross section is as large as 25 mb. We will improve the previous measurements of the J/psi cross section by more than a factor of three in both statistical and systematic errors, which will allow for a significantly more reliable extraction of the J/psi-nucleon cross section and its evolution with beam energy.
This experiment is uniquely suited for:
A fundamental unresolved issue in hadronic physics is how much of the nucleon's spin is carried not by the quarks, but by gluons. This experiment will determine the gluon spin density within the nucleon by measuring the asymmetry of polarized photoproduction of charmed quarks from a polarized target.
Photoproduction of open charm will be tagged by decays of D mesons into high transverse-momentum muons. The parallel/anti-parallel asymmetry for producing open charm is closely related to the fundamental polarized gluon spin density Delta g(x).
The asymmetry for single muons will be measured as a function of muon momentum, muon transverse momentum, and photon beam energies with sufficient precision to discriminate among models of Delta g(x) that differ from each other by as little as 10% in the range 0.1 <x < 0.5. Significant constraints will be placed on both the shape and magnitude of Delta g(x) in this x region. The projected errors are significantly smaller than for other experiments that plan to make direct measurements of Delta g(x). The photon beam will be quasi-mono-chromatic and circularly polarized. It will be produced by polarized electrons hitting an oriented diamond crystal.