Polarization Observables In Low Energy Deuteron Photodisintegration

For the first time, a complete calculation of all 288 polarization observables of deuteron photodisintegration for polarized photons and an oriented deuteron target is presented for energies below +-production threshold. The observables are calculated within a nonrelativistic framework but with inclusion of lowest-order relativistic effects. Explicit meson exchange currents and isobar configurations as manifestation of subnuclear degrees of freedom are included in the calculation. The sensitivity of the various polarization observables with respect to subnuclear degrees of freedom, to electric and magnetic multipole contributions and to a variety of realistic potential models are systematically investigated. Thus this atlas provides the most detailed and systematic survey on polarization observables of this important process. It allows to analyse the different dynamical properties of the np-system as contained in the various observables and, therefore, will be useful for both theoretical studies and for the planning and evaluation of experiments as well. It serves in addition as an important supplement to the recent general review on deuteron photodisintegration by A. Arenhövel and M. Sanzone (Few-Body Systems, Suppl. 3).

We performed high precision measurements of induced and transferred recoil proton polarization in d($\vec{{u03B3}}$, $\vec{p}$)n for photon energies of 277--357 MeV and [theta]cm = 20 ° -- 120 °. The measurements were motivated by a longstanding discrepancy between meson-baryon model calculations and data at higher energies. Moreover, at the low energies of this experiment, theory continues to fail to reproduce the data, indicating that either something is missing in the calculations and/or there is a problem with the accuracy of the nucleon-nucleon potential being used.

More than 50 years ago, in 1934, Chadwick and Goldhaber (ChG 34) published a paper entitled "A 'Nuclear Photo-effect': Disintegration of the Diplon by -y-Rays."l in the introduction: They noted "By analogy with the excitation and ionisation of atoms by light, one might expect that any complex nucleus should be excited or 'ionised', that is, disintegrated, by -y-rays of suitable energy", and furthermore: "Heavy hydrogen was chosen as the element first to be examined, because the diplon has a small mass defect and also because it is the simplest of all nuclear systems and its properties are as important in nuclear theory as the hydrogen is in atomic theory". Almost at the same time, in 1935, the first theoretical paper on the photodisinte gration of the deuteron entitled "Quantum theory of the diplon" by Bethe and Peierls (BeP 35) appeared. It is not without significance that these two papers mark the be ginning of photonuclear physics in general and emphasize in particular the special role the two-body system has played in nuclear physics since then and still plays. A steady flow of experimental and theoretical papers on deuteron photo disintegration and its inverse reaction, n-p capture, shows the continuing interest in this fundamental process (see fig. 1.1).

We measured the angular dependence of the three recoil-proton polarization components in two-body photodisintegration of the deuteron at a photon energy of 2 GeV. These new data provide a benchmark for calculations based on quantum chromodynamics. Two of the five existing models have made predictions of polarization observables. Both explain the longitudinal polarization transfer satisfactorily. Transverse polarizations are not well described, but suggest isovector dominance.

Recoil proton polarization was measured in deuteron photodisintegration at theta{sub cm} = 90{sup o} for photon energies up to 2.4 GeV. The induced polarization py is consistent with vanishing above 1 GeV, in surprising disagreement with meson-baryon calculations. From pQCD it is generally expected that hadron helicity is conserved, but, the polarization transfer observables do not vanish. As the data are inconsistent with existing meson baryon model predictions, and with expectations from pQCD, we suggest that nonperturbative quark models will be needed to understand the underlying reaction dynamics.

We present measurements of the recoil proton polarization for the d(polarized y, polarized p)n reaction at thetac.m. = 90 degrees for photon energies up to 2.4 GeV. These are the first data in this reaction for polarization transfer with circularly polarized photons. The induced polarization py vanishes above 1 GeV, contrary to meson-baryon model expectations, in which resonances lead to large polarizations. However, the polarization transfer Cx does not vanish above 1 GeV, inconsistent with hadron helicity conservation. Thus, we show that the scaling behavior observed in the d(y, p)n cross sections is not a result of perturbative QCD. These data should provide important tests of new nonperturbative calculations in the intermediate energy regime.

Exclusive reactions are becoming one of the major sources of information about the deep structure of nucleons and other hadrons. The 2007 International Workshop held at Jefferson Lab in Newport News, Virginia, USA - the world's leading facility performing research on nuclear, hadronic and quark-gluon structure of matter - focused on the application of a variety of exclusive reactions at high momentum transfer, utilizing unpolarized and polarized beams and targets, to obtain information about nucleon ground-state and excited-state structure at short distances. This is a subject which is central to the programs of current accelerators and especially planned future facilities. This proceedings volume contains, in concentrated form, information about the newest developments, both theoretical and experimental, in the study of hard exclusive reactions.

Exclusive reactions are becoming one of the major sources of information about the deep structure of nucleons and other hadrons. The 2007 International Workshop held at Jefferson Lab in Newport News, Virginia, USA — the world's leading facility performing research on nuclear, hadronic and quark-gluon structure of matter — focused on the application of a variety of exclusive reactions at high momentum transfer, utilizing unpolarized and polarized beams and targets, to obtain information about nucleon ground-state and excited-state structure at short distances. This is a subject which is central to the programs of current accelerators and especially planned future facilities.This proceedings volume contains, in concentrated form, information about the newest developments, both theoretical and experimental, in the study of hard exclusive reactions.