line decor
line decor

 Soft photons and light nuclei
 (INT workshop June 16 - 20, 2008)

  Reported by Daniel Phillips, Walter Gloeckle, Haiyan Gao, and Al Nathan
  Date posted July 8, 2008

About 40 theorists and experimentalists attended the INT workshop (co-sponsored by the Triangle Universities Nuclear Laboratory (TUNL) and the University of Illinois at Urbana-Champaign) on "Soft photons and light nuclei" at the INT from June 16-20, 2008. This meeting focused on theoretical and experimental work pertaining to photo-induced reactions on protons, neutrons, and few-body nuclei at energies below the pion threshold.

At very low energies, the photodisintegration of few-nucleon systems probes the "universal" dynamics that occurs in light nuclei because of the large scattering lengths in the nucleon-nucleon system. These large (compared to the pion Compton wavelength) scattering lengths drive low-energy observables in the A=2 system, as is demonstrated by the concurrent success of effective field theory (EFT) and potential-model calculations in describing data recently obtained at the High-Intensity Gamma-Ray Source (HIGS) at TUNL. The extension of such calculations to the three-body system, and their use to explore correlations, e.g. between neutron-deuteron scattering lengths and thermal capture rates, is an ongoing challenge.

At higher energies, the photon begins to probe dynamics that is specific to the nuclear force. Photo-disintegration of light nuclei and Compton scattering from systems with A=1, 2, and 3 provide a unique window to study the pattern of chiral-symmetry breaking in QCD below the pion threshold.

In particular, nucleon polarizabilities provide the promise of connecting QCD (through, e.g. lattice calculations of polarizabilities described at the workshop) to measurements in the laboratory. The database for unpolarized Compton scattering on the proton is quite good, but the dominant error on the difference of proton electric and magnetic polarizabilities is statistical, which suggests that future, precise experiments, e.g. one that may be carried out at Mainz, could have a sizable impact. There are also detailed future plans for a polarized-beam-polarized-target γp experiment at HIGS that will, for the first time, allow the four different "spin polarizabilities" of the proton to be disentangled from data. Access to neutron polarizabilities is provided by Compton scattering on the deuteron and 3He. Experiments of this character are being carried out at MAX-Lab in Lund, Sweden, and are a high priority for the future program at HIGS. Theoretical approaches to the interpretation of these experiments based on systematic expansions built on QCD's (approximate) chiral symmetry were laid out at the workshop, and compared with methods based on dispersion relations and traditional models of the nuclear force. The conclusion was that the theory of Compton scattering from the proton and deuteron is well under control at energies below the pion threshold, i.e. in the regime where forthcoming experiments will be performed. Work on improvements to the theory of Compton scattering on 3He (which is still in its infancy) was initiated at the meeting.

Three-nucleon forces are another manifestation of the underlying strong-interaction dynamics that can be probed in these experiments. In order to extract reliable information on three-nucleon forces a particular theoretical framework must be chosen. There has been much recent progress as regards the description of nuclear forces in the framework of chiral perturbation theory (ChiPT). Ongoing work is focused on refining the three-nucleon force that is derived from ChiPT and calculating current operators that are consistent with the NN and NNN potentials already obtained. A very rich structure of three-body forces and multi- pion exchange currents seems to be emerging and waits to be fully implemented numerically. It will be interesting to see signatures of these novel mechanisms in the data generated by those three-nucleon forces and multi- pion exchange currents.

At present this is achievable in the two- and three-body systems, where all observables can be computed directly, with continuum wave functions obtained via Faddeev methods. The experimental data in these systems was reviewed, with new polarized-photon data from HIGS contributing to the picture, especially as regards the Gerasimov-Drell-Hearn sum rule on deuterium. The theory for A=2 was agreed to be in excellent shape, with traditional and EFT approaches giving results that agree to high precision. However, unfortunate discrepancies and inconsistencies in the experimental database remain, arguing for renewed efforts to measure γd→np with both precision and well-documented systematic errors so that the highly developed theory that is available there can be put to the test. This would, for instance, provide reassurance that the np→dγ cross section used in simulations of big-bang nucleosynthesis has the 1% accuracy that is claimed for it. The theory for A=3 is not as highly developed: challenges associated with the description of the currents remain, but will hopefully be resolved by the ongoing work on ChiPT already described. In this context, future experiments on three-body photodisintegration (e.g. at HIGS) could provide useful information on three-nucleon forces.

An ongoing technical challenge is to ensure that other, "indirect", methods for computing scattering in N-body systems (e.g. Green's Function Monte Carlo and Lorentz-Integral Transform methods) are completely reliable. Results obtained up until now for four-body photodisintegration and scattering in the A=5 system are very encouraging in this regard. They hold out the promise of ab initio computations for photo-induced reactions in systems ranging up to A=8 and beyond that are based on nuclear forces which are consistent with QCD.

Given this promising landscape on the theory side, the workshop attendees expressed near-universal concern regarding the small number of experimental facilities pursuing this kind of research in the US and Europe. The situation in China and Japan is more promising, with NewSUBARU at the SPring-8 site already providing stimulating data on Helium-4 photodisintegration, and plans for significant future work in this area there, and at the Shanghai Light Source in China. Continued progress at HIGS is crucial to this sub-field staying a part of the US's nuclear-science-research portfolio. A re-invigorated Compton program at Mainz would provide a good boost of high-quality data relevant to that important reaction. Data from all these facilities will stimulate further theoretical advances, as theorists and experimentalists work together to try and unravel the way that the dynamics of QCD manifests itself in low-energy photon reactions on few-nucleon systems.