Haiyan Gao

Walter Glöckle

Alan Nathan

Daniel Phillips

Program Coordinator:
Darlette Powell
(206) 685-4286

Soft Photons and Light Nuclei

June 16-20, 2008

A new generation of photon machines has recently come, or will soon come, online. These machines provide high-intensity beams of quasi-monoenergetic photons with energies of order, or less than, the pion mass. Prominent among these are the HI gS facility in the US, the MAX-lab tagged photon facility in Lund Sweden, the SLEGS facility at the Shanghai light source in China, and the NewSUBARA project in Japan. All these machines except MAX-lab can produce beams of linearly or circularly polarized photons with a high degree of polarization. The facilities in Asia probe the lower-energy part of this region and will be operational in the near future, while the Lund and HIg facilities already exist with known capabilities. Therefore, the planned physics program at these two facilities will be the focus of this workshop. But the workshop will also provide the opportunity to draw out physics issues that the lower-energy machines can address.

At the same time, exciting theoretical developments involving effective field theories (EFTs) are providing new insights into the reactions measured at these machines. This is possible because EFT's character as a systematic expansion in ratios of physical scales provides a means to progressively improve the treatment of both nuclear forces and the currents through which nuclear degrees of freedom interact with photons. Two EFTs are relevant for the energy domain covered by these facilities. At lower energies the so-called "pionless" EFT is applicable. Once the energy is high enough that pionic degrees of freedom are specifically probed, chiral EFTs can be used to describe the interaction of nuclei with photons. In both cases, EFT's description of nuclear structure and reactions involves progressively smaller effects in the nuclear force, e.g. two- and three-pion exchange and consistent three-nucleon forces. In particular, the chiral EFT forces have now been worked out to several orders in the expansion. Further improvement in this description can be expected too, as new three- and four-nucleon forces at higher orders in the expansion are currently under development. While much is known about the electromagnetic properties of the nucleon itself in these chiral EFTs, work on building up the current operators for light nuclei is at a relatively early stage. But such calculations hold the promise of operators for electromagnetic reactions that are derived using techniques analogous to those employed for the nuclear force, and describe data at a concomitant level of accuracy.

Furthermore, for A ≤ 4 modern nuclear Hamiltonians can now be solved essentially exactly for observables involving both bound and scattering states using, e.g. Faddeev-Yakubovsky or Correlated Hyperspherical Harmonics techniques. These theoretical advances give us access to theoretical descriptions of reactions on light nuclei which are systematically improvable, and whose consequences can be reliably computed. Used in concert with accurate experiments, such calculations provide a powerful tool to elucidate critical details of the strong nuclear force.

The combination of theoretical and experimental advances will facilitate examination of three exciting areas: (1) Examination of three-body forces through photo-induced processes on 3He, (2) Low-energy photodisintegration of light nuclei and (3) Compton scattering in systems with A=1, 2, and 3. This workshop will bring together leading theorists involved in calculations in these areas and experimentalists doing experiments at some of the aforementioned photon facilities. The goal of the workshop will be to identify the most promising experiments for access to these three physics goals. This will include detailed collaborative work and discussions between theorists and experimentalists on which particular experiments and kinematics/observables are optimal with respect to both theoretical control and experimental viability.