Scientific Achievements since the 1989 Long Range Plan

In the last 5 years, significant progress has been achieved, both experimentally and theoretically, on two major topics: the polarizabilities of nucleons and threshold -photoproduction. The electric () and magnetic () polarizabilities of hadrons characterize the induced transient dipole moments created by externally applied electromagnetic fields.

A Gell-Mann/Goldberger/Thirring (GGT) dispersion relation provides a sum rule on () in terms of the total photoabsorption cross sections. Recent experiments at the University of Illinois, and by an Illinois-Saskatoon collaboration at the Saskatchewan Accelerator Laboratory (SAL) have combined this sum rule with proton-Compton scattering measurements to extract and . The difference of the proton polarizabilities on () has also been measured at Mainz. The regions of and allowed by these determinations are shown in Figure . They are consistent with one another, but in disagreement with an earlier experiment from the Lebedev Institute in Moscow. The common region of overlap of these new experiments (ALL* in the figure) is in quite reasonable agreement with calculations from PT for the proton.

Recent measurements of small-angle neutron scattering in the Coulomb field of Pb have yielded precise new information on the neutron's electric polarizability. The extracted value for appears to be in reasonable agreement with current PT calculations, but the implied value for is somewhat smaller than current PT predictions. However, considerable cancellations occur in the chiral expansion so there is some concern about the contributions of higher order terms. In addition, further experimental work on neutron polarizabilities and the neutron photoabsorption cross section is clearly warranted.

Threshold pion photoproduction has also been an area of great activity in the last few years. Experiments on the charged pion modes are rather straightforward and the agreement with PT appears to be excellent. (However, the measurements are quite old and a new precision measurement would certainly be welcome.) In contrast, photoproduction has proved much more problematic. After considerable reanalysis, experimental results from Saclay and Mainz seem to be converging on a value for the s- wave multipole at threshold that is close to the leading order predictions. The latter depend only upon gauge invariance and PCAC, and so had been regarded as a theorem. However, detailed 1-loop and leading order 2-loop PT calculations have shown that the Chiral expansions for s-wave production are in fact very slowly converging. In contrast, recent calculations of the threshold p-wave photoproduction multipoles, and of the longitudinal L electroproduction multipole, appear to be converging more rapidly. These are likely to provide a promising new testing ground for PT in future work.