Chiral corrections to the Kroll-Ruderman theorem

Abstract

We calculate the one--loop corrections to the Kroll--Ruderman low--energy theorems for charged pion photoproduction in the framework of heavy baryon chiral perturbation theory. We predict the threshold S--wave multipole $E_{0+}$ to be $E_{0+}^{\mathrm{thr}}(\gamma p\to {\pi }^{+}n)=(28.2\pm 0.6)\cdot 10^{-3}/M_{\pi }$ and $E_{0+}^{\mathrm{thr}}(\gamma n\to {\pi }^{-}p)=(-32.7\pm 0.6)\cdot 10^{-3}/M_{\pi }$, respectively, for a fixed pion--nucleon coupling constant, $g_{\pi N}=13.4$. A comparison to the existing data is also given.

Comments: 7 pp, LaTeX