The chiral filter

Abstract

Thermal neutron capture on a proton, $np\to dγ$, is measured to a fraction of a percent; the impulse approximation misses nine percent of it. This article follows that gap through five decades of an idea of Mannque Rho's, the chiral filter. Chiral symmetry fixes the pion-exchange current that fills the gap, and power counting explains why nothing else competes. The missing nine percent becomes a prediction, correct within errors. On the unprotected side, one constant fixed by tritium beta decay predicted solar $pp$ fusion and the {\it hep} process; $g_A^*$ remains open.