  The forward--backward asymmetry in n p --> d pi^0, which must be zero in the
center-of-mass system if charge symmetry is respected, has been measured to be
[17.2 +/- 8 (stat) +/- 5.5 (sys)] * 10^{-4}, at an incident neutron energy of
279.5 MeV. This charge symmetry breaking observable was extracted by fitting
the data with GEANT-based simulations and is compared to recent chiral
effective field theory calculations, with implications regarding the value of
the u d quark mass difference.
