Threshold neutral pion photoproduction off the tri-nucleon to O(q4)Mark Lenkewitz, Evgeny Epelbaum, H. -W. Hammer and Ulf-G. MeißnerEur. Phys. J. A, 49 2 (2013) 20DOI: https://doi.org/10.1140/epja/i2013-13020-1