Evidence of collinear factorization breaking due to collinear-to-soft Glauber exchanges for a $2\to 3$ exclusive process at leading twist

We exhibit an exclusive process, namely the photoproduction of a ${\pi }^0\gamma$ pair with large invariant mass, which violates collinear factorization. We explicitly demonstrate that this is due to the fact that there exists diagrams with gluon exchange in $t$ channel, contributing at the leading power, for which Glauber gluons are trapped. This is caused by the pinching of the contour integration of both the plus and minus light-cone components of the Glauber gluon momentum. We argue that this leads to the observed ``endpoint-like’’ divergence of the convolution integral at leading order and leading power when collinear factorization is naïvely assumed.