Demonstration of collinear factorization breaking due to collinear-to-soft Glauber exchanges for a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>2</mml:mn><mml:mo stretchy="false">→</mml:mo><mml:mn>3</mml:mn></mml:

We exhibit an exclusive process, namely the photoproduction of a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msup><a:mi>π</a:mi><a:mn>0</a:mn></a:msup><a:mi>γ</a:mi></a:math> 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 <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>t</c:mi></c:math> channel, contributing at the leading power, for which are . This is caused by the pinching of the contour integration of the plus and minus light-cone components of the Glauber gluon momentum. We argue that this leads to the observed “end pointlike” divergence of the convolution integral at leading order and leading power when collinear factorization is naïvely assumed.