The Low theorem for diffractive bremsstrahlung and the soft photon puzzle

Abstract

The anomalous excess of small-kT photons radiated along with multi-hadron production, is challenging the physics community over four decades, but no solution has been proposed so far. We argue that the problem is rooted in the comparison with an incorrect model, usually called bremsstrahlung model. It is believed to be an extension of the Low theorem from the 2 -> 2+gamma process to radiative multi-particle production 2 -> n+gamma, where either initial, or final charged hadrons participate in radiation. We demonstrate that this breaks down unitarity of the S-matrix, so contradicts the optical theorem.