The Low theorem for diffractive bremsstrahlung and the soft photon puzzle

TL;DR

This paper critiques the traditional bremsstrahlung model used to explain the excess of soft photons in multi-hadron production, showing it violates unitarity and the optical theorem, and proposes a need for a revised theoretical approach.

Contribution

It identifies fundamental flaws in extending the Low theorem to multi-particle radiative processes, highlighting the breakdown of unitarity in current models.

Findings

The traditional bremsstrahlung model violates unitarity.

Extending the Low theorem to multi-particle processes is invalid.

A new theoretical framework is needed to resolve 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.