Positivity of the Veneziano Amplitude in $D=4$

TL;DR

This paper demonstrates that the Veneziano amplitude in four dimensions maintains positivity of its coefficients, supporting its consistency with unitarity and the axioms of the S-matrix bootstrap.

Contribution

It provides an exact expression for the Gegenbauer coefficients on the leading Regge trajectory in D=4, showing their positivity and decay with spin.

Findings

Coefficients decay with increasing spin

Coefficients remain positive on the leading trajectory

Supports positivity of the full Veneziano amplitude in D=4

Abstract

The Veneziano amplitude was put forward as a solution to the axioms of the S-matrix bootstrap. However, unitarity, reflected in the positivity of the coefficients in the Gegenbauer expansion of the amplitude is not obvious. In this note we compute the generating function of these coefficients in terms of the Appell hypergeometric function. We use this to read off an exact form of this coefficient on the leading Regge trajectory in $D=4$. We find that it decays with the spin but always remains positive. Since for large spin these coefficients are expected to be smaller than those on the subleading trajectories, our result indicates the positivity of the full Veneziano amplitude in $D=4$.