IR extremality and UV softness in string bootstrap
Whether string amplitudes can be characterized by extremality within the space of consistent S-matrices is an intriguing question. We address this question within the framework of positivity bootstrap. We compute positivity bounds by minimizing the $F^4$ Wilson coefficient while fixing subleading coefficients, and show that the Veneziano amplitude lies close to the boundary of the allowed region. However, this numerical proximity does not imply exact extremality, as demonstrated by explicit counterexamples. When additional KLT-motivated UV softness is imposed, the Veneziano amplitude is found to uniquely minimize the $F^4$ coefficient for $4\leq D\leq10$, connecting the extremality of the Veneziano amplitude to its stringy UV behavior and offering a new perspective on what makes string theory special.