2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/149305QCD strings are color-electric flux tubes between quarks with a finite thickness and thus a finite curvature stiffness. Contrary to an earlier rigid-string by Polyakov and Kleinert, and motivated by the properties of a magnetic flux tubes in type-II superconductors we put forward the hypothesis that QCD strings have a {\em negative\/ stiffness. We set up a new string model with this property and show that it is free of the three principal problems of rigid-strings --- particle states with negative norm, nonexistence of a lowest-energy state, and wrong high-temperature behavior of string tension --- thus making it a better candidate for a string description of quark forces than previous models.LaTeX and figure files tarred and gzipped. New Version contains a number of improvementsHigh Energy Physics - Theorytext