Evidence for Negative Stiffness of QCD Strings

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QCD 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 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.