EVIDENCE FOR NEGATIVE STIFFNESS OF QCD FLUX TUBES IN THE LARGE-N LIMIT OF SU(N)

Quarks are held together by color-electric flux tubes between quarks w ith a finite thickness determined by the dimensionally transmuted coup ling constant. Thick tubes must have a finite curvature stiffness, whi ch led by Polyakov and Kleinert to propose their so-called rigid-strin g model. Contary to their model, and motivated by the properties of a magnetic flux tubes in type-II superconductors, we put forward the hyp othesis that QCD Aux tubes have a negative stiffness. We set up a new string model with this property and show that it is free of two princi pal problems of rigid strings - particle states with negative norm and nonexistence of a lowest-energy state - thus making it a better candi date for a string description of quark forces than previous models. Mo reover, the new model is shown to have the same high-temperature realx T(2)-behavior of the string tension as an SU(N) gluon Aux tube at larg e N, whereas that of rigid strings is complexxT(2), that of Nambu-Goto strings is iT. We concludeon that large-N flux tubes have a negative rather than a positive bending stiffness.