Vector meson dominance and gϱππ at finite temperature from QCD sum rules

Finite Energy QCD sum rule at non-zero temperature is used to determine the q2- and the T-dependence of the ϱππ vertex function in the space-like region. A comparison with an independent QCD determination of the electromagnetic pion form factor Fπ at T ≠ 0 indicates that Vector Meson Dominance holds to a very good approximation at finite temperature. At the same time, analytical evidence for deconfinement is obtained from the result that gϱππ(q2, T) vanishes at the critical temperature Tc, independently of q2. Also, by extrapolating the ϱππ form factor to q2 = 0, it is found that the pion radius increases with increasing T, and it diverges at T = Tc.