Speaker
Description
Our renormalization group consistent version of optimized perturbation, RGOPT, had been used to calculate the nonperturbative QCD spectral density of the Dirac operator and the related chiral quark condensate $\langle \bar q q \rangle$, for $n_f=2$ and $n_f=3$ massless quarks. Sequences of approximations at two-, three-, and four-loop orders gave stable parameter-free determinations
$\langle\bar q q\rangle^{1/3}_{n_f=2}(2\, {\rm GeV}) = -(0.833-0.845) \bar\Lambda_2 $, and $ \langle\bar q q\rangle^{1/3}_{n_f=3}(2\, {\rm GeV}) = -(0.814-0.838) \bar\Lambda_3 $, where the range is our estimated theoretical error and $\bar\Lambda_{n_f}$ the basic QCD scale in the $\overline{MS}$-scheme.
We update our previous analysis up to 5-loop order of (RG optimized) perturbation, thanks to the recently available five-loop order QCD renormalization group functions, and compare our results with other recent determinations from lattice simulations or other approaches.
