Gluon condensate andc-quark mass in pseudoscalar sum rules at 3-loop order

Charmonium sum rules for pseudoscalar 0^{-+} state eta_c(1S) are analyzedwithin perturbative QCD and Operator Product Expansion. The perturbative partof the pseudoscalar correlator is considered at alpha_s^2 order and thecontribution of the gluon condensate is taken into account with alpha_scorrection. The OPE series includes the operators of dimension D=6,8 computedboth in the instanton and factorization model. The method of moments in MS-barscheme allows to establish acceptable values of the charm quark mass and gluoncondensate, using the experimental mass of eta_c. In result of the analisys thecharm quark mass is found to be m_c(m_c)=1.26+-0.02 GeV independently of thecondensate value. The sensitivity of the results to various approximations forthe massive 3-loop pseudoscalar correlator is discussed.Comment: 19 pages, 1 latex + 5 eps files. v2: Operators of dimension D=8 included; the condensate restriction relaxed; c-quark mass is found m_c(m_c)=1.26+-0.02 GeV independently of other sum rules and condensate value. To appear in JHE