Renormalization of quark propagators from twisted-mass lattice QCD at Nf=2

We present results concerning the nonperturbative evaluation of the renormalization constant for the quark field, Zq, from lattice simulations with twisted-mass quarks and three values of the lattice spacing. We use the regularization-invariant momentum-subtraction (RI'-MOM) scheme. Zq has very large lattice spacing artefacts; it is considered here as a test bed to elaborate accurate methods which will be used for other renormalization constants. We recall and develop the nonperturbative correction methods and propose tools to test the quality of the correction. These tests are also applied to the perturbative correction method. We check that the lattice-spacing artefacts indeed scale as a2p2. We then study the running of Zq with particular attention to the nonperturbative effects, presumably dominated by the dimension-two gluon condensate in Landau gauge. We show indeed that this effect is present, and not small. We check its scaling in physical units, confirming that it is a continuum effect. It gives a ∼4% contribution at 2 GeV. Different variants are used in order to test the reliability of our result and estimate the systematic uncertainties. Finally, combining all our results and using the known Wilson coefficient of , we find g2(μ2)μ2CM=2.01(11)(-0.73+0.61)GeV2 at μ=10 GeV, the local operator A2 being renormalized in the MS scheme. This last result is in fair agreement within uncertainties with the value independently extracted from the strong coupling constant. We convert the nonperturbative part of Zq from the regularization-invariant momentum-subtraction (RI'-MOM) scheme to MS. Our result for the quark field renormalization constant in the MS scheme is ZqMSpert((2 GeV)2,gbare2)=0.750(3)(7)-0.313(20)(gbare2-1.5) for the perturbative contribution and ZqMSnonperturbative((2 GeV)2,gbare2)=0.781(6)(21)-0.313(20)(gbare2-1.5) when the nonperturbative contribution is included.