We present the results of an extended scaling test of quenched Wilson twisted mass QCD. We fix the twist angle by using two definitions of the critical mass, the first obtained by requiring the vanishing of the pseudoscalar meson mass m_PS for standard Wilson fermions and the second by requiring restoration of parity at non-zero value of the twisted mass mu and subsequently extrapolating to mu=0. Depending on the choice of the critical mass we simulate at values of beta in [5.7,6.45], for a range of pseudoscalar meson masses 250 MeV < m_PS < 1 GeV and we perform the continuum limit for the pseudoscalar meson decay constant f_PS and various hadron masses (vector meson m_V, baryon octet m_oct and baryon decuplet m_dec) at fixed value of r_0 m_PS. For both definitions of the critical mass, lattice artifacts are consistent with O(a) improvement. However, with the second definition, large O(a^2) discretization errors present at small quark mass with the first definition are strongly suppressed. The results in the continuum limit are in very good agreement with those from the Alpha and CP-PACS Collaborations.
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