Four quark operators in maximally twisted Wilson LQCD
  We discuss how the peculiar properties of maximally twisted Wilson fermions can be exploited to set up a consistent LQCD computational scheme in which the CP-conserving matrix elements of the $\Delta S =1,2$ effective weak Hamiltonian can be evaluated without mixing with wrong-chirality and/or -parity operators. The proposed lattice framework guarantees automatic O($a$) improvement and positive determinant also for pairs of non-degenerate quarks. 
Roberto Frezzotti (INFN-Milano and Universit\`a di Milano ``Bicocca'')   and Giancarlo Rossi (Universit\`a di Roma ``Tor Ver...