Twisted mass QCD and lattice approaches to the ?I = 1/2 rule

Twisted mass lattice QCD (tmQCD), generalised to four Wilson quark flavours, can be used for the computation of some weak matrix elements related toI = 1/2 transitions. Besides eliminating unphysical zero modes, tmQCD may alleviate the four-quark oper- ator renormalisation problems encountered in the calculation of CP-conserving K → � matrix elements with traditional Wilson fermion regularisation. With an active charm quark, the renormalisation of the K → � matrix elements requires at most the sub- traction of a linearly divergent counterterm. Furthermore, in the (partially) quenched approximation the twist angles can be chosen so that only a finite counterterm needs to be subtracted.