An upper limit to the electric dipole moment of the neutron from lattice QCD

Presented the talk. is odd under both of them. This breaking can produce a nonzero value of dn, the electric dipole moment of the neutron [1], dn ≈ 8 · 10 θe · cm. The experimental upper limit is [2] dn|exp < 6 · 10e · cm which implies θ < ∼10. This is a small number and its smallness is not protected by any known symmetry. An alternative possibility for having dn 6= 0 could be a spontaneous breaking of P by a nonzero value 〈0|Q|0〉. This possibility however would be excluded by a theorem by Vafa and Witten [3]. Nevertheless this theorem is based on assumptions whose validity has been questioned. We have then addressed the issue of a spontaneous breaking of P and T by numerical simulations of QCD on the lattice [4].