Neutron electric dipole moment from lattice QCD

CP-PACS Collaboration

Shintani E

Aoki S

Ishizuka N

Kanaya K

Kikukawa Y

Kuramashi Y

Okawa Masanori

Taniguchi Y

Ukawa A

Yoshié T

Physical Review Letters

We carry out a feasibility study for the lattice QCD calculation of the neutron electric dipole moment (NEDM) in the presence of the theta term. We develop the strategy to obtain the nucleon EDM from the CP-odd electromagnetic form factor F3 at small theta, in which NEDM is given by limq2-->0thetaF3(q2)/(2mN), where q is the momentum transfer and mN is the nucleon mass. We first derive a formula which relates F3, a matrix element of the electromagnetic current between nucleon states, with vacuum expectation values of nucleons and/or the current. In the expansion of theta, the parity-odd part of the nucleon-current-nucleon three-point function contains contributions not only from the parity-odd form factors but also from the parity-even form factors multiplied by the parity-odd part of the nucleon two-point function, and, therefore, the latter contribution must be subtracted to extract F3. We then perform an explicit lattice calculation employing the domain-wall quark action with the renormalization group improved gauge action in quenched QCD at a-1~=2 GeV on a 163×32×16 lattice. At the quark mass mfa=0.03, corresponding to mpi/mrho~=0.63, we accumulate 730 configurations, which allow us to extract the parity-odd part in both two- and three-point functions. Employing two different Dirac gamma matrix projections, we show that a consistent value for F3 cannot be obtained without the subtraction described above. We obtain F3(q2~=0.58 GeV2)/(2mN)=-0.024(5)e·fm for the neutron and F3(q2~=0.58 GeV2)/(2mN)=0.021(6)e·fm for the proton.

Physics [ 420 ]

eng

The American Physical Society

Copyright (c) 2005 The American Physical Society

[ISSN] 0031-9007

[DOI] http://dx.doi.org/10.1103/PhysRevD.72.014504 isVersionOf