Calculation of Neutron EDM in quenched and full QCD

TL;DR

This paper presents a lattice QCD calculation of the neutron electric dipole moment (NEDM) in both quenched and full QCD, exploring methodological effects, quark mass dependence, and preliminary results indicating a non-zero NEDM.

Contribution

It provides the first direct lattice calculation of NEDM in full QCD and examines the effects of boundary conditions and quark mass dependence in quenched QCD.

Findings

NEDM remains non-zero toward the chiral limit in quenched QCD due to artifacts.

Preliminary full QCD results suggest a non-zero NEDM at finite quark masses.

Statistical errors are large, preventing confirmation of the expected vanishing of NEDM in the chiral limit.

Abstract

We report on a direct lattice calculation of the neutron EDM(NEDM) using the external electric field method in both quenched and full QCD. In quenched QCD, we use a $24^3\times 32$ lattice at $\beta=2.6$ with the Iwasaki gauge action and the clover fermion action to examine the viability of this method. In particular we investigate possible effects of violation of the periodic boundary condition of the external electric field on the NEDM signal. We also study the quark mass dependence of NEDM in quenched QCD, and observe that NEDM seems to remain non-zero toward the chiral limit because of the quenched artifact. In 2-flavor full QCD we employ configurations generated by the CP-PACS collaboration on a $24^3\times 48$ lattice at $\beta=2.1$ with the same gluon and quark actions as in the quenched case. Since the number of configurations is limited, we employ 8 different source points per one configuration and take an average over them. Our preliminary result at three quark masses ($m_{PS}/m_V\simeq 0.81, 0.76, 0.69$) indicates that non-zero value for NEDM can be obtained in full QCD. Statistical errors, however, are still too large to show the theoretically expected behaviour for NEDM in full QCD that it vanishes in the chiral limit.