The electric dipole moment of the neutron from 2+1 flavor lattice QCD

TL;DR

This paper presents a lattice QCD calculation of the neutron's electric dipole moment using 2+1 flavors and imaginary theta, providing a new upper bound on the CP-violating theta parameter.

Contribution

The study introduces a fully dynamical lattice QCD method with imaginary theta to compute the neutron EDM and extract constraints on CP violation.

Findings

Calculated neutron EDM: -3.8(2)(9) x 10^{-16} e cm

Derived upper bound on theta: < 7.6 x 10^{-11}

Demonstrated feasibility of using imaginary theta in lattice QCD

Abstract

We compute the electric dipole moment d_n of the neutron from a fully dynamical simulation of lattice QCD with 2+1 flavors of clover fermions and nonvanishing theta term. The latter is rotated into the pseudoscalar density in the fermionic action using the axial anomaly. To make the action real, the vacuum angle theta is taken to be purely imaginary. The physical value of d_n is obtained by analytic continuation. We find d_n = -3.8(2)(9) x 10^{-16} [theta e cm], which, when combined with the experimental limit on d_n, leads to the upper bound theta < 7.6 x 10^{-11}.