The electric dipole moment of the neutron from $N_f=2+1+1$ twisted mass fermions

TL;DR

This paper calculates the neutron electric dipole moment using lattice QCD with twisted mass fermions, providing a new estimate of nEDM related to CP violation with consistent results across different methods.

Contribution

The study introduces a lattice QCD approach using twisted mass fermions and position space methods to evaluate the neutron EDM at physical quark masses.

Findings

Consistent nEDM value obtained across multiple smoothing and extraction methods.

nEDM per unit theta found to be approximately -0.045 e·fm.

Results contribute to understanding CP violation in neutron structure.

Abstract

We extract the neutron electric dipole moment (nEDM) $ \vert \vec{d}_n \vert $ on configurations produced with $N_f=2+1+1$ twisted mass fermions with lattice spacing of $a \simeq 0.082$ fm and a light quark mass that corresponds to $M_{\pi} \simeq 373$ MeV. We do so by evaluating the $CP$-odd form factor $F_3$ for small values of the $CP$-violation parameter $\theta$ in the limit of zero momentum transfer. This limit is extracted using the usual parametrization but in addition position space methods. The topological charge is computed via cooling and gradient flow using the Wilson, Symanzik tree-level improved and Iwasaki actions for smoothing. We obtain consistent results for all choices of smoothing procedures and methods to extract $F_3$ at zero momentum transfer. For the ensemble analyzed we find a value of nEDM of $\vert \vec{d}_n \vert / \theta = -0.045(6)(1) {\rm e} \cdot {\rm fm}$.