البوصلة الكهربائية للنيوترون سريع

الملخص الأصلي

كما كتبه المؤلفون، بالإنجليزية arXiv:2607.10606

We present the calculation of the neutron electric dipole moment (nEDM) $$d_n$$ using 2+1 flavor domain wall fermion ensembles with fixed lattice spacing $$a\approx 0.11\,\text{fm}$$ and pion masses of 340, 420, and 576 MeV. We show that the neutron electric dipole moment can be extracted from the energy shift induced by a static uniform external background electric field in the presence of the CP-violating QCD theta-term, $$\bar\theta Q_{top}$$. Motivated by the Feynman-Hellmann theorem, we employ sampling of the topological charge $$q_\text{top}(t)$$ on a single time-slice rather than the global topological charge $$Q_\text{top}=\int q_\text{top}(t) \, dt$$, which dramatically improves the statistical precision of the $$\theta$$-induced nEDM. Key to our method is to calculate the forward matrix element of the topological charge density in the nucleon deformed by a background electric field. We find that calculation with the traditional positive parity-projected nucleon operator is subject to large excited-state contamination. To remove the contamination, we construct the ground state of the deformed nucleon by solving a non-Hermitian generalized eigenvalue problem. With this approach, we find consistent values for the nEDM when using different nucleon interpolating operators, regardless of whether they are covariant or non-covariant under chiral transformations. Finally, after extrapolating to the physical point, we obtain $$d_n=-0.0050(4)^\text{stat}(8)^\text{sys}\bar{\theta}$$ $$e$$ fm, where the systematic uncertainty includes excited-state effects estimated as variation with the Euclidean-time fits and the dependence on the strength of the electric field applied to the neutron. Conventional systematic errors like discretization, finite-volume, and chiral extrapolation effects will be addressed in future work.