Abstract
For the neutron to have an electric dipole moment (EDM), the theory of nature must have T, or equivalently CP, violation. Neutron EDM is a very good probe of novel CP violation in beyond the standard model physics. To leverage the connection between measured neutron EDM and novel mechanism of CP violation, one requires the calculation of matrix elements for CP violating operators, for which lattice QCD provides a first principle method. In this paper, we review the status of recent lattice QCD calculations of the contributions of the QCD Θ-term, the quark EDM term, and the quark chromo-EDM term to the neutron EDM.
Highlights
Electric dipole moment (EDM) of the neutron measures the separation of positive and negative charge in the neutron and is necessarily aligned along the spin axis
The neutron EDM (nEDM) is measured by the change in the spin precession frequency of ultracold-neutrons aligned in a magnetic field under a flip in the direction of a strong background electric field
In the calculation of neutron EDM induced by the QCD Θ-term using θ-expansion or external electric field method, one needs to calculate correlators reweighted by the topological charge
Summary
Electric dipole moment (EDM) of the neutron measures the separation of positive and negative charge in the neutron and is necessarily aligned along the spin axis. There are many BSM scenarios predicting a nEDM between 10−25 – 10−28e·cm, and upcoming experiments will put constraints on them [2,3,4,5,6] This requires both the measurement of, or a bound on, the neutron EDM, and the calculations of the matrix elements of novel CP violating operators within the neutron states. Lattice QCD calculation of the matrix elements of these operators will play an important role in connecting the measured neutron EDM and novel CP violation in BSM scenarios, i.e., knowing the matrix elements along with the bound on (or the value of) the nEDM, one can put bounds on the couplings and on the parameter space of allowed BSM theories. We start by discussing the contribution of the QCD Θ-term to the nEDM and use it to illustrate the various methods used in the calculations
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