Abstract
The isovector time-reversal- and parity-violating pion–nucleon coupling g¯π(1) is uniquely sensitive to dimension-six interactions between right-handed light quarks and the Standard Model Higgs doublet that naturally arises in left-right symmetric models. Recent work has used the g¯π(1)-induced one-loop contribution to the neutron electric dipole moment dn, together with the present experimental dn bound, to constrain the CP-violating parameters of the left-right symmetric model. We show that this and related analyses are based on an earlier meson theory dn computation that is not consistent with the power-counting appropriate for an effective field theory. We repeat the one-loop calculation using heavy baryon chiral perturbation theory and find that the resulting dn sensitivity to g¯π(1) is suppressed, implying more relaxed constraints on the parameter space of the left-right symmetric model. Assuming no cancellations between this loop contribution and other contributions, such as the leading order EDM low-energy constant, the present limit on dn implies |g¯π(1)|≲1.1×10−10.
Highlights
Electric dipole moments (EDMs) of neutral atoms, molecules, and the neutron provide sensitive probes of new sources of timereversal (T) and parity (P) violation
In HBChPT, the loop contributions proportional to κ0,1 arise at nextto-next-to-leading order (NNLO) in the Q /mN expansion, while κ -independent contributions start off at next-to-leading order (NLO)
In terms of the “left-right symmetric” four-quark interaction (4), the NLO loop contributions and the chiral/naïve dimensional analysis (NDA) estimates for the short distance LEC are numerically comparable the loops can be expected to be somewhat smaller due to an mπ /mN suppression
Summary
Electric dipole moments (EDMs) of neutral atoms, molecules, and the neutron provide sensitive probes of new sources of timereversal (T) and parity (P) violation. Even without specifying to the LRSM, the dn limits on Cφud constrain the magnitude of possible effects in T-odd neutron decay correlations In both cases, the dn constraints relied on an earlier pion-loop calculation performed by the authors of Ref. Both the sign and magnitude of NDA and/or model estimates for the short distance contributions are subject to uncertainty, so the most conservative implications will be inferred from the calculable long-distance terms In this respect, we note that the diamagnetic EDMs provide an in principle more robust benchmark than dn, as the nuclear Schiff moment arises from tree-level pion exchange between two nucleons and is relatively free from the uncertainties associated with short-distance contributions.
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