Novel loop-diagrammatic approach to QCD $\theta$ parameter and application to the left-right model

Abstract

When the QCD axion is absent in full theory, the strong $CP$ problem has to be explained by an additional mechanism, e.g., the left-right symmetry. Even though tree-level QCD $\bar{\theta }$ parameter is restricted by the mechanism, radiative corrections to $\bar{\theta }$ are mostly generated, which leads to a dangerous neutron electric dipole moment (EDM). The ordinary method for calculating the radiative $\bar{\theta }$ utilizes an equation $\bar{\theta }=-\text{arg}\text{det}{m}_q^{\mathrm{loop}}$ based on the chiral rotations of complex quark masses. In this paper, we point out that when full theory includes extra heavy quarks, the ordinary method is unsettled for the extra quark contributions and does not contain its full radiative corrections. We formulate a novel method to calculate the radiative corrections to $\bar{\theta }$ through a direct loop-diagrammatic approach, which should be more robust than the ordinary one. As an application, we investigate the radiative $\bar{\theta }$ in the minimal left-right symmetric model. We first confirm a seminal result that two-loop level radiative $\bar{\theta }$ completely vanishes (corresponding to one-loop corrections to the quark mass matrices). Furthermore, we estimate the size of a non-vanishing radiative $\bar{\theta }$ at three-loop level. It is found that the resultant induced neutron EDM is comparable to the current experimental bound, and the expected size is restricted by the perturbative unitarity bound in the minimal left-right model.

32 pages, 5 figures, 2 tables; v2: matches version published in JHEP

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Cited by in corpus (6)

• Closer look at the matching condition for radiative QCD $\theta$ parameter
• CP-violating axion interactions II: axions as Dark Matter
• P and CP solution of the strong CP puzzle
• Impact of the Electroweak Weinberg Operator on the Electric Dipole Moment of Electron
• Two-loop corrections to QCD $\theta$ angle from evanescent operator in the BMHV scheme
• Effective description of Nelson-Barr models and the theta parameter