The Type II Dirac Seesaw Portal to the mirror sector: Connecting neutrino masses and a solution to the strong CP problem

Abstract

We present a version of the Type II Seesaw mechanism for parametrically small Dirac neutrino masses. Our model starts from an $\text{SU}(2)_\text{L} \otimes \text{SU}(2)'\otimes \text{U}(1)_\text{X}$ gauge extension of the Standard Model involving a sector of mirror fermions. A bidoublet scalar with a very small vacuum expectation value connects the SM leptons with their mirror counterparts and we can identify the mirror neutrino with the right-handed neutrino. Similar to the conventional Type II Seesaw, our particle spectrum features singly- and doubly-charged scalars. The strong CP problem is solved by a discrete exchange symmetry between the two sectors that forces the contributions of quarks and mirror quarks to the strong CP phase to cancel each other. We discuss the low-energy phenomenology, comment on the cosmological implications of this scenario and indicate how to realize successful Dirac leptogenesis.