SMART U(1)$$_X$$: standard model with axion, right handed neutrinos, two Higgs doublets and U(1)$$_X$$ gauge symmetryNobuchika Okada, Digesh Raut and Qaisar ShafiEur. Phys. J. C, 80 11 (2020) 1056DOI: https://doi.org/10.1140/epjc/s10052-020-8343-6