Probing general $U(1)'$ models with non-universal lepton charges at FASER/FASER2, COHERENT and long-baseline oscillation experiments

The general anomaly-free $U(1)'$ models allow non-universal lepton charges. We explore the sensitivities of FASER/FASER2, COHERENT and DUNE/T2HK precision experiments to the new gauge boson $Z'$ and the new CP-even scalar $\phi$. With non-universal lepton charges, distinctive reaches at FASER/FASER2 emerge in the regime of low $m_{Z'}$ and small gauge coupling $g_{BL}$ for different $U(1)'$ charge setups. The COHERENT experiment and the future long-baseline experiments DUNE/T2HK also provide complementary probes to the available parameter space. For $m_\phi < 2m_{Z'}$, the search for the scalar $\phi$ at FASER/FASER2 is sensitive to the mixing angle between the scalar singlet and the SM Higgs. In the case of $m_\phi > 2m_{Z'}$, the kinematically allowed decay $\phi\to Z' Z'$ changes the lifetime and decay rates of the scalar $\phi$. The sensitivity reach highly depends on the $Z'$ mass and the gauge coupling $g_{BL}$.