Dark photon and U(1)$_{B-L}$ gauge boson from dark Higgs boson decays at FASER and SHiP

Abstract

We study the sensitivity to dark photons and U(1)$_{B-L}$ gauge bosons produced via dark Higgs boson decays at the FASER and SHiP experiments. In addition to pair production of these vector bosons from both on-shell and off-shell dark Higgs boson decays, a new production process of single vector boson associated with the standard model particles is taken into account. Constraints on the parameter space of dark photon are derived with including the latest results from the FASER experiment. The expected sensitivity regions to the dark photon and U(1)$_{B-L}$ gauge boson of the future FASER2 and SHiP experiments are presented. The sensitivity to the U(1)$_{B-L}$ model with freeze-in sterile neutrino dark matter is also discussed.

Figures (11)

• Figure 1: Feynman diagrams of $\phi \to A' f \bar{f}$
• Figure 2: The decay branching ratios of the dark Higgs boson in the dark photon model. Left: the branching ratios of $\phi \rightarrow A'A'$ (solid) and $\phi \rightarrow A' + \mathrm{SM}$ (dashed) for $m_\phi = 0.5$ GeV (red), $1.2$ GeV (blue), and $3.0$ GeV (green) with $\varepsilon = 10^{-4}$. The vertical dotted lines represent the kinematical thresholds for each $m_\phi$. Right: the branching ratios for $\varepsilon = 10^{-3}$ (red), $10^{-4}$ (blue), $10^{-5}$ (green), and $10^{-6}$ (orange) with $m_\phi = 2.0$ GeV. In both panels, the scalar mixing and the gauge coupling constant are fixed to $\alpha = 10^{-4}$ and $g' = 0.5$, respectively.
• Figure 3: The decay lengths of the dark Higgs boson in the dark photon model. Left: the decay lengths for $m_\phi = 0.5$ GeV (red solid), $1.2$ GeV (blue dashed), and $3.0$ GeV (green dotted) with $\varepsilon = 10^{-4}$. Right: the decay lengths for $\varepsilon = 10^{-3}$ (red solid), $10^{-4}$ (blue dashed), $10^{-5}$ (green dotted), and $10^{-6}$ (orange dashed-dotted) with $m_\phi = 2.0$ GeV. In both panels, the scalar mixing and the gauge coupling constant are fixed to $\alpha = 10^{-4}$ and $g' = 0.5$, respectively.
• Figure 4: The distribution of the dark photons produced by the on-shell dark Higgs decays at FASER (upper), FASER2 (middle) and SHiP (lower) in $(\theta_{A'},~p_{A'})$ plane. The masses of the dark Higgs and dark photon are indicated in panels. The parameters are fixed to $g' = 0.05$ and $\alpha=10^{-3}$ for FASER, and $g' = 0.5$ and $\alpha=10^{-4}$ for FASER2 and SHiP, respectively. The kinetic mixing is taken to $\epsilon=10^{-4}$ for the off-shell dark photon decay.
• Figure 5: The $95$% C.L. excluded regions by FASER for the dark photon decays for $m_\phi=0.5$ (top left), $1.2$ (top right), $2.0$ (bottom left), and $3.0$ (bottom right) GeV. The gauge coupling and the scalar mixing are taken to $g'=0.05$ and $\alpha=10^{-3}$, respectively