Examining scalar portal inelastic dark matter with lepton fixed target experiments

Abstract

Inelastic dark matter scenarios have attracted considerable attention in contemporary particle physics. In this study, we investigate the phenomenology of sub-GeV inelastic dark matter interacting via a lepton-specific scalar portal. By solving the Boltzmann equations, we obtain thermal target curves for several inelastic DM mass splittings in the sub-GeV mediator-mass range. We study the discovery potential of lepton fixed-target experiments, particularly NA64e, LDMX, and NA64$μ$, via their missing-energy signatures. Our analysis focuses on the $φ$-strahlung process, $l N \to l N φ$, followed by the invisible decay of the scalar mediator into Majorana dark matter particles $φ\to χ_1 χ_2$. We use this channel to probe the mediator coupling to charged leptons of the Standard Model. For phenomenologically viable parameters of the inelastic dark matter scenario, we derive projected sensitivities for NA64e, LDMX, and NA64$μ$, assuming that the boosted state $χ_2$ decays visibly via $χ_2 \to χ_1 e^+ e^-$ outside the detector acceptance. Our results demonstrate the complementary roles of electron- and muon-beam experiments in exploring the sub-GeV inelastic dark matter sector interacting via a scalar portal.

Figures (6)

• Figure 1: Feynman diagrams giving rise to $\chi_1 \chi_2 \to \phi \to l^+l^-$ annihilation in the early Universe.
• Figure 2: Feynman diagrams for radiative scalar mediator production $l N \to l N \phi$ followed by invisible decay $\phi \to \chi_1 \chi_2$.
• Figure 3: Feynman diagram describing visible decay of excited state $\chi_2$ to DM particle $\chi_1$ and electron positron pair $e^+ e^-$.
• Figure 4: The projected experimental reach at $90\, \%$ C.L. as a function of $m_\phi$ mass for the benchmark scenario \ref{['eq:EffLagrangianScalarMEDMajoranaiCDM']}. The grey shaded region corresponds to the existing BaBar BaBar:2017tiz monophoton limit from $e^+e^- \to \gamma \phi$. The dashed green, blue, and purple lines show expected limits for NA64e, NA64$\mu$, and LDMX experiments, respectively. The current and projected limits of the NA62 experiment NA62:2021bjiKrnjaic:2019rsv are shown by solid and dashed orange lines, respectively. The red lines correspond to the typical thermal target curves for a set of mass splittings $\Delta_2$.
• Figure 5: The same as in Fig. \ref{['fig:LimitsFTEMajoranaiCDM']} but for the benchmark model \ref{['eq:EffLagrangianScalarMEDMajoranaG5iCDM']}.