Constraints on dark axion portal: missing energy and fermion EDMs

TL;DR

The paper investigates a dark axion portal framework in which ALP–photon–dark-photon interactions connect the SM to a dark sector via $\mathcal{L}_{\text{dark axion portal}} \supset \frac{g_{a\gamma\gamma_D}}{2} a F_{\mu\nu} \widetilde{F}^{\prime\mu\nu}$, with the dark photon $γ_D$ decaying predominantly to dark fermions $χ$. It computes missing-energy signals in electron fixed-target experiments from bremsstrahlung-like $γ^* \to a γ_D$ and vector-meson decays $V \to a γ_D$, assuming $γ_D \to χ\barχ$ and $m_a \ll m_{γ_D}$. The authors derive NA64$e$ bounds using existing data and predict LDMX reach, highlighting that vector-meson channels can dramatically enhance sensitivity for sub-GeV $m_{γ_D}$, while the LDMX reach improves notably with $\text{EOT} \sim 10^{16}$. They also derive EDM-based constraints on CP-violating, fermion-specific ALP couplings from electron, muon, and neutron EDM limits, providing complementary bounds on the portal parameters. Overall, the work maps out a multi-pronged constraint landscape for the dark axion portal and guides future fixed-target searches for invisible dark-sector states.

Abstract

We study a model in which a new interactions between the Standard Model (SM) photon and both the dark photon ($γ_D)$ and an ALP ($a$) are described by the dark axion portal operator. The implications of this dark axion portal scenario for electron fixed-target experiments are presented. In particular, we investigate the missing energy signatures associated with production of dark photons and their subsequent invisible decays into stable dark sector fermions, $γ_D \to χ\barχ$. We discuss the discovery potential for such a scenario and derive projected sensitivity curves for the NA64$e$ and LDMX experiments. Furthermore, novel constraints of the NA64$e$ for $9.37\times 10^{11}$ electrons on target are derived by considering two production mechanisms for invisible states: (i)~the bremsstrahlung-like emission of an $aγ_D$ pair, $e N \to e N γ^* (\to a γ_D)$, and (ii)~the exclusive vector meson photoproduction, $γ^* N \to N V$, followed by the invisible decays of vector mesons, $V \to a γ_D$. Additionally, the constraints on the parameter space of $CP$-violating, fermion-specific ALP and dark photon couplings are established. These constraints are derived from current experimental bounds on the electric dipole moments (EDMs) of SM fermions, incorporating loop-induced contributions to the EDMs of the electron, muon, and neutron.

Figures (6)

• Figure 1: Feynman diagrams illustrating the bremsstrahlung-like production of an ALP and a dark photon in electron-nucleus scattering, $e N \to e N a \gamma_D$. These diagrams correspond to the missing-energy signature within the minimal dark axion portal scenario, governed by the effective Lagrangian $\mathcal{L} \supset \frac{1}{2} g_{a\gamma \gamma_D} a F_{\mu \nu} \widetilde{F}^{\prime \mu \nu}$. In this analysis, the produced dark photon decays predominantly into invisible dark sector fermions, $\gamma_D \to \chi \bar{\chi}$. Diagrams involving photon emission from the nucleus line are omitted, as their contribution is suppressed by a factor of order $(Z m_e / M_N)^2$ for electron-nucleus scattering and is therefore negligible.
• Figure 2: Feynman diagrams for the radiative invisible vector meson decay. A hard photon $\gamma^*$ is produced in the target, $eN\to eN \gamma^*$, and converts to a vector meson $V$ in the exclusive photoproduction process $\gamma^* N \to N V$ in the calorimeter. Then vector meson decays invisibly to the $a\gamma_D$ pair via mixing with the ordinary of-shell photon, $\gamma^*$. The produced dark photon dominantly decays into invisible DS particles, $\gamma_D\to \chi \bar{\chi}$.
• Figure 3: The plot shows the number of $a \gamma_D$ pairs produced versus $\gamma_D$ mass for both LDMX ($10^{16}$ EOT) and NA64e ($5\times 10^{12}$ EOT), with the coupling constant chosen to be $g_{a\gamma\gamma_D}=1~\hbox{GeV}^{-1}$. The hidden particles are produced in bremsstrahlung-like reactions and various meson decays. Left panel: the resulted number of invisible signal events for the LDMX experiment, the $a\gamma_D$ pairs are produced through the bremsstrahlung-like channel $\gamma^*\to a\gamma_D$ (red line), rho meson $\rho \to a\gamma_D$ (brown line), omega meson $\omega \to a\gamma_D$ (orange line), and phi meson decay $\phi \to a\gamma_D$ (green line). The resulted number of $a\gamma_D$ pairs produced at the LDMX is shown (blue line). Right panel: the same as in the left panel, but for the NA64e electron missing momentum facility, implying that the dominant signal mechanisms are due to the virtual photon emission $\gamma^* \to a\gamma_D$ (red line) and $J/\psi\to a \gamma_D$ decays (black line).
• Figure 4: The limits at $90~\%~\hbox{CL}$ on $g_{a\gamma\gamma_D}$ coupling from ATLAS, BaBar, and the fixed-target experiments for the minimal dark ALP portal setup as a function of the dark photon mass $m_{\gamma_D}$. For the accelerator based sensitivity curves we imply that $\hbox{Br}(\gamma_D\to \chi \bar{\chi})\simeq 1$ and $m_a =10\, \hbox{keV}$. Green dashed line is the projected sensitivity for NA64$e$ experiment for $\hbox{EOT}\simeq 5\times 10^{12}$, and blue dashed line corresponds to the projected sensitivity of LDMX facility for $\hbox{EOT}\simeq 10^{16}$. The shaded green region shows the parameter space excluded by the NA64$e$ experiment for $\hbox{EOT}\simeq 9.37\times 10^{11}$. For completeness, by dashed dotted green lines we show the expected reaches of NA64e for $\hbox{EOT}\simeq 10^{13}$ with a finite background, $b\simeq 1$, and background free, $b\simeq 0$, case. We also show the BaBar Zhevlakov:2022vio excluded bounds for the data on monophoton missing energy signature, $e^+e^-\to \gamma a \gamma_D$. The BaBar limits BaBar:2009gco from invisible decays $\Upsilon(1S) \to a\gamma_D$ are shown by brown solid line. The projected bounds of ATLAS ($\mathcal{L}\simeq 3~\hbox{ab}^{-1}$) associated with muon missing momentum process, $\mu N \to \mu N a \gamma_D$ are shown by black dashed line Galon:2019owlATLAS:2016lqx. The region of parameter space excluded by supernova (SN) is shown by shaded red area Hook:2021ous.
• Figure 5: Feynman diagrams which generate EDM terms due to axion coupling with both dark and SM photons (see, e. g., Eq. (\ref{['LagrangianVectorAxionEDM']}) for detail).