Strong Constraints on Sub-GeV Dark Sectors from SLAC Beam Dump E137

We present new constraints on sub-GeV dark matter and dark photons from the electron beam-dump experiment E137 conducted at SLAC in 1980–1982. Dark matter interacting with electrons (e.g., via a dark photon) could have been produced in the electron-target collisions and scattered off electrons in the E137 detector, producing the striking, zero-background signature of a high-energy electromagnetic shower that points back to the beam dump. E137 probes new and significant ranges of parameter space and constrains the well-motivated possibility that dark photons that decay to light dark-sector particles can explain the $\sim 3.6\sigma$ discrepancy between the measured and standard model value of the muon anomalous magnetic moment. It also restricts the parameter space in which the relic density of dark matter in these models is obtained from thermal freeze-out. E137 also convincingly demonstrates that (cosmic) backgrounds can be controlled and thus serves as a powerful proof of principle for future beam-dump searches for sub-GeV dark-sector particles scattering off electrons in the detector.

Figure 1

Top: Layout of the E137 experiment (adapted from Fig. 2 in Ref. [39]). Middle and bottom: An electron beam hits an aluminum target, creating DM particles $\chi$ via bremsstrahlung of $A^{\prime }$ (bottom left). The $\chi$ particles traverse a $\sim 179\mathrm{m}$ deep hill and another $\sim 204\mathrm{m}$-long open region before scattering off electrons (bottom right), which are detected in an electromagnetic shower calorimeter.

Figure 2

Top left: Constraints (95% C.L.) in the $\epsilon \text{−}{m}_{A^{\prime }}$ plane for dark photons $A^{\prime }$ decaying invisibly to light DM $\chi$, with $m_{\chi }<0.5\mathrm{MeV}$. The SLAC E137 experiment excludes a Dirac fermion (red shading and red solid line) or complex scalar (red long dashed line) DM. We fix ${\alpha }_D=0.1$ and assume an electron recoil threshold energy of $E_{\text{th}}=1\mathrm{GeV}$ in the E137 detector (for comparison, the red dotted line shows $E_{\text{th}}=3\mathrm{GeV}$ for a fermionic $\chi$). Also shown are constraints from the anomalous magnetic moment of the electron ($a_e$, $2\sigma$, blue dashed line) and muon ($a_{\mu }$, $5\sigma$, dark-green dashed line), and a light-green dashed line region in which $A^{\prime }$ explains the $a_{\mu }$ discrepancy. Other model-dependent constraints (see text for details) arise from LSND (yellow solid line), SLAC mQ experiment (cyan solid line), BABAR (blue dotted line), and BNL E787 and E949 (brown dotted line). The inset focuses on $m_{A^{\prime }}=100–300\mathrm{MeV}$. Top right and bottom left: Same as top left but for $m_{\chi }=10$ and 50 MeV, respectively. Above the black solid line, the thermal relic abundance of a scalar $\chi$ satisfies ${\mathrm{\Omega }}_{\chi }\le {\mathrm{\Omega }}_{\text{DM}}$; the region above the blue solid line is excluded if $\chi$ can scatter off electrons in the XENON10 experiment, assuming $\chi$ makes up all the DM; the light-gray regions and dotted lines are excluded from searches for $A^{\prime }\to e^{+}{e}^{-}$ (if this mode is available for $m_{A^{\prime }}<2m_{\chi }$) in E141, E774, Orsay, HADES, or A1. Bottom right: 95% C.L. upper limits on ${\alpha }_D$ as a function of $m_{A^{\prime }}$ for a Dirac fermion $\chi$, assuming $\epsilon$ is fixed to the smallest value consistent with explaining the $a_{\mu }$ anomaly. The E137 constraint is shown for $m_{\chi }<0.5\mathrm{MeV}$ (red shading and solid line) and for $m_{\chi }=10,50\mathrm{MeV}$ (dashed red line), while the remaining constraints are only shown for $m_{\chi }<0.5\mathrm{MeV}$. The solid gray curve represents the transition between $A^{\prime }\to \chi \overline{\chi }$ and $A^{\prime }\to visible$ decays.