New Electron Beam-Dump Experiments to Search for MeV to few-GeV Dark Matter

TL;DR

The paper addresses the challenge of probing sub-GeV dark matter that interacts via GeV-scale mediators by proposing a compact detector downstream of an electron beam-dump. Using both analytic scaling and Monte Carlo methods, it shows that such a fixed-target setup can produce and detect χ particles with sensitivities reaching ε ~ 10^{-5}–10^{-3} for $m_{A'}$ up to a few GeV, yielding potentially thousands of events per $10^{22}$ electrons on target. The approach is complementary to B-factory searches, offering access to the MeV–GeV regime where visible-decay searches struggle due to backgrounds, and can be implemented parasitically at existing facilities with strong background suppression strategies. The work also analyzes production mechanisms, geometric acceptance, signal channels, backgrounds, and provides concrete benchmark scenarios, highlighting practical paths toward testing light dark-sector models and the $(g-2)_μ$-motivated parameter space.

Abstract

In a broad class of consistent models, MeV to few-GeV dark matter interacts with ordinary matter through weakly coupled GeV-scale mediators. We show that a suitable meter-scale (or smaller) detector situated downstream of an electron beam-dump can sensitively probe dark matter interacting via sub-GeV mediators, while B-factory searches cover the 1-5 GeV range. Combined, such experiments explore a well-motivated and otherwise inaccessible region of dark matter parameter space with sensitivity several orders of magnitude beyond existing direct detection constraints. These experiments would also probe invisibly decaying new gauge bosons ("dark photons") down to kinetic mixing of ε~ 10^{-4}, including the range of parameters relevant for explaining the (g-2)_μ discrepancy. Sensitivity to other long-lived dark sector states and to new milli-charge particles would also be improved.

Figures (7)

• Figure 1: Schematic experimental setup. A high-intensity multi-GeV electron beam impinging on a beam dump produces a secondary beam of dark sector states. In the basic setup, a small detector is placed downstream so that muons and energetic neutrons are entirely ranged out. In the concrete example we consider, a scintillator detector is used to study quasi-elastic $\chi$-nucleon scattering at momentum transfers $\mathrel{\hbox{$\sim$} \hbox{$>$}} 140$ MeV, well above radiological backgrounds, fast neutrons, and noise. Similar layouts with much smaller detectors or shorter target-detector distances than shown above are similarly sensitive. To improve sensitivity, additional shielding or vetoes can be used to actively reduce high energy cosmogenic and other environmental backgrounds.
• Figure 2: a) $\chi \bar{\chi}$ pair production in electron-nucleus collisions via the Cabibbo-Parisi radiative process (with $A'$ on- or off-shell) and b) $\chi$ scattering off a detector nucleus and liberating a constituent nucleon. For the momentum transfers of interest, the incoming $\chi$ resolves the nuclear substructure, so the typical reaction is quasi-elastic and nucleons will be ejected.
• Figure 3: The $\epsilon^2$ sensitivity of electron-beam fixed-target experiments plotted alongside existing constraints for benchmark values of $m_\chi$, $m_{A'}$, and $\alpha_D$. The solid, dashed, and dot-dashed red curves mark the parameter space for which our basic setup --- a $12~\GeV$ beam impinging on an aluminum beam dump, with a 1 m$^3$ mineral oil detector placed 20 m downstream of the dump --- respectively yields 40, $10^3$, and $2 \cdot 10^4$$\chi$-nucleon quasi-elastic scattering events with $Q^2 \gtrsim (140~\MeV)^2$ per $10^{22}$ electrons on target (EOT). The orange curve shows the 10 event reach for an ILC style 125 GeV beam assuming the same detector and luminosity. Comparable sensitivity can be achieved with much smaller fiducial volumes than we consider, especially for detectors with active muon and neutron shielding and/or veto capabilities. The upper plots show the $\epsilon$ sensitivity for $\alpha_D =0.1$ (left) and $\alpha_D =1$ (right). In these plots LSND may also have sensitivity to $\epsilon^2 \sim 10^{-8} -10^{-6}$ via $\pi^0 \to \gamma \chi \bar{\chi}$ decays for $2 m_{\chi} < m_{A'} < m_{\pi}$deNiverville:2013. The lower left plot shows the reach for $m_\chi = m_{\pi^0} /2 \simeq 68$ MeV where the production from pion decays is kinematically inaccessible and LSND has no significant sensitivity. The lower right plot recasts the $\epsilon^2$ sensitivity for fixed $m_{A'}$ and $\alpha_D$ as a (model-dependent) probe of the $\chi$-electron direct detection cross section $\sigma_{\chi e}$ and includes XENON 10 limits from Essig:2012yx. The black curve assumes $\Omega_\chi = \Omega_{DM}$; the direct detection constraint is weaker when $\chi$ is only a component of the total abundance. The light green band is the region in which an $A^\prime$ resolves the $(g-2)_\mu$ discrepancy to within $2 \sigma$; the dark green curve is the boundary at which contributions to $(g-2)_\mu$ exceed the measured value by $5 \sigma$Pospelov:2008zw. The bound from $e^+ e^- \to \gamma \, +$ invisibles is introduced in detail in section \ref{['ssec:collider']} Other constraints in the literature arise from invisible $J/\psi$ decays Ablikim:2007ek searches Aubert:2008as, rare kaon decays Artamonov:2008qb, and contributions to $(g-2)_e$Giudice:2012ms; for a discussion see section \ref{['ssec:otherlab']}.
• Figure 4: The "luminosity correction" factor $F(q_{min}=m_{A'}^2/(2 E_{beam}),q_{max}=m_{A'})$ defined in \ref{['fdef']}, for use in estimating $A'$ yield. $F$ is proportional to the Weizsacker-Williams effective photon flux $\Phi(q_{min},q_{max})$, multiplied by a material-dependent luminosity factor. The red solid and blue dashed curves correspond to 12 GeV electron beams impinging on a thick Aluminum or Beryllium target (i.e. $q_{max}= \sqrt{2\cdot 12\GeV\cdot q_{min}}$). For the $A'$ mass ranges of interest, $F$ can depend sensitively on $q_{min}$ but far less on $q_{max}$, so that these curves remain approximately valid for other beam energies. For example, scaling beam energy and $m_{A'}^2$ simultaneously by up to a factor of 10 (not shown), so that $q_{min}$ is unchanged but $q_{max}$ scales by $\sqrt{10}$, only affects $F$ at the $\sim 20\%$ level or smaller. The expression for $F$ shown here includes only coherent elastic nuclear scattering and quasi-elastic scattering off nucleons; neglecting inelastic contributions may underestimate the $F$'s for 1--2 GeV $A'$ masses at the highest $q_{min}$ shown by a factor of $\sim 5$.
• Figure 5: Same as Figure \ref{['fig:SummaryReach']} with overlays of curves for $\alpha_D = 1, 0.1$ The gray region is excluded by the same curves shown in \ref{['fig:SummaryReach']} and described in the caption.