New Fixed-Target Experiments to Search for Dark Gauge Forces

TL;DR

The paper analyzes light, kinetically mixed dark gauge bosons $A'$ with mass $m_{A'}$ in the MeV–GeV range and couplings set by $\epsilon$, motivated by anomalies suggesting new U(1)' dynamics. It formalizes the kinetic-mixing framework, derives production and decay properties in fixed-target collisions via bremsstrahlung, and provides simple rate estimates to map experimental sensitivity, including irreducible backgrounds. It then proposes five concrete fixed-target experimental scenarios (A–E) using current GeV-scale electron beams to cover complementary regions of parameter space, balancing beam-dump, thin-target, vertexing, and high-resolution spectrometry approaches. Together with existing constraints, these designs aim to exhaust the natural parameter space for $A'$ in the MeV–GeV range and to provide coverage relevant to dark matter phenomenology and terrestrial/astrophysical hints. The work emphasizes practical implementation at facilities like JLab and outlines extensions to broader dark-sector scenarios and muon-beam opportunities for further reach.

Abstract

Fixed-target experiments are ideally suited for discovering new MeV-GeV mass U(1) gauge bosons through their kinetic mixing with the photon. In this paper, we identify the production and decay properties of new light gauge bosons that dictate fixed-target search strategies. We summarize existing limits and suggest five new experimental approaches that we anticipate can cover most of the natural parameter space, using currently operating GeV-energy beams and well-established detection methods. Such experiments are particularly timely in light of recent terrestrial and astrophysical anomalies (PAMELA, FERMI, DAMA/LIBRA, etc.) consistent with dark matter charged under a new gauge force.

Figures (11)

• Figure 1: Left: Existing constraints on an $A'$. Shown are constraints from electron and muon anomalous magnetic moment measurements, $a_e$ and $a_{\mu}$, the BaBar search for $\Upsilon(3S)\to \gamma\mu^+\mu^-$, three beam dump experiments, E137, E141, and E774, and supernova cooling (SN). These constraints are discussed further in Section \ref{['sec:constraints']}. Right: Existing constraints are shown in gray, while the various lines --- light green (upper) solid, red short-dashed, purple dotted, blue long-dashed, and dark green (lower) solid --- show estimates of the regions that can be explored with the experimental scenarios discussed in Section \ref{['subsec:expA']}--\ref{['subsec:expE']}, respectively. The discussion in \ref{['sec:designs']} focuses on the five points labeled "A" through "E". The orange stripe denotes the "D-term" region introduced in section \ref{['sec:TheoryPrelim']}, in which simple models of dark matter interacting with the $A'$ can explain the annual modulation signal reported by DAMA/LIBRA. Along the thin black line, the $A'$ proper lifetime $c\tau=80 \mu$m, which is approximately the $\tau$ proper lifetime.
• Figure 2: $A'$ production by bremsstrahlung off an incoming electron scattering off protons in a target with atomic number $Z$.
• Figure 3: (a) $\gamma^*$ and (b) Bethe-Heitler trident reactions that comprise the primary QED background to $A'\rightarrow \ell^+\ell^-$ search channels.
• Figure 4: Left: Experimental scenario for benchmark point A ($\epsilon\sim 10^{-5}$, $m_{A'}\sim 50$ MeV). An electron beam is incident on a 10 cm thick tungsten target. Behind the target is a 10 cm (or thicker) shield followed by an instrumented decay region consisting of a combination of tracking planes, electromagnetic calorimetry and scintillator triggers. Right: Reaches of the high- and low-energy dump configurations described in Section \ref{['subsec:expA']}, delineated by regions with 10 or more events and the following configurations --- Blue (inner) Solid Contour:$0.3$ C total charge dumped with a 200 MeV electron beam, a 20 cm shield, and a detector with 5 cm radius 50 cm behind the front of the target. The lepton pair must have total energy exceeding 100 MeV. Blue (inner) Dashed Contour: same configuration, but with no shield. Green (outer) Solid Contour:$0.1$ C (100 nA beam $\times$$10^6$ s) total charge dumped with a 6 GeV electron beam, a 3.9 m shield, and a detector with 10 cm radius 7 m downstream. The lepton pair must have total energy exceeding 3 GeV. Green (outer) Dashed Contour: same configuration, but with 0.9 m of shielding. Gray contours and Orange Stripe: exclusions from past experiments (E137 and E141) and the region that explains DAMA/LIBRA in a simple model --- see Figure \ref{['fig:bigSummary']} for more details.
• Figure 5: Left: Experimental scenario for a small two-arm spectrometer for benchmark point B ($\epsilon\sim 3\times 10^{-5}$, $m_{A'}\sim 200$ MeV). An electron beam is incident upon a thin 0.1 radiation length tungsten target. A small two-arm spectrometer with silicon-strip trackers and a fast calorimeter or scintillator trigger is downstream from the target. Signal events are identified by requiring a displaced vertex $\sim$ 1 cm behind the target. More details are given in the text. Right: Regions corresponding to 10 or more events within acceptance in $10^6$ sec for three different geometries. From right to left: 6 GeV electron beam at 100 nA (0.1 C delivered), with angular acceptance from 20 to 55 mrad and a 1 m long detector (solid red line); 6 GeV beam at 5 nA ($5\times 10^{-3}$ C delivered), with angular acceptance from 10 to 27 mrad in a 2 m-long detector region (dashed darker red line); and 2 GeV beam at 0.5 nA ($5\times 10^{-4}$ C delivered) with the same geometry as the dashed red line (solid dark red line). In all cases, we require that the $A'$ carry at least 83% of the beam energy, the track impact parameters at the target exceed 50 $\mu$m, and the reconstructed vertex displacement exceed 1 cm. We assume 50% $\phi$ coverage. Gray contours and Orange Stripe: exclusions from past experiments (E137 and E141) and the region that explains DAMA/LIBRA in a simple model --- see Figure \ref{['fig:bigSummary']} for more details.