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In each setting, the proposed experiment will accumu- late between 70 and 300 million trident events. With these statistics, it will be possible to search offline for small resonances comprising a few thousandths of the collected data in a resolution-limited window. This will allow sensitivity to new gauge boson couplings α ′ /α as low as 10 − 7 over the broad mass range probed by APEX, as summarized in Figure 10. This sensitivity would im- prove on the cross-section limits from past experiments by a factor of ∼ 10 − 1000. As a specific example, we have illustrated the expected 

sensitivity of setting “A” to A ′ signals with different ϵ in Figure 11. Each component of the target populates a different invariant mass distribution; for simplicity we consider only the contribution from the front planes of the target, with θ$_{eff}$ ≈ 5 . 5 ◦ (recall that the target is extended along the beam line and consists of 4–5 planes in a zig-zag configuration). The top panel illustrates the absolute size of A ′ signals at m$_{A}$ ′ = 200 MeV compared to the continuum trident background (gray line) and the size of 2 and 5-sigma statistical fluctuations (blue and green dashed lines), while the bottom panel illustrates how the same signals would appear after subtracting a smooth parameterization of the background. The purple curves in each panel corresponds to an A ′ signal with α ′ /α = 7 × 10 − 6 at 200 MeV, which according to the estimates in § II D would not be seen or excluded at 2 σ by a future KLOE search in φ → ηA ′ . The red curve 

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has α ′ /α = 1 . 3 × 10 − $^{7}$, corresponding to the expected “5 σ ” sensitivity (not accounting for the trials factor) in APEX. 

This paper summarized a new experiment (“ A ′ experi- ment ”, or “APEX”) that has been proposed to the Jeffer- son Laboratory’s PAC 35 [1]. The experiment proposes to use 30 days of beam to measure the electron-positron 

## VIII. CONCLUSIONS 

