An Electron Fixed Target Experiment to Search for a New Vector Boson A' Decaying to e+e-

TL;DR

This work proposes the A' Experiment (APEX), a fixed-target electron-scattering search for a new GeV-scale vector boson A' that couples to electrons via kinetic mixing. By using CEBAF, long tilted tungsten wire-mesh targets, and High Resolution Spectrometers to measure $e^+e^-$ invariant masses with excellent mass resolution, APEX aims to detect a narrow resonance across 65–550 MeV with $\alpha'/\alpha$ as small as $\sim (6-8)\times 10^{-8}$ (down to ~2×10^{-7} at higher masses). The approach leverages high statistics, sophisticated background control of QED trident processes, and detailed MC-based reach calculations to significantly improve existing constraints on electron-coupled new forces. If realized, the experiment would probe dark matter–related scenarios and provide a versatile framework for similar searches at other facilities, with broad implications for beyond-Standard-Model physics.

Abstract

We describe an experiment to search for a new vector boson A' with weak coupling alpha' > 6 x 10^{-8} alpha to electrons (alpha=e^2/4pi) in the mass range 65 MeV < m_A' < 550 MeV. New vector bosons with such small couplings arise naturally from a small kinetic mixing of the "dark photon" A' with the photon -- one of the very few ways in which new forces can couple to the Standard Model -- and have received considerable attention as an explanation of various dark matter related anomalies. A' bosons are produced by radiation off an electron beam, and could appear as narrow resonances with small production cross-section in the trident e+e- spectrum. We summarize the experimental approach described in a proposal submitted to Jefferson Laboratory's PAC35, PR-10-009. This experiment, the A' Experiment (APEX), uses the electron beam of the Continuous Electron Beam Accelerator Facility at Jefferson Laboratory (CEBAF) at energies of ~1-4 GeV incident on 0.5-10% radiation length Tungsten wire mesh targets, and measures the resulting e+e- pairs to search for the A' using the High Resolution Spectrometer and the septum magnet in Hall A. With a ~1 month run, APEX will achieve very good sensitivity because the statistics of e+e- pairs will be ~10,000 times larger in the explored mass range than any previous search for the A' boson. These statistics and the excellent mass resolution of the spectrometers allow sensitivity to alpha'/alpha one to three orders of magnitude below current limits, in a region of parameter space of great theoretical and phenomenological interest. Similar experiments could also be performed at other facilities, such as the Mainz Microtron.

Figures (12)

• Figure 1: Anticipated 2$\sigma$ sensitivity in $\alpha'/\alpha = \epsilon^2$ for the $A'$ experiment (APEX) at Hall A in JLab (thick blue line), with existing constraints on an $A'$ from electron and muon anomalous magnetic moment measurements, $a_e$ and $a_{\mu}$ (see Pospelov:2008zw), the BaBar search for $\Upsilon(3S)\to \gamma\mu^+\mu^-$:2009cp, and three beam dump experiments, E137, E141, and E774 Bjorken:1988asRiordan:1987awBross:1989mp (see Bjorken:2009mm). The $a_{\mu}$ and $\Upsilon(3S)$ limits assume equal-strength couplings to electrons and muons. The red region indicates the region of greatest theoretical interest, as described in the text. The gray dashed line indicates the scale used for other plots in this paper. The irregularity of the reach is an artifact of combining several different run settings (see Table \ref{['tab:bigBGtable']}). The precise mass range probed by this type of experiment can be varied by changing the spectrometer angular settings and/or the beam energies. We stress this point as other experimental facilities may be able to perform experiments similar to APEX, but targeting complementary regions of parameter space.
• Figure 2: Left: Dark matter annihilation into the dark photon $A'$, which decays into charged leptons such as electrons and/or muons, can explain the cosmic-ray electron and/or positron excesses seen by PAMELA, Fermi, ATIC, HESS, and other experiments. Right: Dark matter scattering into an excited state off nuclei through $A'$ exchange in direct dark matter detection experiments can explain the annual modulation signal observed by DAMA/LIBRA, and the null results of other direct detection experiments.
• Figure 3: Anticipated 2$\sigma$ sensitivity in $\alpha'/\alpha = \epsilon^2$ for the $A'$ experiment (APEX) at Hall A in JLab (thick blue line), compared with current limits and estimated potential $2\sigma$ sensitivity for $A'$ searches in existing data (dashed lines), assuming optimal sensitivity as described in the text. From left to right: KTeV $\pi^0 \rightarrow \gamma A' \rightarrow \gamma e^+e^-$ (orange dashed curve), KLOE $\phi \rightarrow \eta A' \rightarrow \eta e^+e^-$ (green dashed curve) and Belle $e^+e^- \rightarrow \gamma A' \rightarrow \gamma \mu^+\mu^-$ (gray dashed curve). Existing constraints are as in Figure \ref{['fig:bigSummary']}.
• Figure 4: $A'$ production by bremsstrahlung off an incoming electron scattering off protons in a target with atomic number $Z$.
• Figure 5: The layout of the experimental setup --- see text for details.