New experimental results for a vector boson A

The search for a new vector boson, $A'$, in the test run of A' EXperiment (APEX) results in a limit for a weak coupling $\alpha' \ga 10^{-6} \alpha$ to electrons ($\alpha=e^2/4\pi$) in the mass range 175 MeV $< m_{A'} <$ 250 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 plan to search for the $A'$ by using the CEBAF electron beam at energies of $\approx$ 1--4 GeV incident on $0.5-10\%$ radiation length multi-foil Tungsten targets, and measure the resulting $e^+e^-$ pairs using the High Resolution Spectrometers and a septum magnet in Hall A at Jefferson Lab. With a 33-day run, the experiment will achieve very good sensitivity because the statistics of $e^+e^-$ pairs will be $\sim 10,000$ times larger in the explored mass range than in any previous search for the $A'$ boson. This talk will discuss the experiment and present the