SLAC-PUB-13882 SU-ITP-10/01 An Electron Fixed Target Experiment to Search for a New Vector Boson A ′ Decaying to e $^{+}$e − Rouven Essig, 1 Philip Schuster, 1 Natalia Toro, 2 and Bogdan Wojtsekhowski 3 $^{1}$Theory Group, SLAC National Accelerator Laboratory, Menlo Park, CA 94025 $^{2}$Theory Group, Stanford University, Stanford, CA 94305 $^{3}$Thomas Jefferson National Accelerator Facility, Newport News, VA 23606 (Dated: October 23, 2018) We describe an experiment to search for a new vector boson A ′ with weak coupling α ′ ≳ 6 × 10 − $^{8}$α to electrons ( α = e $^{2}$/ 4 π ) 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 [1]. 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 α $^{′}$/α 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. The development of the Standard Model of particle interactions is the culmination of a century of searches and analyses with fixed-target and colliding beam exper- iments. Interactions with new forces beyond the Stan- dard Model are currently limited by well-tested gauge symmetries to a handful of possibilities. One of the few remaining ways for interactions with new sub-GeV vector-like forces to arise is for charged particles to ac- quire millicharges, ϵe , under these forces. This occurs through a simple and generic mechanism proposed by Holdom [2], in which a new vector particle A ′ µ mixes via quantum loops with the Standard Model photon. MeV– GeV masses for the A ′ gauge boson are particularly well- motivated in this context. Such sub-GeV forces are a common feature of extensions of the Standard Model, but existing constraints are surprisingly weak, with lim- its at ϵe ≲ (0 . 3 − 1) × 10 − $^{2}$e . Fixed-target experiments with high-intensity electron beams and existing precision spectrometers are ideally suited to explore sub-GeV forces by probing reactions in which a new A ′ vector particle is produced by radiation off an electron beam [3, 4]. The A ′ can decay to an elec- tron and positron pair and appears as a narrow resonance of small magnitude in the invariant mass spectrum. The production rate of A ′ s , the luminosity, and the mass res- In this paper, we shall focus on a search for new vec- tor bosons. However, it should be emphasized that this Fixed-target experiments of this form are particularly timely in light of a series of recent anomalies from terres- trial, balloon-borne, and satellite experiments that sug- gest that dark matter interacts with Standard Model par- ticles. Much of this data sharply hints that dark matter is directly charged under a new force mediated by an A ′ and not described by the Standard Model. Theoretical as well as phenomenological expectations suggest an A ′ mass m$_{A}$ ′ ≲ 1 GeV and ϵe ≲ 10 − $^{2}$e . olution attainable at, for example, Jefferson Laboratory (JLab), the Mainz Microtron, and the SLAC National Accelerator Laboratory vastly exceeds what is currently available using colliding electron beam facilities. In [3], several fixed-target experimental strategies were outlined to search for new sub-GeV vector interactions. In this pa- per, we summarize a concrete A ′ search using Jefferson Laboratory’s Continuous Electron Beam Accelerator Fa- cility (CEBAF) and the High Resolution Spectrometers (HRS) in Hall A [1], highlighting the features that are applicable to similar experimental facilities. This exper- iment, the A ′ Experiment (APEX), can probe charged particle couplings with new forces as small as 2 × 10 − $^{4}$e and masses between 65 MeV and 550 MeV — an im- provement by more than two orders of magnitude in cross section sensitivity over all previous experiments. arXiv:1001.2557v1 [hep-ph] 15 Jan 2010 I. INTRODUCTION