Strong Constraints on Sub-GeV Dark Sectors from SLAC Beam Dump E137

Abstract

We present new constraints on sub-GeV dark matter and dark photons from the electron beamdump experiment E137 conducted at SLAC in [1980][1981][1982]. Dark matter interacting with electrons (e.g., via a dark photon) could have been produced in the electron-target collisions and scattered off electrons in the E137 detector, producing the striking, zero-background signature of a high-energy electromagnetic shower that points back to the beam dump. E137 probes new and significant ranges of parameter space, and constrains the well-motivated possibility that dark photons that decay to light dark-sector particles can explain the ∼ 3.6σ discrepancy between the measured and SM value of the muon anomalous magnetic moment. It also restricts the parameter space in which the relic density of dark matter in these models is obtained from thermal freeze-out. E137 also convincingly demonstrates that (cosmic) backgrounds can be controlled and thus serves as a powerful proof-ofprinciple for future beam-dump searches for sub-GeV dark-sector particles scattering off electrons in the detector.INTRODUCTION. Dark matter (DM) with mass below ∼ 1 GeV and interacting with Standard Model (SM) particles through a light mediator is a viable and natural possibility consistent with all known data (see e.g. [1][2][3][4][5][6][7][8][9][10][11][12]). High-intensity fixed-target experiments have impressive sensitivity to such light DM [3]. The basic experimental strategy begins with the production of a relativistic DM beam out of electron or proton collisions with a fixed target, followed by detection via DM scattering in a detector positioned downstream of the target. The prospects of proton fixed-target experiments, including several ongoing neutrino oscillation experiments, have been investigated in [3,[13][14][15][16], and the MiniBooNE experiment at FNAL is presently conducting the first dedicated search [17]. More recently, the potential of electron beam-dump experiments has been explored [18][19][20] [15]. These proposals complement the ongoing efforts to probe sub-GeV DM with low-energy e + e − colliders [21] and direct detection experiments via DM-electron scattering [4,9,22], as well as broader efforts to search for low-mass dark sectors that are weakly coupled to the SM [23,24]. MODELS. We focus on a motivated class of DM models based on a new 'dark' gauge symmetry, U(1) D [25-27], although our discussion applies to any scenario in which DM interacts with electrons. In this framework, the DM χ is charged under U(1) D , which is kinetically mixed with the SM hypercharge, U(1) Y , allowing for DM interactions with the SM [28,29]. If the U(1) D is spontaneously broken, its gauge boson (the 'dark photon' A ) is massive. The low energy effective Lagrangian is

Detecting superlight dark matter with Fermi-degenerate materials

“…ref. [75]). For sub-MeV DM and mediator couplings this large, vanilla DM models may be brought into thermal equilibrium with the photon plasma and affect N eff and/or BBN [78].…”

“…Restricting to processes that do not require coupling to additional SM particles beyond electrons, the relevant experimental bounds come from measurements of the anomalous magnetic moment of the electron (g − 2) e [72][73][74], beam dump experiments such as E137 [75], and low energy e + e − machines such as BaBar [76,77].…”

MeV-scale dark matter deep underground

“…In fact, for an electron beam with energy of 100 MeV, neutrinos originating from muons or pions are no longer kinematically accessible. One of the strongest constraints on light DM comes in fact from E137 in a manner analogous to neutrino experiments [21] where DM production in the target proceeds via eN → eN A /χχ/ϕϕ * . E137 sets a constraint on DM from the non-observation of anomalous electronrecoil events in a detector ∼ 300 meters downstream of the target.…”

“…Indeed, the MeV-to-GeV DM mass window remains an elusive blind spot in the current search effort [1], despite the existence of viable models [2][3][4][5][6][7][8] -including those invoked to explain the excess 511 keV photon line from the galactic bulge [9] with MeV scale DM annihilation into electron-positron pairs [3,4]. Recent progress in our understanding of the status of MeV-scale DM has come from a combination of re-interpretation of surface-level proton-beam neutrino experiments results [10][11][12][13], rare meson decays [14][15][16][17][18], electron beam dump experiments [19][20][21][22], Bfactories [19,23], precision measurements [5,19,24], the CMB [25][26][27][28][29], and DM-electron scattering in direct detection experiments [30].…”

Muong−2anomaly and dark leptonic gauge boson

“…There may be more constraints using the LDM detection such as the E137 beam dump experiments [45], but they depend on other parameters such as the LDM coupling, and we do not consider them in this paper. Part of the unconstrained parameters in this scenario can be probed by the proposed invisibly-decaying dark gauge boson searches at DarkLight [46] and SPS [47,48].…”