New Limits on Hidden Photons from Past Electron Beam Dumps

TL;DR

Hidden photons with mass $m_{\gamma'}$ and kinetic mixing $\chi$ are motivated by extensions of the SM and can be probed in electron beam dumps. The paper develops a framework for production via bremsstrahlung, scattering, and decay behind a fixed target, incorporating thick-target approximations and Monte Carlo acceptance corrections, to derive updated exclusion limits. It adds two new bounds from KEK and Orsay that tighten the GeV-scale region of the $(m_{\gamma'},\chi)$ parameter space and integrates these with existing constraints from other experiments and precision measurements. The results refine the hidden-photon parameter space and inform future fixed-target and beam-dump searches, including upcoming experiments like HPS and APEX.

Abstract

Hidden sectors with light extra U(1) gauge bosons, so-called hidden photons, have recently attracted some attention because they are a common feature of physics beyond the Standard Model like string theory and supersymmetry and additionally are phenomenologically of great interest regarding recent astrophysical observations. The hidden photon is already constrained by various laboratory experiments and presently searched for in running as well as upcoming experiments. We summarize the current status of limits on hidden photons from past electron beam dump experiments including two new limits from such experiments at the High Energy Accelerator Research Organization in Japan (KEK) and the Laboratoire de l'accelerateur lineaire (LAL, Orsay) that have so far not been considered. All our limits take into account the experimental acceptances obtained from Monte Carlo simulations.

Figures (3)

• Figure 1: Sketch of the setup of an electron beam dump experiment illustrating the definitions of the lengths $L_\mathrm{sh}$, $L_\mathrm{dec}$ and $L_\mathrm{tot}$ used in the text. An incoming electron beam of energy $E_0$ hits the target and produces in bremsstrahlung a hidden photon ${\gamma'}$ with energy $E_{\gamma'}$ that decays behind the shield e.g. into $e^+e^-$ which can then be observed in the detector.
• Figure 2: Limits on the hidden photon mass $m_{\gamma'}$ and kinetic mixing $\chi$ from different electron beam dump experiments. In addition to the limits from E141 (magenta dotted line), E137 (red dashed line) and E774 (orange long-dashed line) presented already in Ref. Bjorken:2009mm, the regions labeled KEK (green dash-dotted line) and Orsay (blue solid line) have been excluded in the present work.
• Figure 3: Collection of all current limits on hidden photons: from the electron beam dump experiments of the present work (colored lines, cf. Fig. \ref{['fig-limits']}, all other limits as gray lines), Standard Model precision measurements, muon and electron anomalous magnetic moment, a reinterpretation of the BaBar search $e^+ e^- \rightarrow \gamma \mu^+ \mu^-$ for pseudoscalars, the electron fixed target experiments A1 and APEX, the $\nu$-Cal I experiment at the Serpukhov proton beam dump, the KLOE experiment, the neutrino experiments NOMAD, PS191 and CHARM, and from the Kaon decay $K \rightarrow \mu \nu \gamma'$, cf. text for details.