Dark Photon Searches with Initial-State Radiation at Fixed-Target Configurations

TL;DR

This paper studies how initial-state radiation affects dark photon searches at fixed-target experiments, showing it can significantly enhance sensitivity in certain mass ranges.

Contribution

It introduces the impact of initial-state radiation on dark photon detection at fixed-target setups, improving sensitivity estimates for Belle II and NA64.

Findings

Initial-state radiation can dominate the annihilation process around specific dark photon masses.

Sensitivity to the kinetic mixing parameter can be increased by up to 2.7 times at Belle II.

Multi-bin spectral analysis improves background separation and detection sensitivity.

Abstract

In this work, we investigate the contribution of the annihilation process with initial-state radiation ($e^+ e^- \to \gamma A'$) to the invisible dark photon ($A'$) searches at the electron fixed-target configurations. For illustration, we consider both the disappearing positron track signature at Belle II and the large missing energy search at NA64. When the dark photon has a narrow decay width, the effect of the initial-state radiation to the annihilation process can dominate over its $s$-channel and bremsstrahlung counterparts around $m_{A'} \simeq 60\,\rm{MeV}$ ($m_{A'} \simeq 200\,\rm{MeV}$) for Belle II (NA64), to enhance the corresponding sensitivity on the kinetic mixing parameter $\epsilon$ by a factor of up to approximately 2.7 (1.3). For Belle II, we further perform a multi-bin analysis with the spectrum information to better separate the background and signal channels for significant improvement of the sensitivity.