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

Abstract

In this work, we investigate the contribution of the annihilation process with initial-state radiation ($e^{+}{e}^{-}\to {\hat{I}}^3{A}^{\prime }$) to the invisible dark photon ($A^{\prime }$) 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 $m_{A^{\prime }}\simeq 60\mathrm{MeV}$ ($m_{A^{\prime }}\simeq 200\mathrm{MeV}$) for Belle II (NA64), to enhance the corresponding sensitivity on the kinetic mixing parameter $\hat{I}\mu$ 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.

11 pages, 9 figures, 1 table