Impact of QED backgrounds on light-by-light scattering measurements at $e^{+}{e}^{-}$ and $e^{-}{e}^{-}$ colliders

Abstract

The process of elastic scattering of photons ($\gamma \gamma \to \gamma \gamma$, light-by-light scattering) has attracted significant interest in recent years as a loop-induced process sensitive to all charged particles. To date, this process has been studied through Delbrück scattering, photon splitting in the nuclear Coulomb field, and ultra-peripheral heavy-ion collisions at the LHC. Future precision measurements are anticipated at high-luminosity $e^{+}{e}^{-}$ colliders (SuperKEKb, FCC, CEPC, ILC, CLIC), their $e^{-}{e}^{-}$ options, and $\gamma \gamma$ colliders based on the backscattering of laser photons. In this paper, we show that background QED processes severely limit the study of elastic light-by-light scattering at $e^{+}{e}^{-}$ colliders. For the $e^{+}{e}^{-}$ case, the main background arises from $e^{+}{e}^{-}$ annihilation into two photons within the detector acceptance after the initial electron and positron emit hard ISR photons at small angles. Since only two photons with a small total transverse momentum are registered in the detector in both cases, this process effectively mimics the $\gamma \gamma \to \gamma \gamma$ signal with a significantly larger cross section at high invariant masses. Other relevant background QED processes for both types of collisions are also comprehensively analyzed and discussed.

Comments: 10 pages, 15 figures