On direct observation of millicharged particles at $c$-$\ddot{I}„$ factories and other $e^{+}{e}^{-}$-colliders

Abstract

Hypothetical particles with tiny electric charges (millicharged particles or MCPs) can be produced in electron-positron annihilation if kinematically allowed. Typical searches for them at $e^{+}{e}^{-}$ colliders exploit a signature of a single photon with missing energy carried away by the undetected MCP pair. We put forward an idea to look alternatively for MCP energy deposits inside a tracker, which is a direct observation. The new signature is relevant for non-relativistic MCPs, and we illustrate its power on the example of the $c$-$\ddot{I}„$ factory, where we argued that the corresponding searches may be background-free. We find that it can probe the MCP charge down to $3\times 10^{-3}$ of the electron charge for the MCP masses in $\mathcal{O}(5)$ MeV vicinity of each energy beam value where the factory will collect a luminosity of 100 fb${}^{-1}$ in one year. This mass region is unreachable with the searches for missing energy and single photon.

12 pages, 3 figures; v2: the background-free signature is introduced