Probing dark-state electromagnetic form factors at future linear electron-positron colliders with polarized beams

We study the electromagnetic form factors of electrically neutral dark-sector particles in this paper. Concretely speaking, we focus on operators that can lead to interactions of fermionic dark states with the Standard-Model (SM) photon, including both dimension-5 (the magnetic and electric dipole moments) and dimension-6 (the anapole moment and charge radius) operators. Correspondingly, instead of the SM-photon form factors, we employ hypercharge gauge-field form factors that can induce additional couplings to the SM $Z$-boson. Utilizing the mono-photon production channel at future linear electron-positron colliders, the International Linear Collider (ILC) and the Compact Linear Collider (CLIC), we demonstrate that tuning beam polarization allows for simultaneous enhancement of signal-event rates and suppression of dominant SM background events, i.e. neutrino-induced processes, and hence improvement of sensitivity reach. Our analysis reveals that ILC and CLIC can probe electromagnetic form factors of dark-sector particles up to about two orders of magnitude beyond existing experimental limits.