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

TL;DR

This paper investigates how future linear colliders can detect neutral dark-sector particles through their electromagnetic form factors by using polarized beams to enhance signals and suppress backgrounds, extending current experimental limits.

Contribution

It introduces a method to probe dark-sector electromagnetic form factors at future colliders using polarized beams, improving sensitivity beyond existing bounds.

Findings

ILC and CLIC can probe form factors two orders of magnitude beyond current limits.

Beam polarization enhances signal detection and background suppression.

EFT validity is maintained for dimension-5 operators within sensitive regions.

Abstract

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. We also estimate, at the order-of-magnitude level, the validity range of the effective-field-theory (EFT) approach we adopt, finding that for the dimension-5 operators the EFT remains valid in most of the parameter regions to which the ILC and CLIC can be sensitive, while it breaks down in the entire sensitivity region for the dimension-6 operators.