Dark Moments for the Standard Model ?

TL;DR

This paper explores the possibility that loop-induced dark magnetic dipole moments for Standard Model fermions, especially electrons, could arise from portal matter in dark photon models, with implications for experimental searches.

Contribution

It introduces a phenomenological analysis of dark magnetic dipole moments induced by portal matter, highlighting potential experimental signatures and constraints.

Findings

Dark magnetic dipole moments can be compatible with current constraints.

Parameter space favors invisible dark photon decays under certain conditions.

Future experiments like Belle II can strongly constrain this scenario.

Abstract

If dark matter (DM) interacts with the Standard Model (SM) via the kinetic mixing (KM) portal, it necessitates the existence of portal matter (PM) particles which carry both dark and SM quantum numbers that will appear in vacuum polarization-like loop graphs. In addition to the familiar $\sim e\epsilon Q$ strength, QED-like interaction for the dark photon (DP), in some setups different loop graphs of these PM states can also induce other coupling structures for the SM fermions that may come to dominate in at least some regions of parameter space regions and which can take the form of `dark' moments, eg, magnetic dipole-type interactions in the IR, associated with a large mass scale, $\Lambda$. In this paper, motivated by a simple toy model, we perform a phenomenological investigation of a possible loop-induced dark magnetic dipole moment for SM fermions, in particular, for the electron. We show that at the phenomenological level such a scenario can not only be made compatible with existing experimental constraints for a significant range of correlated values for $\Lambda$ and the dark $U(1)_D$ gauge coupling, $g_D$, but can also lead to quantitatively different signatures once the DP is discovered. In this setup, assuming complex scalar DM to satisfy CMB constraints, parameter space regions where the DP decays invisibly are found to be somewhat preferred if PM mass limits from direct searches at the LHC and our toy model setup are all taken seriously. High precision searches for, or measurements of, the $e^+e^- \to \gamma +{\rm DP}$ process at Belle II are shown to provide some of the strongest future constraints on this scenario.