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 ∼ eϵQ 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, e.g., magnetic dipole-type interactions in the IR, associated with a large mass scale, Λ. 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 Λ and the dark U(1)D gauge coupling, gD, 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− → γ + DP process at Belle II are shown to provide some of the strongest future constraints on this scenario.
Highlights
JHEP11(2021)035 as it will be assumed here, the simplest possibility is that this occurs via the vev(s) of at least one dark Higgs field in analogy with the spontaneous symmetry breaking in the Standard Model (SM)
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
What else could happen in such a situation to generate a suitably large interaction so that, e.g., the DM annihilation to SM fields via the dark photon (DP) can yield the observed relic density while satisfying other experimental constraints? One possibility is to take the PM approach a bit more seriously, as we do here, and consider an augmented new gauge structure corresponding to a group, G, under which the PM fields, here considered to be vector-like fermions, and the SM fermion fields occur in the same representation; many scenarios of this form are possible [35, 37]
Summary
It is well-known that the coupling of a photon to a neutral massive Dirac fermion, such as a Dirac neutrino or fermionic dark matter itself, can be decomposed into a small finite set of various (effective) interaction terms of increasing dimension. Since we will be concerning ourselves with a sub-GeV DM and DP, in what follows we will mostly be restricting ourselves to processes wherein the typical mass scales will be ∼ a few GeV of less, i.e., values Λf This being the case, we can set the constants ci,f → 0 and drop the two terms in the effective coupling proportional to q2/Λ2f 10−4, corresponding to the charge and anapole form factors, in comparison to the two dipole moment terms. Employing a slight change in both notation and normalization, and explicitly including a factor of the U(1)D gauge coupling, gD, in the dark dipole moment coupling It is the effect of this dark magnetic dipole term on the couplings of the SM fermions to the DP that we will investigate further below under the assumption that the KM vector coupling
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