Dark Matter (DM) and its potential interactions with the Standard Model (SM) continue to present a rich framework for model building. In the case of thermal DM of a mass between a few MeV and a few GeV, a compelling and much-explored framework is that of a dark photon/vector portal, which posits a new U (1) "dark photon" that undergoes small kinetic mixing (KM) with the SM hypercharge. This mixing can be mediated at the one-loop level by portal matter (PM) fields which are charged under both the dark U (1) and the SM gauge group. From requirements for a lack of gauge anomalies and lifetime constraints from early-universe cosmology, it is favored that fermionic portal matter take the form of vector-like copies of SM particles, albeit non-trivially charged under the dark U (1), which have delicate cancellations of dark U (1) charges and SM hypercharges in order to yield a finite and calculable KM. The distinctive particle content of PM models then presents an intriguing framework with which one may postulate on the UV completions of a simplified model with the dark photon portal, including those which may embed the dark U (1) in a larger group structure. We construct a model in which the dark U (1) is extended to SU (4)F × U (1)F , incorporating a local SU (3) flavor symmetry with PM appearing as a vector-like "fourth generation" to supplement the three generations of the SM. To ensure finite contributions to KM, the SM gauge group is arranged into Pati-Salam multiplets. The new extended dark gauge group presents a variety of interesting experimental signatures, including non-trivial consequences of the flavor symmetry being unified with the dark sector.

The Future Circular Collider (FCC-ee) offers the unique opportunity of studying the Higgs Yukawa coupling to the electron, $$y_\mathrm {e}$$ y e , via resonant s-channel production, $$\mathrm {e^+e^-}\rightarrow \mathrm {H}$$ e + e - → H , in a dedicated run at $$\sqrt{s} = m_\mathrm {H}$$ s = m H . The signature for direct Higgs production is a small rise in the cross sections for particular final states, consistent with Higgs decays, over the expectations for their occurrence due to Standard Model (SM) background processes involving $$\mathrm {Z}^*$$ Z ∗ , $$\gamma ^*$$ γ ∗ , or t-channel exchanges alone. Performing such a measurement is remarkably challenging for four main reasons. First, the low value of the e$$^\pm $$ ± mass leads to a tiny $$y_\mathrm {e}$$ y e coupling and correspondingly small cross section: $$\sigma _\mathrm {ee\rightarrow H} \propto m_\mathrm {e}^2 = 0.57$$ σ ee → H ∝ m e 2 = 0.57  fb accounting for initial-state $$\gamma $$ γ radiation. Second, the $$\mathrm {e^+e^-}$$ e + e - beams must be monochromatized such that the spread of their centre-of-mass (c.m.) energy is commensurate with the narrow width of the SM Higgs boson, $$\varGamma _\mathrm {H} = 4.1$$ Γ H = 4.1  MeV, while keeping large beam luminosities. Third, the Higgs mass must also be known beforehand with a few-MeV accuracy in order to operate the collider at the resonance peak, $$\sqrt{s} = m_\mathrm {H}$$ s = m H . Last but not least, the cross sections of the background processes are many orders-of-magnitude larger than those of the Higgs decay signals. A preliminary generator-level study of 11 Higgs decay channels using a multivariate analysis, which exploits boosted decision trees to discriminate signal and background events, identifies two final states as the most promising ones in terms of statistical significance: $$\mathrm {H}\rightarrow gg$$ H → g g and $$\mathrm {H}\rightarrow \mathrm {W}\mathrm {W}^*\!\rightarrow \ell \nu $$ H → W W ∗ → ℓ ν  + 2 jets. For a benchmark monochromatization with 4.1-MeV c.m. energy spread (leading to $$\sigma _\mathrm {ee\rightarrow H} = 0.28$$ σ ee → H = 0.28  fb) and 10 ab$$^{-1}$$ - 1 of integrated luminosity, a $$1.3\sigma $$ 1.3 σ signal significance can be reached, corresponding to an upper limit on the e$$^\pm $$ ± Yukawa coupling at 1.6 times the SM value: $$|y_\mathrm {e}|<1.6|y^\mathrm {\textsc {sm}}_\mathrm {e}|$$ | y e | < 1.6 | y e S M | at 95% confidence level, per FCC-ee interaction point per year. Directions for future improvements of the study are outlined.

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The vector portal/kinetic mixing simplified model of dark matter, in which thermal dark matter of a mass ranging from a few MeV to a few GeV can be realized in association with a dark-sector U (1), relies on the appearance of a small kinetic mixing term between this dark U (1) and the Standard Model (SM) hypercharge. It is well-known that kinetic mixing of the appropriate magnitude can be generated at one loop by the inclusion of "portal matter" fields which are charged under both the dark U (1) and the SM hypercharge, and it has been previously argued on phenomenological grounds that fermionic portal matter fields must exhibit a very specific set of characteristics: They must be vector-like and have the same Standard Model group representations as existing SM fermions. A natural explanation for the presence of dark U (1)-charged copies of SM fermions would be to enlarge the dark gauge group and embed the SM and the portal matter into a single dark multiplet, however, previous attempts at accomplishing this task require significant ad hoc additions to the model to ensure that the portal matter fields are vector-like while the SM fields are chiral, and rely on complicated dark Higgs sectors to realize the appropriate symmetry breaking. In this paper, we argue that embedding a portal matter model in extra dimensions can easily generate chiral SM fermions and vector-like portal matter from a single dark bulk multiplet, while allowing for a far simpler dark Higgs sector. We present a minimal construction of this "Kaluza-Klein portal matter" and explore its phenomenology at the LHC, noting that even in our simple realization of the paradigm such a construction exhibits phenomenology unique from that of more conventional theories of large extra dimensions and 4-dimensional models of portal matter.