Identifying Charged Lepton-like Portal Matter at Future Colliders

In the Kinetic Mixing (KM) portal scenario, the interaction of dark matter (DM) with the particles of the Standard Model (SM) is generated by diagrams connecting the familiar photon with its dark sector analog, the dark photon (DP), via loops of particles carrying both dark and SM quantum numbers, \ie, Portal Matter (PM). For the case of sub-GeV DM and DP, these PM states may lie in the $\sim 1-10$ TeV range and be potentially accessible at the HL-LHC as well as at other future lepton and hadron colliders. In perhaps the simplest scenario of this kind, PM consists of just a pair of electrically charged, iso- and color-singlet, vector-like (VL) fermions having opposite dark charges, with an $O(1)$ mass splitting, yielding a finite value for the strength of the KM, \ie, $ε\sim a~ few \times10^{-4}$. The dark Higgs induced mixing of PM states with their SM analogs allows for their decay but can also lead to significant distortions in the expected production properties for the PM at future lepton colliders due to $t$-channel dark Higgs exchange, potentially confusing PM identification. We show that the large set of clean observables available at lepton colliders is more that sufficient to resolve any of these ambiguities. The possibility of the production of like-sign PM fields via $t/u$-channel exchange of the same dark Higgs is also briefly explored.

💡 Research Summary

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The paper investigates a concrete realization of the kinetic‑mixing (KM) portal in which dark matter (DM) and a dark photon (DP) communicate with the Standard Model (SM) through loops of “portal matter” (PM) particles. In the simplest incarnation, the PM sector consists of just two vector‑like fermions that are electrically charged (Q = ±1), color‑ and weak‑isospin singlets, and carry opposite dark‑U(1) charges. Their masses are taken to be in the 1–10 TeV range, giving a kinetic‑mixing parameter ε ≈ few × 10⁻⁴, a value that is both theoretically natural and phenomenologically viable for sub‑GeV DM.

A single dark Higgs field h_D breaks the dark U(1)D, giving mass to the DP and simultaneously providing the Yukawa coupling that mixes the lighter PM fermion (denoted E) with the corresponding SM right‑handed lepton ℓR. This mixing allows E to decay promptly via E → ℓ S or ℓ V, where S is the physical dark Higgs scalar and V is the longitudinal mode of the DP (the Goldstone boson). For O(1) Yukawa couplings the partial widths are essentially Γ ≈ y² M_E/(32π), so the decays are prompt even for very small y (down to ≈ 10⁻¹). Current LHC searches (13 TeV, 139 fb⁻¹) recast to this scenario set lower bounds of about 0.9 TeV for electron‑mixing and 0.85 TeV for muon‑mixing PM.

The core of the study focuses on future lepton colliders (e⁺e⁻ or μ⁺μ⁻) with center‑of‑mass energies of a few TeV. Because the initial state consists of leptons, the same Yukawa interaction that mediates the decay also generates a t‑channel exchange of the dark Higgs in the production process e⁺e⁻ → E⁺E⁻. This t‑channel contribution interferes with the usual s‑channel γ/Z exchange, leading to pronounced distortions in the total cross section, the angular distribution dσ/dcosθ, the energy spectrum of the final‑state leptons, and various polarization observables. In particular, the forward region can be significantly enhanced while the backward region is suppressed, a pattern that is absent for ordinary vector‑like charged leptons without dark‑Higgs exchange.

To disentangle these effects, the authors propose a suite of clean observables that are readily accessible at lepton colliders with high luminosity (hundreds of fb⁻¹) and excellent detector capabilities:

1. Differential angular distribution dσ/dcosθ – sensitive to the interference pattern and to the mass of the exchanged dark Higgs.
2. Lepton energy spectrum – the t‑channel modifies the shape, especially at low energies.
3. Beam polarization asymmetries (A_LR) – by flipping the longitudinal polarization of the e⁻/e⁺ (or μ⁻/μ⁺) beams, one can isolate the chiral structure of the t‑channel coupling.
4. Final‑state lepton polarization (P_L) – the dark‑Higgs exchange preferentially produces left‑handed or right‑handed leptons depending on the Yukawa chirality.
5. Spin‑correlation observables – correlations between the polarizations of the two produced E’s provide an additional handle on the underlying dynamics.

Monte‑Carlo estimates (not presented in detail) suggest that with realistic integrated luminosities the statistical uncertainties on these observables are well below the size of the predicted deviations, allowing a clear discrimination between pure s‑channel VL lepton production and the mixed s + t‑channel scenario. The authors stress that such a discrimination is essentially impossible at hadron colliders (HL‑LHC, FCC‑hh) because the initial state partons are quarks and gluons, which do not couple to the dark Higgs at tree level; there the PM would appear indistinguishable from a generic heavy charged lepton.

A further novel signature discussed is the production of like‑sign PM pairs via t‑ or u‑channel dark‑Higgs exchange: e⁻e⁻ → E⁻E⁻ (or μ⁻μ⁻ → E⁻E⁻). This process is practically background‑free in the SM, as lepton number is conserved. The cross section scales as σ ∼ y⁴/(16π s) and is highly sensitive to the dark‑Higgs mass and the Yukawa coupling strength. For y ≈ 1 and a light dark Higgs (m_S ≲ few × 100 GeV), the rate can be sizable even at a 1 TeV collider, offering a striking confirmation of the portal‑matter framework.

In summary, the paper demonstrates that (i) a minimal portal‑matter sector consisting of vector‑like charged leptons with opposite dark charges naturally yields the required kinetic‑mixing parameter; (ii) the same Yukawa interaction that enables their decay also modifies their production at lepton colliders via t‑channel dark‑Higgs exchange; (iii) a comprehensive set of angular, energy, and polarization observables at future e⁺e⁻/μ⁺μ⁻ machines can unambiguously identify these effects and confirm the portal‑matter nature of the new states; and (iv) the like‑sign production channel provides an additional, essentially background‑free probe of the dark‑Higgs coupling. These results highlight the unique power of precision lepton colliders to explore dark‑sector portals that are otherwise inaccessible at hadron machines.