Disappearing Track Signals from a Light Charged Higgs in the Alternative Left-Right Model

We study the phenomenology of a light charged Higgs boson in the framework of the Alternative Left–Right Symmetric Model (ALRM). In this model, stringent flavor constraints are evaded due to a non-conventional fermion spectrum in which the right-handed up-type quarks are paired with the exotic down-type quarks rather than the Standard Model down-type quarks, leading to the absence of tree-level flavor-changing neutral currents. Furthermore, a specific assignment of the global $U(1)_S$ symmetry and the resulting emergent $R$-parity prevent mixing between the right- and left-handed charged gauge bosons, $W_R$ and $W_L$, providing additional suppression of flavor-violating effects. The ALRM accommodates potentially viable dark matter candidates, both fermionic and scalar ones. In this context, an associated charged Higgs state, $H_2^\pm$, belonging to the dark sector can naturally acquire a sub-TeV to TeV-scale mass without conflicting with any experimental constraints. We focus on scenarios in which $H_2^\pm$ behaves as a long-lived particle due to a sub-GeV mass splitting with the dark matter candidate. We identify regions of parameter space consistent with the observed dark matter relic density and other experimental constraints. A detailed analysis of disappearing track signatures is performed, including realistic tracklet reconstruction efficiencies, and the existing ATLAS search are recast to assess the current limits and future sensitivities. We find that the HL-LHC has limited sensitivity to TeV-scale charged Higgs bosons in this scenario, while the 27 TeV HE-LHC can effectively probe the relevant parameter space, with a 100 TeV collider offering substantially enhanced discovery potential.

💡 Research Summary

In this work the authors investigate the phenomenology of a light charged Higgs boson, (H_2^\pm), that belongs to the dark sector of the Alternative Left‑Right Model (ALRM). The ALRM differs from the conventional left‑right symmetric model in two crucial ways. First, the right‑handed up‑type quarks are paired with exotic down‑type quarks ((d’)) rather than the Standard Model (SM) down‑type quarks, which eliminates tree‑level flavor‑changing neutral currents (FCNCs). Second, a global (U(1)_S) symmetry is introduced; after spontaneous breaking a residual (Z_2) remains, giving rise to an R‑parity that forbids mixing between the left‑ and right‑handed charged gauge bosons, (W_L) and (W_R). Consequently, the usual LHC limits on (W_R) do not apply, while the heavy neutral gauge boson (Z’) remains constrained.

The scalar sector contains a bi‑doublet (\Phi) and left‑ and right‑handed doublets (\chi_{L,R}). After electroweak and right‑handed symmetry breaking, the charged scalar spectrum splits into an R‑parity even state (H_1^\pm) (which behaves like a conventional charged Higgs) and an R‑parity odd state (H_2^\pm). Because (H_2^\pm) is odd under R‑parity, it cannot couple directly to SM particles; it only interacts with the dark‑sector fields, namely the lightest R‑odd neutral scalar (H_1^0/A_1^0) or the fermionic dark matter candidate (n). The mass of (H_2^\pm) is controlled by the parameter (\mu_3) and the quartic couplings (\alpha_{2,3}); in the limit (\alpha_2=\alpha_3) the mass simplifies to a function of the right‑handed VEV (v_R).

A key phenomenological feature arises when the mass splitting (\Delta m = m_{H_2^\pm} - m_{\text{DM}}) is of order 0.1–1 GeV. In this regime the two‑body decay (H_2^\pm \to \text{DM} + \pi^\pm) is phase‑space suppressed, giving the charged Higgs a macroscopic lifetime (cτ of several centimeters to meters). The charged pion from the decay is extremely soft and often fails reconstruction, so the observable signature is a disappearing track: a high‑(p_T) charged track that abruptly terminates inside the inner detector.

The authors perform a comprehensive analysis of this signature. They first map the viable parameter space that satisfies the observed dark‑matter relic density, using MicrOMEGAs to compute annihilation cross sections for both scalar and fermionic dark matter. The allowed region includes a wide range of (m_{H_2^\pm}) from a few hundred GeV up to several TeV, with (\Delta m) naturally lying in the sub‑GeV window thanks to the model’s mass relations. Direct‑detection limits (LUX‑Zeplin, XENONnT) and indirect constraints (gamma‑ray, AMS‑02) are shown to be evaded because the dark matter is R‑odd and couples only weakly to SM quarks and leptons.

Production of (H_2^\pm) at hadron colliders proceeds mainly via Drell‑Yan processes (pp \to \gamma^/Z^ \to H_2^+ H_2^-) and, to a lesser extent, via (W_R) exchange. The cross section depends on the right‑handed gauge coupling (g_R) and the scale (v_R); the authors adopt (g_R = g_L) and explore (v_R) in the 5–10 TeV range, yielding cross sections of order 10–100 fb for (m_{H_2^\pm}) around 1 TeV.

To assess current experimental sensitivity, the ATLAS 13 TeV disappearing‑track search with 36 fb(^{-1}) is recast. The authors implement realistic tracklet reconstruction efficiencies (track length 30–80 cm, (p_T > 55) GeV) and model the dominant backgrounds (fake tracks from electrons/muons and hadronic interactions) using ATLAS public data. Their reinterpretation shows that existing data exclude only (m_{H_2^\pm} \lesssim 600) GeV for the chosen (\Delta m) range. The High‑Luminosity LHC (HL‑LHC) with 3 ab(^{-1}) would extend the reach to roughly 1 TeV, but would still miss TeV‑scale charged Higgs bosons.

Projections for future colliders are then presented. At the 27 TeV High‑Energy LHC (HE‑LHC) with 15 ab(^{-1}), the increased parton luminosities and improved detector capabilities raise the discovery potential to (m_{H_2^\pm}) of 1–2 TeV at the 5σ level. At a 100 TeV proton‑proton collider (FCC‑hh) with 30 ab(^{-1}), the reach extends to about 3 TeV, and even heavier states become accessible if the mass splitting remains below 0.2 GeV, preserving a long lifetime. The authors also discuss how dedicated upgrades to inner‑tracker granularity and timing could further enhance sensitivity to very short tracklets.

In summary, the paper demonstrates that the ALRM provides a theoretically motivated framework where a light charged Higgs, protected by R‑parity, can be long‑lived and give rise to disappearing‑track signatures. Current LHC data place modest limits, but the next generation of high‑energy, high‑luminosity hadron colliders will be able to probe the bulk of the parameter space consistent with dark‑matter relic density and other constraints. The study highlights disappearing‑track searches as a powerful probe of dark‑sector charged scalars and underscores the complementarity between collider experiments and dark‑matter phenomenology.