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

TL;DR

The paper investigates the phenomenology of a light, long-lived charged Higgs $H_2^{\pm}$ in the Alternative Left-Right Symmetric Model (ALRM), where a global $U(1)_S$ symmetry and a nonstandard fermion spectrum suppress flavor constraints and yield a residual $Z_2$ that stabilizes dark matter. It analyzes two dark-matter realizations—scotino and scalar—highlighting how a compressed dark sector drives a suppressed $H_2^{\pm}$ width and a disappearing-track collider signature. The study develops a realistic LLP framework, computes production and decay rates, and recasts the ATLAS disappearing-track search, finding that the HL-LHC has limited sensitivity to TeV-scale $H_2^{\pm}$, while the 27 TeV HE-LHC and especially a 100 TeV collider can probe much of the viable parameter space. The results emphasize the complementarity of dark-matter constraints and LLP collider searches in testing extended left–right frameworks and guide future collider strategies for LLP discovery.

Abstract

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.

Figures (24)

• Figure 1: Left: Variation of dark matter relic density with dark matter mass $m_{n_{e}}$. The dark black band shows the values within $1\sigma$ range of the observed relic density. Right: Variation of direct detection cross section $\sigma_{SI}~(cm^2)$ with DM mass $m_{n_e}$. The dashed red line represents the XENONnT limit on the spin-independent direct detection cross-section of dark matter which excludes the whole region above it. The gradient color indicates mass splitting between the dark matter and $H_2^\pm$.
• Figure 2: Left: The variation of darkmatter relic density with darkmatter mass $M_{H_1^0/A_1}$. The dark solid lines represent the $1\sigma$ range of the observed relic density. Right: Spin Independent cross section as a function of dark matter mass with XENONnT XENON:2023cxc exclusion as the shaded region. The black points respect the observed relic density within $1\sigma$ limit. The color gradient denotes the mass splitting between the charged scalar $H^\pm_2$ and dark matter.
• Figure 3: Left: Variation of the pair production cross-section of $H_2^\pm$ with its mass at the LHC. Right: Variation of the associated production cross section of $H_2^\pm$ along with $A_1/H^0_1$ with its mass $m_{H^\pm_2}$. To generate these plots, we have considered $v_R$ = 15 TeV, tan$~\beta$=10, $\lambda_2=-0.01$, $\lambda_3=0.01$, ($m_{d'},m_{s'},m_{b'}$)=(3 TeV, 4 TeV, 5 TeV) and $m_{Z^\prime}=5.87$ TeV.
• Figure 4: Feynman diagrams of $H_2^\pm$ decays with mass splitting between the charged and neutral scalars, $\Delta M <1$ GeV. The $R$-parity odd fermions, $n$ and $d'$ are taken to be heavier than $H_2^\pm$.
• Figure 5: Variation of the total decay width $\Gamma$ of $H^\pm_2$ as a function of its mass $m_{H^\pm_2}$ and the mass splitting $0.2<\Delta M=(m_{H^\pm_2}-m_{\rm DM})<15$ GeV. For this scan, we set $\lambda_2$ in the range [-$10^{-5}$, -0.1] so as to get the $\Delta M$ in the desired range. Other parameters are taken to be: $\alpha_{1,2,3}=0.1,~\lambda_3=0.1, ~ \text{tan}\beta=10, {~\rm and} ~ v_R=20$ TeV.