Doubly charged Higgs boson at same-sign lepton colliders

TL;DR

The paper studies the production and detection of the Georgi-Machacek model's doubly charged Higgs $H_5^{++}$ at same-sign lepton colliders, focusing on the $W^+W^+$ fusion channel $\ell^+\ell^+ \to H_5^{++}\bar{\nu}\bar{\nu}$ with $H_5^{++}\to W^+W^+$ decays when the triplet VEV $v_\Delta$ is sizable. It develops a mass-determination strategy based on the invariant mass distribution of the dilepton final state and a $\chi^2$ fit to extract $m_{H_5}$, taking into account SM backgrounds and kinematic cuts; the method relies on the Lorentz-invariant nature of the invariant mass $M$. The study also compares single production at same-sign colliders to pair production at opposite-sign colliders, showing that single production gains prominence at higher energies and provides a complementary discovery channel, especially for $\sqrt{s}$ in the multi-TeV range. The results indicate that for moderate $m_{H_5}$ and $v_\Delta$, high-luminosity, high-energy same-sign lepton colliders can achieve meaningful sensitivity and mass measurements, strengthening the case for such facilities as part of the future collider program.

Abstract

We investigate the search for doubly charged Higgs bosons in the Georgi-Machacek (GM) model at same-sign lepton colliders. The dominant production mode is vector boson fusion, through which the particle is singly produced and decays into a pair of same-sign $W$ bosons if the triplet vacuum expectation value is sufficiently large, as allowed in the GM model. Considering the leptonic decays of the $W$ bosons, we discuss the discovery reach of the signal and a method to extract the mass of the doubly charged Higgs boson. It is impossible to construct the transverse mass of the decay product to obtain the mass because of the neutrinos radiated from the initial-state leptons. Alternatively, we propose to make use of the invariant mass of the same-sign leptons. We perform $χ^2$ fitting using the analytical formula for the invariant mass distribution and demonstrate that this approach is effective in extracting the information on the mass. We also compare the process with pair production processes at the opposite-sign lepton colliders.

Figures (8)

• Figure 1: The dependence of the cross section $\sigma$ on (a) $\sqrt{s}$, (b) $m_{H_5}$, and (c) $v_\Delta$. In each figure, the parameters not on the horizontal axis are fixed at $\sqrt{s}=2~\mathrm{TeV}$, $m_{H_5}=400~\mathrm{GeV}$, and $v_{\Delta}=15~\mathrm{GeV}$ ($\cos \beta \simeq 0.17$). The gray regions are excluded by the experimental constraints discussed in Sec. \ref{['sec: GM_model']}.
• Figure 2: Feynman diagrams of new physics contributions for the signal process.
• Figure 3: Comparisons between the cross section of $\mu^+ \mu^+ \to \bar{\nu}_\mu \bar{\nu}_\mu W^+W^+$ in the SM and the GM model as functions of (a) $\sqrt{s}$, (b) $m_{H_5}$, and (c) $v_\Delta$. In each figure, the parameters not in the horizontal axis are fixed at $\sqrt{s}=2~\mathrm{TeV}$, $m_{H_5}=400~\mathrm{GeV}$, and $v_{\Delta}=15~\mathrm{GeV}$ ($\cos \beta \simeq 0.17$).
• Figure 4: Contours for the statistical significance with $\sqrt{s} = 2~\mathrm{TeV}$ and $\mathcal{L} = 1000~\mathrm{fb^{-1}}$.
• Figure 5: The distributions of (a) the transverse mass of $e^+ e^+ \cancel{E}$ and (b) the invariant mass of $e^+e^+$. The red and blue curves are contour for $m_{H_5} = 600~\mathrm{GeV}$ and $m_{H_5} = 400~\mathrm{GeV}$, respectively. Other parameters are fixed as $\sqrt{s} = 2~\mathrm{TeV}$ and $v_\Delta = 15~\mathrm{GeV}$ ($\cos \beta \simeq 0.17$). The black curve represents the SM prediction. For all curves, we use the kinematical cuts in Eq. (\ref{['eq: kinematica_cut']}).