Model-independent probes of CP violation in the heavy scalar sector at muon colliders

TL;DR

This work introduces a model-independent strategy to test CP violation in an extended scalar sector using a heavy scalar $h_2$ with tree-level $h_2VV$ and $h_2Zh_1$ couplings alongside the SM-like $h_1$. By focusing on vector-boson-fusion production at a future muon collider and the decay $h_2\to Z h_1$, the paper shows that observing $V V\to h_2\to Z h_1$ directly requires both couplings to be nonzero, thereby signaling CP violation. The authors implement an effective Lagrangian, simulate signal and backgrounds at $\sqrt{s}=3$ and 10 TeV with realistic detector effects and beam-induced background, and perform a mass-based analysis using $m_{b\bar{b}}$ and $m_{b\bar{b}\ell^+\ell^-}$ to extract discovery potentials. They find that CP violation could be discovered at $5\sigma$ up to $m_{h_2}\approx 1$ TeV for $\sqrt{s}=3$ TeV with $L=0.9~\text{ab}^{-1}$, and up to $m_{h_2}\approx 4.5$ TeV for $\sqrt{s}=10$ TeV with $L=10~\text{ab}^{-1}$, assuming $c_2,c_{12}\lesssim 0.2$. This channel provides a direct, collider-based probe of CP violation in the scalar sector, complementary to EDM and Yukawa-sector studies, and demonstrates the strong potential of multi-TeV muon colliders for BSM Higgs phenomenology.

Abstract

We propose a model-independent test of CP violation in the scalar sector. We consider a heavy neutral scalar $h_2$ with tree-level couplings at the $h_2 V V$ and $h_2 h_1 Z$ vertices (with $V=W^{\pm},Z$), alongside the 125~GeV SM-like Higgs boson $h_1$. At future muon colliders (MuC), we exploit vector-boson-fusion (VBF) production of $h_2$ followed by the decay $h_2 \to Z h_1$. In our framework, observing the single process $V V \to h_2 \to Z h_1$ implies both relevant couplings are nonzero, which is sufficient to establish CP violation in the scalar sector. We simulate signal and backgrounds at $\sqrt{s}=3~(10)$ TeV with integrated luminosity $L=0.9~(10)~\mathrm{ab}^{-1}$. We then present the expected discovery sensitivites across the $(c_2,c_{12})$ parameter space (with the coupling parameters $c_{2}$ and $c_{12}$ defined in the text) for multiple $m_{h_2}$ hypotheses.

Figures (7)

• Figure 1: Feynman diagrams for the VBF processes $V V \to h_2 \to Z h_1$ (with $V = W^{\pm}, Z$).
• Figure 2: Left: total cross sections $\sigma$ of $V V \to h_2$ at MuC with $\sqrt{s} = 3$ and $10$ TeV as a function of $m_{h_2}$. Right: $\mathrm{Br}_{h_2\to Z h_1}$ as a function of $m_{h_2}$ for different $c_{12}/c_2$.
• Figure 3: Normalized distributions of $m_{b\bar{b}}$ (left), $m_{b\bar{b}\ell^-\ell^+}$ (middle), and $m\mkern-10.5mu/$ (right), for both signal and background processes after pre-selection ($m_{h_2}=700~\mathrm{GeV}$ with $\sqrt{s}=3~\mathrm{TeV}$).
• Figure 4: Normalized distributions of $m_{b\bar{b}}$ (left), $m_{b\bar{b}\ell^-\ell^+}$ (middle), and $m\mkern-10.5mu/$ (right), for both signal and background processes after preselection ($m_{h_2}=3000~\mathrm{GeV}$ with $\sqrt{s}=10~\mathrm{TeV}$).
• Figure 5: Expected discovery potential for significance for points in $(c_2, c_{12})$-plane with $\sqrt{s}=3$ TeV and $L=0.9~\mathrm{ab}^{-1}$. Benchmark points: $m_{h_2}\in\{700,1000\}$ GeV. The colored lines correspond to the boundaries with expected $2\sigma$ (green), $3\sigma$ (blue), and $5\sigma$ (red) significances, respectively.