V-Associated Production & Vector Boson Fusion as an LHC Signature of CP Violation

TL;DR

The paper tackles the question of revealing CP violation in extended Higgs sectors through LHC processes, focusing on VBF and V-AP production of BSM Higgses and the decay $H_a \rightarrow Z h$ where $h$ is CP-even. It proposes a framework of multi-channel, joint-signature sets (including both VBF and $V$-associated production) and analyzes their viability within the complex two-Higgs doublet model (C2HDM) with a softly broken $\mathbb{Z}_2$ symmetry at the HL-LHC, accounting for potential gluon-fusion contamination and the need to confirm $H_a$ couplings to electroweak gauge bosons. The authors perform comprehensive parameter scans (Types I), impose theoretical and experimental constraints including EDM bounds, and identify set (vi) as the most promising pathway to establish Higgs-sector CP violation, with significant portions of viable parameter space accessible at the HL-LHC under optimistic sensitivity assumptions. The study emphasizes the synergy between collider probes and EDM measurements, and suggests that a multi-channel, possibly future lepton collider, program will be essential to decisively confirm CP violation in the scalar sector.

Abstract

We investigate the role of vector boson fusion (VBF) and associated production with an electroweak boson (V-AP) of beyond-the-Standard-Model Higgses at the LHC in probing CP violation in extended Higgs sectors. Resonant production of a new Higgs boson through V-AP/VBF subsequently decaying into a Z boson and a 125 GeV Higgs boson h would be a robust sign of CP violation, since h has been measured to be (predominantly) a CP-even state. After identifying this and other sets of signatures which rely on V-AP/VBF to jointly uncover CP violation, we analyze the prospects to measure CP violation in this way for the complex two-Higgs-doublet-model at the High-Luminosity LHC.

Figures (12)

• Figure 1: Feynman diagram for the CP-violating processes with $V$-AP production (left) and VBF production (right) with the subsequent $H_a \rightarrow Z h$ decay.
• Figure 2: Left: Feynman diagram for VBF production of $H_a$. Right: Feynman diagram for ggF production of $H_a$ with two jets. In both cases, the final-state LHC signature is $H_a + j j$.
• Figure 3: $W$-AP cross sections (in pb) for $H_a$ -- with $H_a \rightarrow Zh \rightarrow \ell \ell b \bar{b}$ -- at $\sqrt{s}=14$ TeV for the C2HDM scans $S_1$ (left and central panels) and $S_2$ (right panel), as a function of $m_{H_a}$. Left panel: $H_a=H_2$. Central and right panels: $H_a=H_3$. We also depict the theoretical upper bound on the cross section (dashed-red line, see text for details) and the cross section yielding 1 event (including the decay of the $W$-boson) at the HL-LHC (solid-red line).
• Figure 4: Current (solid red) and projected HL-LHC (dashed-red, grey band) sensitivity lines of $pp \rightarrow W H_b$ production at $\sqrt{s} = 14$ TeV normalized to the SM associated production $\sigma^{\rm W-AP}_{SM}$ of a SM Higgs of mass $m_{H_b}$ as a function of $m_{H_b}$, see text for details. Orange squares (black crosses) correspond to HL-LHC observable (unobservable) C2HDM scan points when no EDM constraints are applied, and blue circles (red crosses) when EDM bounds are strictly imposed, respectively for scan $S_1$ (for $m_{H_b} > 125$ GeV) and $S_2$ (for $m_{H_b} < 125$ GeV). Also shown (solid green) are the LEP limits on $\sigma(W H_b)/\sigma(W H_{\rm SM}^{m_{H_b}})$ assuming $c_{H_bZZ} = c_{H_bWW}$.
• Figure 5: $p p \rightarrow W H_3$ cross section times branching fraction BR$(H_3 \rightarrow Z H_1) \times$BR$(Z \rightarrow \ell\ell)$ for $S_2$ scan points within the HL-LHC grey band of Fig. \ref{['fig:AssociatedW']}. In dotted-red (dotted-black) we show the current CMS (HL-LHC strongest projection) limits from $p p \rightarrow H_3 \rightarrow Z H_1 \rightarrow \ell\ell b\bar{b}$CMS:2019ogx, see text for details.