[Paper Review] Search for a CP-odd Higgs boson decaying into a heavy CP-even Higgs boson and a $Z$ boson in the $\ell^+\ell^- t\bar{t}$ and $ u\bar{ u}b\bar{b}$ final states using 140 fb$^{-1}$ of data collected with the ATLAS detector

Georges Aad, Abbott, Braden Keim+2981 more|arXiv (Cornell University)|Jan 1, 2023

Particle physics theoretical and experimental studies2 citations

TL;DR

This paper presents a search for a CP-odd Higgs boson (A) decaying into a Z boson and a heavy CP-even Higgs boson (H) in the ℓ⁺ℓ⁻t̄t and νν̄bb̄ final states using 140 fb⁻¹ of proton-proton collision data at √s = 13 TeV with the ATLAS detector. No significant deviation from the Standard Model is observed; upper limits on the A→ZH production cross-section are set, with improved sensitivity in the mH > 350 GeV and mA > 800 GeV region, and model-independent limits are provided on reconstructed m(t̄t) and m(bb̄) distributions.

ABSTRACT

A search for a heavy CP-odd Higgs boson, A, decaying into a Z boson and a heavy CP-even Higgs boson, H, is presented. It uses the full LHC Run 2 dataset of pp collisions at $ \sqrt{s} $ = 13 TeV collected with the ATLAS detector, corresponding to an integrated luminosity of 140 fb$^{−1}$. The search for A → ZH is performed in the $ {\ell}^{+}{\ell}^{-}t\overline{t} $ and $ u \overline{ u}b\overline{b} $ final states and surpasses the reach of previous searches in different final states in the region with m$_{H}$ > 350 GeV and m$_{A}$ > 800 GeV. No significant deviation from the Standard Model expectation is found. Upper limits are placed on the production cross-section times the decay branching ratios. Limits with less model dependence are also presented as functions of the reconstructed m($ t\overline{t} $) and m($ b\overline{b} $) distributions in the $ {\ell}^{+}{\ell}^{-}t\overline{t} $ and $ u \overline{ u}b\overline{b} $ channels, respectively. In addition, the results are interpreted in the context of two-Higgs-doublet models.[graphic not available: see fulltext]

Motivation & Objective

• To search for a heavy CP-odd Higgs boson (A) decaying into a Z boson and a heavy CP-even Higgs boson (H) in the context of extended Higgs sectors beyond the Standard Model.
• To probe the parameter space where mH > 2mtop, which is inaccessible to previous searches relying on H→bb̄, WW, or ττ decays.
• To improve sensitivity in the high-mass regime (mA > 800 GeV, mH > 350 GeV) by targeting the ℓ⁺ℓ⁻t̄t and νν̄bb̄ final states.
• To provide model-independent limits on the reconstructed m(t̄t) and m(bb̄) distributions, reducing theoretical dependence.
• To interpret the results in the framework of two-Higgs-doublet models (2HDM), particularly in scenarios with mA ≠ mH and alignment limit.

Proposed method

• The analysis uses 140 fb⁻¹ of pp collisions at √s = 13 TeV collected by the ATLAS detector during LHC Run 2.
• Events are selected in two final states: ℓ⁺ℓ⁻t̄t (with one top quark decaying semileptonically and the other hadronically) and νν̄bb̄ (with Z→νν̄ and H→bb̄).
• Reconstructed top-quark and b-quark pair masses (m(t̄t) and m(bb̄)) are used as key kinematic variables to enhance sensitivity.
• Backgrounds are estimated using control regions and simulation, with careful treatment of multijet, Drell-Yan, and top-quark contributions.
• Systematic uncertainties are evaluated for object reconstruction, trigger efficiency, and modeling of jets and missing transverse momentum.
• Statistical significance is assessed using a profile-likelihood ratio test, and upper limits on the A→ZH production cross-section are set at 95% confidence level.

Experimental results

Research questions

• RQ1 What is the sensitivity of the ATLAS experiment to a CP-odd Higgs boson (A) decaying into a Z boson and a heavy CP-even Higgs boson (H) in the ℓ⁺ℓ⁻t̄t final state?
• RQ2 How does the sensitivity of the νν̄bb̄ final state compare to previous searches, especially in the high-mass regime (mA > 800 GeV)?
• RQ3 Can model-independent limits be set on the reconstructed m(t̄t) and m(bb̄) distributions to reduce theoretical dependence?
• RQ4 What are the constraints on the A→ZH signal strength in two-Higgs-doublet models (2HDM), particularly in the alignment limit and for mA ≠ mH?
• RQ5 How do the results compare to previous searches in other final states, especially in the region where mH > 350 GeV and mA > 800 GeV?

Key findings

• No significant excess above the Standard Model background is observed in either the ℓ⁺ℓ⁻t̄t or νν̄bb̄ final states.
• Upper limits on the A→ZH production cross-section are set at 95% confidence level, with improved sensitivity in the mH > 350 GeV and mA > 800 GeV region compared to previous searches.
• Model-independent upper limits are presented as functions of the reconstructed m(t̄t) and m(bb̄) distributions, providing robust constraints across the phase space.
• In the context of 2HDM, the results constrain the parameter space where mA > mH, particularly for tanβ values between 1 and 10.
• The search achieves sensitivity beyond previous searches in the ℓ⁺ℓ⁻t̄t and νν̄bb̄ channels, especially in the high-mass regime.
• The results are interpreted in models with hierarchical 2HDMs and dark matter mediators, extending the reach of existing LHC searches.