[Paper Review] Study of Electroweak Phase Transition in Exotic Higgs Decays at the CEPC

Zhen Wang, Xuliang Zhu+8 more|arXiv (Cornell University)|Mar 18, 2022

Particle physics theoretical and experimental studies4 citations

TL;DR

This study investigates exotic Higgs decays into a pair of light spin-0 scalars (H→ss→4b) at the Circular Electron Positron Collider (CEPC), using 5000 fb⁻¹ of e⁺e⁻ data at √s = 240 GeV. It demonstrates that the CEPC can probe a strong first-order electroweak phase transition with singlet scalars as light as 20 GeV, achieving significantly better sensitivity than the LHC, especially in the 15–60 GeV mass range.

ABSTRACT

A strong first-order electroweak phase transition (EWPT) can be induced by light new physics weakly coupled to the Higgs. This study focuses on a scenario in which the first-order EWPT is driven by a light scalar $s$ with a mass between 15-60 GeV. A search for exotic decays of the Higgs boson into a pair of spin-zero particles, $h o ss$, where the $s$-boson decays into $b$-quarks promptly is presented. The search is performed in events where the Higgs boson is produced in association with a $Z$ boson, giving rise to a signature of two charged leptons (electrons or muons) and multiple jets from $b$-quark decays. The analysis is considering a scenario of analysing 5000 fb$^{-1}$ $e^+ e^-$ collision data at $\sqrt{s} = 240 $ GeV from the Circular Electron Positron Collider (CEPC). This study with $4b$ final state conclusively tests the expected sensitivity of probing the light scalars in the CEPC experiment. The sensitivity reach is significantly larger than that can be achieved at the LHC.

Motivation & Objective

• To investigate the potential of the CEPC to probe a strong first-order electroweak phase transition (SFOEWPT) driven by a light scalar singlet coupled to the Higgs boson.
• To assess the sensitivity of the CEPC to exotic Higgs decays into a pair of light spin-zero scalars (H→ss), where each s decays promptly into b-quarks.
• To compare the performance of cut-based and multivariate (BDT) analysis techniques in identifying the H→ss→4b final state in ZH associated production.
• To establish expected upper limits on the signal cross-section times branching ratio at 95% CL for scalar masses between 15–60 GeV.

Proposed method

• The analysis focuses on ZH associated production in e⁺e⁻ collisions at √s = 240 GeV, with the Higgs decaying into two light scalars (s) that each decay into b-quarks, yielding a 4b final state.
• Kinematic variables such as invariant mass, angular distributions, and b-jet tagging are used to distinguish signal from background, with a boosted decision tree (BDT) employed to enhance signal sensitivity.
• A binned profile-likelihood fit is performed to set 95% CL upper limits on the signal cross-section times branching ratio, incorporating both statistical and systematic uncertainties via nuisance parameters.
• Systematic uncertainties—such as jet energy scale, b-tagging efficiency, and lepton reconstruction—are modeled using Gaussian, log-normal, or Poisson priors in the likelihood function.
• The study uses a reference detector simulation to model realistic CEPC conditions, including detector resolution and acceptance effects.
• The analysis combines electron and muon final states from Z→ℓ⁺ℓ⁻ decays to improve statistics and sensitivity.

Experimental results

Research questions

• RQ1 Can the CEPC achieve sensitivity to exotic Higgs decays H→ss→4b for light scalar masses down to 15 GeV?
• RQ2 How does the CEPC's sensitivity to light scalar-mediated SFOEWPT compare to that of the LHC in the 15–60 GeV range?
• RQ3 What is the relative performance of cut-based versus multivariate (BDT) analysis in identifying the 4b final state from H→ss decays?
• RQ4 To what extent do realistic detector effects and systematic uncertainties limit the sensitivity reach of the CEPC in this channel?
• RQ5 Can the CEPC conclusively probe the parameter space where a strong first-order electroweak phase transition is induced by a light scalar singlet?

Key findings

• The CEPC achieves expected upper limits on the signal cross-section times branching ratio (σ_ZH × B(H→ss→4b)) ranging from 0.0006 fb at m_s = 15 GeV to 0.0008 fb at m_s = 60 GeV, using the BDT analysis.
• For m_s = 15 GeV, the 95% CL upper limit is 0.0006⁺⁰·⁰⁰⁰³⁺⁰·⁰⁰⁰⁰⁷₋₀·⁰⁰⁰²₋₀·⁰⁰⁰⁰² fb, demonstrating high sensitivity to light scalars.
• The BDT analysis outperforms the cut-based method, reducing the upper limit by a factor of 5–6 across most masses, especially below 30 GeV.
• The CEPC's sensitivity reaches down to m_s = 20 GeV with a 95% CL upper limit of 0.0005 fb, significantly exceeding the LHC's projected reach in this mass range.
• The study confirms that the CEPC can conclusively probe the SFOEWPT parameter space for light scalars, particularly in the 15–60 GeV window, where the LHC has limited sensitivity.
• The realistic simulation, including detector effects and systematic uncertainties, yields a weaker exclusion limit than projected in prior studies, underscoring the importance of detailed detector modeling.