[Paper Review] Strong Symmetry Breaking at e+e- Linear Colliders

Timothy L. Barklow|ArXiv.org|Dec 21, 2001

Particle physics theoretical and experimental studies3 references5 citations

TL;DR

This paper investigates the sensitivity of an e⁺e⁻ linear collider (LC) with √s = 0.5–1.5 TeV to strong electroweak symmetry breaking via high-energy gauge boson and top quark pair production. By measuring anomalous triple and quartic gauge couplings in processes like e⁺e⁻ → νν̄W⁺W⁻ and e⁺e⁻ → W⁺W⁻, the LC enables precise determination of chiral Lagrangian parameters α₄, α₅, L₉ᴸ, L₉ᴿ, and vector/resonance parameters, with superior resolution for vector resonance masses and widths compared to the LHC, especially at higher energies and with polarization.

ABSTRACT

The study of strong symmetry breaking at an e+e linear collider with sqrt(s)=0.5-1.5 TeV is reviewed. It is shown that processes such as e+e- --> nu nubar W+ W-, e+e- --> nu nubar t tbar, and and e+e- --> W+ W- can be used to measure chiral Lagrangian and strong resonance parameters. The linear collider results are compared with those expected from the LHC.

Motivation & Objective

• To assess the capability of an e⁺e⁻ linear collider (LC) with √s = 0.5–1.5 TeV to probe strong electroweak symmetry breaking when the Higgs boson is not discovered.
• To compare the sensitivity of the LC to strong dynamics signals with those expected at the LHC, particularly in the context of anomalous gauge couplings and resonances.
• To evaluate the measurement precision of chiral Lagrangian parameters α₄, α₅, L₉ᴸ, L₉ᴿ from processes such as e⁺e⁻ → νν̄W⁺W⁻, e⁺e⁻ → W⁺W⁻, and e⁺e⁻ → νν̄t̄t.
• To determine the LC's ability to measure the mass and width of strong vector and scalar resonances, even when above the center-of-mass energy.
• To explore the unique access of the LC to W⁺W⁻ → t̄t scattering, a channel dominated by QCD backgrounds at the LHC.

Proposed method

• Utilizes effective chiral Lagrangian formalism to describe strong interactions of gauge bosons below resonance thresholds, with terms involving coefficients α₄, α₅, L₉ᴸ, L₉ᴿ that induce anomalous quartic and triple gauge couplings.
• Analyzes e⁺e⁻ → νν̄W⁺W⁻, νν̄ZZ, νν̄t̄t, and W⁺W⁻ processes at √s = 0.5–1.5 TeV to extract sensitivity to chiral Lagrangian parameters via angular distributions and kinematic cuts.
• Applies cuts to isolate longitudinally polarized W boson (WₗWₗ) scattering, reducing background from transverse modes and non-resonant processes.
• Uses K-matrix unitarization in the LET model to simulate strong scattering amplitudes and compute signal significances and 95% confidence level (C.L.) mass scale limits.
• Performs Monte Carlo simulations to estimate signal-to-background ratios, statistical uncertainties, and systematic errors in resonance mass and width measurements.
• Compares results from the LC with those expected from the LHC, particularly in the I=0,1,2 isospin channels, using signal significance and mass scale limits as metrics.

Experimental results

Research questions

• RQ1 Can an e⁺e⁻ linear collider with √s = 0.5–1.5 TeV measure chiral Lagrangian parameters α₄ and α₅ with higher precision than the LHC?
• RQ2 How does beam polarization and center-of-mass energy affect the signal-to-background ratio in e⁺e⁻ → νν̄W⁺W⁻ and e⁺e⁻ → νν̄t̄t processes?
• RQ3 What is the sensitivity of the LC to strong vector and scalar resonances in WW scattering, and how does it compare to the LHC in terms of mass and width resolution?
• RQ4 Can the LC uniquely probe W⁺W⁻ → t̄t scattering, which is overwhelmed by QCD backgrounds at the LHC?
• RQ5 How do theoretical systematic uncertainties in signal and background calculations compare between the LC and the LHC for strong symmetry breaking signals?

Key findings

• At √s = 1.5 TeV with 1000 fb⁻¹ and 80%/0% beam polarization, the signal-to-background ratio (S/√B) for e⁺e⁻ → νν̄t̄t reaches 22, significantly higher than at the LHC.
• The LC measures the mass and width of a 1.5 TeV vector resonance with a width uncertainty of only 6% at √s = 0.5 TeV, improving to sub-percent levels at √s = 1.0 and 1.5 TeV when on resonance.
• For scalar resonances, the LC achieves a 95% C.L. mass scale limit of 4.3 TeV in the I=0 channel at √s = 1.5 TeV, outperforming the LHC in the same channel.
• The LC provides a 7σ signal significance for I=1 structure in νν̄W⁺W⁻/ZZ at √s = 1.0 TeV, increasing to 20σ at √s = 1.5 TeV, demonstrating superior sensitivity at higher energies.
• The LC measures the vector resonance mass M₁ with a 95% C.L. lower limit of 6.4 TeV in e⁺e⁻ → W⁺W⁻ at √s = 1.5 TeV, matching or exceeding the LHC's sensitivity in the I=2 channel.
• The LC achieves a 3σ signal significance for vector resonance production in e⁺e⁻ → W⁺W⁻ at √s = 0.5 TeV, with a 95% C.L. mass limit of 4.8 TeV, outperforming the LHC for resonances up to 2.5 TeV.