[Paper Review] Physics Potential of the Next Generation Colliders

Rohini M. Godbole|ArXiv.org|May 12, 2002

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper evaluates the physics potential of next-generation colliders—specifically the LHC and future $e^+e^-$ linear colliders—in probing the mechanism of electroweak symmetry breaking. It argues that these colliders will either discover the Higgs boson directly, test its properties to confirm the Standard Model, or reveal new physics such as supersymmetry, composite Higgs, or large extra dimensions, thereby resolving the hierarchy problem and completing the electroweak sector of particle physics.

ABSTRACT

In this article I summarize some aspects of the current status of the field of high energy physics and discuss how the next generation of high energy colliders will aid in furthering our basic understanding of elementary particles and interactions among them, by shedding light on the mechanism for the spontaneous breakdown of the Electroweak Symmetry.

Motivation & Objective

• To assess the role of future colliders in resolving the mechanism of electroweak symmetry breaking, a central open question in high-energy physics.
• To evaluate whether the Higgs boson exists as a fundamental scalar or if new physics—such as supersymmetry, composite Higgs, or large extra dimensions—must exist to solve the hierarchy problem.
• To determine the sensitivity of upcoming colliders like the LHC and $e^+e^-$ linear colliders to deviations from the Standard Model in gauge boson couplings, Higgs properties, and new resonances.
• To compare the discovery reach of hadronic (LHC) and leptonic ($e^+e^-$, $\gamma\gamma$) colliders for new physics beyond the Standard Model.
• To establish that precision measurements at $e^+e^-$ colliders (LEP/SLC) already constrain the Higgs mass indirectly, highlighting the need for direct discovery at future machines.

Proposed method

• Analyzing precision electroweak measurements from LEP and SLC to extract indirect constraints on the Higgs boson mass, using the $\chi^2/\text{dof} = 22.9/15$ fit quality as a benchmark.
• Evaluating the reach of the LHC and future $e^+e^-$ linear colliders in detecting deviations in trilinear and quartic gauge boson couplings from Standard Model predictions.
• Using effective field theory and chiral Lagrangian approaches (e.g., parameters $L_9$) to model new physics effects in Higgs and vector boson self-couplings.
• Assessing the discovery potential for spin-2 graviton resonances in dilepton final states ($gg \to G \to l^+l^-$) at the LHC, with angular distributions used to distinguish spin-2 from spin-1 signals.
• Applying polarization and detector acceptance simulations to optimize sensitivity to new physics in $\gamma\gamma$ and $e^+e^-$ colliders, particularly for large extra dimensions.
• Comparing the discovery reach of different collider types—hadronic (LHC) and leptonic (linear $e^+e^-$, $\gamma\gamma$)—for new physics scenarios such as composite Higgs and large extra dimensions.

Experimental results

Research questions

• RQ1 Can future colliders like the LHC and $e^+e^-$ linear colliders directly discover the Higgs boson and confirm its properties as predicted by the Standard Model?
• RQ2 What is the sensitivity of the LHC and $e^+e^-$ colliders to deviations in trilinear and quartic gauge boson couplings that would signal new physics beyond the Standard Model?
• RQ3 Can the LHC detect spin-2 graviton resonances from large extra dimensions, and can angular distributions in dilepton final states distinguish them from spin-1 resonances?
• RQ4 To what extent can the LHC and $e^+e^-$ colliders probe models of composite Higgs or large extra dimensions that avoid the need for supersymmetry to solve the hierarchy problem?
• RQ5 How do precision measurements at LEP/SLC constrain the Higgs mass indirectly, and what does this imply for the discovery potential of next-generation colliders?

Key findings

• Precision measurements at LEP and SLC have constrained the Higgs mass indirectly, with the $\chi^2/\text{dof} = 22.9/15$ indicating a high-precision fit to the Standard Model, allowing indirect extraction of the Higgs mass.
• The top quark mass was confirmed independently via direct observation at the Tevatron ($m_t = 174.3 \pm 5.1$ GeV) and indirect precision measurements at LEP/SLC ($m_t = 180.5 \pm 10.0$ GeV), validating the consistency of the Standard Model at loop level.
• The LHC is expected to cover the full parameter space of warped extra dimensions with a TeV-scale compactification scale, enabling discovery of graviton resonances in the dilepton channel.
• The angular distribution of $l^+l^-$ pairs in the center-of-mass frame at the LHC can distinguish spin-2 graviton signals from spin-1 resonances, even with reduced detector acceptance at large $\cos\theta^*$.
• The joint reach of the LHC and future $e^+e^-$ linear colliders will cover the full range of viable models for large extra dimensions and composite Higgs, including those that avoid the need for supersymmetry.
• The $e^+e^-$ and $\gamma\gamma$ colliders offer superior sensitivity to new physics via polarization, particularly for probing trilinear and quartic gauge couplings and chiral Lagrangian parameters like $L_9$.