[Paper Review] The role of polarized positrons and electrons in revealing fundamental interactions at the Linear Collider

Gudrid Moortgat‐Pick, K. Abe+72 more|arXiv (Cornell University)|Jul 1, 2005

Particle physics theoretical and experimental studies312 references152 citations

TL;DR

This paper demonstrates that polarized electron and positron beams at the International Linear Collider (ILC) significantly enhance the discovery potential and precision measurement capabilities for new physics beyond the Standard Model. By leveraging longitudinal and transverse beam polarization, the ILC can suppress backgrounds, improve sensitivity to new particles, and probe fundamental interactions such as those in supersymmetry, extra dimensions, and anomalous couplings with high precision.

ABSTRACT

The proposed International Linear Collider (ILC) is well-suited for discovering physics beyond the Standard Model and for precisely unraveling the structure of the underlying physics. The physics return can be maximized by the use of polarized beams. This report shows the paramount role of polarized beams and summarizes the benefits obtained from polarizing the positron beam, as well as the electron beam. The physics case for this option is illustrated explicitly by analyzing reference reactions in different physics scenarios. The results show that positron polarization, combined with the clean experimental environment provided by the linear collider, allows to improve strongly the potential of searches for new particles and the identification of their dynamics, which opens the road to resolve shortcomings of the Standard Model. The report also presents an overview of possible designs for polarizing both beams at the ILC, as well as for measuring their polarization.

Motivation & Objective

• To establish the critical role of polarized electron and positron beams in maximizing the physics reach of the International Linear Collider (ILC).
• To address key limitations of the Standard Model, such as the hierarchy problem, CP violation, and the nature of the Higgs sector, through precision measurements.
• To enable improved identification of new physics dynamics by reducing background contributions and enhancing signal sensitivity.
• To provide a comprehensive physics case for beam polarization, covering both discovery and precision physics at the ILC.
• To evaluate and compare the performance of longitudinally and transversely polarized beams across diverse new physics scenarios.

Proposed method

• Utilizes formalism for polarized cross sections in $e^+e^-$ collisions, including effective polarization and left-right asymmetry to extract physics information.
• Applies the Blondel scheme to extract $\sin^2\theta_W^{\text{eff}}$ from Z-boson decays at GigaZ, enabling constraints on Higgs mass and SUSY parameters.
• Analyzes reference reactions in supersymmetry, contact interactions, and models with extra dimensions to quantify the impact of beam polarization.
• Evaluates the use of transverse polarization for probing CP violation and spin correlations in processes like chargino/neutralino production.
• Reviews and compares existing event generators (e.g., SHERPA, HERWIG++, MADGRAPH, WHIZARD) for their capability to model polarization and spin correlations.
• Assesses technical designs for beam polarization and polarization measurement systems at the ICD, including polarized sources and diagnostic tools.

Experimental results

Research questions

• RQ1 How does beam polarization improve the sensitivity to new physics beyond the Standard Model at the ILC?
• RQ2 What is the impact of longitudinal and transverse beam polarization on the measurement of Higgs couplings and triple gauge boson vertices?
• RQ3 In what ways does polarization enhance the discovery and characterization of supersymmetric particles, including their masses and couplings?
• RQ4 How can transverse polarization be used to probe CP-violating effects in new physics processes?
• RQ5 What are the most effective simulation tools for modeling polarized processes, and how do they compare in accuracy and capability?

Key findings

• Polarized positron beams at the ILC significantly suppress backgrounds in new physics searches, improving signal-to-noise ratios.
• Longitudinal polarization enables precise determination of $\sin^2\theta_W^{\text{eff}}$ at GigaZ, leading to constraints on the Higgs boson mass and supersymmetric parameters.
• Transverse beam polarization enhances sensitivity to CP-violating effects in Z-boson decays and in chargino/neutralino production, allowing for the detection of non-minimal CP phases.
• Polarization allows for the separation of production mechanisms in Higgs boson searches, particularly for $ZZH$ and $Z\gamma H$ couplings.
• Event generators such as SHERPA, HERWIG++, and MADGRAPH with helicity amplitude techniques are capable of modeling polarization effects with high accuracy.
• The use of polarized beams enables the measurement of selectron and smuon masses above production thresholds with improved precision, aiding in the reconstruction of SUSY spectra.