[Paper Review] Positron Polarization at the International Linear Collider

P. Osland, N. Paver|ArXiv.org|Jul 15, 2005

Particle physics theoretical and experimental studies3 references3 citations

TL;DR

This paper advocates for polarized positron beams at the International Linear Collider (ILC) to enhance sensitivity to new physics, particularly through longitudinal and transverse beam polarization. It demonstrates that positron polarization significantly improves discovery reach for flavor-changing neutral couplings, CP-violating interactions, and extra-dimensional gravity models, with transverse polarization enabling discrimination between competing gravity scenarios like ADD and RS.

ABSTRACT

We review some recent arguments supporting the upgrade of the International Linear Collider by a polarized positron beam, in addition to the polarized electron beam. The examples presented here mainly focus on the impact of positron polarization on items relevant to new physics searches, such as the identification of novel interactions in fermion-pair production and the formulation of new CP-sensitive observables. In particular, in addition to the benefits from positron and electron longitudinal polarizations, the advantages in this field of having transverse polarization of both beams are emphasized.

Motivation & Objective

• To establish the physics case for implementing polarized positron beams at the ILC beyond the already planned polarized electron beam.
• To investigate how positron polarization enhances the detection of new physics signals, especially in rare processes and CP-violating observables.
• To evaluate the advantages of transverse beam polarization in probing novel sources of CP violation and distinguishing between competing gravity models in extra dimensions.
• To quantify the improvement in discovery and exclusion reach for key new physics signatures such as flavor-changing top couplings and anomalous triple gauge couplings.
• To demonstrate that combined longitudinal and transverse polarization enables more model-independent, precise, and sensitive measurements of new physics parameters.

Proposed method

• Uses effective field theory to model flavor-changing neutral couplings (FCNC) in top quark decays via $ar{t}q$ and $t\bar{t}$ production in $e^+e^-$ collisions.
• Applies a model-independent effective Lagrangian for CP-violating $Z \to \bar{b}bg$ and $Z \to \bar{b}bgg$ decays, parameterized by $h_{Vb}$ and $h_{Ab}$ coupling constants.
• Employs transverse beam polarization to define azimuthal angular asymmetries sensitive to CP violation and anomalous triple gauge couplings.
• Utilizes differential cross-section distributions convoluted with Legendre polynomials to probe spin-2 graviton exchange in ADD and RS models.
• Applies polarization-dependent asymmetries and angular distributions to distinguish between the continuous KK graviton spectrum (ADD) and discrete resonances (RS) in $e^+e^- \to \bar{f}f$.
• Combines data from multiple fermion final states ($f = \mu, \tau, c, b, t$) to improve sensitivity and statistical significance.

Experimental results

Research questions

• RQ1 How does positron beam polarization improve the sensitivity to flavor-changing neutral couplings in top quark decays?
• RQ2 To what extent can transverse polarization enhance the detection of CP-violating signals in $Z$ boson decays to $\bar{b}bg$ final states?
• RQ3 Can transverse beam polarization enable discrimination between the ADD and RS models of extra-dimensional gravity?
• RQ4 What is the quantitative improvement in discovery reach for graviton masses when both beams are transversely polarized compared to unpolarized or longitudinally polarized beams?
• RQ5 How do combined longitudinal and transverse polarizations enhance the measurement of anomalous triple gauge couplings?

Key findings

• With $E_{\rm c.m.} = 0.5$ TeV and $\mathcal{L}_{\rm int} = 300$ fb$^{-1}$, the 3$\sigma$ reach for $\sigma_{\mu\nu}$-coupling BR($t \to Zq$) improves from 0.48 to 0.17 when positron polarization increases from 0% to 60%.
• For $\gamma_{\mu}$-coupling BR($t \to \gamma q$), the 3$\sigma$ reach drops from 0.30 to 0.038 with $P_{e^+} = 60\%$, demonstrating significant suppression of SM backgrounds.
• Transverse polarization of $P^{T}_{e^-} = 80\%$ and $P^{T}_{e^+} = 60\%$ improves the five-$\sigma$ identification reach for $M_H$ in the ADD model from 2.2 TeV (unpolarized) to 2.8 TeV at $\sqrt{s} = 1.0$ TeV and $\mathcal{L}_{\rm int} = 1000$ fb$^{-1}$.
• The five-$\sigma$ discrimination power between ADD and RS models reaches 2.8 TeV at $\sqrt{s} = 1.0$ TeV with transverse polarization, compared to 2.2 TeV with longitudinal polarization.
• Imaginary parts of anomalous couplings $\lambda_1$ and $\lambda_2$ are constrained to $3.8 \times 10^{-3}$ and $3.0 \times 10^{-2}$ at 90% C.L. when both $P^{T}_{e^-} = 80\%$ and $P^{T}_{e^+} = 60\%$, showing improved sensitivity over unpolarized beams.
• The azimuthal asymmetry $A_i^T$ is non-zero in the ADD model due to an imaginary component in the graviton coupling, enabling clear discrimination from the SM and RS models, which remain symmetric.