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Single-spin measurements and heavy new physics in the $e^+e^- \to t\bar{t}$ process at an FCC-ee

Haotian Cao, Frank Petriello

TL;DR

This work analyzes the potential of the FCC-ee to uncover heavy new physics in $e^+e^- \to t\bar{t}$ using the Standard Model Effective Field Theory. By exploiting the full spin-density-matrix formalism, including single-spin observables, the authors show that measurements of $A_{FB}$, $A_2$, and spin-dependent coefficients can outperform spin-correlation observables in constraining SMEFT Wilson coefficients, and they demonstrate how single-spin data help resolve flat directions in the coefficient space. Focusing on the $t\bar{t}$ threshold region ($\sqrt{s} \approx 345$–$365$ GeV) with realistic luminosities, they compute $1/\Lambda^2$-interference-dominated bounds (and assess $1/\Lambda^4$ effects) for operator classes including four-fermion interactions, top-Z vertex corrections, and electroweak dipoles, with bounds often in the multi-TeV to above $10$ TeV range. The results emphasize the unique role of single-spin observables in SMEFT probes at a future lepton collider and motivate extending the analysis to loop-induced operators and broader collider scenarios.

Abstract

We investigate the potential of single-spin components of the spin-density matrix in the $e^+ e^- \to t\bar{t}$ process at a future FCC-ee for probing heavy new physics parametrized using the SMEFT framework. We consider the full spectrum of spin observables and the complete angular decomposition of the $t\bar{t}$ production process in our study. We find that single-spin measurements generically provide stronger probes of SMEFT Wilson coefficients than measurements where the $t\bar{t}$ spins are correlated, and that single-spin observables are important for resolving flat directions that can appear in the Wilson-coefficient parameter space.

Single-spin measurements and heavy new physics in the $e^+e^- \to t\bar{t}$ process at an FCC-ee

TL;DR

This work analyzes the potential of the FCC-ee to uncover heavy new physics in using the Standard Model Effective Field Theory. By exploiting the full spin-density-matrix formalism, including single-spin observables, the authors show that measurements of , , and spin-dependent coefficients can outperform spin-correlation observables in constraining SMEFT Wilson coefficients, and they demonstrate how single-spin data help resolve flat directions in the coefficient space. Focusing on the threshold region (– GeV) with realistic luminosities, they compute -interference-dominated bounds (and assess effects) for operator classes including four-fermion interactions, top-Z vertex corrections, and electroweak dipoles, with bounds often in the multi-TeV to above TeV range. The results emphasize the unique role of single-spin observables in SMEFT probes at a future lepton collider and motivate extending the analysis to loop-induced operators and broader collider scenarios.

Abstract

We investigate the potential of single-spin components of the spin-density matrix in the process at a future FCC-ee for probing heavy new physics parametrized using the SMEFT framework. We consider the full spectrum of spin observables and the complete angular decomposition of the production process in our study. We find that single-spin measurements generically provide stronger probes of SMEFT Wilson coefficients than measurements where the spins are correlated, and that single-spin observables are important for resolving flat directions that can appear in the Wilson-coefficient parameter space.

Paper Structure

This paper contains 11 sections, 19 equations, 9 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Formalism
  3. Standard Model results
  4. SMEFT results
  5. Top threshold setup
  6. Discussion of observables
  7. Numerical results
  8. Two-coefficient bounds
  9. Conclusions
  10. Common factors for spin correlation coefficients
  11. Plots

Figures (9)

  • Figure 1: Regions in the $(C_{VV},C_{VA})$ parameter space expected to be constrained at $\sqrt{s} = 365~\text{GeV}$, based on the observables shown. The allowed regions are shown as shaded areas. The fit in the left panel includes terms up to linear order in $1/\Lambda^2$, while the right panel also accounts for quadratic contributions.
  • Figure 2: Regions in the $(C_{VA},C_{AV})$ parameter space expected to be constrained at $\sqrt{s} = 365~\text{GeV}$, based on the observables shown. The allowed regions are shown as shaded areas. The fit in the left panel includes terms up to linear order in $1/\Lambda^2$, while the right panel also accounts for quadratic contributions.
  • Figure 3: Regions in the $(C_{AV},C_{AA})$ parameter space expected to be constrained at $\sqrt{s} = 365~\text{GeV}$, based on the observables shown. The allowed regions are shown as shaded areas. The fit in the left panel includes terms up to linear order in $1/\Lambda^2$, while the right panel also accounts for quadratic contributions.
  • Figure 4: Regions in the $(C_{tZ},C_{t\gamma})$ parameter space expected to be probed at $\sqrt{s} = 365~\text{GeV}$, based on the observables considered in this study. The allowed regions are shown as shaded areas. The fit in the left panel includes terms up to linear order in $1/\Lambda^2$, while the right panel also accounts for quadratic contributions.
  • Figure 5: Comparison of bounds from the $\bar{C}_{ij}$ observables (dashed lines) with other observables in the parameter space $(C_{VA},C_{AA})$.
  • ...and 4 more figures