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Probing nonstandard bosonic interactions via W-boson pair production at lepton colliders

K. Hagiwara, T. Hatsukano, S. Ishihara, R. Szalapski

TL;DR

This paper develops a comprehensive, gauge-invariant EFT framework to probe nonstandard bosonic interactions in e^+e^- → W^+W^-, using a nine-tensor form-factor decomposition. It systematically treats both linear (with a light Higgs) and nonlinear (no light Higgs) realizations of electroweak symmetry breaking, deriving how dimension-6 linear and dimension-4 nonlinear operators modify WWV couplings and gauge-boson propagators. A detailed numerical analysis with LEP II and projected LEP II/LC data quantifies constraints on the operator coefficients, showing how WW production provides complementary information to four-fermion processes and how high-energy measurements enhance sensitivity to running form factors in the linear scenario. The study demonstrates that future linear colliders can significantly tighten bounds and help discriminate between linear and nonlinear realizations, with particular emphasis on operators like f_WWW and α_11 as discriminants.

Abstract

The process e+e- --> W+W- provides a valuable laboratory to test the Standard Model (SM) and to search for new physics. The most general helicity amplitudes for this process require the introduction of nine form-factors which we calculate in the context of SU(2) X U(1) gauge-invariant extensions of the SM. The contributions of new physics are parametrized via an effective Lagrangian constructed from the light fields. Because the mechanism of electroweak symmetry-breaking remains an open problem we consider both the effective Lagrangian with a linearly realized Higgs sector, i.e. with a light physical Higgs boson, and the effective Lagrangian which utilizes a nonlinear realization of the Higgs mechanism. The use of an effective Lagrangian allows one to calculate consistently nonstandard contributions to e+e- --> W+W- amplitudes as well as the nonstandard contributions to other processes. We study the interplay of the low-energy and Z-pole measurements with measurements via the processes e+e- --> fermion pairs and e+e- --> W+W- at LEP II or a future linear e+e- collider. Concrete relationships between operators of the linear and nonlinear realizations are presented where possible.

Probing nonstandard bosonic interactions via W-boson pair production at lepton colliders

TL;DR

This paper develops a comprehensive, gauge-invariant EFT framework to probe nonstandard bosonic interactions in e^+e^- → W^+W^-, using a nine-tensor form-factor decomposition. It systematically treats both linear (with a light Higgs) and nonlinear (no light Higgs) realizations of electroweak symmetry breaking, deriving how dimension-6 linear and dimension-4 nonlinear operators modify WWV couplings and gauge-boson propagators. A detailed numerical analysis with LEP II and projected LEP II/LC data quantifies constraints on the operator coefficients, showing how WW production provides complementary information to four-fermion processes and how high-energy measurements enhance sensitivity to running form factors in the linear scenario. The study demonstrates that future linear colliders can significantly tighten bounds and help discriminate between linear and nonlinear realizations, with particular emphasis on operators like f_WWW and α_11 as discriminants.

Abstract

The process e+e- --> W+W- provides a valuable laboratory to test the Standard Model (SM) and to search for new physics. The most general helicity amplitudes for this process require the introduction of nine form-factors which we calculate in the context of SU(2) X U(1) gauge-invariant extensions of the SM. The contributions of new physics are parametrized via an effective Lagrangian constructed from the light fields. Because the mechanism of electroweak symmetry-breaking remains an open problem we consider both the effective Lagrangian with a linearly realized Higgs sector, i.e. with a light physical Higgs boson, and the effective Lagrangian which utilizes a nonlinear realization of the Higgs mechanism. The use of an effective Lagrangian allows one to calculate consistently nonstandard contributions to e+e- --> W+W- amplitudes as well as the nonstandard contributions to other processes. We study the interplay of the low-energy and Z-pole measurements with measurements via the processes e+e- --> fermion pairs and e+e- --> W+W- at LEP II or a future linear e+e- collider. Concrete relationships between operators of the linear and nonlinear realizations are presented where possible.

Paper Structure

This paper contains 16 sections, 72 equations, 2 figures, 10 tables.

Table of Contents

  1. Introduction
  2. A form-factor-based analysis of $e^+e^- \to W^+W^-$
  3. Calculation of the form-factors
  4. The linear representation
  5. The nonlinear realization
  6. Processes with four external fermions
  7. The linear realization of the symmetry-breaking sector
  8. The nonlinear realization of the symmetry-breaking sector
  9. Expectations for improved measurements
  10. The process $e^+e^- \to W^+W^-$
  11. The linear realization of the symmetry-breaking sector
  12. The nonlinear realization of the symmetry-breaking sector
  13. The 'phenomenological Lagrangian'
  14. Numerical Analysis
  15. Discussion
  16. ...and 1 more sections

Figures (2)

  • Figure 1: The process $e^-e^+ \rightarrow W^-W^+$ with momentum and helicity assignments.The momenta $k$ and $\overline{k}$ are incoming, but $p$ and $\overline{p}$ are outgoing. The arrows along the $W$-boson lines indicate the flow of negative electronic charge.
  • Figure 2: The SM Feynman graphs for the process $e^-e^+ \rightarrow W^-W^+$. Momentum and helicity assignments coincide with those of Fig. \ref{['fig-eeww-blob']}.