[Paper Review] Model independent search for Z'-boson signals

A. V. Gulov, V. V. Skalozub|ArXiv.org|May 15, 2009

Particle physics theoretical and experimental studies3 references3 citations

TL;DR

This paper presents a model-independent method to search for a $Z^\prime$ boson as a virtual state in $e^+e^-$ scattering by leveraging renormalization group (RG) relations among low-energy couplings to fermions. By reducing the number of free parameters through RG constraints, the authors define unique observables that isolate $Z^\prime$ signals, leading to $95\%$ confidence level estimates of $Z^\prime$ couplings to leptons and quarks from LEP data, with hints of a $1-1.2$ TeV $Z^\prime$ boson.

ABSTRACT

An approach to the model-independent searching for the Z' gauge boson as a virtual state in scattering processes is developed. It accounts for as a basic requirement the renormalizability of underlying unspecified in other respects model. This results in a set of relations between low energy couplings of Z' to fermions that reduces in an essential way the number of parameters to be fitted in experiments. On this ground the observables which uniquely pick out the Z' boson in leptonic processes are introduced and the data of LEP experiments analyzed. The Z' couplings to leptons and quarks are estimated at 95% confidence level. These estimates may serve as a guide for experiments at the Tevatron and/or LHC. A comparison with other approaches and results is given.

Motivation & Objective

• To develop a model-independent approach for detecting a $Z^\prime$ boson as a virtual state in high-energy scattering processes.
• To reduce the number of free parameters in $Z^\prime$ coupling fits by deriving low-energy constraints from renormalization group (RG) equations.
• To define unique observables that correlate exclusively with $Z^\prime$ exchange, enabling unambiguous signal identification in $e^+e^- \to \ell^+\ell^-$ and Bhabha processes.
• To analyze LEP2 data using these observables to set $95\%$ confidence level bounds on $Z^\prime$ couplings to fermions.
• To provide a guide for $Z^\prime$ searches at the Tevatron and LHC based on LEP constraints.

Proposed method

• Derive RG relations between low-energy couplings of the $Z^\prime$ to fermions by analyzing the renormalization group evolution of fermion scattering amplitudes.
• Use the RG relations to reduce the number of independent parameters in $Z^\prime$ coupling fits, from many to only 2–3 for leptonic processes.
• Define kinematic observables that are uniquely sensitive to $Z^\prime$ exchange by exploiting the correlations enforced by RG relations.
• Perform one-parameter and many-parameter fits to LEP2 data for $e^+e^- \to \mu^+\mu^-$, $\tau^+\tau^-$, and $e^+e^-$ processes to extract $Z^\prime$ signal hints.
• Apply neural network analysis to detect $Z^\prime$ hints in Bhabha scattering, particularly for chiral $Z^\prime$ couplings.
• Use the RG-improved effective field theory approach to relate high-scale $Z^\prime$ couplings to low-energy observables, eliminating dependence on heavy particle details.

Experimental results

Research questions

• RQ1 Can a model-independent search for a $Z^\prime$ boson be formulated using only low-energy effective couplings and RG constraints?
• RQ2 How do RG relations reduce the number of free parameters in $Z^\prime$ coupling fits for leptonic $e^+e^-$ processes?
• RQ3 What unique observables can be constructed to isolate $Z^\prime$ signals from other new physics contributions in $e^+e^- \to \ell^+\ell^-$ processes?
• RQ4 What are the $95\%$ confidence level bounds on $Z^\prime$ couplings to leptons and quarks derived from LEP2 data using this method?
• RQ5 Can this approach detect $Z^\prime$ hints in Bhabha scattering, especially for chiral couplings, and how do the results compare to standard model fits?

Key findings

• The RG relations between $Z^\prime$ couplings to fermions reduce the number of independent parameters in $Z^\prime$ fits to 2–3, significantly improving parameter constraints.
• A one-parameter observable in $e^+e^- \to \mu^+\mu^-$ process shows a $1\sigma$ confidence level hint for a $Z^\prime$ boson with mass $1-1.2$ TeV.
• In Bhabha scattering, a $2\sigma$ confidence level hint for a $Z^\prime$ boson with mass $1-1.2$ TeV is observed, supporting the existence of a $Z^\prime$ at the TeV scale.
• The many-parameter fit of LEP2 data, incorporating all experimental information and RG constraints, shows a reduced uncertainty compared to standard fits, confirming the robustness of the method.
• The $Z^\prime$ couplings to leptons and quarks are estimated at $95\%$ confidence level, providing a benchmark for future searches at the Tevatron and LHC.
• Neural network analysis confirms the presence of $Z^\prime$ hints in Bhabha process, particularly for chiral $Z^\prime$ couplings, reinforcing the signal significance.