[Paper Review] Off-shell W-pair production - universal versus non-universal corrections

Ansgar Denner, S. Dittmaier+2 more|ArXiv.org|Jan 23, 2001

Metallurgy and Material Forming4 citations

TL;DR

This paper investigates universal and non-universal electroweak radiative corrections to off-shell W-pair production in e⁺e⁻ collisions using the RacoonWW Monte Carlo generator. It shows that while universal corrections dominate at LEP2 energies (1–2% impact on total cross section), non-universal effects grow significantly at higher energies, reaching 10% in angular distributions and distorting W-boson invariant-mass peaks due to final-state radiation, which universal approximations fail to capture.

ABSTRACT

Electroweak radiative corrections to e+e- scattering processes typically amount to O(10%) at LEP energies. Their logarithmic increase with energy renders them even more important at future colliders. Although the bulk of these corrections is due to universal process-independent effects, the remaining non-universal corrections are nevertheless phenomenologically important. We describe the structure of the universal corrections to e+e- --> WW --> 4f in detail and discuss the numerical size of universal and non-universal effects using the Monte Carlo generator RACOONWW.

Motivation & Objective

• To analyze the structure and numerical impact of universal and non-universal electroweak radiative corrections in off-shell W-pair production at e⁺e⁻ colliders.
• To quantify the limitations of universal corrections—such as running couplings, Coulomb singularity, and leading-order initial-state radiation—in describing full radiative corrections.
• To assess the importance of non-universal corrections, particularly in differential distributions and W-invariant-mass spectra, where they significantly distort resonance shapes.
• To validate the accuracy of the RacoonWW Monte Carlo generator in including full O(α) corrections via the double-pole approximation for precision physics at LEP2 and future linear colliders.
• To demonstrate that photon recombination schemes strongly affect W-invariant-mass distributions, highlighting the need for explicit treatment of final-state radiation in precision simulations.

Proposed method

• Uses the RacoonWW Monte Carlo generator to compute full O(α) radiative corrections in the double-pole approximation (DPA), which isolates dominant two-resonance contributions.
• Compares results from full DPA calculations with simplified universal approximations, including effective couplings, Coulomb enhancement, and leading-logarithmic initial-state radiation.
• Employs two photon recombination schemes—'calo' (inclusive) and 'bare' (exclusive)—to assess sensitivity of W-invariant-mass distributions to final-state radiation.
• Performs numerical comparisons at CM energies of 200 GeV (LEP2) and 500 GeV (LC), analyzing total cross sections, angular distributions, and invariant-mass spectra.
• Evaluates the improved Born approximation (IBA) as a proxy for universal corrections, testing its sensitivity to the QED splitting scale Q and its deviation from full results.
• Analyzes the distortion of the W-boson resonance line shape due to final-state radiation, which is not captured by universal corrections.

Experimental results

Research questions

• RQ1 How do universal electroweak corrections—such as running couplings and Coulomb singularity—contribute to W-pair production at LEP2 and future linear collider energies?
• RQ2 To what extent do non-universal corrections, arising from explicit diagrammatic calculations, affect total cross sections and differential distributions in e⁺e⁻ → WW → 4f?
• RQ3 Why do W-invariant-mass distributions show strong dependence on photon recombination schemes, and what does this imply for resonance shape reconstruction?
• RQ4 How do non-universal corrections grow with energy, and at what point do they become dominant over universal effects?
• RQ5 Can the improved Born approximation (IBA) reliably approximate full radiative corrections, and what are its limitations in describing final-state radiation effects?

Key findings

• Universal corrections dominate at LEP2 energies (200 GeV), contributing approximately 1–2% to the total W-pair cross section, with deviations from full results up to 5%.
• Non-universal corrections grow significantly with energy, reaching 10% in angular distributions at 500 GeV, especially in backward and intermediate-angle regions.
• The W-invariant-mass distribution is strongly distorted by final-state radiation, with shifts of up to 10 MeV in resonance position depending on the photon recombination scheme (e.g., 'calo' vs. 'bare').
• The improved Born approximation (IBA) fails to describe these line-shape distortions, as it neglects radiation from W bosons and final-state fermions.
• The sensitivity of the IBA to the QED splitting scale Q reaches 5% at 500 GeV, indicating a significant uncertainty in universal-only approximations at high energies.
• At energies above 170 GeV, the DPA-based RacoonWW generator achieves a theoretical accuracy of about 0.5%, making it suitable for precision tests at future linear colliders.