[Paper Review] Electroweak physics in six-fermion final states at future e+e- colliders

Fabrizio Gangemi, G. Montagna+3 more|ArXiv.org|Jan 10, 2000

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents a Monte Carlo event generator for full electroweak tree-level calculations of six-fermion final states at future e⁺e⁻ linear colliders, enabling precision studies of intermediate-mass Higgs bosons, top-quark pair production, and anomalous quartic gauge couplings. It demonstrates that complete calculations, including initial-state radiation and finite-width effects, are essential for detecting Higgs signals and measuring particle properties via invariant mass distributions and kinematic cuts with sensitivities down to ∼10⁻² for anomalous couplings at 1 TeV with 1000 fb⁻¹ luminosity.

ABSTRACT

Three studies on six-fermion production processes are presented, in which the production of an intermediate-mass Higgs boson, the top-quark physics and the analysis of possible anomalous quartic gauge couplings are considered. A Monte Carlo event generator has been developed for full electroweak tree-level calculations on six-fermion processes at the Linear Collider. The Monte Carlo procedure has been adapted to deal with a large variety of diagram topologies, so as to keep under control the relevant signals and all the backgrounds of such processes. The effects of initial-state-radiation and beamstrahlung are also taken into account. The relevance of electroweak backgrounds and finite-width effects is discussed and several analyses of final-state distributions are presented, both for the detection of the signals of interest and for the study of properties of the particles involved, thus showing the importance of complete calculations for precision studies at the Linear Collider.

Motivation & Objective

• To develop a Monte Carlo event generator capable of handling complex six-fermion processes at future e⁺e⁻ linear colliders with full electroweak tree-level accuracy.
• To study the relevance of six-fermion final states for detecting intermediate-mass Higgs bosons (140–250 GeV), top-quark pair production, and anomalous quartic gauge couplings.
• To assess the impact of electroweak backgrounds, finite-width effects, initial-state radiation (ISR), and beamstrahlung on signal detection and measurement precision.
• To identify kinematic distributions and cuts that enhance sensitivity to new physics signals, particularly anomalous quartic gauge couplings.

Proposed method

• A custom Monte Carlo event generator was developed using the ALPHA algorithm for automated amplitude calculation and adapted from four-fermion codes (HIGGSPV, WWGENPV) to handle the large number of diagram topologies in six-fermion processes.
• The generator includes full electroweak tree-level matrix elements, incorporating both charged and neutral current interactions, and accounts for ISR and beamstrahlung effects via dedicated radiation models.
• The simulation employs unweighted event generation to study final-state distributions such as invariant masses of four-fermion systems, angular correlations, and event shapes.
• Anomalous quartic gauge couplings were modeled using dimension-four effective Lagrangians (L₄ and L₅), with parameters α₄ and α₅ introduced to probe deviations from the Standard Model.
• Kinematic cuts were optimized to enhance sensitivity to anomalous couplings, focusing on variables like M(WW), W-boson decay angle θ(W), and low-transverse-momentum jet pairs.
• Cross-sections were calculated at √s = 1 TeV with 1000 fb⁻¹ luminosity, and statistical uncertainties were estimated to determine sensitivity limits at 1σ.

Experimental results

Research questions

• RQ1 How do finite-width effects and electroweak backgrounds impact the detection of intermediate-mass Higgs bosons in six-fermion final states at e⁺e⁻ colliders?
• RQ2 What kinematic distributions in six-fermion final states are most sensitive to the presence of a Higgs boson with mass around 185 GeV?
• RQ3 To what extent can initial-state radiation and beamstrahlung distort key observables such as invariant masses and angular distributions in six-fermion processes?
• RQ4 What kinematic cuts maximize sensitivity to anomalous quartic gauge couplings α₄ and α₅ in six-fermion final states at 1 TeV center-of-mass energy?
• RQ5 Can complete tree-level calculations improve the precision of Higgs and top-quark property measurements compared to narrow-width approximations?

Key findings

• At √s = 800 GeV and 500 fb⁻¹ luminosity, more than 2000 six-fermion events are expected for a Higgs boson with mass 185 GeV, demonstrating high sensitivity for intermediate-mass Higgs studies.
• Finite-width effects cause deviations of 10–15% in cross-section predictions compared to the narrow-width approximation, highlighting the need for full width treatment in precision physics.
• The invariant mass of the qqe⁺e⁻ system at 800 GeV provides a clean signal for Higgs boson production via vector boson fusion, largely unaffected by ISR and beamstrahlung.
• Kinematic cuts on M(WW), θ(W), and low-transverse-momentum jet pairs enhance sensitivity to anomalous coupling α₅, achieving a sensitivity of approximately 10⁻² at 1 TeV with 1000 fb⁻¹.
• The cross-section for six-fermion processes with anomalous couplings shows a measurable dependence on α₅ in the range (−0.01, 0.01), with 1σ experimental uncertainty bands clearly resolving deviations from the Standard Model at α₅ = 0.
• The inclusion of ISR and beamstrahlung does not significantly degrade sensitivity when using variables not involving missing momentum, confirming robustness of the selected kinematic observables.