[Paper Review] Effective K-factors for $gg o H o WW o l u l u$ at the LHC

G. Davatz, G. Dissertori+3 more|arXiv (Cornell University)|Feb 20, 2004

Particle physics theoretical and experimental studiesPhysics and Astronomy168 citations

TL;DR

This paper proposes a reweighting technique to incorporate higher-order QCD corrections into Monte Carlo simulations for the Higgs boson decay channel $gg \to H \to WW \to \ell\!\nu\ell\!\bar{\nu}$ at the LHC. By reweighting PYTHIA-generated events using precise NLO/NNLO predictions for Higgs and WW background transverse momentum spectra, the method computes an effective experimental K-factor of 2.04 for a 165 GeV Higgs, increasing discovery potential significantly—enabling 5σ significance with just 0.4 fb$^{-1}$ luminosity.

ABSTRACT

A simulation of the search for the Standard Model Higgs boson at the LHC, in the channel gg -> H -> WW -> lnu lnu, is described. Higher-order QCD corrections are taken into account by using a reweighting procedure, which allows us to combine event rates obtained with the PYTHIA Monte Carlo program with the most up-to-date theoretical predictions for the transverse-momentum spectra of the Higgs signal and its corresponding WW background. With this method the discovery potential for Higgs masses between 140 and 180 GeV is recalculated and the potential statistical significance of this channel is found to increase considerably. For a Higgs mass of 165 GeV a signal-to-background ratio of almost 2:1 can be obtained. A statistical significance of five standard deviations might already be achieved with an integrated luminosity close to 0.4 fb^{-1}. Using this approach, an experimental effective K-factor of about 2.04 is obtained for the considered Higgs signature, which is only about 15 % smaller than the theoretical inclusive K-factor.

Motivation & Objective

• To improve the simulation of the $gg \to H \to WW \to \ell\!\nu\ell\!\bar{\nu}$ Higgs search channel at the LHC by incorporating higher-order QCD corrections.
• To address the limitation of standard Monte Carlo programs that fail to accurately model transverse momentum spectra due to missing higher-order corrections.
• To compute an effective experimental K-factor that accounts for realistic selection cuts, including a jet veto, which reduces the impact of QCD corrections compared to inclusive K-factors.
• To assess the statistical discovery potential of the Higgs boson in the 140–180 GeV mass range under realistic experimental conditions.

Proposed method

• A reweighting procedure is applied to PYTHIA-generated events using $p_T$-dependent K-factors derived from NLO/NNLO theoretical predictions for the Higgs boson and non-resonant WW background.
• The method recalibrates the transverse momentum spectra of signal and background events to match high-precision theoretical calculations, preserving event kinematics under cuts.
• Selection criteria are applied based on isolated opposite-sign leptons, missing transverse momentum, and a jet veto (30 GeV), mimicking realistic LHC detector conditions.
• The effective experimental K-factor $K_{\text{eff}}$ is computed as the ratio of reweighted accepted events to unweighted accepted events in PYTHIA simulations.
• Signal and background event rates are reweighted separately, and statistical significance is evaluated using Poisson-based criteria for 5σ discovery.
• The approach accounts for kinematic correlations and spin effects in $W$-boson decays by using differential cross sections as a function of $p_T$.

Experimental results

Research questions

• RQ1 How do higher-order QCD corrections affect the Higgs signal and WW background in the $gg \to H \to WW \to \ell\!\nu\ell\!\bar{\nu}$ channel when realistic cuts are applied?
• RQ2 Can a reweighting technique based on $p_T$-dependent K-factors improve the accuracy of Monte Carlo simulations for Higgs searches beyond simple inclusive K-factor scaling?
• RQ3 What is the effective experimental K-factor for the Higgs signal and WW background after applying a jet veto and other kinematic cuts?
• RQ4 What integrated luminosity is required to achieve 5σ statistical significance for a Higgs boson in the 140–180 GeV mass range using this method?

Key findings

• The effective experimental K-factor for the Higgs signal at 165 GeV is 2.04, which is about 15% smaller than the inclusive K-factor of 2.37.
• For a Higgs mass of 165 GeV, a signal-to-background ratio of nearly 2:1 is achieved after selection cuts, significantly enhancing discovery potential.
• A statistical significance of five standard deviations can be reached with an integrated luminosity of approximately 0.4 fb$^{-1}$ for a 165 GeV Higgs boson.
• The effective K-factor for the non-resonant WW background is 1.36 when integrating over the full WW mass spectrum, and 1.44 for the 165–175 GeV mass window.
• The reweighting method improves the accuracy of event rate predictions by incorporating differential $p_T$ spectra from NLO/NNLO calculations, reducing reliance on naive scaling with inclusive K-factors.
• The method demonstrates that systematic uncertainties on the background must be controlled to better than 10–20% to preserve discovery sensitivity, especially when signal-to-background ratios are low.