[Paper Review] Higgs couplings at the LHC

Dieter Zeppenfeld|ArXiv.org|Mar 13, 2002

Particle physics theoretical and experimental studies15 citations

TL;DR

This paper evaluates the precision with which the Large Hadron Collider (LHC) can measure Standard Model Higgs boson couplings to gauge bosons and fermions using 100–200 fb⁻¹ of integrated luminosity. By analyzing gluon fusion and weak boson fusion (WBF) production channels, it demonstrates that Higgs couplings—particularly $H\to WW^*$, $H\to \gamma\gamma$, and $H\to \tau\tau$—can be determined with 5–10% accuracy, enabling direct tests of the Higgs mechanism and fermion mass generation.

ABSTRACT

The observation of a SM-like Higgs boson in multiple channels at the LHC allows the extraction of Higgs couplings to gauge bosons and fermions. The precision achievable at the LHC, for an integrated luminosity of 200 fb^{-1}, is reviewed and updated.

Motivation & Objective

• To assess the LHC's capability to measure Higgs boson couplings to fermions and gauge bosons with high precision.
• To determine the sensitivity of various Higgs decay channels—especially $H\to \gamma\gamma$, $H\to ZZ^*$, $H\to WW^*$, and $H\to \tau\tau$—to coupling extraction.
• To evaluate the role of weak boson fusion (WBF) in reducing backgrounds and enabling precise partial width measurements.
• To quantify the accuracy of indirect extraction of partial widths (e.g., $\Gamma_W$, $\Gamma_\tau$) from signal cross sections and ratios.
• To establish the robustness of coupling ratio measurements against theoretical and experimental uncertainties, particularly in the 115–150 GeV Higgs mass range.

Proposed method

• Uses parton-level and full detector simulation analyses to model signal and background rates for $gg\to H\to \gamma\gamma$, $H\to ZZ^*\to 4\ell$, $H\to WW^*\to \ell\ell\ell\ell$, and WBF processes.
• Applies double forward jet tagging and central jet vetoing techniques to isolate WBF $qq\to qqH$ events with high signal-to-background ratios.
• Employs statistical error estimation via $\Delta\sigma_H/\sigma_H = \sqrt{N_S + N_B}/N_S$ to quantify precision on signal cross sections.
• Derives partial widths indirectly using measured ratios of signal rates and the $H\to WW^*$ width as a reference, via equations such as $\tilde{\Gamma}_W = \Gamma_W(1 - \epsilon)$.
• Uses the $H\to WW^*$ width as a reference to extract $\Gamma_\tau$, $\Gamma_t$, and $\Gamma_g$ through ratios like $\tilde{\Gamma}_\tau = (X_\tau / X_W)\tilde{\Gamma}_W$, minimizing dependence on absolute cross-section normalization.
• Validates results against full detector simulations and accounts for QCD corrections, initial/final state radiation, and jet reconstruction inefficiencies in the forward region.

Experimental results

Research questions

• RQ1 What is the expected precision in measuring Higgs couplings to gauge bosons and fermions at the LHC with 100–200 fb⁻¹ of data?
• RQ2 How do weak boson fusion (WBF) channels improve the measurement of Higgs couplings compared to inclusive gluon fusion modes?
• RQ3 To what extent can partial widths such as $\Gamma_W$, $\Gamma_\tau$, and $\Gamma_t$ be extracted indirectly from signal cross sections and ratios?
• RQ4 What is the impact of experimental and theoretical uncertainties—especially QCD corrections and jet reconstruction inefficiencies—on coupling measurements?
• RQ5 How do the measured coupling ratios ($g_{Htt}/g_{HWW}$, $g_{H\tau\tau}/g_{HWW}$) test the Higgs mechanism and fermion mass generation in the Standard Model?

Key findings

• For $m_H \geq 110$ GeV, the $qq\to qqH\to WW^*\to ll'\!/p_T$ WBF channel yields 102–1460 signal events in 200 fb⁻¹, with statistical errors on the signal cross section ranging from 16.0% at 110 GeV to 2.8% at 160 GeV.
• The $H\to WW^*$ partial width can be extracted with 5–10% accuracy in the 120–140 GeV mass range, enabling direct measurement of the $HWW$ coupling and testing of the Higgs mechanism.
• The $H\to \gamma\gamma$ and $H\to ZZ^*\to 4\ell$ modes provide signal-to-background ratios of 1:1 to 6:1 and 1:4 to 1:1, respectively, with high statistical significance in 100 fb⁻¹.
• The top quark Yukawa coupling ($g_{Htt}$) can be probed at the 15–20% level for $m_H \leq 130$ GeV via $t\bar{t}H$ production with $H\to b\bar{b}$ decay.
• Indirect extraction of $\Gamma_\tau$, $\Gamma_t$, and $\Gamma_g$ via ratios of measured signal rates achieves 5–10% accuracy, with $\Gamma_{\gamma\gamma}$ and $\Gamma_{gg}$ dominated by $W$ and top-quark loops, respectively.
• Systematic uncertainties—especially from higher-order QCD effects—are small (≤5%) and do not limit precision in most channels, indicating potential for further improvement with higher luminosity.