Bounding the Higgs width at the LHC using full analytic results for gg -> 2e 2μ

Abstract: We revisit the hadronic production of the four-lepton final state, e^- e^+ \mu^- \mu^+, through the fusion of initial state gluons. This process is mediated by loops of quarks and we provide first full analytic results for helicity amplitudes that account for both the effects of the quark mass in the loop and off-shell vector bosons. The analytic results have been implemented in the Monte Carlo program MCFM and are both fast, and numerically stable in the region of low Z transverse momentum. We use our results to study the interference between Higgs-mediated and continuum production of four-lepton final states, which is necessary in order to obtain accurate theoretical predictions outside the Higgs resonance region. We have confirmed and extended a recent analysis of Caola and Melnikov that proposes to use a measurement of the off-shell region to constrain the total width of the Higgs boson. Using a simple cut-and-count method, existing LHC data should bound the width at the level of 25-45 times the Standard Model expectation. We investigate the power of using a matrix element method to construct a kinematic discriminant to sharpen the constraint. In our analysis the bound on the Higgs width is improved by a factor of about 1.6 using a simple cut on the MEM discriminant, compared to an invariant mass cut m_{4l} > 300 GeV.

• The paper develops complete analytical helicity amplitudes for gg fusion into four-lepton final states, including full quark mass effects.
• It quantifies interference between Higgs-mediated and continuum processes, which is crucial for accurate predictions in off-shell regions.
• The study constrains the Higgs width to 25–45 times the SM expectation using a simple cut-and-count method, enhancing measurement sensitivity.

Analytical Constraints on Higgs Width Using LHC Data

This paper addresses the significant challenge of constraining the width of the Higgs boson using data from the Large Hadron Collider (LHC). The authors provide a comprehensive analysis of the hadronic production of four-lepton final states, specifically $e^- e^+ \mu^- \mu^+$, through the fusion of initial state gluons. This process is mediated by loops of quarks, and their work includes novel analytical results for helicity amplitudes, accounting for both quark mass effects and off-shell vector bosons.

Key Contributions

• Analytical Formulation: The work develops complete analytical expressions for helicity amplitudes involving gluon fusion and four-lepton-final states. This includes the full mass dependence for quarks in the loop. Analytical results are implemented into MCFM (Monte Carlo for FeMtobarn processes), ensuring fast and numerically stable computations, especially in regions with low transverse momentum of $Z$ bosons.
• Interference Effects: The authors study interference between Higgs-mediated and continuum production processes. This interference is crucial to accurate theoretical predictions, especially in off-shell regions away from the Higgs resonance.
• Higgs Width Constraints: By examining the off-shell region, the paper extends previous analyses suggesting constraints on the total width of the Higgs boson. With existing LHC data, they propose bounding the width to levels of 25--45 times the Standard Model (SM) expectation using a simple cut-and-count method.

Numerical Results

The study confirms that the impact of interference is substantial. In the off-shell region, particularly above the Higgs resonance, the inclusion of interference reduces the number of expected events. This phenomenon affects the extraction of the Higgs width directly from LHC data. For instance, the destructive interference between Higgs-related diagrams and continuum background results in negative contributions in the SM, where the Higgs boson unitarizes the continuum $gg \rightarrow ZZ$ cross section.

Implications and Future Directions

The method proposed here utilizes off-shell regions to constrain the Higgs width without assuming detector resolution limitations. This innovative approach, which leverages matrix element methods (MEM), improves the sensitivity to off-shell Higgs events. The authors demonstrate that using MEM can provide sharper constraints on the Higgs width compared to conventional invariant mass cuts. With advancements in computation and further data from the LHC, these methods potentially offer a path to more precise determinations of Higgs boson attributes.

Conclusion

This analytical framework for constraints on Higgs width using LHC data marks an essential milestone in particle physics research. The paper's rigorous approach to interference considerations and the deployment of MEM highlight the complexity and advancement in collider-based measurements. As particle physics continues its efforts to refine and verify the SM, analyses such as these provide critical insights and methodology, paving the way for future theoretical and experimental developments in high-energy physics.