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

John M. Campbell, R. Keith Ellis, Ciaran Williams

TL;DR

This work provides the first full analytic, mass-dependent one-loop amplitudes for gg -> 4l including off-shell vector bosons, enabling precise predictions for Higgs-mediated and continuum contributions and their interference in the off-shell region. By implementing these amplitudes in MCFM, the authors quantify the Higgs off-shell cross section and demonstrate its utility for constraining the Higgs width via off-shell measurements, following and extending the Caola–Melnikov approach. They show that current LHC data can bound Γ_H at O(10-40) Γ_H^SM in off-shell regions, and that a matrix element method can improve these bounds by roughly a factor of 1.6 relative to a simple m_4l cut. The study highlights the importance of interference effects and p_T stability, and proposes MEM-based strategies and future NLO improvements to sharpen Higgs width constraints.

Abstract

We revisit the hadronic production of the four-lepton final state, e^- e^+ μ^- μ^+, 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.

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

TL;DR

This work provides the first full analytic, mass-dependent one-loop amplitudes for gg -> 4l including off-shell vector bosons, enabling precise predictions for Higgs-mediated and continuum contributions and their interference in the off-shell region. By implementing these amplitudes in MCFM, the authors quantify the Higgs off-shell cross section and demonstrate its utility for constraining the Higgs width via off-shell measurements, following and extending the Caola–Melnikov approach. They show that current LHC data can bound Γ_H at O(10-40) Γ_H^SM in off-shell regions, and that a matrix element method can improve these bounds by roughly a factor of 1.6 relative to a simple m_4l cut. The study highlights the importance of interference effects and p_T stability, and proposes MEM-based strategies and future NLO improvements to sharpen Higgs width constraints.

Abstract

We revisit the hadronic production of the four-lepton final state, e^- e^+ μ^- μ^+, 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.

Paper Structure

This paper contains 54 sections, 106 equations, 12 figures, 7 tables.

Table of Contents

  1. Introduction
  2. Glue-Glue initiated and Quark-gluon initiated Higgs amplitudes
  3. Process $(a):gg\rightarrow H\rightarrow e^-e^+\mu^-\mu^+$
  4. Process $(d):qg\rightarrow H \rightarrow e^-e^+\mu^-\mu^+ + q$ amplitudes
  5. Calculation of the non-Higgs boson mediated amplitudes
  6. Process $(c): q \bar{q} \rightarrow e^-e^+\mu^-\mu^+$
  7. Process $(b):$ Calculation of the gluon induced continuum amplitude $gg\rightarrow e^-e^+\mu^-\mu^+$
  8. Process $(d): q g \to e^-e^+\mu^-\mu^+ + q$
  9. Phenomenology
  10. Constraining the Higgs width using the matrix element method
  11. The Kinematic Discriminant
  12. Measuring the Higgs width using the MEM
  13. Future Theoretical Improvements
  14. Conclusions
  15. Definition of scalar integrals
  16. ...and 39 more sections

Figures (12)

  • Figure 1: Representative diagrams for the partonic processes considered in this paper.
  • Figure 2: A representation of the $\mathcal{O}(g_s^4 e^8)$ interferences between Higgs and continuum production of four leptons considered in this paper. The interference contributions are obtained by cutting this diagram across a single dashed line.
  • Figure 3: Diagrams for $gg \to \gamma^*\gamma^*$
  • Figure 4: Overall picture at 8 TeV, (colour online). In this and the following figure the CMS cuts described in the text have been imposed, but the constraint $m_{4\ell}>100~{\rm GeV}$ has been removed to extend the range of the plot.
  • Figure 5: Overall picture at 13 TeV, (colour online).
  • ...and 7 more figures