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Measuring Higgs CP and couplings with hadronic event shapes

Christoph Englert, Michael Spannowsky, Michihisa Takeuchi

TL;DR

The paper demonstrates that hadronic event shapes in Higgs+2 jets provide powerful discrimination between CP-even and CP-odd Higgs hypotheses and can help disentangle gluon fusion from weak boson fusion production. By combining MLM-matched simulations with a jet-based selection and a binned likelihood approach, the study shows that event shapes can surpass the traditional ΔΦ_{jj} observable in CP sensitivity and offer a pathway to more precise Higgs coupling extractions. The results indicate practical CP measurements are feasible at 14 TeV with sufficient luminosity, though pile-up and experimental systematics must be carefully addressed. Overall, event shapes emerge as a theoretically robust and experimentally viable tool to probe the CP structure and production mechanisms of the Higgs boson at the LHC.

Abstract

Experimental falsification or validation of the Standard Model of Particle Physics involves the measurement of the CP quantum number and couplings of the Higgs boson. Both ATLAS and CMS have reported an SM Higgs-like excess around m_H=125 GeV. In this mass range the properties of the Higgs boson can be extracted from an analysis of the azimuthal angle distribution of the two jets in pp->Hjj events. This channel is also important to measure the couplings of the Higgs boson to electroweak gauge bosons and fermions, hereby establishing the exceptional role of the Higgs boson in the Standard Model. Instead of exploiting the jet angular correlation, we show that hadronic event shapes exhibit substantial discriminative power to separate a CP even from a CP odd Higgs. Some event shapes even show an increased sensitivity to the Higgs CP compared to the azimuthal angle correlation. Constraining the Higgs couplings via a separation of the weak boson fusion and the gluon fusion Higgs production modes can be achieved applying similar strategies.

Measuring Higgs CP and couplings with hadronic event shapes

TL;DR

The paper demonstrates that hadronic event shapes in Higgs+2 jets provide powerful discrimination between CP-even and CP-odd Higgs hypotheses and can help disentangle gluon fusion from weak boson fusion production. By combining MLM-matched simulations with a jet-based selection and a binned likelihood approach, the study shows that event shapes can surpass the traditional ΔΦ_{jj} observable in CP sensitivity and offer a pathway to more precise Higgs coupling extractions. The results indicate practical CP measurements are feasible at 14 TeV with sufficient luminosity, though pile-up and experimental systematics must be carefully addressed. Overall, event shapes emerge as a theoretically robust and experimentally viable tool to probe the CP structure and production mechanisms of the Higgs boson at the LHC.

Abstract

Experimental falsification or validation of the Standard Model of Particle Physics involves the measurement of the CP quantum number and couplings of the Higgs boson. Both ATLAS and CMS have reported an SM Higgs-like excess around m_H=125 GeV. In this mass range the properties of the Higgs boson can be extracted from an analysis of the azimuthal angle distribution of the two jets in pp->Hjj events. This channel is also important to measure the couplings of the Higgs boson to electroweak gauge bosons and fermions, hereby establishing the exceptional role of the Higgs boson in the Standard Model. Instead of exploiting the jet angular correlation, we show that hadronic event shapes exhibit substantial discriminative power to separate a CP even from a CP odd Higgs. Some event shapes even show an increased sensitivity to the Higgs CP compared to the azimuthal angle correlation. Constraining the Higgs couplings via a separation of the weak boson fusion and the gluon fusion Higgs production modes can be achieved applying similar strategies.

Paper Structure

This paper contains 11 sections, 16 equations, 10 figures, 1 table.

Table of Contents

  1. Introduction
  2. Event Shape observables and $\Delta\Phi_{jj}$
  3. Elements of the Analysis
  4. Event generation
  5. Selection Cuts and Analysis Strategy
  6. Results
  7. $\cal{CP}$ even vs. $\cal{CP}$ odd
  8. Higgs-lookalike $\cal{CP}$ odd
  9. Toward discriminating gluon fusion and weak boson fusion contributions
  10. Impact of pile-up
  11. Conclusions and Outlook

Figures (10)

  • Figure 1: Normalized distributions of $\Delta\Phi_{jj}$ and of the event shape observables of Sec. \ref{['sec:shapes']}. The cuts of Sec. \ref{['sec:selection']} have been applied.
  • Figure 2: Correlation of the thrust event shape with $\Delta\Phi_{jj}$ angle as defined in Eq. (\ref{['eq:phijj']}) in terms of the 2d differential probability distribution $1/\sigma\, {{\text{d}}^2 \sigma/({\text{d}} \Delta\Phi_{jj}\, {\text{d}} T_{\perp,g} )}$
  • Figure 3: Correlation of the cone thrust minor event shape with $\Delta\Phi_{jj}$ angle as defined in Eq. (\ref{['eq:phijj']}) in terms of the 2d differential probability distribution $1/\sigma\, {{\text{d}}^2 \sigma/({\text{d}} \Delta\Phi_{jj}\, {\text{d}} T_{C,m} )}$
  • Figure 4: Distributions of the event shape observables of Sec. \ref{['sec:shapes']} including the background after the cuts of Sec. \ref{['sec:selection']}.
  • Figure 5: $\Delta\Phi_{jj}$ distribution including the background after the cuts of Sec. \ref{['sec:selection']}.
  • ...and 5 more figures