Constraining CP-violating Higgs Sectors at the LHC using gluon fusion

TL;DR

This study targets CP violation in the Higgs sector by allowing a CP-even/CP-odd admixture parameterized by a mixing angle α and focusing on gluon-fusion Higgs production with two jets followed by h→ττ. By combining LO simulations of signal (GF and VBF with full top-mass dependence) and major backgrounds, detector effects, and a multivariate analysis around CP-sensitive jet correlations, the authors show that 8 TeV LHC data can nearly exclude a pure CP-odd Higgs at ~95% CL, with stronger constraints projected for 14 TeV as luminosity increases. The key discriminant is the azimuthal separation Δφ_jj of the tagging jets, supported by analytic insights into the CP structure of GF and WBF production. The results provide a robust, differential approach to constraining CP properties that complements EDM limits and could be extended with additional decay modes and likelihood-based techniques to improve sensitivity further.

Abstract

We investigate the constraints that the LHC can set on a 126 GeV Higgs boson that is an admixture of CP eigenstates. Traditional analyses rely on Higgs couplings to massive vector bosons, which are suppressed for CP-odd couplings, so that these analyses have limited sensitivity. Instead we focus on Higgs production in gluon fusion, which occurs at the same order in the strong coupling for both CP-even and -odd couplings. We study the Higgs plus two jet final state followed by Higgs decay into a pair of tau leptons. We show that using the 8 TeV dataset it is possible to rule out the pure CP-odd hypothesis in this channel alone at nearly 95\% C.L, assuming that the Higgs is CP-even. We also provide projected limits for the 14 TeV LHC run.

Figures (4)

• Figure 1: Observable distributions for the signal and background. From the top-left and proceeding clockwise: $m_{jj}$, $\Delta\phi_{jj}$, sin($|\Delta\phi_{jj}|/2$) and $\Delta \eta_{jj}$. For each figure the yields are normalized to the expected yields at 8 TeV for the gluon fusion channel at 20 fb$^{-1}$ with $\alpha=0$. Samples have been passed through the detector pseudo-simulation and subjected to the full selection on all channels. The loose WBF selection and the additional category selections are applied in all cases.
• Figure 2: RoC curves for our boosted decision tree for 8 TeV (left) and 14 TeV (right). The red lines are for $\alpha=1.5$ and the blue lines for $\alpha=0.6$. The dashed curves show the results only including the $\sin\left(|\Delta\phi_{jj}| \right)$ variable, and the solid curves those for the full BDT with all 18 observables included, as described in the text.
• Figure 3: Expected limits that can be achieved with our analysis using the $20~{\rm{fb}}^{-1}$ 8 TeV dataset (left) and using a $50~{\rm{fb}}^{-1}$ dataset at 14 TeV (right). The dashed lines show the estimated significance of the total signal over the Standard Model backgrounds and the solid lines show the exclusion significance computed using the $\mathrm{CL}_s$ method relative to the $\alpha=0$ case. See text for details.
• Figure 4: The projected 95% exclusion confidence limit on the mixing angle $\alpha$ that can be set as a function of the integrated luminosity at the 14 TeV run of the LHC.