Test of $CP$ Invariance in Higgs Boson Vector-Boson-Fusion Production Using the $H\toγγ$ Channel with the ATLAS Detector

Abstract

A test of $CP$ invariance in Higgs boson production via vector-boson fusion has been performed in the $H\rightarrowγγ$ channel using 139 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13\,\mathrm{TeV}$ collected by the ATLAS detector at the LHC. The Optimal Observable method is used to probe the $CP$ structure of interactions between the Higgs boson and electroweak gauge bosons, as described by an effective field theory. No sign of $CP$ violation is observed in data. Constraints are set on the parameters describing the strength of the $CP$-odd component in the coupling between the Higgs boson and the electroweak gauge bosons in two effective field theory bases: $\tilde{d}$ in the HISZ basis and $c_{H\tilde{W}}$ in the Warsaw basis. The results presented are the most stringent constraints on $CP$ violation in the coupling between Higgs and weak bosons. The 95% CL constraint on $\tilde{d}$ is derived for the first time and the 95% CL constraint on $c_{H\tilde{W}}$ has been improved by a factor of 5 compared to the previous measurement.

Figures (3)

• Figure 1: Distribution of the output of $\mathrm{BDT_{VBF/ggF}}$ (left) and $\mathrm{BDT_{VBF/continuum}}$ (right). The comparison between the continuum background and the sideband data indicates the continuum background used in the BDT training is well modeled.
• Figure 2: Distribution of the optimal observable $\mathit{OO}$ for events with $m_{\gamma\gamma}\in[118, 132]~\text{Ge V}\xspace$. Contributions in three signal regions are summed together with a weight of $\ln(1+S/B)$ for each signal region, where $S$ and $B$ are the expected yields of signal and background events with $m_{\gamma\gamma}\in[118, 132]~\text{Ge V}\xspace$. The overflow and underflow are included in the highest and lowest bin, respectively. The uncertainty band shown includes all systematic uncertainties. The weighted summed $m_{\gamma\gamma}$ distribution of data events is shown in the inner panel along with the signal and background contributions. The lower panel is the $\mathit{OO}$ distribution in data after subtraction of all backgrounds, in comparison with the SM VBF process, and VBF processes with $\tilde{d}=0.06$ and $\tilde{d}=-0.06$. The sensitivity to $\tilde{d}$ is dominated by the tails of the $\mathit{OO}$ distribution.
• Figure 3: $\Delta\mathrm{NLL}$ curves as a function of (a) $\tilde{d}$ and (b) $c_{H\tilde{W}}$. In figure (a), the $\Delta\mathrm{NLL}$ of $\tilde{d}$ considers the interference-plus-quadratic terms, whereas in figure (b) the $\Delta\mathrm{NLL}$ of $c_{H\tilde{W}}$ considers the interference-only term. The solid lines are the observed results, while the dashed lines are the expected results. In figure (a), the blue lines represent the results of this analysis, while the red lines represent the results from the $H\rightarrow \tau\tau$ analysis HIGG-2018-14. The black lines show the combination of these two analyses. For all figures, the dashed horizontal lines show the values of $\Delta\mathrm{NLL}$ used to define the 68% and 95% confidence intervals.