Higgs CP Properties From Early LHC Data

TL;DR

The paper investigates CP violation in the Higgs sector by fitting early Higgs-rate data to a CP-mixed resonance model, allowing a CP-even component H and CP-odd component A to mix via angle $\alpha$ and to have modified Yukawa couplings and possible dimension-5 couplings to gauge bosons. It shows that a pure CP-odd state is excluded at ~3σ, but large mixing ( $\alpha$ around 1) remains allowed within current uncertainties; both a near-degenerate two-resonance scenario and an effective-theory approach with dimension-5 operators can reproduce the observed signal strengths, often with enhanced di-photon rates. The work highlights that future high-luminosity LHC data and angular analyses will meaningfully constrain CP properties, potentially distinguishing CP-even/odd admixtures and clarifying the Higgs sector's structure.

Abstract

In this paper, we constrain CP violation in the Higgs sector using the measured signal strengths in the various Higgs search channels. To this end, we introduce a general parameterization for a resonance which is an admixture of a CP-even Higgs-like state and a CP-odd scalar. By performing a fit to the available data from the Tevatron and LHC experiments one obtains constraints on the mixing angle and the couplings of the resonance to Standard Model fields. Depending on the couplings, sizable mixing angles are still compatible with the data, but small mixing is in general preferred by the fit. In particular we find that a pure CP-odd state is disfavored by the current data at the 3 sigma level. Additionally we consider a mixed fermiophobic resonance and a model with two degenerate mixed resonances and find that both scenarios can successfully fit the data within current errors. Finally, we estimate that the mixing angle can be constrained to α< 1.1 (0.7) in the full 8 TeV (14 TeV) run of the LHC.

Figures (5)

• Figure 1: Quality of the fit to experimental data in the $x_{\rm u}-\alpha$ plane, for $x_{\rm d}=0$ and $y_{\rm u}=y_{\rm d}=1$. The orange (grey) shaded areas agree with the data at the $1\sigma$ (dark), $2\sigma$ (medium) and $3\sigma$ (light) level. The blue star shows the best fit point, while the black dot corresponds to the SM. The solid (dashed) lines are contours of constant $r_{\gamma \gamma}$$(r_{ZZ})$.
• Figure 2: Quality of the fit for the single resonance model in the $\alpha$--$x_{\rm u}$ (top left), $\alpha$--$y_{\rm d}$ (top-right), $x_{\rm u}$--$y_{\rm u}$ (bottom left), and $x_{\rm d}$--$y_{\rm d}$ (bottom right) plane. The remaining free parameters are set to their best fit values (see text for details). Colors and contours are as in Fig. \ref{['fig:res1']}. In addition, the blue (very dark) shaded region indicates $\Delta \chi^2<1$ relative to the best fit, and the dotted lines in the top-right plot are contours of constant $r_{\tau\tau}$.
• Figure 3: Quality of the fit for the double-resonance model, in the $\alpha$--$x_{\rm u}$ (left) and $x_{\rm u}$--$y_{\rm d}$ (right) plane. The remaining free parameters are set to their best-fit values (see text for details). Colors and contours are as in Fig. \ref{['fig:res1']} and \ref{['fig:res2']}.
• Figure 4: Quality of the fit in the effective operator model, in the $c_W$--$c_B$ (left) and $c_W$--$\alpha$ (right) plane. The remaining free parameters are set to their best fit values, see text for details. As in the previous figures, the shaded areas agree with the data at the level of $3\sigma$ (light), $2\sigma$ (medium), $1\sigma$ (dark), and $\Delta \chi^2 < 1$ (blue/very dark). The solid lines are contours of constant $r_{\gamma\gamma}$, while the dashed lines denote constant $r_{Z\gamma}$ (left) and $r_{ZZ}$ (right).
• Figure 5: Projected sensitivity on the CP mixing angle $\alpha$ and CP-odd top Yukawa coupling $x_{\rm u}$ from upcoming LHC data on Higgs rate measurements. The plot is similar to Fig. \ref{['fig:res1']}, under the assumption that all rate measurements have a central value consistent with the SM. Shown are the 95% C.L. limits for current errors but SM-like central values (light solid), quadrupled statistics per experiment at the end of the 8 TeV run (light dashed), and expected errors for 300 fb$^{-1}$ at 14 TeV including only the channels discussed in atlashigh and combining the two experiments (green, thick).