Higgs Couplings at the End of 2012

TL;DR

This study performs a global fit of Higgs couplings using public LHC and Tevatron data up to end-2012, parameterizing tree-level couplings with $C_V$, $C_U$, $C_D$ and loop-induced couplings with $\overline{C}_g$, $\overline{C}_\gamma$ plus possible new-physics shifts $\Delta C_g$, $\Delta C_\gamma$. It analyzes both a general coupling framework and Two-Higgs-Doublet Models (Type I/II), including scenarios where extra loop contributions arise from Beyond-the-Standard-Model physics. The results show a preference for non-SM contributions to the $\gamma\gamma$ coupling or a sign flip in the top coupling relative to the $W$ coupling, with the SM still providing a reasonable fit but not optimal; in 2HDMs, viable but non-SM-like minima exist, though no clear improvement over the SM is achieved in the $\tan\beta>1$ region. The analysis highlights the need for more complete channel-by-channel information and full likelihoods to robustly discriminate between the SM and new-physics Higgs scenarios.

Abstract

Performing a fit to all publicly available data, we analyze the extent to which the latest results from the LHC and Tevatron constrain the couplings of the Higgs boson-like state at ~ 125 GeV. To this end we assume that only Standard Model (SM) particles appear in the Higgs decays, but tree-level Higgs couplings to the up-quarks, down-quarks and vector bosons, relative to the SM are free parameters. We also assume that the leptonic couplings relative to the SM are the same as for the down-quark, and a custodial symmetry for the V=W,Z couplings. In the simplest approach, the effective Higgs couplings to gluons and photons are computed in terms of the previous parameters. This approach is also applied to Two-Higgs-Doublet Models of Type I and Type II. However, we also explore the possibility that the net Higgs to gluon-gluon and gamma-gamma couplings have extra loop contributions coming from Beyond-the-Standard Model physics. We find that the SM p-value ~ 0.5 is more than 2 sigma away from fits in which: a) there is some non-SM contribution to the gamma-gamma coupling of the Higgs; or b) the sign of the top quark coupling to the Higgs is opposite that of the W coupling. In both these cases p-values ~ 0.9 can be achieved. Since option b) is difficult to realize in realistic models, it would seem that new physics contributions to the effective couplings of the Higgs are preferred.

Figures (11)

• Figure 1: Two parameter fit of $\Delta C_\gamma$ and $\Delta C_g$, assuming $C_U=C_D=C_V=1$ (Fit I). The red, orange and yellow ellipses show the 68%, 95% and 99.7% CL regions, respectively. The white star marks the best-fit point $\Delta C_\gamma=0.426$, $\Delta C_g=-0.086$.
• Figure 2: One-dimensional $\chi^2$ distributions for the three parameter fit, Fit II, of $C_U$, $C_D$, $C_V$ with $C_\gamma=\overline C_\gamma$ and $C_g=\overline C_g$ as computed in terms of $C_U,C_D,C_V$.
• Figure 3: Two-dimensional $\chi^2$ distributions for the three parameter fit, Fit II, of $C_U$, $C_D$, $C_V$ with $C_\gamma=\overline C_\gamma$ and $C_g=\overline C_g$ as computed in terms of $C_U,C_D,C_V$. The red, orange and yellow ellipses show the 68%, 95% and 99.7% CL regions, respectively. The white star marks the best-fit point. Details on the minima in different sectors of the ($C_U$, $C_D$) plane can be found in Table \ref{['tab:fit2']}.
• Figure 4: One-dimensional $\chi^2$ distributions for the three parameter fit, Fit II, but imposing $C_U>0$, $C_D>0$; the left two plots allow for $C_V>1$ ($\chi^2_{\rm min}=18.66$), while in the right two plots $C_V\le1$ ($\chi^2_{\rm min}=18.89$).
• Figure 5: Two-dimensional $\chi^2$ distributions for the three parameter fit, Fit II, as in Fig. \ref{['fit2-2d']} but with $C_U>0$, $C_D>0$, $C_V>0$. The upper row of plots allows for $C_V>1$, while in the lower row of plots $C_V\le1$ is imposed.