Fingerprinting Higgs Suspects at the LHC

TL;DR

The paper tests whether current LHC Higgs-like signals are compatible with the Standard Model by formulating an effective chiral Lagrangian with a light scalar $h$ and two couplings $a$ and $c$ to gauge bosons and fermions. It performs a global fit to LHC signal strengths and exclusions near $m_h\approx124$–$126$ GeV, incorporating EW precision data through oblique parameters $\Delta S$, $\Delta T$, and $\Delta U$, and finds two near-degenerate minima in the $(a,c)$ plane with the SM point near the allowed region $\sim$82% CL (rising to ~$94\%$ CL with Moriond 2012 data). The analysis highlights channel-dependent interference, especially in $\gamma\gamma$, which sustains degeneracy, and proposes using ratios of signal strengths, such as $\mu^{\gamma\gamma}/\mu^{ZZ}$ and $\mu^{\gamma\gamma}_{VBF}/\mu^{\gamma\gamma}$, to lift this degeneracy and tighten tests of EWSB. Overall, the work provides a structured EFT framework to quantify deviations from SM Higgs predictions and to guide future measurements toward decisively distinguishing SM and beyond-SM scenarios.

Abstract

We outline a method for characterizing deviations from the properties of a Standard Model (SM) Higgs boson. We apply it to current data in order to characterize up to which degree the SM Higgs boson interpretation is consistent with experiment. We find that the SM Higgs boson is consistent with the current data set at the 82 % confidence level, based on data of excess events reported by CMS and ATLAS, which are interpreted to be related to the mass scale mh = 124-126 GeV, and on published CL_s exclusion regions. We perform a global fit in terms of two parameters characterizing the deviation from the SM value in the gauge and fermion couplings of a Higgs boson. We find two minima in the global fit and identify observables that can remove this degeneracy. An update for Moriond 2012 data is included in the Appendix, which finds that the SM Higgs boson is now consistent with the current data set at only the 94 % confidence level (which corresponds to ~ 2 sigma tension compared to the best fit point).

Figures (10)

• Figure 1: Global fit results in the $(a,c)$ plane for all best fit $\sigma/\sigma_{SM}$ values given by ATLAS and CMS. The SM Higgs boson with a linear realization of the EW chiral Lagrangian corresponds to the point $(1,1)$ and is within the yellow $90 \%$ CL region. The $65 \%$ and $99\%$ CL regions correspond to the green and light gray shaded regions. The plot on the left includes all production channels, the plot on the right only includes the $gg$ production channel in the signals with inclusive Higgs production, rescaled as described in the text. The $\chi^2/{\rm d.o.f} \simeq 0.99$ for the left plot, while $\chi^2/{\rm d.o.f} \simeq 1.03$ for the right plot. We also show two lines characterizing the relationship between the parameters a,c in the minimal composite Higgs scenarios of Refs. Agashe:2004rsContino:2006qr, red dashed (for MCHM4) and blue dashed (for MCHM5). In Eq. (\ref{['eq:efflag']}), rephasing of the Higgs field $h \rightarrow - h$ maps the best fit regions shown to other physically equivalent regions in the $(a,c)$ space.
• Figure 2: Global fit results in the $(a,c)$ plane for all best fit $\sigma/\sigma_{SM}$ values given by ATLAS and CMS, taking into account all production channels. Also shown are the exclusion contours in the $(a,c)$ plane determined by mapping the SM exclusion into an effective exclusion of $(\sigma/\sigma_{SM})[a,c] <1$. Again the $65,90,99 \%$ CL regions correspond to the green, yellow and gray regions in the plots. The exclusion curve derived from ATLAS data is given by the red (dashed) line (with the region to the right of the line excluded), the CMS exclusion curve is the solid blue line.
• Figure 3: Global fit results in the $(a,c)$ plane for all reported best fit $\sigma/\sigma_{SM}$ values given by ATLAS and CMS using subsets of the data. The conventions for the plots are the same as in the previous figures. Fitting the data in subsets degrades the fit. The left figure testing the $b \, \bar{b}$ and $\tau^+ \, \tau^-$ data essentially is unconstrained with $\chi^2/{\rm d.o.f} \simeq 0$, the middle plot with the combination of $\rm W^+ \, W^-$, $\rm Z \, Z$ and vector boson fusion based $\gamma \gamma$ has a $\chi^2/{\rm d.o.f} \simeq 0.28$ while the right plot based on all $\gamma \gamma$ excesses has a $\chi^2/{\rm d.o.f} \simeq 0.62$.
• Figure 4: Global fit results in the $(a,c)$ plane for all best fit $\sigma/\sigma_{SM}$ values given by ATLAS and CMS combined with EWPD. The plot follows the same convention as the previous plots. The $\chi^2/{\rm d.o.f} = 1.3$. As before the blue (red) curve is the exclusion region from CMS (ATLAS). Again the shown exclusion curves are determined from inclusive production.
• Figure 5: Contours of constant signal production in the $(a,c)$ plane. We have also included the exclusion curves which we have derived from ATLAS and CMS data.