First Glimpses at Higgs' face

TL;DR

The paper analyzes 7+8 TeV LHC Higgs search data to determine whether the observed ~125 GeV scalar behaves as the SM Higgs or points to New Physics, using a model‑independent effective chiral Lagrangian with parameters $a$ and $c_j$ and higher‑dimensional operators ($c_g$, $c_{\gamma}$) plus a possible invisible width BR$_{inv}$. It performs joint fits to Higgs signal strengths and electroweak precision data, deriving best‑fit regions in the $(a,c)$ plane and showing EWPD strongly constrains $a$ to be near 1; BR$_{inv}$ is constrained to be small, while $c_g$ and $c_{\gamma}$ show potential NP sensitivity under certain UV assumptions. The main results indicate the SM point is roughly $2\sigma$ away from the global best fit, though data remain compatible with the SM when considering uncertainties and higher‑dimensional operators; the analysis also highlights tensions among a few channels, and introduces tension‑based exclusion techniques as complementary to conventional fits. Overall, the work provides a framework to bound or exclude NP scenarios that would significantly alter the Higgs properties and demonstrates how evolving data can sharpen or relax these constraints.

Abstract

The 8 TeV LHC Higgs search data just released indicates the existence of a scalar resonance with mass ~ 125 GeV. We examine the implications of the data reported by ATLAS, CMS and the Tevatron collaborations on understanding the properties of this scalar by performing joint fits on its couplings to other Standard Model particles. We discuss and characterize to what degree this resonance has the properties of the Standard Model (SM) Higgs, and consider what implications can be extracted for New Physics in a (mostly) model-independent fashion. We find that, if the Higgs couplings to fermions and weak vector bosons are allowed to differ from their standard values, the SM is ~ 2 sigma from the best fit point to current data. Fitting to a possible invisible decay branching ratio, we find BR_{inv} = 0.05\pm 0.32\ (95% C.L.) We also discuss and develop some ways of using the data in order to bound or rule out models which modify significantly the properties of this scalar resonance and apply these techniques to the global current data set.

Figures (7)

• Figure 1: Global fit results in the $(a,c)$ plane for all reported best fit values given by ATLAS and CMS, left (right) without EWPD (with EWPD). In both plots we take $m_h = 125 \, {\rm GeV}$ for the Tevatron and CMS7/8 and $m_h = 126.5 \, {\rm GeV}$ for ATLAS7/8. The green, yellow, gray regions corresponds to the allowed $1,2,3 \, \sigma$ spaces for a two parameter fit. The best fit point in each region is also labeled with a point. The thicker point indicates the one with the smaller $\chi^2_{min}$.
• Figure 2: Best fit regions (at 68%, 95% and 99.9% C.L.) in the $(a,c)$ plane for a fit to all reported signal-strength values given by ATLAS ($m_h = 126.5 \, {\rm GeV}$), CMS ($m_h = 125 \, {\rm GeV}$) and the Tevatron ($m_h = 125 \, {\rm GeV}$) collaborations individually. We plot the same best fit contours over the same domain of parameter space to allow a direct comparison amongst experimental results. The significant change in the ATLAS results from version one of this paper is due to the use of the ATLAS diphoton data broken into subcategories.
• Figure 3: Global fit to ${\rm Br}_{inv}$ for the SM Higgs using only CMS data (left figure) for $m_h = 125 \, {\rm GeV}$ with two methods as a check of our fit procedure. Blue curve - global signal strength based fit, Red curve - individual channel fit. See text for further explanation. The middle figure shows the $\chi^2$ distribution developed from the combined best fit $\hat{\mu}_c$ supplied by the four experiments, including the 7 and 8 TeV LHC results for two mass values. In the right figure we show the discovery potential for ${\rm Br}_{inv}$, updating a result from the analysis in Ref. Espinosa:2012vu with the new global signal strength data.
• Figure 4: (Left) Results of fitting to $c_g, c_{\gamma}$ when the SM is assumed. Note that in these results we have assumed that $c_g,c_\gamma$ are real. (Middle) Results of fitting to $c_g, c_{\gamma},a,c$ and marginalizing over (a,c) subject to the constraint $a>0$ and $0 < c < 3$. (Right) Results of fitting to $c_g, c_{\gamma},{\rm BR}_{inv}$ and marginalizing over ${\rm BR}_{inv}$.
• Figure 5: (Left) Results of fitting to $c_g, c_{\gamma},a,c$ and marginalizing over the higher dimensional operators. (Right) Allowed space when $c_t$ is varied independently. See text for more details.