Determining Higgs couplings with a model-independent analysis of h ->gamma gamma

TL;DR

This work addresses the problem of determining Higgs couplings in a model-independent way by employing an electroweak chiral Lagrangian with two free parameters, $a$ and $c$, and focusing on the exclusive $h\to\gamma\gamma$ channel. It demonstrates that fully exclusive event categorization, including categories sensitive to VBF and VH production, provides stronger constraints on $(a,c)$ than inclusive analyses, and derives a Bayesian framework to infer these couplings from category-by-category data. A key finding is a degeneracy in $(a,c)$ space arising from interference in $h\to\gamma\gamma$, which can be alleviated by incorporating $ZZ\to4l$ data; projecting to higher luminosity shows potential to break the degeneracy and measure $a$ with ~25% precision and $c$ with ~100% precision at $m_h=120$ GeV. The study emphasizes the need for model-independent reporting and richer information sharing to interpret LHC results and test the dynamics of electroweak symmetry breaking.

Abstract

Discovering a Higgs boson at the LHC will address a major outstanding issue in particle physics but will also raise many new questions. A concerted effort to determine the couplings of this new state to other Standard Model fields will be of critical importance. Precise knowledge of these couplings can serve as a powerful probe of new physics, and will be needed in attempts to accommodate such a new boson within specific models. In this paper, we present a method for constraining these couplings in a model-independent way, focusing primarily on an exclusive analysis of the gamma gamma final state. We demonstrate the discriminating power of fully exclusive analyses, and discuss ways in which information can be shared between experimentalists and theorists in order to facilitate collaboration in the task of establishing the true origins of any new physics discovered at the LHC.

Figures (4)

• Figure 1: Expected exclusion limits from $\gamma\gamma$ at $\sqrt{s} = 7\,$TeV with $L=5\,\text{fb}^{-1}$ and $m_h=120\,$GeV. Purple solid curve: exclusive analysis with 8+2 categories; Dashed blue curve: inclusive analysis with 8 categories; Dotted red curve: inclusive analysis with 4 categories. The area on the right of each curve is excluded at $95\%$ probability.
• Figure 2: Contours of constant probability for $\gamma\gamma$ in the plane $(a,c)$ obtained by injecting the SM signal $(a=1,c=1)$. Left plot: 68% contours for the $jj$, $1l$ and inclusive categories. Right plot: 68%, 90%, 95% contours in the exclusive analysis with 8+2 categories and 95% contour in the inclusive analysis with 4 categories. Both plots are for $\sqrt{s} = 7\,$TeV with $L=20\,\text{fb}^{-1}$ and $m_h=120\,$GeV.
• Figure 3: Contours of constant probability in the plane $(a,c)$ for $\gamma\gamma$, $ZZ$ and $WW$ obtained by injecting the SM signal $(a=1,c=1)$. Left plot: 68% contours for individual $\gamma\gamma$ (10-categories exclusive analysis, red area), $WW\to l\nu l\nu$ (5-categories exclusive analysis, green area) and $ZZ\to4l$ (inclusive analysis, blue area) channels. Right plot: 68%, 90%, 95% contours for their combination. For $WW$ and $ZZ$ the probability function has been constructed by rescaling the number of events reported by CMS respectively in Ref. Chatrchyan:2012ty and Ref. Chatrchyan:2012dg; see text.
• Figure 4: As for Fig. \ref{['fig:SMinjected']} with injected fermiophobic signal $(a=1/\sqrt{2},c=0)$.