Composite Higgs Search at the LHC

TL;DR

The paper analyzes how a composite Higgs, realized as a pseudo-Goldstone boson, modifies Higgs couplings and collider phenomenology. Using the SILH framework and two explicit 5D realizations (MCHM4, MCHM5), it derives how production cross-sections and branching ratios scale with the parameter $\xi=(v/f)^2$ and examines the resulting LEP/Tevatron bounds and LHC search significances across key channels, including $H\to\gamma\gamma$, $H\to ZZ\to 4l$, $H\to WW\to 2l2 u$, $H\to WW\to l\nu jj$, and $H\to \tau\tau$. The study finds that in MCHM4 all couplings are suppressed, generally weakening LHC sensitivities, while in MCHM5 a large $\xi$ can enhance gluon-fusion production and some BRs, yielding higher significances in certain mass ranges. The results underscore that LHC discovery prospects for a composite Higgs are highly model-dependent and hinge on the interplay of production mechanisms and decay channels, with EW precision and Tevatron constraints shaping the viable parameter space.

Abstract

The Higgs boson production cross-sections and decay rates depend, within the Standard Model (SM), on a single unknown parameter, the Higgs mass. In composite Higgs models where the Higgs boson emerges as a pseudo-Goldstone boson from a strongly-interacting sector, additional parameters control the Higgs properties which then deviate from the SM ones. These deviations modify the LEP and Tevatron exclusion bounds and significantly affect the searches for the Higgs boson at the LHC. In some cases, all the Higgs couplings are reduced, which results in deterioration of the Higgs searches but the deviations of the Higgs couplings can also allow for an enhancement of the gluon-fusion production channel, leading to higher statistical significances. The search in the H to gamma gamma channel can also be substantially improved due to an enhancement of the branching fraction for the decay of the Higgs boson into a pair of photons.

Figures (14)

• Figure 1: Higgs branching ratios as a function of the Higgs boson mass in the SM ($\xi=0$, upper left) and MCHM5 with $\xi=0.2$ (upper right), 0.5 (bottom left) and 0.8 (bottom right).
• Figure 2: The branching ratios of MCHM5 as a function of $\xi$ for $M_H=120$ GeV (left) and $M_H=180$ GeV (right).
• Figure 3: The Higgs total width $\Gamma_H$ (in GeV) vs. $M_H$ (in GeV) in the SM (continuous line) and for $\xi=0.2$ (dashed), $\xi=0.5$ (dot-dashed) and $\xi=0.8$ (dotted) in MCHM4 (left) and MCHM5 (right).
• Figure 4: Contour plots of the Higgs total width, $\Gamma_H$, in the plane $(M_H,\xi)$ for MCHM4 (left) and MCHM5 (right). The contours correspond to $\Gamma_H=10^{-3}, 10^{-2}, 10^{-1}$ and $0.5$ GeV.
• Figure 5: Experimental limits from Higgs searches at LEP (blue/dark gray) and the Tevatron (green/light gray) in the plane $(M_H,\xi)$ for MCHM4 (left) and MCHM5 (right). EW precision data prefer low value of $\xi$: the red continuous line delineates the region favored at 99% CL (with a cutoff scale fixed at 2.5 TeV) while the region below the red dashed line survives if there is an additional 50% cancellation of the oblique parameters.