Dilaton constraints and LHC prospects

TL;DR

The paper investigates a dilaton arising from spontaneously broken conformal dynamics as an alternative mechanism for electroweak symmetry breaking. It derives the dilaton’s couplings, decay widths, and branching ratios, showing that most SM-like rates scale as $(v^2/f^2)$ while loop-induced $χgg$ and $χγγ$ couplings receive beta-function–driven enhancements captured by $R_g$ and $R_{ ext{γ}}$. By reinterpreting LEP and LHC Higgs searches, the authors derive constraints in the $(M_{ m\chi},f)$ plane, finding $f$ must be above about 1 TeV for many high-mass scenarios and that very light dilatons remain viable for modest $f$. They propose LHC discovery channels—diphoton for low-mass and $ZZ o 4 ext{l}$ for high-mass dilatons—with distinctive features such as an enhanced diphoton rate and a narrow width that aid discrimination from a SM Higgs. The work also discusses ILC prospects and outlines how measurements of rising beta-function–driven couplings ($R_g$, $R_{ ext{γ}}$) could characterize the dilaton and extract the conformal breaking scale, providing concrete guidance for future searches and model testing.

Abstract

The Standard Model Higgs searches using the first 1-2 fb-1 of LHC data can be used to put interesting constraints on new scalar particles other than the Higgs. We investigate one such scenario in which electroweak symmetry is broken via strongly coupled conformal dynamics. This scenario contains a neutral scalar dilaton---the Goldstone boson associated with spontaneously broken scale invariance---with a mass below the conformal symmetry breaking scale and couplings to Standard Model particles similar (but not identical) to those of the Standard Model Higgs boson. We translate the LEP and LHC Higgs limits to constrain the dilaton mass and conformal breaking scale. The conformal breaking scale f is constrained to be above 1 TeV for dilaton masses between 145 and 600 GeV, though it can be as low as 400 GeV for dilaton masses below 110 GeV. We also show that (i) a dilaton chi with mass below 110 GeV and consistent with the LEP constraints can appear in gg --> chi --> gamma gamma with a rate up to ~10 times the corresponding Standard Model Higgs rate, and (ii) a dilaton with mass of several hundred GeV is much narrower than the corresponding Standard Model Higgs, leading to improved search sensitivity in chi --> ZZ --> 4l.

Figures (12)

• Figure 1: The scaling functions $R_g$ and $R_{\gamma}$ as a function of the dilaton mass. Note that $R_g$ is divided by 100 in the plots to allow the two functions to be displayed on the same axes.
• Figure 2: Dilaton branching ratios as a function of the dilaton mass for masses below 200 GeV. The right-hand plot compares the dilaton branching ratios into final states important for LHC Higgs searches in inclusive production modes (solid lines) to the corresponding SM Higgs branching ratios (dotted lines).
• Figure 3: Dilaton branching ratios as a function of the dilaton mass for masses up to 1000 GeV.
• Figure 4: Total decay width of the dilaton as a function of the dilaton mass, for various values of the conformal symmetry breaking scale $f$. We plot only $M_{\chi} < f$. The corresponding total width of the SM Higgs is shown for comparison (solid line).
• Figure 5: Constraints on the dilaton mass and conformal scale $f$ from LEP and the LHC. Regions below the solid and dot-dashed blue lines are excluded by the LEP SM Higgs search and a flavor-independent LEP search for a hadronically-decaying Higgs, respectively. Regions below the solid and dot-dashed red lines are excluded by the combined LHC SM Higgs search and the LHC Higgs search in the $H \to \gamma\gamma$ channel, respectively. For the LHC exclusion, we take the stronger of the ATLAS and CMS limits at each mass point. The dotted black lines show contours of $\sigma(gg \to \chi \to \gamma\gamma)/\sigma(gg \to H_{\rm SM} \to \gamma\gamma)$.