Warped Gravitons at the LHC and Beyond

TL;DR

This work analyzes warped extra dimensions with bulk SM fields to search for Kaluza-Klein gravitons at the LHC. It shows that KK gravitons predominantly couple to top/Higgs and to longitudinal W/Z, making gluon-fusion production with G→Z_LZ_L (ZZ) decays a promising discovery channel, while vector-boson fusion is subdominant. The authors quantify the 4-lepton ZZ signature as a clean probe, estimating LHC reach of about 2 TeV and SLHC reach around 3 TeV for moderate curvature k/M_P, and discuss the validity of the effective theory when c ≈ 1–2. They conclude that heavier gravitons (>4 TeV) would require higher luminosity or energy, but the proposed channel provides a robust test of bulk RS scenarios and their flavor solutions.

Abstract

We study the production and decay of Kaluza-Klein (KK) gravitons at the Large Hadron Collider (LHC), in the framework of a warped extra dimension in which the Standard Model (SM) fields propagate. Such a scenario can provide solutions to both the Planck-weak hierarchy problem and the flavor puzzle of the SM. In this scenario, the production via $q \bar{q}$ annihilation and decays to the conventional photon and lepton channels are highly suppressed. However, we show that graviton production via gluon fusion followed by decay to longitudinal $Z/W$ can be significant; vector boson fusion is found to be a sub-dominant production mode. In particular, the ``golden'' $ZZ$ decay mode offers a distinctive 4-lepton signal that could lead to the observation at the LHC with 300 fb$^{-1}$ (SLHC with 3 ab$^{-1}$) of a KK graviton with a mass up to $\sim 2$ ($\sim 3$) TeV for the ratio of the AdS$_5$ curvature to the Planck scale modestly above unity. We argue that (contrary to the lore) such a size of the curvature scale can still be within the regime of validity of the framework. Upgrades beyond the SLHC luminosity are required to discover gravitons heavier than $\sim 4$ TeV, as favored by the electroweak and flavor precision tests in the simplest such models.

Figures (5)

• Figure 1: The cross-sections (integrated over one width) for $gg \rightarrow ZZ$ via KK graviton (solid lines) and the corresponding SM background (dashed lines). We show the cross-sections for $c \equiv k / {\bar{M}}_{P} = 0.5, 1, 1.5, 2$ (from bottom to top). See the text for an explanation of the upper limit on $c$. The yellow region shows where we expect the KK graviton mass to be in the simplest models according to relation in Eq \ref{['KKgKKG']} and the limit on gauge KK mass from precision tests.
• Figure 2: Same as fig. \ref{['nocuts+b']}, but with $\eta<2$.
• Figure 3: The total number of expected events for the purely leptonic decay mode for $Z$ pairs from KK graviton decay using 300 fb$^{-1}$ with $\eta<2$. See also Fig. (\ref{['nocuts+b']})
• Figure 4: Significance for the purely leptonic decay mode for $Z$ pairs from KK graviton using 300 fb$^{-1}$. See also Fig. (\ref{['nocuts+b']})
• Figure 5: Same as FIG. \ref{['StoRBnoet']}, but with $\eta<2$.