Kaluza-Klein Gluons as a Diagnostic of Warped Models

TL;DR

This paper investigates the first KK gluon, $g^{1}$, in various Randall-Sundrum constructions with SM fields in the bulk. By analyzing production and decay patterns, including $g^{1}$ couplings to light quarks and to tops, as well as the resonance width and potential interference with SM backgrounds, the authors identify robust discriminants among models with custodial symmetry, large IR brane kinetic terms, holographic Higgs, and Higgsless scenarios. They show that observables such as $g^{1}$-initiated production rates (including $b g^{1}$-fusion), branching fractions, and an interference-based asymmetry $A_i$ can reveal the localization of fermions and the presence of additional light KK states, even before electroweak precision tests. The work highlights the potential for dijet or $t\overline t$ channels, constraints from Tevatron data, and the practical utility of $g^{1}$ as a diagnostic for RS realizations at the LHC.

Abstract

We study the properties of $g^{1}$, the first excited state of the gluon in representative variants of the Randall Sundrum model with the Standard Model fields in the bulk. We find that measurements of the coupling to light quarks (from the inclusive cross-section for $pp\to g^{1} \to t\bar t$), the coupling to bottom quarks (from the rate of $pp\to g^{1} b$), as well as the overall width, can provide powerful discriminants between the models. In models with large brane kinetic terms, the $g^1$ resonance can even potentially be discovered decaying into dijets against the large QCD background. We also derive bounds based on existing Tevatron searches for resonant $t \bar{t}$ production and find that they require $M_{g^{1}} \gtrsim 950$ GeV. In addition we explore the pattern of interference between the $g^1$ signal and the non-resonant SM background, defining an asymmetry parameter for the invariant mass distribution. The interference probes the relative signs of the couplings of the $g^{1}$ to light quark pairs and to $t\bar t$, and thus provides an indication that the top is localized on the other side of the extra dimension from the light quarks, as is typical in the RS framework.

Figures (10)

• Figure 1: The 1st KK gluon mass in units of $1/z_v$ and coupling of the first KK gluon to a fermion zero mode localized at UV brane as a function of brane kinetic term $\kappa r_{IR}$.
• Figure 2: Coupling of the first KK gluon (with respect to the zero mode gluon coupling) with $\kappa r_{IR} = 0,1,5,10,20$ (descending) to a fermion zero mode as a function of bulk mass parameter $c$.
• Figure 3: Coupling of the First KK mode of the gluon to the light KK modes of the custodial partners of the right-handed up-type quark as a function of the bulk mass parameter $c$. The left panel shows the left-handed coupling whereas the right panel the right-handed coupling.
• Figure 4: Cross section for $pp \rightarrow g^1$ at the LHC, for standard RS with the SM in the bulk ($\kappa r_{IR}=0$), three models with large brane kinetic terms ($\kappa r_{IR} = 5, 10, 20$) and the model with a larger custodial symmetry, in the cases when $N=0$ or $1$, of the additional KK custodial partner quarks are light enough that $g^1$ can decay into them.
• Figure 5: Cross section for $pp \rightarrow b g^1$ at the LHC, for standard RS with the SM in the bulk ($\kappa r_{IR}=0$), a model with a large brane kinetic term ($\kappa r_{IR} = 5$) and the model with a larger custodial symmetry, in the cases when $0$ ($1$) of the additional KK custodial partner quarks are light enough that $g^1$ can decay into them.