The Bulk RS KK-gluon at the LHC

TL;DR

The paper investigates the bulk RS KK-gluon in a scenario where Standard Model fields propagate in the extra dimension and KK-gluon decays predominantly to ttbar, focusing on highly boosted top signatures at the LHC. It analyzes production cross sections, decay widths, and the challenging top-jet phenomenology required to identify the signal, proposing benchmark background rejection levels to assess discovery potential. It demonstrates that resonances up to about 5 TeV could be discovered with sufficient top-tagging efficiency, and shows how angular distributions and top polarization can reveal the spin and chirality of the KK-gluon coupling. The work highlights the role of t_R partial compositeness in shaping couplings and argues that energetic-top final states offer broad applicability to other top-rich new-physics scenarios.

Abstract

We study the possibility of discovering and measuring the properties of the lightest Kaluza-Klein excitation of the gluon in a Randall-Sundrum scenario where the Standard Model matter and gauge fields propagate in the bulk. The KK-gluon decays primarily into top quarks. We discuss how to use the $t \bar{t}$ final states to discover and probe the properties of the KK-gluon. Identification of highly energetic tops is crucial for this analysis. We show that conventional identification methods relying on well separated decay products will not work for heavy resonances but suggest alternative methods for top identification for energetic tops. We find, conservatively, that resonances with masses less than 5 TeV can be discovered if the algorithm to identify high $p_T$ tops can reject the QCD background by a factor of 10. We also find that for similar or lighter masses the spin can be determined and for lighter masses the chirality of the coupling to $t\bar t$ can be measured. Since the energetic top pair final state is a generic signature for a large class of new physics as the top quark presumably couples most strongly to the electroweak symmetry breaking sector, the methods we have outlined to study the properties of the KK-gluon should also be important in other scenarios.

Figures (13)

• Figure 1: Left: Wavefunction of the first excited gauge KK mode. Note that it is extremely flat near the UV brane, with $AdS_5$ bulk coordinate defined by $y=\pi \phi$. Right: Coupling of the first gauge KK mode to a zero mode fermion as a function of the fermion localization parameter, $\nu$. Notice that this coupling asymptotes quickly to a constant value for $\nu < -0.5$
• Figure 2: Total cross-section for production of the first KK gluon, as a function of KK mass.
• Figure 3: Invariant mass distribution of $t\bar{t}$ pairs coming from the KK gluon resonance, and SM $t\bar{t}$ production. The errors shown on the background curve are the statistical errors assuming 100 $fb^{-1}$ of luminosity.
• Figure 4: Invariant mass distribution of the decay products for several masses of the KK gluon. This assumes all $t\bar{t}$ events are fully collimated. "BG" is QCD dijet production. All jets are required to have pseudo-rapidities $|\eta| < 0.5$, and at least one to have $p_T > 500$ GeV. The errors shown on the background curve are the statistical errors assuming 100 $fb^{-1}$ of luminosity.
• Figure 5: Left: Fraction of events with certain numbers of distinct objects for events from decay of a KK gluon, with mass (top to bottom) 2, 3, and 4 TeV as a function of invariant mass of the $t\bar{t}$ pair, after imposing a cut on the top $p_T$: 500 GeV, 1 TeV, and 1.5 TeV. Right: SM $t\bar{t}$ production using the same cuts as the corresponding plot on the right. The line labeled "1 coll." is the fraction of events where at least one of the tops has all three decay products within the same cone. A cone size of 0.4 has been used.