Kaluza--Klein excitations of gauge bosons at the LHC

Abstract

We consider the Randall-Sundrum extra dimensional model with fields propagating in the bulk based on an extended electroweak gauge symmetry with specific fermion charges and localizations that allow to explain the LEP anomaly of the forward--backward asymmetry for $b$--quarks, $A_{FB}^b$. We study the manifestations of the strongly--interacting and electroweak gauge boson Kaluza--Klein excitations $\vkk$ at the LHC, with masses of the order of a few TeV, which dominantly decays into top and bottom quark pairs. We first analyze the two--body tree--level production processes $pp \to t\bar t$ and $b\bar b$ in which the Kaluza--Klein (KK) excitations of gauge bosons are exchanged. We find that the additional channels can lead to a significant excess of events with respect to the Standard Model prediction; characteristic top quark polarization and angular asymmetries are quantitatively studied and turn out to probe the chiral structure of couplings to excited states. We then analyze higher order production processes for the gauge boson excitations which have too weak or no couplings to light quarks and, in particular, the loop induced process $gg \to \vkk \to t\bar t$ and $b\bar b$ in which the anomalous $gg \vkk $ four--dimensional vertex has to be regulated. The RS effects in this process, as well as in the four--body reactions $pp \to t\bar t b\bar b$, $t\bar t t\bar t$, $b\bar b b\bar b$ and in the related three--body reactions $gb \to b t \bar t$, $b b\bar b$, in which the $\vkk$ excitations are mainly radiated off the eavy quarks, are shown to be potentially difficult to test at LHC, due to small hase space and low parton density for $\mkk \gsim 3$ TeV.

Figures (9)

• Figure 1: Feynman diagram for heavy $Q=t,b$ quark pair production at the LHC through $q\bar{q}$ annihilation, where $q$ stands for initial state quarks and KK for the KK excitations of neutral gauge bosons, namely $\gamma^{(n)}$, $g^{(n)}$, $Z^{(n)}$ and $Z^{\prime (n)}$ with $n=0,1,2,\dots$ being the KK level; mainly the zero--modes $n=0$, except for $Z'$, and the first KK excitations $n=1$ contribute to the signal. For bottom quark production there is an additional $t$--channel diagram exchanging neutral KK gauge bosons with bottom partons in the initial state (b).
• Figure 2: The invariant mass distribution of the cross section for the $pp\to t\bar{t}$ process for all four scenarios $E_{1,2,3,4}$ defined in Table \ref{['tab:par']}. We use the CTEQ5M1 set of parton distribution functions and include the cuts of eq. (\ref{['eq:ptcut']}) on the $p_T^{t,\bar{t}}$ and $\eta_{t,\bar{t}}$ of the final quarks. The solid curve is for the SM background and the long--dashed one for the RS model with contributions of all the first KK excitations of the gauge bosons $g^{(1)}, \gamma^{(1)}, Z^{(1)}$ included; the short--dashed curve is for the SM case along with only $g^{(1)}$ exchange; the dotted curve is for the SM case along with only $\gamma^{(1)}$ exchange; the dashed--dotted curve is for the SM case along with only $Z^{(1)}$ exchange.
• Figure 3: The invariant mass distribution of the cross section for the $pp\to b\bar{b}$ process for all four parameter sets $E_{1,2,3,4}$ defined in Table \ref{['tab:par']}. We use the CTEQ5M1 set of parton distribution functions and include the cuts of eq. (\ref{['eq:ptcut']}) on the $p_T^{b,\bar{b}}$ and $\eta_{b,\bar{b}}$ of the final quarks. The solid curve is for the SM background and the long--dashed one for the RS model with contributions of all the first KK excitations of the gauge bosons $g^{(1)}, \gamma^{(1)}, Z^{(1)}, Z^{\prime (1)}$ included.
• Figure 4: The top quark polarization as a function of the invariant mass $m_{t \bar{t}}$ in the reaction $pp \to t \bar{t}$ in the SM (solid) and in the RS model (dashed) for the set $E_1$ of parameters.
• Figure 5: The lepton asymmetry distribution versus the $m_{t\bar{t}}$ invariant mass for the process $pp \to t \bar{t}$ in SM and in the RS scenario $E_1$.