Top production at the Tevatron/LHC and nonstandard, strongly interacting spin one particles

TL;DR

This work analyzes how nonstandard, strongly interacting spin-1 bosons—axigluons and flavour universal colorons—modify top-quark pair production at the Tevatron and LHC. It derives the modified qq̄→tt̄ amplitudes, evaluates Tevatron Run II data to set mass bounds, and studies their impact on the tt̄ mass spectrum at the LHC, including interference effects and broad resonances. The results show Tevatron excludes m_A up to about 910 GeV and place cot ξ–dependent bounds on m_C, while the LHC observables such as the tt̄ spectrum and tt polarization correlations can distinguish axigluons from colorons in wide-mass regions. Together, the work highlights the importance of interference and width effects in constraining extended color gauge sectors and informs experimental search strategies.

Abstract

In this note, we consider possible constraints from $t \bar t$ production on the gauge bosons of theories with an extended strong interaction sector such as axigluons or flavour universal colorons. Such constraints are found to be competitive with those obtained from the dijet data. The current $t \bar t$ data from the Tevatron rule out axigluon masses ($m_A$) up to 900 GeV and 850 GeV at 2 $σ$ and 4 $σ$ levels respectively. For the case of flavour universal colorons the data rule out a mass ($m_C$) below 800 GeV (780 GeV) at the $2 (4) σ$ level and also the mass range between 900 GeV to 2.1 TeV at 2 $σ$ level, for $\cot ξ= 1$, where $ξ$ is the mixing angle. For $\cot ξ=2$ on the other hand, the excluded range is $m_C \lsim 950 (920)$ GeV and $m_C \gsim 1.02 (1.15 \lsim m_C \lsim 1.8)$ TeV at $2 σ$ ($4 σ$) level. We point out that for higher axigluon/coloron masses, even for the dijet channel, the limits on the coloron mass, for $\cot ξ= 1$, may be different than those for the axigluon. We also compute the expected forward-backward asymmetry for the case of the axigluons which would allow it to be discriminated against the SM as also the colorons. We further find that at the LHC, the signal should be visible in the $t \bar t$ invariant mass spectrum for a wide range of axigluon and coloron masses that are still allowed. We point out how top polarisation may be used to further discriminate the axigluon and coloron case from the SM as well as from each other.

Figures (6)

• Figure 1: (a) The branching fraction of axigluon/coloron to a $t \bar{t}$ pair as a function of the boson mass; (b) A comparison of the deviation of the total $t \bar{t}$ cross section caused by the presence of an axigluon (solid line) with the resonant production followed by decay.
• Figure 2: $t \bar{t}$ production cross-section at the Tevatron as a function of the axigluon (coloron mass) and constraints on it from the current $t \bar{t}$ production data. The solid (green) line corresponds to the axigluon case. The short- (blue) and long-dashed (red) lines correspond to the flavour universal coloron for $\cot \xi = 1$ and $\cot \xi=2$ respectively. The horizontal lines correspond to the current central value from the CDF experiment Cabrera:2006ya and the $95 \%$ confidence level bands. We have used CTEQ-6L1 parton distribution functions evaluated at $Q = m_t$ and included the appropriate $K$--factor Campbell:2006wx.
• Figure 3: Exclusion region in the $\cot \xi$ -- $m_C$ plane using the $t \bar{t}$ data at Tevatron Cabrera:2006ya. The solid curve shows the constraint imposed by the $\rho$ parameter $m_C/\cot \xi \mathrel{\hbox{$>$} {\hbox{$\sim$}}} 450$. We restrict the plot to $\cot \xi < 4.0$ for reasons mentioned in the text.
• Figure 4: Forward backward asymmetry expected in the $t \bar{t}$ production due to axigluon contribution as a function of the axigluon mass $m_A$.
• Figure 5: The expected $m_{t \bar{t}}$ spectrum at the LHC in presence of either axigluons or colorons of a specified mass. Also shown, for comparison, are the SM expectations.