Abstract
New colour octets stand out among the new physics proposals to explain the anomalous forward-backward asymmetry measured in $t \bar t$ production by the CDF experiment at the Tevatron. We perform a fit to $t \bar t$ observables at the Tevatron and the LHC, including total cross sections, various asymmetries and the top polarisation and spin correlations, to find the most likely parameters of a light colour octet to be consistent with data. In particular, an octet coupling only to right-handed quarks gives a good fit to all measurements. The implications from the general fit are drawn in terms of predictions for top polarisation observables whose measurements are yet not very precise, and observables which simply have not been measured.
Highlights
New colour octets stand out among the new physics proposals to explain the anomalous forward-backward asymmetry measured in ttproduction by the CDF experiment at the Tevatron
We perform a fit to ttobservables at the Tevatron and the LHC, including total cross sections, various asymmetries and the top polarisation and spin correlations, to find the most likely parameters of a light colour octet to be consistent with data
In order to answer that, we plot these four observables (Pz,x at the Tevatron; Pz and Chel at the LHC) in figure 4 with three colour codes according to the size of the new physics contribution to the ttasymmetry ∆AFB: (i) red for ∆AFB ≤ 0.03, as is the case of the latest D0 measurement [17]; (ii) orange for 0.03 ≤ ∆AFB ≤ 0.06, as favoured by the current Tevatron average in table 1; (iii) green for 0.06 ≤ ∆AFB, as it corresponds to the CDF measurement [16]
Summary
In addition to its mass and width, a colour octet exchanged in uu, dd → G → tthas vector and axial couplings to the up, down and top quarks, gAu ,V , gAd ,V , gAt ,V totalling eight parameters. For each parameter space point, a Monte Carlo calculation for pp → ttis run using Protos [50] to find the new physics corrections to the observables considered. For the parameter space points where the overall agreement is of 2σ or slightly above, a refined calculation of the ttobservables is made with higher statistics (2 × points for Tevatron and 2 × points for LHC at each CM energy), and the fit is repeated with these values. Prediction 7.16 ± 0.21 pb [52] 176.3 ± 6.9 pb [52] 251.7 ± 9.6 pb [52] 0.088 ± 0.006 [6] 0.038 ± 0.003 [6] 0.048 ± 0.004 [6] 0.0123 ± 0.0005 [6] 0.0111 ± 0.0004 [6] 0.0070 ± 0.0003 [6]
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