...though it's not BICEP2 this time :) This is a long overdue update on the forward-backward asymmetry of the top quark production.
The final straw is two recent updates from Tevatron's D0 experiment. Earlier this year, D0 published the measurement of the forward-backward asymmetry of the direction of the leptons
from top quark decays. The top quark sometimes decays leptonically, to a b-quark, a neutrino, and a charged lepton (e+, μ+). In this case, the momentum of the lepton is to some extent correlated with that of the parent top, thus the top quark asymmetry may come together with the lepton asymmetry (although some new physics models affect the top and lepton asymmetry in a completely different way). The previous D0 measurement showed a large, more than 3 sigma, excess in that observable. The new refined analysis using the full dataset reaches a different conclusion: the asymmetry is Al=(4.2 ± 2.4)%, in a good agreement with the Standard Model. As can be seen in the picture, none of the CDF and D0 measurement of the lepton asymmetry in several final states shows any anomaly at this point. Then came the D0 update of the regular ttbar forward-backward asymmetry in the semi-leptonic channel. Same story here: the number went down from 20% down to Att=(10.6 ± 3.0)%, compared to the Standard Model prediction of 9%. CDF got a slightly larger number here, Att=(16.4 ± 4.5)%, but taken together the results are not significantly above the Standard Model prediction of Att=9%.
So, all the current data on the top quark, both from the LHC and from the Tevatron, are perfectly consistent with the Standard Model predictions. There may be new physics somewhere at the weak scale, but we're not gonna pin it down by measuring the top asymmetry. This one is a dead parrot:
Graphics borrowed from this talk.