Another one bites the dust...

...though it's not BICEP2 this time :) This is a long overdue update on the forward-backward asymmetry of the top quark production.

Recall that, in a collision of a quark and an anti-quark producing a top quark together with its antiparticle, the top quark is more often ejected in the direction of the incoming quark (as opposed to the anti-quark). This effect can be most easily studied at the Tevatron who was colliding protons with antiprotons, therefore the direction of the quark and of the anti-quark could be easily inferred. Indeed, the Tevatron experiments observed the asymmetry at a high confidence level. In the leading order approximation, the Standard Model predicts zero asymmetry, which boils down to the fact that gluons mediating the production process couple with the same strength to left- and right-handed quark polarizations. Taking into account quantum corrections at 1 loop leads to a small but non-zero asymmetry.

Intriguingly, the asymmetry measured at the Tevatron appeared to be large, of order 20%, significantly more than the value  predicted by the Standard Model loop effects. On top of this, the distribution of the asymmetry as a function of the top-pair invariant mass, and the angular distribution of leptons from top quark decay were strongly deviating from the Standard Model expectation. All in all, the ttbar forward-backward anomaly has been considered, for many years, one of our best hints for physics beyond the Standard Model. The asymmetry could be interpreted, for example, as  being due to new heavy resonances with the quantum numbers of the gluon, which are predicted by models where quarks are composite objects. However, the story has been getting less and less  exciting lately. First of all, no other top quark observables  (like e.g. the total production cross section) were showing any deviations, neither at the Tevatron nor at the LHC. Another worry was that the related top asymmetry was not observed at the LHC. At the same time, the Tevatron numbers have been evolving in a worrisome direction: as the Standard Model computation was being refined the prediction was going up; on the other hand, the experimental value was steadily going down as more data were being added. Today we are close to the point where the Standard Model and experiment finally meet...

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.