D0: top forward-backward asymmetry continues to intrigue

While the LHC has been depressingly confirming the predictions of the Standard Model, the good old Tevatron remains the only light in the tunnel. The only 2 lights actually: almost 4 sigma anomaly of the dimuon charge symmetry measured by D0, and over 3 sigma anomaly of the top quark forward-backward asymmetry at high t-tbar invariant mass (yikes) measured by CDF. Future will tell whether these lights should be interpreted as the way out or as the oncoming train. As for the present, the D0 collaboration just provided an important update concerning the forward-backward asymmetry that puts the CDF result in a slightly different... light.

For the latest update D0 used 5.4fb−1 and focused on semi-leptonic top decays. The idea of the measurement is simple: one picks the top decay products, reconstructs the original top and anti-top momenta, and check for an excess of top over antitop quarks moving forward (that is along the proton beam) in the t-tbar rest frame. The Standard Model predicts such an excess should be very small, of order 5%. Instead, after unfolding detector effects from the measured asymmetry, D0 finds the "unfolded" or "production" asymmetry to be 19.6 ± 6.5 %. This kicks in very nicely with the analogous CDF result of 15.8 ± 7.4 % (or 20% when combined with the asymmetry in the dilepton channel). Both results are about 2 sigma away from the Standard Model and both point in the same direction, which is intriguing and almost exciting.

However, not everything agrees perfectly between D0 and CDF. Most importantly, D0 does not see any significant dependence of the asymmetry on the t-tbar invariant mass. On the other hand, CDF sees a dramatic dependence: it was actually the abnormally high asymmetry in the mtt > 450 GeV bin that allowed them to claim a 3-sigma anomaly at the beginning of this year. The situation is thus a bit volatile: the good matching of the inclusive asymmetry between the two experiments is obtained after integrating over discrepant results in the low and high mtt bins.

D0 shares one more important result which I find more exciting. D0 measured the leptonic asymmetry of the leptons originating from top quark decays. The observable is defined as an excess of positive charge leptons moving forward + negative charge leptons moving backward over negative (positive) leptons moving forward (backward). For experimentalists it is more user-friendly than the top asymmetry : it is defined in the laboratory frame and one can avoid tedious and uncertain reconstruction of the momenta of the top quarks. From the theoretical point of view, the lepton asymmetry is tightly related to the top forward-backward asymmetry but not identical. It is related, because the direction of the lepton is clearly correlated with the direction of the mother top or antitop. It is not identical, because that direction is also correlated with the polarization of the mother top. In fact, if the top quark is polarized along some axis, for example in the direction of its motion, the lepton prefers to fly along that direction. See for example this paper for more details. All in all, D0 finds this leptonic asymmetry to be 15.2 ± 4.0%, compared to the Standard Model prediction of 2%. This is more than 3 sigma discrepancy! Not only we get a novel 3 sigma anomaly to cherish, but we also get a hint of anomalous top polarization.

To wrap up , D0 has brought some exciting news and some worrying news too (see the paper for more worries concerning modeling additional QCD radiation that I didn't mention here). The new results will somewhat shake the hierarchy of new physics models that address Tevatron's anomalies, but we have to wait for the next load of theory papers for quantitative details. On the experimental front, next year Tevatron will update all these measurements with twice as much data. About the same time, the LHC will be seriously joining in the game too. Although the LHC cannot measure the top asymmetry directly, due to the symmetric p-p initial state at the LHC, they can access the same physics by constructing more fancy observables. For example, a recent CMS note investigates whether top quarks move closer to the beam axis than anti-top quarks more often than the other way around. Such an effect would be a consequence of a positive forward-backward asymmetry of the t-tbar pair production in quark-antiquark collisions. No effect has been observed in CMS or ATLAS who studied a similar observable. However this doesn't mean much yet: most models addressing Tevatron's top anomaly predict the CMS observable to be the edge of their current sensitivity. It is more worrying that LHC observes no anomalous effects in other top quark observables, like the production cross section or the invariant mass distribution. Yeah, the obstinate lack of new physics at the LHC is utterly worrying. Guys, you better find something fast, otherwise there'll be nothing but darkness.

The D0 paper is available on arXiv. See also Tommaso's comments on the CMS note.