Top quarks as forward as expected

The Standard Model predicts that the forward-backward asymmetry of top-antitop events changes depending on the mass of the top-antitop system. For alternative models, this dependence can be quite different. A recent DZero paper on the lepton-plus-jets channel finds good agreement between the data (the points in the above plot) and various calculations of Standard Model predictions (the horizontal lines).

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Of the hundreds of publications released by the Tevatron experiments in recent years, few have provoked as much interest, from both theorists and experimentalists, as the measurements of the directional asymmetry in the production of top-antitop quark pairs. This parameter expresses the tendency for produced top quarks to follow the initial proton direction, and top antiquarks the antiproton direction, and is sensitive to many possible theories beyond the Standard Model. Previous measurements from both the DZero and CDF collaborations found asymmetries appreciably larger than were expected from Standard Model processes, giving a tantalizing hint that there may be some new physics at play and prompting a vast body of theoretical work predicting different models to explain this behavior.

Significantly, this forward-backward asymmetry parameter is unique to the Tevatron, because it is defined only for matter-antimatter collisions. A similar quantity can be measured at the LHC experiments, but the sensitivity is hampered by the different production mechanism of top-antitop quarks. As such, further improved measurements from the Tevatron are very important to help resolve the anomaly. This week the DZero collaboration released a much anticipated new publication on this subject, in which the full data set is analyzed for the first time using the lepton-plus-jets channel.

The precision of previous measurements was constrained by the limited size of the signal sample, and so a major focus of this latest analysis was to increase the signal efficiency. In the lepton-plus-jets channel, the top-antitop quarks decay to a single charged lepton (a muon or electron), four quark jets and an undetected neutrino. By accepting events in which one jet remains unreconstructed in the detector, the detection efficiency for signal events is almost doubled. The cost of this change is an increase in the background contamination; this is mitigated by analyzing the data in separate subsamples of differing signal purity.

Two experimental challenges merit a brief explanation here. First, it is nontrivial to determine the original direction of the top quark because the undetected neutrino carries away an unknown momentum. For events with a missing jet, there is an additional loss of information. To get around this, other properties of the event are used to statistically evaluate the most likely direction of the initial top (and antitop) quark. Using this method, the direction of the top quark is correctly assigned for more than 75 percent of cases.

A second challenge lies in the fact that the major background process (W boson plus jets) itself has a large directional asymmetry, which needs to be accurately determined and corrected for in the final measurement.

With all of the improvements, the new analysis had the sensitivity required to reach the milestone 5-sigma discovery criterion, if the asymmetry had remained at the value reported in previous Tevatron papers. However, the final measured forward-backward asymmetry is in fact consistent with the Standard Model prediction, with a value of 10.6 ± 3.0 percent.

The asymmetry is also determined in terms of the mass of the top-antitop system, which is especially useful for distinguishing between various new physics models. Again, the observed dependence versus mass is in good agreement with the expectations.

While much of the community might have been cheering for an observation of new physics in this measurement, it is important to remember that when we ask Nature the profound questions, we should listen to the answers. In this case, once again, the Standard Model has come out on top!

Mark Williams

These DZero members, all from the University of Rochester, made significant contributions to this publication, as well as a similar paper exploring the lepton asymmetry of top-antitop quarks.
The University of DZero has been active since 2010 and, in this time, has provided almost 50 lectures targeted to the interests of the particle physics community. Topics range from technical tutorials on experimental and theoretical aspects of particle physics to presentations of careers outside of academia, often given by former DZero collaborators. The University Deans are Jenny Holzbauer (University of Mississippi) and Hang Yin (Fermilab), who arrange and coordinate talks and, perhaps most importantly, ensure that pizza is available for attendees. For more information, please visit the DZero Web page.