Angling for new physics with leptons from top quark pairs

In the Standard Model, pairs of top-antitop quarks are produced with a known directional bias in proton-antiproton collisions, with the top (antitop) quark following the (anti)proton direction slightly more than half of the time. By measuring the angular distributions of leptons (muons and electrons) from top quark decays, this dependence can be rigorously tested. This measurement finds good agreement between the data and Standard Model predictions (MC@NLO), as shown by the plot on the left. 

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The Tevatron data set, collected from proton-antiproton collisions, gives access to some unique precision tests of the Standard Model. One helpful property of the collision environment is the existence of distinct proton and antiproton directions, allowing the directional preferences of matter and antimatter production to be measured. One can classify particle production as “forward” (for example, positively charged particles following the proton direction and negatively charged particles following the antiproton direction) or “backward” (vice versa). One can then create a class of “forward-backward” asymmetry observables, which can be explored in searches for new physics.

The DZero collaboration recently published a new measurement of one such directional asymmetry, from top-antitop quark pair production. To first order, the Standard Model predicts top-antitop production to be forward-backward symmetric. However, higher-order effects introduce a small asymmetry, of a few percent, meaning that (anti)top quarks tend to be produced along the (anti)proton beam direction. Other theories beyond the Standard Model, such as so-called axigluon models, can give significant departures from the Standard Model values, generally increasing the asymmetries to larger values.

For this publication, the top and antitop quarks are both selected in the lepton channel, in particular, in their decays to a muon or electron. Rather than try to infer the direction of the original (anti)top quark, we use the directions of the charged leptons originating from the decay products of the original top and antitop quarks. For this analysis, we looked at original (anti)top quark decaying into either a muon or electron (its charge depending on whether it decayed from a top or an antitop), an undetected neutrino and a b quark jet. This decay pattern is very distinctive, giving a sample signal purity exceeding 80 percent.  Both muons and electrons leave extended signatures in the detector, meaning that their trajectories are known with excellent precision, and the correspondence between the lepton and top quark direction is well-understood.

In fact, this paper measures two separate asymmetry parameters: a single lepton asymmetry (for example, how often the positive lepton follows the proton direction versus the antiproton direction) and a similarly defined dilepton asymmetry. The two quantities are highly correlated, but both their individual values and their ratio are precisely predicted by the Standard Model. After subtracting the contributions from various background processes and accounting for apparent asymmetries caused by detector effects, the final results are found to agree with the Standard Model predictions. Two possible axigluon models in particular are disfavored by the data. Stay tuned for more measurements of similar top quark asymmetries from DZero in the coming weeks.

Mark Williams

These DZero members, all from CEA, Saclay, France, made significant contributions to this publication.
The Clued0 administration team provides essential computing support for the Clued0 network of computers at DZero. This is a collection of several hundred nodes, comprising both desktop computers and dedicated processors used for parallel computing. Their duties range from fixing hardware and software issues with individual machines to coordinating software upgrades across the entire network.