Symmetries are important

The image above shows the top-antitop rapidity difference (top) and the forward-backward asymmetry as a function of the top quark pair invariant mass (bottom) after correcting for event selection and detector effects. The data, shown in black, contains an asymmetry that exceeds what is predicted by current Standard Model calculations, shown in red.

Past analyses by scientists at CDF and DZero have shown hints of an unexpectedly large forward-backward asymmetry in the production of top quark pairs during Tevatron collisions. A new measurement at CDF using the full Tevatron data set confirms this asymmetry and uses new techniques to study the dependence of the asymmetry on the properties of the top-antitop system, providing new information about the effect and how it compares to theoretical predictions.

Symmetry is fundamental to the theories that describe elementary particle interactions. A discrepancy from a predicted symmetry or asymmetry can be an important clue pointing to the presence of new physical processes or the need for revision of existing theoretical calculations.

The new result from CDF looks at the direction of motion of the outgoing top quark using a variable called rapidity and determines how often the top quark travels along the direction of the colliding proton (forward) or antiproton (backward).

Only a portion of the top quark pair events produced during collisions inside the CDF detector are actually observed and recorded by the detector, and even in events that do get recorded, sometimes the measured direction of motion of a top quark does not correspond exactly to its true direction of motion. This analysis introduces new techniques to correct for these effects in order to measure the “parton level” asymmetry—that is, the true asymmetry in all top quark events before the influence of experimental effects. The size of the asymmetry is found to be 16.4 ± 4.5 percent, compared to a Standard Model prediction of approximately 7 percent. It is seen to grow approximately linearly as Mtt and |Δy| increase.

The slopes of linear fits to the data are used to compare this mass- and rapidity-dependent behavior to the Standard Model expectations. CDF finds that the slopes exceed the predictions with a significance above 2 sigma, confirming the previous measurements and providing important information about possible new physical interactions or improvements that must be made to existing theoretical predictions.

edited by Andy Beretvas

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These CDF physicists contributed to this data analysis. Top row from left: Dan Amidei, Ryan Edgar, Dave Mietlicki, Tom Schwarz, all from the University of Michigan. Bottom row from left: Jon Wilson, Tom Wright, both from the University of Michigan, and Joey Huston from Michigan State University.