This work was supported by the U.S. Department of Energy and National Science Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Education, Culture, Sports, Science, and Technology of Japan; the Natural Sciences and Engineering Research Council of Canada; the National Science Council of the Republic of China; the Swiss National Science Foundation; the A. P. Sloan Foundation; the Bundesministerium fur Bildung und Forschung, Germany; the Korean World Class University Program and the National Research Foundation of Korea; the Science and Technology Facilities Council and the Royal Society, U.K.; the Russian Foundation for Basic Research; the Ministerio de Ciencia e Innovación and Programa Consolider-Ingenio 2010, Spain; the Slovak R&D Agency; the Academy of Finland; the Australian Research Council (ARC); and the EU Community Marie Curie Fellowship Contract No. 302103.

We report a measurement of the differential cross section dσ/d(cosθt) for top-quark pair production as a function of the top-quark production angle in proton-antiproton collisions at √s=1.96 TeV. This measurement is performed using data collected with the CDF II detector at the Tevatron, corresponding to an integrated luminosity of 9.4 fb-1. We employ the Legendre polynomials to characterize the shape of the differential cross section at the parton level. The observed Legendre coefficients are in good agreement with the prediction of the next-to-leading-order standard-model calculation, with the exception of an excess linear-term coefficient a1=0.40±0.12, compared to the standard-model prediction of a1=0.15+0.07−0.03. © 2013 American Physical Society.

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