8 [20] E. E. Boos et al. , Phys. Atom. Nucl. 69 , 1317 (2006); Yad. Fiz. 69 , 1352 (2006). [21] R. Brun and F. Carminati, CERN Program Library Long Writeup W5013, 1993 (unpublished). [22] V. M. Abazov et al. , D0 collaboration, Phys. Rev. D 75 , 092001 (2007); [23] S. Snyder, Ph.D. thesis, State University of New York at Stony Brook (1995), [Institution Report No. FERMILAB-THESIS-1995-27 ]. [24] A. Hoecker and V. Kartvelishvili, Nucl. Instrum. Meth- ods in Phys. Res. A 372 , 469 (1996). [25] V. Kartvelishvili, [26] M. Mangano, P. Nason, and G. Ridolfi, Nucl. Phys. B 373 , 295 (1992). [27] P. Nason, S. Dawson, and R. K. Ellis, Nucl. Phys. B 327 , 49 (1989) [Erratum-ibid. B 335 , 260 (1990)]. [28] D. Stump et al. , J. High Energy Phys. 0310 , 046 (2003). [29] N. Kidonakis and R. Vogt, Phys. Rev. D 78 , 074005 (2008). [30] A.D. Martin, W.J. Stirling, R.S. Thorne, G. Watt, Eur. Phys. J. C 63 189 (2009). [31] S. Frixione and B. R. Webber, J. High Energy Phys. 0206 , 029 (2002); S. Frixione, P. Nason, and B. R. Web- ber, J. High Energy Phys. 0308 , 007 (2003). [32] N. D. Gagunashvili, Nucl. Instrum. Methods in Phys. Res. A 596 , 439 (2008). [8] I. Bigi et. al. , Phys. Lett. B 181 , 157 (1986). [9] C. T. Hill and S. J. Parke, Phys. Rev. D 49 , 4454 (1994). [10] D. Atwood et al. , Phys. Rev. D 52 , 6264 (1995). [11] T. Affolder et al. , CDF Collaboration, Phys. Rev. Lett. 87 , 102001 (2001); B. Abbott et al. , D0 Collaboration, Phys. Rev. D 58 , 052001 (1998). [12] V. M. Abazov et al. , D0 Collaboration, Nucl. Instrum. Methods in Phys. Res. A 565 , 463 (2006). [13] G. C. Blazey et al. , in Proceedings of the Workshop: QCD and Weak Boson Physics in Run II, edited by U. Baur, R. K. Ellis, and D. Zeppenfeld, Fermilab-Pub- 00/297 (2000). [14] V. M. Abazov et al. , D0 Collaboration, Nucl. Instrum. Methods in Phys. Res. A 620 , 400 (2010). [15] V. M. Abazov et al. , D0 Collaboration, Phys. Rev. D 80 (RC), 071102 (2009). [16] Pseudorapidity is defined as η = − ln tan( θ/ 2) where θ is the angle measured with respect to the proton beam coinciding with the positive z axis of a right-handed co- ordinate system at the center of the detector. [17] M. L. Mangano, et al. , J. High Energy Phys. 0307 , 001 (2003). [18] T. Sj¨ ostrand et al. , Comput. Phys. Commun. 135 , 238 (2001); R. Field and R. C. Group, arXiv:hep-ph/0510198. [19] J. Pumplin et al. , J. High Energy Phys. 0207 , 012 (2002). Culiac´ an, Mexico. [g] Visitor from Universit¨ at Bern, Bern, Switzerland.  [GeV/c] T top quark p 0 50 100 150 200 250 300 350 400 Shape ratio to NLO pQCD 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0  -1 DØ data, 1 fb NLO pQCD scale uncertainty Approx. NNLO pQCD MC@NLO PYTHIA ALPGEN FIG. 6: Ratio of (1 /σ ) d σ/ d p T for top quarks in t ¯ t production (two entries per event) to the expectation from NLO pQCD. The gray band encompasses uncertainties on the scale of pQCD and parton distribution functions. Also shown are ra- tios relative to NLO pQCD for an approximate NNLO pQCD calculation and of predictions for several event generators. Inner and outer error bars represent statistical and total (sta- tistical and systematic added in quadrature) uncertainties, respectively.