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folded result on the top-quark mass is 2 − 6% for m$_{t}$ in the 170-175 GeV/ c 2 range. This additional uncer- tainty does not need to be considered for comparisons with models in which m$_{t}$ is set to 170 GeV/ c $^{2}$. For the main background sources, W/Z +jets, we have also con- sidered the variations of the background shape caused by uncertainties in the k-factors and in additional scale factors for heavy-flavour jets. Other systematic uncer- tainties [15] account for uncertainties in the modeling of the signal, estimated from the difference between alpgen and pythia , for uncertainties in the PDFs and in the b - quark fragmentation. The uncertainty on the integrated luminosity is 6 . 1%. The systematic uncertainties quoted in the following combine the uncertainty on the normal- ization (independent of p$_{T}$ ) with the shape-dependent systematics. The total correlated systematic uncertainty is 9 . 6% (including the uncertainty on luminosity) and the total systematic uncertainty on the cross section, in- tegrating over p$_{T}$ , is 10 . 7%. 

Results from NLO pQCD [26, 27] calculations obtained using CTEQ61 [28] PDFs (using the scale µ = m$_{t}$ = 170 GeV /c $^{2}$) are overlaid on the measured differential cross section in Fig. 4. Also shown are results from an approximate next-to-NLO (NNLO) pQCD calcula- tion [29] computed using MSTW2008 NLO PDFs [30] and same scales choices as the NLO result, and from the mc@nlo [31] (using CTEQ61 PDFs), alpgen , and pythia event generators. The QCD scale uncertainty was evaluated for the NLO pQCD calculation [26, 27] 

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by varying µ = m$_{t}$ = 170 GeV /c 2 by factors of 2 and 1 / 2, and the PDF uncertainty by the approximate NNLO code [29]. The total uncertainty is < 4% with only a small ( < 1%) shape variation. A comparison of the ratio of d σ/ d p$_{T}$ relative to a NLO pQCD calculation is shown in Fig. 5. The NLO pQCD calculations agree with the measured cross section, however, results from alpgen ( pythia ) have a normalization shift of about 45% (30%) with respect to data. A shape comparison of the ratio of (1 /σ ) d σ/ d p$_{T}$ relative to NLO pQCD is shown in Fig. 6. All of the calculations reproduce the observed shape. The χ 2 and corresponding χ 2 probabilities [32] for the com- parisons in Figs. 5 and 6 of predictions to data are given in Table IV. 

In conclusion, we have presented a 1 fb $_{−}$1 measure- ment of the differential cross section of the top-quark p$_{T}$ for t ¯ t production in the ℓ +jets channel using p ¯ p col- lisions at √$_{s}$ = 1 . 96 TeV. Results from NLO and NNLO pQCD calculations and from the mc@nlo event genera- tor agree with the normalization and shape of the mea- sured cross section. Results from alpgen+pythia and pythia describe the shape of the data distribution, but not its normalization. We thank the staffs at Fermilab and collaborating institutions, and acknowledge support from the DOE and NSF (USA); CEA and CNRS/IN2P3 (France); FASI, Rosatom and RFBR (Russia); CNPq, 

