4 by the b NN algorithm. Additionally, we require E$_{T}$ > 20 GeV (25 GeV) for the e +jets ( µ +jets) channel and electrons (muons) with | η | < 1 . 1 (2 . 0). ment of its total production rate, such as the measure- ment of its quantum numbers and of its couplings to other SM particles, may indicate whether the top quark plays a privileged role in the symmetry breaking. Focusing on details of the t ¯ t production, measurements of differential cross sections in the t ¯ t system test perturbative QCD (pQCD) for heavy-quark production and can constrain potential new physics beyond the SM [9], e.g., by mea- suring the transverse momentum of the top quark [10]. Our measurement uses the alpgen [17] event gener- ator, with pythia [18] for parton showering, hadroniza- tion, and modeling of the underlying event, to simulate the inclusive t ¯ t signal. A pythia sample serves as a cross check. The CTEQ6L1 set of parton distribution functions (PDFs) [19] was used with a common factor- ization and renormalization scale set to µ = m$_{t}$ $_{+}$∑$_{p}$ jets T for m$_{t}$ = 170 GeV /c $^{2}$. Backgrounds are modeled with alpgen+pythia for W +jets and Z +jets production, pythia for diboson ( W W , W Z , and ZZ ) production, and comphep [20] for single top-quark production. The detector response is simulated using geant [21]. The simulated t ¯ t signal is normalized to the cross section measured by a dedicated likelihood fit in the same fi- nal state using the same event selections (including the b -tagging requirement) and data as Ref. [15], namely to 8 . 46 +1 . 09 $_{−}$0 . 97 pb at a top-quark mass m$_{t}$ = 170 GeV /c 2 (in good agreement with the value extracted in this study by integrating the differential cross section). The dibo- son and single top-quark backgrounds are normalized to their SM predictions, Z +jets to the prediction from next- to-leading-order (NLO) pQCD, and W +jets such that the predicted number of events matches the data before applying b tagging. In this Letter, we present a new measurement of the inclusive differential cross section for p ¯ p → t ¯ t + X pro- duction at √$_{s}$ = 1 . 96 TeV as a function of the p$_{T}$ of the top quark. The measurement is corrected for detec- tor efficiency, acceptance and resolution effects, making it possible to perform direct comparisons with different theoretical predictions. The data were acquired with the D0 detector at the Fermilab Tevatron Collider and cor- respond to an integrated luminosity of ≈ 1 fb $^{$_{−}$1}$. This measurement was performed in the ℓ +jets decay channel of t ¯ t → W bW ¯ b → ℓν + b ¯ b + ≥ 2 jets, where ℓ repre- sents an e or µ from the decay of the W boson or from W → τ → ℓ . The dependence of the cross section on the p$_{T}$ of the top quark was examined previously using ≈ 100 pb $_{−}$1 of Tevatron Run I data at √$_{s}$ = 1 . 8 TeV [11], where no deviations from the SM were reported. The small multijet background, in which a jet is misidentified as an isolated lepton, is non-negligible only in the e +jets channel. Its rate is estimated from data using the large difference in the probability of electro- magnetic showers of real electrons or misidentified jets to satisfy the electron selection criteria. The details of the sample composition and the observed yields before and after requiring the jets to be tagged as b -jet are pre- sented in Table I. The D0 detector [12] is equipped with a 2 T solenoidal magnet surrounding silicon-microstrip and scintillating- fiber trackers. These are followed by electromagnetic (EM) and hadronic uranium/liquid argon calorimeters, and a muon spectrometer consisting of 1 . 8 T iron toroidal magnets and wire chambers and scintillation counters. Electrons are identified as track-matched energy clus- ters in the EM calorimeter. Muons are identified by matching tracks in the inner tracking detector with those in the muon spectrometer. Jets are reconstructed from calorimeter energies using the Run II iterative seed-based midpoint cone algorithm with a radius of 0.5 [13]. Jets are identified as originating from a b quark using an artifi- cial neural network ( b NN) which combines several track- ing variables [14]. Large missing transverse energy, E$_{T}$ (the negative of the vector sum of transverse energies of calorimeter cells, corrected for reconstructed muons) signifies the presence of an energetic neutrino. Events are selected using a three-level trigger system, which has access to tracking, calorimeter, and muon information, and assures that only events with the desired topology or with objects above certain energy thresholds are kept for further analysis. The selection yields 145 and 141 events in the e +jets and µ +jets decay channels, respectively. The measured t ¯ t signal fraction is 0 . 79, indicating that this sample is suitable for detailed studies of t ¯ t production. A con- strained kinematic fit to the t ¯ t final state, which takes The analysis uses similar data samples, event selec- tion, and corrections as used in the inclusive t ¯ t → ℓ +jets cross-section measurements detailed in Ref. [15]. Events accepted by lepton+jets triggers are subject to additional selection criteria including exactly one isolated lepton with p$_{T}$ > 20 GeV /c and ≥ 4 jets with p$_{T}$ > 20 GeV /c and | η | < 2 . 5 [16]; at least one jet must have p$_{T}$ > 40 GeV /c . At least one jet is also required to be tagged sample e +jets µ +jets t ¯ t 131 108 W +jets 10 15 Z +jets 3.0 3.1 single top 2.7 2.0 diboson 1.3 1.3 multijet 9.0 0.0 summed prediction 156 130 total background uncertainty 3.0 2.8 predicted signal uncertainty 11 9.0 data 145 141 TABLE I: Expected yields for signal and backgrounds samples and observed event counts in e +jets and µ +jets channels.