A search for the pair production of scalar top quarks,t, has been performed in 360 pb −1 of data from pp collisions at a center-of-mass energy of 1.96 TeV, collected by the DØ detector at the Fermilab Tevatron collider. Thet decay mode considered ist → cχ 0 1 , whereχ 0 1 is the lightest supersymmetric particle. The topology analyzed therefore consists of a pair of acoplanar heavy-flavor jets with missing transverse energy. The data and standard model expectation are in agreement, and a 95% C.L. exclusion domain in the (mt , mχ0 ciated with the left-and right-handed degrees of freedom of each ordinary quark. The mass eigenstates result from the diagonalization of a mass matrix, with elements determined by the specific SUSY-breaking pattern. A light SUSY partner of the top quark, or stop, is a generic prediction of models in which the scalar quark masses are equal at the grand unification scale. A first reason is that, due to the impact of the large top quark Yukawa coupling in the renormalization group equations, DØ Collaboration / Physics Letters B 645 (2007) 119-127 123 the diagonal elements of the mass matrix are driven to values smaller than those for the other scalar quarks at the electroweak scale [2]. A second reason is that the off-diagonal terms are proportional to the relevant quark mass, and hence are much larger in the case of the top quark. The mass eigenstates are therefore broadly split, with the mass of the lighter stopt thus driven to an even lower value [3]. Finally, a light stop is a necessary ingredient in the context of electroweak baryogenesis [4].In models with R-parity conservation [5], the lightest SUSY particle (LSP) is stable, and cosmological constraints imply that it should be neutral and colorless [6]. In a large class of SUSY models, the lightest of the neutralinos-the mass eigenstates resulting from the mixing of the SUSY partners of the neutral gauge and Higgs bosons-is the LSP, which furthermore appears as a viable dark matter candidate. In the following, it will be assumed that R-parity is conserved and that the LSP is the lightest neutralinoχ is considered, and it is assumed that t → cχ 0 1 , a flavor-changing loop decay [7], is the only relevant decay mode, i.e., that the tree-level four-body decays [8] t → bff χ 0 1 can be neglected. In pp collisions, stop pair production proceeds via qq annihilation and gluon-gluon fusion. The cross section has very little dependence on SUSY parameters other than the stop mass. At the center-of-mass energy of 1.96 TeV available in Run II of the Fermilab Tevatron collider, it ranges from 15 to 2.25 pb for stop masses from 100 to 140 GeV, as calculated at nextto-leading order (NLO) with PROSPINO [9], for equal renormalization and factorization scales μ rf = mt and using the CTEQ6.1M parton distribution functions (PDFs) [10]. The final state topology resulting from thet → cχ 0 1 decay is a pair of acoplanar jets, with large missing transverse energy / E T carried away by the two weakly interacting LSPs. Previous searches in this...