SEARCH FOR DIRECT TOP SQUARK PAIR PRODUCTION IN FINAL STATES WITH TWO LEPTONS WITH ATLAS IN SQRT(S) = 7 TEV AND SQRT(S) = 8 TEV PP COLLISIONS

Two searches for direct top squark production in final states with two leptons (electrons or muons) are presented, analyzing data collected by the ATLAS experiment at the Large Hadron Collider. The first search targets the $\tilde t_1 \rightarrow t \tilde \chi^0_1$ decay using 4.7 fb$^{-1}$ of pp collision data at $\sqrt{s} = 7$ TeV. No excess over Standard Model expectation is found and limits at 95\% CL have been derived considering either spin-1/2 heavy top-quark partners or top squarks. A spin-1/2 top-quark partner mass between 300 GeV and 480 GeV is excluded for a heavy neutral particle mass below 100 GeV. A top squark with a mass of 300 GeV decaying to a top quark and a massless neutralino is also excluded. The second search targets the $\tilde t_1 \rightarrow b \tilde \chi^\pm_1$ decay, or the three body decay $\tilde t_1 \rightarrow b W \tilde \chi^0_1$ via an off-shell top quark, using 20.3 fb$^{-1}$ of pp collision data at $\sqrt{s} = 8$ TeV. The observed events has been found to be consistent with the Standard Model expectation and limits at 95\% CL have been derived in different families of signal models. For the case of a massless neutralino, a top squark with a mass between 150 and 435~GeV decaying to a $b$-quark and a chargino is excluded for a chargino approximately degenerate with the top squark. For a 300~GeV top squark decaying to a $b$-quark and a chargino, chargino masses between 150 and 290~GeV are excluded for a lightest neutralino with mass below 50~GeV. Limits are also set on the direct 3-body decay mode, $\tilde{t}_1\rightarrow W\tilde{\chi}^0_1 b$, excluding a top squark mass between 90 and 170~GeV, under the assumption of a massless neutralino.