  The top squark (stop) may dominantly decay to a bottom quark and a chargino
if the mass difference between the stop and the lightest neutralino is
comparable or less than the top quark mass. Such a moderately compressed
spectrum is a challenging scenario for the stop search at the Large Hadron
Collider, because it is difficult to separate the signals from the top and
anti-top background. In this paper we focus on the di-leptonic decay channel,
and consider many kinematic variables as possible discriminators. These include
several MT2 variables and new "compatible-masses" variables which fully utilize
all kinematic information of the background. We use several sample spectra with
different characteristics to study the efficiencies of these variables in
distinguishing the signal from the background. The finding is that different
combinations of variables or strategies should be used for different spectra to
maximally enhance the signal significance and expand the reach of the stop
search in this scenario. The new variables that we proposed in this paper are
also useful for other new physics searches with di-leptonic top and anti-top
events as the dominant background.
