  In supersymmetric extensions of the Standard Model, the superpartners of the
top quark (stops) play the crucial role in addressing the naturalness problem.
For direct pair-production of stops with each stop decaying into a top quark
plus the lightest neutralino, the standard stop searches have difficulty
finding the stop for a compressed spectrum where the mass difference between
the stop and the lightest neutralino is close to the top quark mass, because
the events look too similar to the large $t\bar{t}$ background. With an
additional hard ISR jet, the two neutralinos from the stop decays are boosted
in the opposite direction and they can give rise to some missing transverse
energy. This may be used to distinguish the stop decays from the backgrounds.
In this paper we study the semileptonic decay of such signal events for the
compressed mass spectrum. Although the neutrino from the $W$ decay also
produces some missing transverse energy, its momentum can be reconstructed from
the kinematic assumptions and mass-shell conditions. It can then be subtracted
from the total missing transverse momentum to obtain the neutralino
contribution. Because it suffers from less backgrounds, we show that the
semileptonic decay channel has a better discovery reach than the fully hadronic
decay channel along the compressed line $m_{\tilde{t}} -
m_{\tilde{\chi}}\approx m_t$. With 300 $\text{fb}^{-1}$, the 13 TeV LHC can
discover the stop up to 500 GeV, covering the most natural parameter space
region.
