  We study methods of extracting new physics signals in final states with a
top-quark pair plus large missing energy at the LHC. We consider two typical
examples of such new physics: pair production of a fermionic top partner (a
$T'$ in Little Higgs models for example) and of a scalar top partner (a
$\tilde{t}$ in SUSY). With a commonly-adopted discrete symmetry under which non
Standard Model particles are odd, the top partner is assumed to decay
predominantly to a top quark plus a massive neutral stable particle $A^0$. We
focus on the case in which one of the top quarks decays leptonically and the
other decays hadronically, $pp \to {\tt} A^0A^0 X \to bj_1j_2 \bar b\ell^-
\bar\nu A^0A^0 X + c.c.$, where the $A^0$s escape detection. We identify a key
parameter for the signal observation: the mass splitting between the top
partner and the missing particle. We reconstruct a transverse mass for the
lepton-missing transverse energy system to separate the real $W$ background
from the signal and propose a definition for the reconstructed top quark mass
that allows it to take unphysical values as an indication of new physics. We
perform a scan over the two masses to map out the discovery reach at the LHC in
this channel. We also comment on the possibility of distinguishing between
scalar and fermionic top partners using collider signatures.

High Energy Physics - Phenomenology