  A search for the associated production of the Higgs boson with a top quark
pair is performed in multilepton final states using 20.3 fb$^{-1}$ of
proton-proton collision data recorded by the ATLAS experiment at $\sqrt{s}=8$
TeV at the Large Hadron Collider. Five final states, targeting the decays $H\to
WW^*$, $\tau\tau$, and $ZZ^*$, are examined for the presence of the Standard
Model (SM) Higgs boson: two same-charge light leptons ($e$ or $\mu$) without a
hadronically decaying $\tau$ lepton; three light leptons; two same-charge light
leptons with a hadronically decaying $\tau$ lepton; four light leptons; and one
light lepton and two hadronically decaying $\tau$ leptons. No significant
excess of events is observed above the background expectation. The best fit for
the $t\bar t H$ production cross section, assuming a Higgs boson mass of 125
GeV, is $2.1 ^{+1.4}_{-1.2}$ times the SM expectation, and the observed
(expected) upper limit at the 95% confidence level is 4.7 (2.4) times the SM
rate. The $p$-value for compatibility with the background-only hypothesis is
$1.8\sigma$; the expectation in the presence of a Standard Model signal is
$0.9\sigma$.
