  This Letter presents a search for a heavy neutral particle decaying into an
opposite-sign different-flavor dilepton pair, $e^\pm \mu^\mp$, $e^\pm
\tau^\mp$, or $\mu^\pm \tau^\mp$ using 20.3 fb$^{-1}$ of $pp$ collision data at
$\sqrt{s} = 8$ TeV collected by the ATLAS detector at the LHC. The numbers of
observed candidate events are compatible with the Standard Model expectations.
Limits are set on the cross section of new phenomena in two scenarios: the
production of $\tilde{\nu}_{\tau}$ in $R$-parity-violating supersymmetric
models and the production of a lepton-flavor-violating $Z'$ vector boson.
