Search for a Heavy Neutral Particle Decaying to $e\mu$, $e\tau$, or $\mu \tau$ in $pp$ Collisions at $\sqrt{s}=8\mathrm{TeV}$ with the ATLAS Detector

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 $20.3{\mathrm{fb}}^{-1}$ of $pp$ collision data at $\sqrt{s}=8\mathrm{TeV}$ 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 $Z^{\prime }$ vector boson.

Figure 1

Observed and predicted $e\mu$, $e{\tau }_{\mathrm{had}}$, $\mu {\tau }_{\mathrm{had}}$ invariant mass distributions. The contributions of the different processes are also shown: “Others” includes diboson and single-top while “Jet fake” refers to $W+\text{jets}$ and multijet. All overflows are included in the rightmost bin. Signal simulations are shown for $m_{{\tilde{\nu }}_{\tau }}=1\mathrm{TeV}$ and $m_{Z^{\prime }}=0.75\mathrm{TeV}$. The couplings ${\lambda }_{311}^{\prime }=0.11$ and ${\lambda }_{i3k}=0.07$ ($Q_{\ell {\ell }^{\prime }}=1$) are used for the RPV ($Z^{\prime }$) model. The uncertainty bands include both the statistical and systematic uncertainties.

Figure 2

The 95% C.L. limits on cross section times branching ratio as a function of ${\tilde{\nu }}_{\tau }$ mass (top plots) and $Z^{\prime }$ mass (bottom plots) for $e\mu$ (left), $e\tau$ (middle), and $\mu \tau$ (right). Theory curves are for the arbitrary choice of couplings ${\lambda }_{311}^{\prime }=0.11$ and ${\lambda }_{i3k}=0.07$ for ${\tilde{\nu }}_{\tau }$ and $Q_{\ell {\ell }^{\prime }}=1$ for $Z^{\prime }$. The gray band around the theory curve represents the theoretical uncertainty from the PDFs and factorization and renormalization scales.