OPERA tau neutrino candidate event 9234119599

A kink is a sudden change of the track direction, as it happens when a particle decays. Therefore, the kink is the topological evidence for a particle decay when it produces only a charged particle in the final state. All tracks were classified as hadrons using their overall track length reconstructed along all the downstream available bricks, the topology at their end-point and the correlation between their momentum and range.

One of these particles was identified as a proton and another one as a pion.</p><p>Two electromagnetic showers induced by ${\gamma}$-rays were located. The first one was originated from the secondary vertex, 2.2~mm downstream of it, and its energy was reconstructed to be 5.6~GeV. The probability that it pointed to the secondary vertex was estimated to be ${32\%}$, several orders of magnitude larger than any other interpretation.

The second $\gamma$ induced shower, with a reconstructed energy of 1.2~GeV, was compatible with pointing to the secondary vertex, with a probability of ${82\%}$, 8 times larger than any other interpretation. The reconstructed energy of the neutrino interaction is $24.3$~GeV/c.</p><p>The invariant mass of the two $\gamma$-rays is $(120\pm 20(\textit{stat.})\pm 35(\textit{syst.}))$~MeV/c$^2$, supporting the hypothesis that they originate from a $\pi^0$ decay.

Similarly the invariant mass of the charged decay product assumed to be a $\pi^-$ and of the two $\gamma$-rays amounts to $(640^{+125}_{-80}(stat.)^{+100}_{-90}(syst.))$~MeV/c$^2$, which is compatible with the $\rho$ particle mass. The $\rho$ particle has an average lifetime as short as $4 \times 10^{-24}$~s and it decays through strong interactions, without producing any visible displacement. The branching ratio of the decay mode $\tau\rightarrow\rho^{-}\nu_\tau$ is about 25$\%$.</p><p>This event was thus interpreted as a $\nu_{\tau}$ charged-current interaction with the $\tau$ lepton decaying into a $\rho^- \nu_{\tau}$ and the subsequent $\rho^- \rightarrow \pi^{-} \pi^0$ decay.

A detailed description of the event is given in <a href="doi.org/10.1016/j.physletb.2010.06.022">10.1016/j.physletb.2010.06.022</a>.</p>