OPERA tau neutrino candidate event 10123059807

One backward track was found in films 18-19 and a black segment in film 20. No $\gamma$ was detected. All tracks were identified as hadrons using their overall track length reconstructed along their path, the topology at their end-point and the correlation between their momentum and range.</p><p>It turned out that the 7 tracks did not originate from the same vertex because their impact parameters exceed the values expected according to the resolution: therefore, a particle decay must have occurred over a short flight length since it happened in the same lead plate.

Looking at the impact parameters of the tracks, two hypotheses were made on the event topology: a primary vertex with 4 tracks and a trident decay or a primary vertex composed by 3 tracks and a secondary vertex with 4 tracks attached. Other hypotheses were discarded given their probability lower than $0.01\%$. The most probable hypothesis is the first one, with a vertex topological probability of $72.3\%$, according to the impact parameters, and corresponds to a $\nu_{\tau}$ charged-current interaction with the $\tau$ lepton decaying into three hadrons.

In this configuration, the kink angle, evaluated as the mean kink angle for each daughter track, is 231~mrad. The hypothesis of a short decay of a heavy particle is supported also by the invariant mass measurement done with the three daughter tracks, 1.2~GeV, assuming the ${\pi}$ mass for all daughters.</p><p>The rejected hypothesis could be explained only by a $\nu_\mu$ charged-current interaction with charmed hadron production that would require the presence of a muon at the primary vertex, strongly suppressed by the hadron identification of the tracks.</p><p>The neutrino energy was measured to be $16.4$~GeV/c.

This event was thus interpreted as a $\nu_{\tau}$ charged-current interaction with the $\tau$ lepton decaying into three hadrons.</p>