OPERA tau neutrino candidate event 12227007334

Its vertex consisted of two charged tracks, one of which exhibits a kink topology with ${\theta_{kink}=(90\pm 2)}$~mrad, after a flight length of ${(960 \pm 30)}$~${\mu}$m. The distance of closest approach between the ${\tau}$ candidate and the other primary track is 0.1~${\mu}$m.</p><p>In addition to these two tracks, four forward-going and two backward-going nuclear fragments pointing to the primary vertex were observed.</p><p>The ${\tau}$ daughter interacted after having crossed 22 plates, producing four charged particles and a photon, and it could thus be unambiguously identified as a hadron.

Its measured momentum is ${11.0^{+13.9}_{-3.9}}$~GeV/c, while its impact parameter w.r.t.~the primary vertex is ${(83 \pm 5)}$~${\mu}$m.</p><p>The second primary track measured momentum is ${(1.1\pm 0.2)}$~GeV/c and it was classified as a proton from its energy loss in the emulsion target. The linear density of grains along the track left by a particle, indeed, is correlated with its energy loss. The ratio between the grain density of this track and that of the ${\tau}$ daughter track is ${(1.45 \pm 0.06)}$, to be compared with ${(1.38 \pm 0.14)}$ expected for a proton/m.i.p.~ratio.

Therefore, the second primary track was likely produced by a proton.</p><p>This event was thus interpreted as a $\nu_{\tau}$ charged-current interaction with the $z\tau$ lepton decaying into a single hadron. A detailed description of the event is given in <a href="doi.org/10.1103/PhysRevLett.115.121802">10.1103/PhysRevLett.115.121802</a>.</p>