Discovery of $\tau$ Neutrino Appearance in the CNGS Neutrino Beam with the OPERA Experiment

The OPERA experiment was designed to search for ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations in appearance mode, i.e., by detecting the $\tau$ leptons produced in charged current ${\nu }_{\tau }$ interactions. The experiment took data from 2008 to 2012 in the CERN Neutrinos to Gran Sasso beam. The observation of the ${\nu }_{\mu }\to {\nu }_{\tau }$ appearance, achieved with four candidate events in a subsample of the data, was previously reported. In this Letter, a fifth ${\nu }_{\tau }$ candidate event, found in an enlarged data sample, is described. Together with a further reduction of the expected background, the candidate events detected so far allow us to assess the discovery of ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations in appearance mode with a significance larger than $5\sigma$.

Figure 1

Display of the ${\nu }_{\tau }$ candidate event as seen by the electronic detectors in the $x\text{−}z$ projection (top panel) and $y\text{−}z$ projection (bottom panel). The OPERA (right-handed) reference frame is oriented such that the $y$ axis is perpendicular to the hall floor and pointing up; the $z$ axis is orthogonal to the brick walls and is oriented as the incoming neutrinos. The angle between the neutrino direction and the $z$ axis projected into the $yz$ plane is 58 mrad. The brick containing the neutrino interaction is highlighted in magenta. The solid line shows the direction of the primary track P1 (see the text) at its most upstream point as reconstructed in the emulsion detectors.

Figure 2

Event display of the fifth ${\nu }_{\tau }$ candidate event in the horizontal projection longitudinal to the neutrino direction. The primary and secondary vertices are indicated as $V_0$ and $V_1$, respectively. The black stubs represent the track segments as measured in the films.

Figure 3

Monte Carlo distributions of the kinematical variables for ${\nu }_{\tau }$ events passing all the location and decay search chain with $\tau \to 1h$ decay topology. Red lines show the measured values for the candidate event and the corresponding errors. Grey areas show the regions excluded by the selection criteria.