Evidence for ${\nu }_{\mu }\to {\nu }_{\tau }$ appearance in the CNGS neutrino beam with the OPERA experiment

The OPERA experiment is designed to search for ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations in appearance mode, i.e., through the direct observation of the $\tau$ lepton in ${\nu }_{\tau }$-charged current interactions. The experiment has taken data for five years, since 2008, with the CERN Neutrino to Gran Sasso beam. Previously, two ${\nu }_{\tau }$ candidates with a $\tau$ decaying into hadrons were observed in a subsample of data of the 2008–2011 runs. Here we report the observation of a third ${\nu }_{\tau }$ candidate in the ${\tau }^{-}\to {\mu }^{-}$ decay channel coming from the analysis of a subsample of the 2012 run. Taking into account the estimated background, the absence of ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations is excluded at the $3.4\sigma$ level.

Figure 1

Electronic detector display of the new ${\nu }_{\tau }$ candidate event. The blue solid lines represent the linear extrapolation of tracks measured in the emulsion films of the vertex brick. The dashed blue lines show the fit of the most downstream hits according to the model: $x(z)=p_{0x}+p_{1x}(z-z_0)+p_{2x}(z-z_0{)}^2$ with $z_0=-267.826\mathrm{cm}$. The quadratic term parameter is $p_{2x}=(-0.00389\pm 0.00069){\mathrm{cm}}^{-1}$, and the fit ${\chi }^2/ndf$ is $2.6/4$.

Figure 2

Distributions of $p_{2x}$ for a Monte Carlo sample of ${\mu }^{-}$ and ${\mu }^{+}$. The vertical solid line represents the measured value, and the vertical band corresponds to the $1\sigma$ confidence interval. The visible structures are due to the pitch of the RPC readout which introduces a discretization effect.

Figure 3

Display of the new ${\nu }_{\tau }$ candidate event in the $xz$ projection: tracks $\tau$ and $p_0$ come from the primary vertex; the $\tau$ candidate decays in the plastic base of film 39, and track $d_1$ is the $\tau$ decay daughter identified as a muon. The starting point of the shower generated from the $\gamma$ is visible in film 41. The inset contains a zoomed view of the primary and decay vertex region.

Figure 4

The $\tau$ lepton direction (dashed line) and the momenta of the other primary particles ($p_0$ and $\gamma$ ray) in the plane transverse to the CNGS beam. The blue arrow is the sum of the transverse momenta of the $p_0$ track and of the $\gamma$ ray.

Figure 5

Monte Carlo distribution of the reconstructed kinematical variables (see the text) for the $\tau \to \mu$ decay channel. Red lines show the measured values and red bands their uncertainty. Grey areas cover the regions excluded by the selection cuts.