Observation of a first $\nu_\tau$ candidate in the OPERA experiment in the CNGS beam

TL;DR

The OPERA experiment reported the first candidate event for tau neutrino appearance from muon neutrino oscillations over a 730 km baseline, providing direct evidence of neutrino flavor change.

Contribution

This paper presents the first observation of a tau neutrino candidate in a long-baseline experiment, demonstrating direct detection of neutrino oscillations in appearance mode.

Findings

First tau neutrino candidate observed in CNGS beam

Detailed analysis confirms the event's consistency with tau neutrino interaction

Background estimates support the significance of the candidate event.

Abstract

The OPERA neutrino detector in the underground Gran Sasso Laboratory (LNGS) has been designed to perform the first detection of neutrino oscillations in direct appearance mode through the study of the $\nu_\mu\rightarrow\nu_\tau$ channel. The hybrid apparatus consists of an emulsion/lead target complemented by electronic detectors and it is placed in the high energy long-baseline CERN to LNGS beam (CNGS) 730 km away from the neutrino source. Runs with CNGS neutrinos were successfully carried out in 2008 and 2009. After a brief description of the beam, the experimental setup and the procedures used for the analysis of the neutrino events, we describe the topology and kinematics of a first candidate $\nu_\tau$ charged-current event satisfying the kinematical selection criteria. The background calculations and their cross-check are explained in detail and the significance of the event is assessed.