Search for Tau Neutrinos in the τ → e Decay Channel in the OPERA Experiment

Abstract

The OPERA (Oscillation Project with Emulsion tRacking Apparatus) experiment is a long baseline neutrino oscillation experiment that was designed to perform a conclusive test of the νμ → ντ oscillations hypothesis. The main aim of this experiment is a direct observation of τ leptons in ντ charged-current interactions. A good electromagnetic shower reconstruction is important for the τ detection in the τ → e decay channel. So far, 4 ντ candidates have been observed in the OPERA detector with a background of 0.23 events. Given the number of analyzed events and the low background, νμ → ντ oscillations are established with a significance of 4.2 σ. Reconstruction of electromagnetic showers is one of the fundamental tools needed by neutrino experiments and the estimation of their energy is a crucial issue. This is also true for the OPERA experiment where we have developed and tested an algorithm, in order to evaluate the energy of electron through the identification of its shower for the τ → e decay channel as well as νe interactions by using the OPERA bricks. Normal reconstruction of a shower event depends upon the shower being contained in one brick. However, when a shower is initiated in the upstream portion of the brick, the shower will begin in the first brick, pass through the Electronic Detector (ED) and end in the second brick; making normal reconstruction methods unusable. Nearly half of the electromagnetic shower events detected take place across two bricks, meaning that previously, half of detected events had to be discarded. I have expanded my code in such a way to analyze showers with one or two brick distributions, allowing us to include 50 percent more events in our data sets. This approach is using for the first time the information of the downstream brick to reconstruct and estimate the energy of electromagnetic showers in the OPERA experiment.