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Poster excerpt 10: A New Method for the Energy Measurement of Electromagnetic Showers for the Detection of the tau->e Decay in the Opera Search

March 3, 2015

(The following text has been submitted by Behzad Hosseini on behalf of the Opera collaboration)

Introduction

The OPERA experiment is a long baseline neutrino oscillation experiment that was designed to provide a conclusive proof of the νμ → ντ oscillations hypothesis. Having observed 4 ντ candidates with a background of 0.23 events, νμ → ντ oscillations are established at 4.2σ level.

The electromagnetic shower reconstruction is mandatory for the
τ detection in the τ → e decay channel. A new algorithm has been developed to evaluate the energy of electrons through the identification of the associated shower in the OPERA bricks.

By exploiting the electron identification capability, OPERA is able to perform also a nm → ne oscillation search. In the CNGS beam the expected ne contamination is small compared to the dominant nm component (ne/ νμ = 0.8%). The search for the νμ  → ne oscillations is made by seeking for an excess of ne charged current events.

Energy Estimation

The ECC brick is the basic unit of the OPERA target. Each brick consists of 57 emulsion films interleaved by 56 lead plates, packed together in a box with transverse dimensions of 10.2 × 12.7 cm2 and thickness of ∼ 7.5 cm (corresponding to about 10 X0) and a total weight of 8.3 kg.

The standard reconstruction of an electromagnetic shower can be applied only to those fully contained in one brick. We present a new algorithm aimed to analyze showers involving one or two bricks along the beam direction.

hosseini1

Figure 1: Electromagnetic shower reconstruction within the brick.

The energy of showering electrons is determined by its correlation with the number of reconstructed base-tracks in the bricks’ emulsion foils. This number does not depend only on the electron’s energy but it’s also affected by the position in the brick of the decay point of τ lepton. Therefore, it is needed to divide the ECC brick in different zones.

Showers initiated in the upstream 21 plates are fully reconstructed within that brick. However, for showers initiated in the downstream emulsion foils (22nd to 57th), the full shower reconstruction has to extend to two bricks.

As an application of the algorithm to data, an electromagnetic shower associated to a νe interaction was studied. The event 9197043461, occurred on 16 July 2009, was located in the Napoli scanning lab long ago. Its electromagnetic shower had been studied in detail.

The primary vertex of this event is formed by three particles: an electron, a hadron and a π0, see Figure 2. Since the electron track is attached to the primary vertex, the event was classified as a νe interaction.

hosseini2

Figure 2: Vertex display of the electron neutrino interaction.

The energy of the shower estimated with the new algorithm is: EREC = 17.04 ± 0.21 GeV, which is in agreement with the measurement obtained with the standard algorithm. Moreover the new method would allow to increase by about 50% the reconstructed electron shower data set.

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