Charged Pion Energy Reconstruction in the ATLAS Barrel Calorimeter

The intrinsic performance of the ATLAS barrel and extended barrel calorimeters for the measurement of charged pions is presented. Pion energy scans (E = 20, 50, 200, 400 and 1000 GeV) at two pseudo-rapidity points (η = 0.3 and 1.3) and pseudorapidity scans (−0.2 < η < 1.8) with pions of constant transverse energy (ET = 20 and 50 GeV) are analysed. A simple approach, that accounts in first order for non-compensation and dead material effects, is used for the pion energy reconstruction. The intrinsic performances of the calorimeter are studied: resolution, linearity, effect of dead material, tails in the energy distribution. The effect of electronic noise, cell energy cuts and restricted cone size are investigated. 1 ATLAS Calorimetry A view of the ATLAS calorimeters [1] is presented in Figure 1. The calorimetry consists of an electromagnetic (EM) calorimeter covering the pseudorapidity region |η| < 3.2, a hadronic barrel calorimeter covering |η| < 1.7, hadronic end-cap calorimeters covering 1.5 < |η| < 3.2, and forward calorimeters covering 3.1 < |η| < 4.9. The EM calorimeter is a lead/liquid-argon (LAr) detector with accordion geometry [2]. Over the pseudorapidity range |η| < 1.8, it is preceded by a presampler detector, installed immediately behind the cryostat cold wall, and used to correct for the energy lost in the material upstream of the calorimeter. The hadronic calorimetry of ATLAS, presented in Figure 1, consists of three main devices. In the barrel region (|η| < 1.7) there is the scintillating Tile Calorimeter [3]. The Hadronic End-cap LAr Calorimeter (HEC) extends up to |η| = 3.2. The range 3.1 < |η| < 4.9 is covered by the high density Forward Calorimeter (FCAL). Up to |η| = 2.5 the basic granularity of the hadron calorimeters is ∆η × ∆φ = 0.1 × 0.1. This region is used for precise measurements of the energy and angles of jets. In the region |η| > 2.5, the basic granularity is approximately ∆η ×∆φ = 0.2× 0.2. A more detailed description of all ATLAS calorimeters is given in the Calorimeter TDRs ([1], [2] and [3]). The performance of the barrel and extended barrel sections of the ATLAS hadronic calorimeter for the measurement of charged pion energy is studied. The intrinsic energy resolution, the effects of dead material, electronic noise and limited cone size are discussed. 2 Energy Resolution In the barrel region, the response of the calorimeter was studied at two pseudorapidity values: η = 0.3 (central barrel) and η = 1.3 (extended barrel). First, the energy sampled in the different calorimeter compartments is converted to the total deposited energy using the electromagnetic energy scale (EM scale). The intrinsic performance of the calorimeter is studied: the energy considered is not restricted to a cone and electronic noise is not added. These effects are discussed later in Section 4. The algorithm to reconstruct the pion energy is similar to the ”Benchmark Method” used