Charged Pion Energy Reconstruction in the ATLAS Barrel Calorimeter

Intrinsic performance of the ATLAS calorimeters in the barrel region with respect to charged pions was studied. For this the following simulated data were used: pion energy scans ($E = 20, 50, 200, 400$ and $1000$ GeV) at two pseudo-rapidity points ($eta = 0.3$ and $1.3$) and pseudo-rapidity scans ($-0.2 < eta < 1.8$) with pions of constant transverse energy ($E_T = 20$ and $50$ GeV). For pion energy reconstruction the benchmark approach was used. Performance was estimated for cases, when energy and rapidity dependent and independent calibration parameters were applied. The best results were obtained with energy and rapidity dependent parameters. Studies done for pions enabled optimization of the cone size and of the cut to obtain the best energy resolution. Energy dependence of the resolution can be parameterized as: $(50pm4)%/sqrt{E} oplus (3.4pm0.3)% oplus 1.0/E$ at $eta = 0.3$ and $(68pm8)%/sqrt{E} oplus (3.0pm0.7)% oplus 1.5/E$ at $eta = 1.3$. Larger constant term at $eta=0.3$ can be explained by longitudinal leakage from calorimeters in this region. Resulted deviation from linearity in response is within $pm0.5$%. Resolution, obtained for pseudo-rapidity scans, can be described as: $(39pm1)%/sqrt{E} oplus (1pm5)%$ for $E_T = 20$ GeV and $(49pm9)%/sqrt{E} oplus (2pm6)%$ for $E_T = 50$ GeV, everywhere, except region around $eta = 1.5$, where resolution is spoiled by significant energy losses in dead materials. Distributions of response ($sigma/E_{beam}$) have 2.2% (1.1%) for $E_T = 20$ GeV ($50$ GeV). Analysis of tails of energy distribution was also presented.