Performance of the ATLAS Hadronic Endcap Calorimeter Modules to Electrons and Pions from 1999 Beam Test Data

During the summer of 1999, the first six production modules of the Hadronic Endcap Calorimeter were assembled and installed in a beam test cryostat at CERN. In this note, the performance of the calorimeter is assessed in terms of its response and resolution to electrons and pions. The calorimeter is evaluated at five impact points and over an energy range of 10 to 200 GeV. The linearity of the response to electrons is observed to be within approximately one percent, and the average electromagnetic scale constant is measured to be $\\alpha_{em}=(3.82\\pm0.04)$~GeV/nA.\\linebreak The intrinsic (i.e.\\ electronic noise subtracted) energy resolution obtained is $\\frac{\\sigma}{E}=\\frac{(23.29\\pm 0.09)\\%}{\\sqrt{E_{0}(\\mbox{\\scriptsize{GeV}})}} \\oplus (0.00\\pm 0.13)\\%$ for electrons, and $\\frac{\\sigma}{E} = \\frac{(76.2\\pm 09)\\%}{\\sqrt{E_{0}(\\mbox{\\scriptsize{GeV}})}} \\oplus (6.68\\pm 0.09)\\%$ for pions, where $E_0$ is the incident particle energy. Comparison with Monte Carlo simulations and the effect of the electronics calibration procedure are discussed. Finally, the ratio of electromagnetic to hadronic response, $e/h$, is measured to be $e/h=(1.509\\pm0.021)$.