ALICE experience with GEANT4

Since its release in 1999, the LHC experiments have been evaluating Geant4 in view of adopting it as a replacement for the obsolescent Geant3 transport MonteCarlo. The ALICE collaboration has decided to perform a detailed physics validation of elementary hadronic processes against experimental data already used in international benchmarks. In one test, proton interactions on different nuclear targets have been simulated, and the distribution of outgoing particles has been compared to data. In a second test, penetration of quasi-monoenergetic low energy neutrons through a thick shielding has been simulated and again compared to experimental data. In parallel, an effort has been put on the integration of Geant4 in the AliRoot framework. An overview of the present status of ALICE Geant4 simulation and the remaining problems will be presented. This document will describe in detail the results of these tests, together with the improvements that the Geant4 team has made to the program as a result of the feedback received from the ALICE collaboration. We will also describe the remaining problems that have been communicated to Geant4 but not yet addressed.