Particle tracking and energy loss measurements with the LCTPC : A comparison to simulation models

The large prototype TPC (LCTPC), instrumented with GEM readout and front-end electronics based on the ALICE ALTRO chip was tested at the DESY T24 electron beam. Data was collected with and without an external magnetic field of 1T, and in both the standard configuration where the tracks are along the long edge (5.2 mm) of the pads and in a perpendicular configuration where the tracks are along the short edge (1.2 mm) of the pads. For electrons with momentum p = 5 GeV/c the straggling function (charge distribution), the specific energy loss (dE/dx), and the spatial resolution was extracted and compared to simulations based on GEANT4 energy loss calculations. The specific energy loss resolution was in general found to be better in the simulation than in the data, due to a narrower straggling function, raising questions about the energy loss modelling of shell effects. The spatial resolution was found to be well described by the simulations when there was no magnetic field. For magnetic field runs the large E × B effect due to inhomogeneities in the electric field, caused by the support structure of the GEMs, is found to deteriorate the resolution from 70 μm to 100 μm even when an average correction is applied. This suggests that a redesign of the GEMs would significantly improve the spatial resolution.