An invariant mass method for the energy calibration ofneutral pions

Data collected in the 1990's from the ALEPH experiment at the LEP collider still has potential to provide new or improved precision measurements in particle physics. In the ALEPH data neutral pions suffer from poor calibration. In this study, an energy calibration based on a global mass constraint is applied to neutral pions in order to further improve the quality of ALEPH data. The calibration is obtained by modifying photon energies with a scale such that the mean reconstructed mass is equal to the nominal pion mass. Since the reconstructed mass of neutral pions is observed to run with energy, individual scales are determined for 50 energy intervals independently for both simulated and real data. The calibration is also performed independently for four π 0 topologies. We show that this procedure improves pion energy resolution and the agreement between simulated and real data. This calibration is expected to lead to improved statistical significance in measurements of particle production and help to reduce systematic errors in those analyses.