Absolute momentum calibration of the HARP TPC

In the HARPexperiment the large-angle spectrometer is using a cylindrical TPC as main tracking and particle identificationdetector. The momentumscale of reconstructed tracks in the TPC is the most important systematic error for the majority of kinematic bins used for the HARPmeasurements of the double-differential production cross-section of charged pions in proton interactions on nuclear targets at large angle. The HARPTPC operated with a number of hardware shortfalls and operational mistakes. Thus it was important to control and characterize its momentumcalibration. While it was not possible to enter a direct particle beaminto the sensitive volume of the TPC to calibrate the detector, a set of physical processes and detector properties were exploited to achieve a precise calibration of the apparatus. In the following we recall the main issues concerning the momentummeasurement in the HARPTPC, and describe the crosschecks made to validate the momentumscale. As a conclusion, this analysis demonstrates that the measurement of momentumis correct within the published precision of 3%.