Abstract
In the HARP experiment the large-angle spectrometer is using a cylindrical TPC as main tracking and particle identification detector. The momentum scale of reconstructed tracks in the TPC is the most important systematic error for the majority of kinematic bins used for the HARP measurements of the double-differential production cross-section of charged pions in proton interactions on nuclear targets at large angle. The HARP TPC operated with a number of hardware shortfalls and operational mistakes. Thus it was important to control and characterize its momentum calibration. While it was not possible to enter a direct particle beam into 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 momentum measurement in the HARP TPC, and describe the cross-checks made to validate the momentum scale. As a conclusion, this analysis demonstrates that the measurement of momentum is correct within the published precision of 3%.
Highlights
The HARP experiment makes use of a large-acceptance spectrometer consisting of a forward and large-angle detection system
The forward spectrometer — based on large area drift chambers [6] and a dipole magnet complemented by a set of detectors for particle identification (PID): a time-offlight wall [7] (TOFW), a large Cherenkov detector (CHE) and an electromagnetic calorimeter — covers polar angles up to 250 mrad which is well matched to the angular range of interest for the measurement of hadron production to calculate the properties of conventional neutrino beams
Asserting the correctness of the momentum reconstruction in the HARP Time Projection Chamber (TPC) has not been easy, as can be expected from a chamber affected by a large number of dead channels, cross-talk, static and dynamic distortions in the absence of the possibility to use a direct particle beam for calibration
Summary
The HARP TPC was designed and built in a record time of about 1.5 years. Its main design features are an almost full solid angle acceptance and high-event rate capabilities. We recall here that in large angle cross section results published so far [8, 9], only the first part of the spill (about 30% of the total events), where the dynamic distortions are negligible were used (as discussed in Appendix A the distortions can be monitored by a physical parameter named d◦′ ). This provides very little penalty in measuring cross sections because already with this statistics systematic errors dominate in most of kinematic bins [8, 9]. In the absence of an appropriate calibration system and without the possibility of exposing the TPC to test-beams, a wide range of experimental crosschecks has been employed to assess the momentum scale in the HARP TPC, as described in the following
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