The HARP Time Projection Chamber: Characteristics and physics performance

Abstract

The HARP spectrometer that took data at the CERN Proton Synchrotron in 2001 and 2002 had as large-angle detector system a Time Projection Chamber (TPC) surrounded by Resistive Plate Chambers. The design of the TPC, experience with its operation, and its good physics performance are described. The successful recovery from track distortions arising from inhomogeneities of the electric and magnetic fields in the TPC volume is discussed.A. Bolshakova INTRODUCTIONThe HARP experiment arose from the realization that the differential cross-sections of hadron production in the collisions of low-momentum protons with nuclei were known only within a factor of two to three. Consequently, the HARP spectrometer was designed to carry out a programme of systematic and precise measurements of hadron production by protons and pions with momenta from 1.5 to 15 GeV/c. The experiment was performed at the CERN Proton Synchrotron in 2001 and 2002 with a set of targets ranging from hydrogen to lead. With a view to achieving nearly 4π acceptance, the HARP detector combined a large-angle spectrometer with a forward spectrometer. The latter consisted of a dipole magnet and drift chambers for momentum measurement, and a threshold Cherenkov counter, a time-of-flight wall, and an electromagnetic calorimeter for particle identification. The large-angle spectrometer comprised a cylindrical Time Projection Chamber (TPC) and an array of Resistive Plate Chambers (RPCs) around and behind the TPC. The purpose of the TPC was track reconstruction and particle identification by dE/dx. The purpose of the RPCs was to complement the particle identification by dE/dx by time of flight, especially to distinguish between electrons and pions in the momentum range 150-250 MeV/c where the specific ionization of electrons and pions is too close for their separation.This paper describes the HARP TPC, the experience with its operation, and its physics performance.The TPC had to be designed, constructed and commissioned within a brief period of 17 months. Under this time pressure a few per se minor mishaps occurred which, however, had quite some consequences for the quality of the raw data. Part of this paper is dedicated to the-so we hope-interesting discovery of, and the recovery from, these errors. This paper also sets the record straight with respect to the TPC performance reported in Refs.[1] and [2]. It justifies and details our criticism thereof [3,4]. PERFORMANCE: OBJECTIVES AND RESULTSThe experiment was carried out in the T9 beam of the CERN Proton Synchrotron that had a maximum momentum of p = 15 GeV/c. Thus in the large-angle region, the detector was to handle comfortably transverse momenta up to p T ∼ 2 GeV/c. A, say, 2σ separation (2.3% misidentification probability) between positive and negative charges at p T = 2 GeV/c called for a resolution σ(1/p T ) not worse than about 0.25 (GeV/c) −1 . Table 1 summarizes the key parameters of the TPC, and compares objectives with the achieved results. In later sections, the table entries will be discuss...

Cross-sections of large-angle hadron production in proton– and pion–nucleus interactions IV: copper nuclei and beam momenta from ±3 GeV/c to ±15 GeV/c

“…Our calibration work on the HARP TPC and RPCs is described in detail in [4] and [5], and in references cited therein. In particular, we recall that static and dynamic TPC track distortions up to 10 mm have been corrected to better than 300 µm.…”

“…Another contribution comes from the removal of events with an abnormally large number of TPC hits. 4 The systematic error of the track finding efficiency is taken as 1% which reflects differences between results from different persons who conducted eyeball scans. We also take the statistical errors of the parameters of a fit to scan results as systematic error into account [1].…”

“…The systematic error of the p T scale is taken as 2% as discussed in [4]. For the data from the +15 GeV/c beams, this error was doubled to account for a larger than usual uncertainty of the correction for dynamic TPC track distortions.…”

“…We stress that errors on abundances only lead to cross-section errors in case of a strong overlap of the resolution functions of both identification variables, dE/dx and β. The systematic error of the correction for migration, absorption of secondary protons and pions in materials, and for pion decay into muons, is taken as 20% of the correction, or 1% of the cross-section, whichever 4 In less than 0.5% of the number of good events, because of apparatus malfunction, the number of TPC hits was much larger than possible for a physics event. Such events were considered unphysical and eliminated.…”

“…Our work involves only the HARP large-angle spectrometer, the characteristics of which are described in detail in [4] and [5].…”