Shifting Standards: Experiments in Particle Physics in the Twentieth Century

Chapter 13 “Measurement of the Antineutron-Proton Cross Section at Low Energy”

• 149 The purpose of Gunderson et al.’s (1981)1 experiment was to investigate the nucleon-antinucleon interaction at low energy to see if there were resonances or bound states near threshold, which was, at the time, an open question: “A strong motivation for the study of the N̄-N [nucleon-antinucleon ] cross section at low energy is the possible existence of resonance and bound states near threshold” (587). As the authors noted, “despite considerable experimental effort, the question of the range of the annihilation interaction at low momentum remains open” (587). The authors remarked that there were difficulties associated with the use of antiprotons in such investigations. These included the energy loss caused by ionization, which limited the path length available at a given energy, and also the need to use a deuterium target to separate the isospin = 0 and the isospin = 1 contributions.2 The authors concluded that “both of these difficulties can be circumvented by the use of antineutrons as projectiles ” (587). There were, however, also technical problems associated with using antineutrons. Charged-particle beams can be manipulated by using electric and magnetic fields to select particles with a particular charge, mass, and momentum. Because neutrons and antineutrons are electrically neutral, such techniques are not available for beams of those particles. One needs a way of tagging the antineutrons and determining their energy. The experimenters stated that antineutrons could be obtained from either proton-nucleus collisions or from a separated antiproton beam by charge exchange (p̄p → n̄n). They chose to use the latter interaction: “In the case of charge exchange, the n̄ momentum can be determined from the geometry of the charge-exchange interaction, . . . [and] at low energy the n̄ momentum can be accurately determined by measurement of the time of flight” (587).3 Figure 13. 1 is “a schematic representation of the apparatus showing the CHAPTER 13 “Measurement of the Antineutron-Proton Cross Section at Low Energy” 150 • The Antineutron-Proton Cross Section beam-defining counters, charge-exchange target, transmission target, and calorimeter” (587). A second figure presented a schematic block diagram of the signal-processing electronics. The incoming beam was a separated, negative beam with a momentum of 1 GeV/c. It consisted primarily of pions and muons, with a small percentage of antiprotons: “Counters T1, T2, and T3 defined the incident antiproton beam, with T2 serving as a reference for all time-of-flight measurements” (587). The beam telescope also included a water Cerenkov counter veto, which served to reduce the number of pions and muons, which are less massive than the antiproton and thus have a higher velocity in a momentum-selected beam.4 The time-of-flight spectrum of beam particles is shown in figure 13.2.5 The beam contained approximately 100 times more pions and muons than antiprotons but, as shown in the figure, the use of the water Cerenkov counter veto reversed this ratio: “The time difference between p̄’s [antiprotons] and the lighter particles, together with the water Cherenkov counter in veto, enabled us to reduce the light-particle contamination of the antiproton signal in the trigger to ≤ 2%. A further selection on the time of flight at the analysis stage reduced the contamination to a negligible level” (588). As shown in figure 13.2, a time cut on the beam particles also helped to identify the antiprotons . An incoming antiproton was signaled by a T1T2T3 coincidence with no count in the Cerenkov counter and an appropriate time of flight. The charge-exchange region, in which the charge-exchange reaction Figure 13.1. Schematic view of the experiment to measure the antineutron-proton cross section. From Gunderson et al. (1981). The Antineutron-Proton Cross Section • 151 occurred, consisted of a copper degrader to reduce the antiproton energy, counters S1, S2, and S3, and polyethylene charge-exchange material. It was surrounded by an anticounter, and “a p̄ signal unaccompanied by a signal in the anticounter defined an n̄ in flight” (588). The anticounter eliminated those events in which proton-antiproton annihilation produced charged particles. There was an additional anticounter AA, behind the transmission target, to shield the calorimeter from particles produced by other antiproton -proton annihilations. This anticounter was not part of the event trigger but the time of the track in AA was recorded, and this allowed the elimination of events that occurred during the time of flight of the antineutron . The three counters...