Shifting Standards: Experiments in Particle Physics in the Twentieth Century

Chapter 14 “New Measurements of Properties of the Ω− Hyperon”

• 157 In 1964 Barnes et al. (1964) reported the observation of a hyperon with strangeness minus three, the Ω− particle.1 The existence of this particle, and its mass, had been predicted theoretically and, as the experimenters remarked, “consequently, the existence of the Ω− has been cited as a crucial test of the theory of unitary symmetry of strong interactions. The mass is predicted by the Gell-Mann-Okubo mass formula to be about 1680 MeV/ c2” (Barnes et al. 1964, 204). This was the mass they observed, and the experimenters felt that, “in view of the properties of charge (Q = −1), strangeness (S = −3), and mass (M = 1686 ± 12 MeV/c2) established for particle 3, we feel justified in identifying it with the sought for Ω−” (206). The experiment by Luk et al. (1988), the subject of this chapter, was designed to provide a measurement of the lifetime of the Ω− and also to measure various polarization parameters of the Ω− decay. In this experiment selectivity was applied not only to the entire data set to find a sample that included Ω− candidates but also to that selected sample in order to separate the Ω− particles from the far more numerous Ξ− particles, which were produced at the same time and whose decays, as observed in the apparatus, resembled those of the Ω−. The plan and elevation views of the experimental apparatus are shown in figure 14.1. The apparatus was designed to collect samples of both Ω− and Ξ− particles through the decay processes (14.1) Ω− → Λ + K−, followed by Λ → p + π−, and (14.2) Ξ− → Λ + π−, followed by Λ → p + π−. Now, “because the proton is the most massive of the final-state particles, it usually carries most of the momentum of the parent hyperon. Thus, each of these decay sequences has the distinctive property of having a high-momentum positive particle emerging from a negatively charged beam” (Luk et al. 1988, 20). The final state would also contain two negatively charged particles (figure 14.1). CHAPTER 14 “New Measurements of Properties of the Ω− Hyperon” 158 • “New Measurements of Properties of the Ω– Hyperon” The hyperons were produced by a beam of 400 GeV protons striking a beryllium target and a negatively charged beam was selected by a combination of collimators and two magnets, M1 and M2. Data were taken with the central momentum of the beam equal to either 196 GeV/c or 154 GeV/c. The negatively charged particles decayed in a 12.5-m-long decay volume, of which 8.5 m was in vacuum. The decay volume was followed by a spectrometer, which consisted of a magnet M3 and eight multiwire proportional chambers (MWPC’s) (C1–C8) to measure the momenta of the decay particles: “Prompt signals were available from the MWPC’s for use in the trigger electronics. In particular, C7 and C8 (downstream of the spectrometer magnet) had separate signals from the left and right halves to distinguish particles of negative (subscript N) and positive (subscript P) electrical charges, respectively” (20). The apparatus also contained three scintillation counters: S1, to signal a particle in the beam channel; S2, which contained a hole for the beam and served to eliminate particles in the beam halo; and S3 which was placed downstream of C8 and detected high-momentum particles of either electric charge, as “it [S3] was struck Figure 14.1. Plan (a) and elevation (b) views of the spectrometer with a typical event topology shown. From Luk et al. (1988). “New Measurements of Properties of the Ω– Hyperon” • 159 by beam particles, e. g. π−, which did not decay, and the high-momentum decay products, e. g. protons, π− and K−” (20). The event trigger consisted of a coincidence between S1·S̄2·S3·C7N C7P·C8N, and “this trigger selected events with at least one positive and one negative particle. . . . The geometry of S3 favored events where the positive particle had reasonably high momentum. This signature was appropriate for both Ξ− and Ω− hyperons” (20; emphasis added).2 The experiment had a total of 6,250,000 triggers. The next problem was to select those events with a topology consistent with either a Ω− or Ξ− decay from this sample. As one can see from equations (14.1) and (14.2) above, these events would have the same topology, a negatively charged track and a neutral V decay that pointed back to the decay vertex. The difference was that the Ω− would decay into a Λ and a K−, whereas the Ξ− would decay into a Λ and...