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98 • In Bowen et al. (1967) the analysis of the data was an exercise in selectivity.1 There was a large experimental background that masked or mimicked the desired signal. It was so large that the signal for the phenomenon under investigation could not initially be seen in the data. In order to isolate that signal, various selection criteria, or cuts, had to be applied to the data so that the Ke2 + branching ratio could be measured. One question that arises in such a case is whether the result obtained is an artifact of those selection criteria. Perhaps the simplest and most straightforward strategy used to argue for the correctness of a result when selection criteria are used in its production is to vary the values of those cuts. If the result remains constant under that variation, then this argues that it is not an artifact. If it is a real effect, then reasonable changes in the cuts used should not significantly affect the result. If the result changes substantially when the values are changed, then this suggests, although it does not definitively demonstrate, that the result is an artifact of the cuts. This strategy is clearly illustrated in this experiment designed to measure the Ke2 + branching ratio, the fraction of all K+ mesons that decayed into a positron and an electron neutrino (K+ → e+ + νe).2 The reasons for the experiment opened the paper: “A spark-chamber and counter experiment has been performed at the Princeton-Pennsylvania Accelerator (PPA) to measure the branching ration for K+ → e+ + ν (Ke2 + decay). The pure leptonic decays of the charged π and K mesons provide stringent tests of the V-A theory of weak interactions” (Bowen et al. 1967, 1314).3 At the time of this experiment, the V-A theory had strong experimental support, although it had not been severely tested in strangeness -changing decays, which included Ke2 + decay.4 The theoretical predictions of the Ke2 + branching ratio were explicit. If the interaction was pure axial-vector (A), the predicted ratio of Ke2 + to Kμ2 + CHAPTER 10 “Measurement of the Ke2 + Branching Ratio” “Measurement of the Ke2 + Branching Ratio” • 99 decays was 2.6 × 10−5, corresponding to a branching ratio of 1.6 × 10−5.5 Pure pseudoscalar (P) coupling, on the other hand, predicted a Ke2 + to Kμ2 + ratio of 1.02. If even only a small amount of pseudoscalar interaction were present, along with the dominant axial-vector interaction, then the Ke2 + branching ratio would be much larger. For example, adding only 1 part in 1,000 of pseudoscalar interaction to the axial-vector interaction would increase the expected branching ratio by a factor of 4; “thus, even a rough measurement of the Ke2 + branching ratio can provide an additional test of the applicability of the V-A theory to strangeness nonconserving weak interactions” (1314). The best previous measurement of the Ke2 +/Kμ2 + ratio had set an upper limit of 2.6 × 10−3, a factor of 100 larger than that predicted by V-A theory, and the experimenters felt that they could not only improve on this limit but also actually measure the branching ratio.6 In principle this was a simple experiment. The positron from Ke2 + decay has a momentum of 246.9 MeV/c in the kaon center of mass. This is larger than that of any other charged particle produced in the direct decay of the kaon, the closest being the muon from Kμ2 + decay, 235.6 MeV/c. Thus, all one had to do, in principle, to measure the branching ratio was to stop the K meson, identify the decay particle as a positron, and measure its momentum . If it was approximately 247 MeV/c, then it was due to Ke2 + decay and one could count the candidates. One would then compare that number to the total number of kaon decays to obtain the branching ratio. In practice, however, the experiment was far more difficult. The Experimental Apparatus The experimental apparatus is shown in figure 10.1. The incoming beam was positively charged, unseparated, and had a momentum distribution centered on 530 MeV/c. It consisted primarily of pions and protons and also contained small numbers of kaons, muons, and positrons. The desired kaons were separated from the more numerous pions and protons by range in matter and by time of flight,7 and they were stopped in counter C3, the stopping region. A stopped K meson was signaled by the...

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