Shifting Standards: Experiments in Particle Physics in the Twentieth Century

Chapter 19 The Case of the Disappearing Sigmas

212 • In the prologue I discussed the idea that the use of the five-sigma criterion required both knowledge and judgment and was not merely the application of a formula to the data. In this chapter I present a case study that illustrates this. In 2003 the CLAS Collaboration at the Jefferson Physical Laboratory published a paper titled “Observation of an Exotic S = +1 Baryon in Exclusive Photoproduction from the Deuteron” (Stepanyan et al. 2003). This was the “pentaquark,” an elementary particle composed of five quarks.1 Such a particle had been reported twice previously, by the LEPS Collaboration (Nakano et al. 2003) and by the Diana Collaboration (Barmin et al. 2003), with statistical significances of 4.6 ± 0.1 σ and 4.4 σ, respectively. In addition, theorists predicted the existence of such a particle with a mass of approximately 1.53 GeV/c2 (Diakonov, Petrov, and Polyakov 1997). The CLAS experiment claimed a significance of (5.2 ± 0.6) σ, which accounted for the “Observation of” title. The particle was observed in the K+n invariant mass spectrum obtained from events from the reaction γd → K+K−pn. Its mass was 1.542 ± 0.005 GeV/c2, in good agreement with both the previous experimental results and the theoretical prediction. The experiment was carefully done. The missing mass spectrum for the K+K−p fit the known neutron mass very well, as it should (figure 19.1). In addition, the experimenters were able to detect the known particles, φ → K+K−, and the Λ(1520) → pK−. (Events with these particles were excluded from the final data sample to avoid spurious signals.) The evidence for the Θ+ particle is shown in figure 19.2, the invariant mass of the nK+ system produced in the reaction. The group estimated a signal of 43 events above background. The claimed result was robust against changes in the event selection criteria: “The sensitivity of the peak to the placement of event selection cuts was CHAPTER 19 The Case of the Disappearing Sigmas The Case of the Disappearing Sigmas • 213 Figure 19.2. Invariant mass of the nK+ system. A fit (solid line) to the peak on top of the smooth background (dashed line) gives a statistical significance of 5.8 σ. From Stepanyan et al. (2003). Figure 19.1. Missing mass spectrum for the γd → pK+K−X reaction, which shows a peak at the neutron mass. The inset shows the neutron peak with a tighter requirement on the timing between the protons and kaons. From Stepanyan et al. (2003). 214 • The Case of the Disappearing Sigmas studied, and the conclusion that the peak at 1.542 GeV/c2 is very robust” (Stepanyan et al. 2003, 252001-5). In addition, the group varied the fitting functions and found that the significance of the result was robust against such changes: “In all we tried ten variations of event cut placement and/ or different fitting functions. All fits reproduce the measured data with reduced χ2 in the range between 0.6 and 1.2 The estimated statistical significance in those ten cases ranges from 4.6 σ to 5.8 σ” (252001-5). Thus, the stated result was (5.2 ± 0.6) σ. The group concluded that “these results from CLAS, together with other experiments, now provide convincing evidence for the existence of an S = + 1 baryon state at 1.542 GeV/c2 with a small intrinsic width” (252001-5; emphasis added). In 2006 the CLAS Collaboration published “Search for the Θ+ Pentaquark in the Reaction γd → pK−K+n” (McKinnon et al. 2006).3 They remarked that, “considering the important implications of a possible pentaquark state, it was necessary to test the reproducibility of our previous result. In addition, there are several experiments that see evidence for the Θ+ and many that do not” (McKinnon et al. 2006, 212001-2). The new experiment had almost six (5.92) times the data of the first experiment. Their result is shown in figure 19.3. No peak is visible. Once again the group was able to detect the neutron in the missing mass spectrum and the Λ(1520) in the pK− invariant mass plot. They concluded that “the current null result shows that the nK+ invariant mass peak of [their earlier experiment] could not be reproduced” (212001-5).4 Nevertheless the group did not completely repudiate their earlier result. They used their new null result as the best estimate of the background, normalized the two curves to...