APEX: An Unexplained Event

Letters from: [ Michael E. Zeller and Jack Sandweiss ][1] [ Walter Greiner ][1] In Gary Taubes' article “The one that got away?” (News & Comment, [10 Jan., p. 148][2]), we are portrayed as being at odds with “virtually every nuclear physicist who has worked on the [APEX] experiments.” While that may be true, the article could be interpreted as casting doubt on our scientific judgment and implying that somehow our support was “enlisted” by Jack Greenberg for other-than-scientific reasons. Thus, we outline here the evidence for structure in the electron positron sum-energy spectrum from the APEX experiment that Greenberg has shown us which makes us believe further investigation is warranted. The top drawing in the accompanying figure shows the sum-energy spectrum as analyzed by Greenberg and his colleague Guangsheng Xu, overlayed on a background spectrum. The bottom spectrum is the background alone. The peaks between 680 and 800 thousand electron volts (keV) are in the same sum-energy region as seen in the EPOS I experiment (see the EPOS, 1990 spectrum from Taubes' article, p. 149). ![Figure][3] We ask three questions about this structure and these peaks. First, is the deviation from the background statistically significant? By standard statistical analysis, the probability that a statistical fluctuation from the background of this magnitude would occur in the sum-energy region previously seen in the EPOS I experiment is 3.5 × 10−5. The probability that such a deviation would occur by chance at any value of sum-energy in the spectrum is 3 × 10−3. Thus, we consider the structure to be statistically significant. Second, is the structure somehow manufactured by the cuts? The background spectrum shown is made with the use of the same analysis, but electrons and positrons from different events are chosen; that is, they are randomly associated. Such a background can also be generated by electrons and positrons in the same event, but out of time coincidence, and no structure is seen. We conclude from this that whatever the cause, the structure only occurs with correlated electrons and positrons in the same event and in time. Third, is there adequate motivation for the cuts? Greenberg's analysis imposed only two additional cuts on the data in the spectrum shown in the article from APEX, 1995: (i) the opening angle between the electron and positron was to be greater than 135°, and (ii) there were to be only one electron and one positron in the event. The primary reason for the first cut was that the EPOS I data suggested that the electrons and positrons observed in the peaks were emitted back to back. Thus, a cut was made that would have high acceptance for such events. The second cut was intended to yield a cleaner sample. Both these cuts appear to have been well motivated. The spectrum shown can be divided into three statistically independent samples: (i) events where both heavy ions were observed and electrons went to the right and positrons to the left of the apparatus, (ii) events where both ions were observed and the electrons went to the left and positrons to the right, and (iii) events where only one ion was observed. All three spectra show the same structure, albeit with the expected reduction in statistical significance. With regard to Taubes' description of the generation of such structure by randomly generating events, the data shown are the total sample and have not been selected beyond the cuts shown. Thus if the peaks were a statistical fluctuation such as was generated randomly in a computer, they would have occurred with at most a 3 in 1000 probability. The structures have been reproduced by a member of the APEX collaboration and have been seen by many in the collaboration. To our knowledge, they have not been explained in a scientifically consistent way. Nothing in Taubes' article refutes this result. # {#article-title-2} Because of my involvement in the development of the theory of strong fields referred to in Taubes' article, I followed the details of the electron + positron (e+e−) experiments closely from their inception. There are clearly anomalous aspects of the observation of so-called e+e−-resonances that puzzled me right after the apparent observation of correlated e+e− emission. In particular, a particle interpretation seemed unlikely because it was in sharp contradiction with well-established physics (for example, the Lamb shift, g-2, Delbruck scattering). However, this does not detract from the possibility that positron lines and later the e+e− sharp sum-energy lines have been observed for which an explanation might be found in the context of known physics, such as quasiatomic phenomena correlated with nuclear effects. This is a separate consideration. Greenberg was extremely cautious when making public statements and consistently insisted on further clarification of the positive findings in detail, as he now does for the negative results from APEX that are based on incomplete measurements and analysis. When I doubted the existence of a new particle in an opening address at the German Physical Society meeting in Berlin in 1988, I was assailed by many experimentalists because of “my discouragement of their work,” which they obviously believed in at that time, just as strongly as they now propose that all the narrow structures previously observed were statistical fluctuations. The retreat to this explanation appears to me (I have seen the EPOS II data discussed in seminars but not in publication), to have been influenced by premature publication of the APEX results. I have difficulties with this attitude because there exist identical observations in the literature for which the data were taken months apart and for which the analysis was carried out by physicists other than Tom Cowan and Greenberg. These data appear to represent a reproducible result in detail and not a statistical fluctuation. The measurements that are required now seem to be resisted, even though they are of paramount importance. Although I am a theorist, it seems clear to me that the experiments are still incomplete on both sides of the Atlantic, not having really addressed (contrary to some statements) the important thin-target excitation-function studies that may be the key to demonstrating reproducibility. Until these are done, I do not see how the issue of e+e− peaks can be resolved. [1]: /lookup/doi/10.1126/science.275.5306.1401b [2]: /lookup/doi/10.1126/science.275.5297.148 [3]: pending:yes