"The Rise and Fall of Anomalons -- a Cautionary Tale"
by Wm. C. McHarris
Anomalons were all the rage in nuclear science for about a decade in the 1980's to 1990's. First observed in cosmic rays, they were heavy ions -- fragments of atomic nuclei -- that exhibited anomalously short interaction paths; hence, the name, "anomalons."
There is much we don't know about nuclei, but one thing we do think we know pretty well is how charged heavy ions lose energy when interacting with matter. If we know their charge, mass, and energy, then we can predict rather precisely the lengths of their paths as they pass through a particular medium. This is because they lose energy primarily through a multitude of "small" collisions with the electrons of the atoms and molecules in the medium. There are enough collisions to make good statistical predictions of the lengths of their paths.
However, in some cosmic-ray experiments, secondary fragments were observed to have much shorter paths than predicted. These "anomalons" were never the primary cosmic rays but only occasional secondary fragments produced after the primary heavy ions had collided with nuclei in the photographic emulsions. The difficulty with cosmic-ray experiments, however, is reproducibility -- one has to be content with whatever events have occurred, and these tend to be few and far-between.
When the Berkeley Bevelac came on line in the early1980's, it replaced cosmic rays as a reliable source of high-energy heavy ions. Heavy ions were accelerated to moderate energies by the HILAC -- Heavy Ion Linear ACcelerator -- at the top of the hill, transported by an "umbilical cord" beamline halfway down the hill to the Bevatron, and there reaccelerated to GeV energies. This rather Rube Goldberg arrangement was originally proposed to extend the lives of two aging accelerators, but it served well for almost twenty years. And one of its first successes was the production of anomalons "in abundance." "In abundance" must be used advisedly, however, for the primary beams never acted anomalously, and a fair amount of statistics had to be applied to separate the relatively few anomalous secondary fragments from all the other debris.
Two "World Conferences on Anomalons" were held at Berkeley in the 1980's, and I was present at both. Explanations for the phenomenon were numerous -- and at times highly imaginative. The most lauded explanations had to do with "color seepage." Just as the short-range, saturated chemical bond can be thought of a result of the "remnants" of the long-range electromagnetic (QED) force, so might the short-range, saturated strong nuclear force be thought of a a result of the "remnants" of the long-range color (QCD) force between quarks. And just as polar chemical bonds can result in dipoles that interact with outside matter, so might "color seepage" cause anomalons to interact more strongly with the material they were passing through, resulting in an anomalously short path. The concept of color-seepage became rather fashionable.
I was working with a group at the Bevalac involved with the mechanisms of pion production in relativistic collisions between nuclei, so I had pions on my mind. While listening to some of these explanations, I suddenly had the idea of a much more mundane explanation: These could instead be the result of negative pions (produced copiously in such collisions) loosely bound (with a velocity dependent force) to neutrons -- "pi-neuts," a term we coined. One of the group leaders and I decided to follow up on this, and over the next month or so we gave ourselves a cram course about the region where pion physics meets nuclear physics. We kept pretty quiet about what we were doing, for the subject was bound to be both exciting and controversial, releasing a preprint only after our resulting paper had been accepted for publication. ["Anomalons as Pineuts Bound to Nuclear Fragments: A Possible Explanation," Wm. C. McHarris and J. O. Rasmussen, Phys. Lett. B 126, 49 (1983)]. In this we were proven wise, for one senior researcher accused us of stealing his ideas, another told me that he, too, had come up with the same idea but discarded it as impossibly mundane, and a senior faculty member approached me asking how much I would charge for him to be included on the next publication!
Anomalons remained in vogue for a few years longer, and many more experiments were performed. But alas, eventually they were shown to be artifacts of the very involved statistics involved in analyzing the experiments. I emphasize that there was no fudging of data involved in any of these experiments-- no hanky-panky whatsoever. It was simply that the (large) groups of experimentalists wanted and believed so earnestly that anomalons be real that they inadvertently blinded themselves to the uncertainties involved with the statistics. Our paper in "Scientific American" ["High-Energy Collisions between Atomic Nuclei," WCM and JOR, Sci. Am. 250, No. 1, p.58 (Jan. 1984)] gives an overall view of the situation, and I hope that the measured skepticism in it might have contributed to the downfall of anomalons.
So anomalons disappeared, but pineuts remained. Several subsequent studies have indicated that they are indeed produced in high-energy heavy-ion collisions. They are actually a sort of "penta-quark," and pentaquarks have more recently been seen in elementary particle experiments. I must confess, however, that even though anomalons are currently considered beneath contempt -- I confess that I still have a soft spot for those old cosmic-ray experiments, which are more difficult to explain away.Attachment #1: Rise_and_Fall_of_Anomalons.pages
