The “Death Cries” of Dark Matter?

The cosmic ray energy spectrum is in the news! The Alpha Magnetic Spectrometer experiment (AMS-02), mounted on the International Space Station, is reporting results about the prevalence of positrons in the cosmic radiation, which otherwise comprises mostly protons. This is being touted as newsworthy, because, if there is a drop-off in that prevalence at higher energies, it will corroborate certain theories of dark matter, which propose that the mutual annihilation of dark-matter particles generates positrons of energies up to but not exceeding levels corresponding to those particles’ high mass.   Similarly enticing results were reported by the PAMELA (Payload for Anti-Matter Matter Exploration and Light-Nuclei Astrophysics) experiment in 2008.  The sophistication of AMS-02 will hopefully be able to take those measurements further, but, unfortunately, we will have to wait a while for more definitive results from higher-energy parts of the spectrum.

AMS-02 mounted on the International Space Station. Credit: NASA

What is intriguing about this story is that it really brings us back to where particle physics began over 80 years ago.  In 1930 Robert Millikan (1868-1953), the doyen of physics at the California Institute of Technology, set postdoctoral researcher Carl Anderson (1905-1991) to work on building a cloud chamber in order to measure the same thing AMS-02 is designed to measure, the energy spectrum of cosmic rays.  Millikan believed that measuring the spectrum would confirm his controversial (and incorrect) theory that cosmic rays originated as photons produced in the interstellar synthesis of elements, which then created secondary radiation when they encountered atmospheric nuclei.  Much in the way that every element emits a characteristic spectrum of light, Millikan figured that the energy spectrum of this secondary radiation would cluster into characteristic bands, observing which would, in effect, be like listening to the “birth cries” of the elements.¹

In the early 1930s, measuring the energy spectrum of the cosmic rays was an unprecedented and extremely difficult task.  Tracks of cosmic rays had only been seen in cloud chambers for the first time a few years earlier by Dmitri Skobeltsyn (1892-1990), and the chambers were only rarely used to study electrons, which were evidently a major component of the secondary cosmic radiation.  

As I show in my paper, “Strategies of Detection: Interpretive Practices in Experimental Particle Physics, 1930-1950,” (free pdf here) Anderson had to resort to expedient means of identifying particles and measuring their energy.  In most cases, he was able to distinguish between positively and negatively charged particles by seeing which way they curved in a magnetic field, although very high-energy particles did not curve appreciably in his magnet, and it was always possible a wayward particle was traveling up rather than down through his chamber.  Beyond that, he felt he could safely assume that all particles in his chamber were either electrons or protons, so if he felt he could distinguish between the two particles, he could calculate their energy based on those particles’ mass, and measurements of their momentum.

It was in the course of these experiments that Anderson detected positively charged particles that could not be either protons or upward-traveling electrons.² He  interpreted this new particle to be a positive electron, or “positron,” which, of course, is the same thing that AMS-02 is trying to pick out from a background of protons. Anderson’s discovery—made when he was 27 years old—won him a share of the 1936 Nobel Prize.

Anderson and his cloud chamber. Note how similarly it looks to the AMS (except turned on its side). This is because surrounding instrumentation with a coil magnet remains a central part of distinguishing charged particles. Source: National Archives and Records Administration and California Institute of Technology, digital file from the AIP Emilio Segrè Visual Archives

The discovery of the positron, and, around the same time the neutron, followed a few years later by the “mesotron” (later reconceptualized as the muon), is the standard story of the origins of particle physics. However, that story tends to shift historians’ attention away from the cosmic radiation as a subject of interest in itself, and toward the numerous particles that were discovered, at first in cosmic rays, and later in large numbers in high-energy particle accelerators.  But, I would argue, there are some interesting points to be picked out if we keep our attention on the cosmic ray energy spectrum.

Without knowing how hard it was to identify particles in cloud chambers, it is not apparent how badly the positron discovery might have hindered cosmic ray physics and particle physics, since it might have suddenly become necessary to distinguish reliably between protons and positrons, which was something that limitations in cloud chamber instrumentation made it difficult to do.  Fortunately, this need was obviated by the work of Patrick Blackett and Giuseppe Occhialini (pdf) at Cambridge.  

Blackett and Occhialini’s work (for which Blackett would win the 1948 Nobel Prize) is usually noted for 1) coming in second to Anderson in the positron discovery, 2) for using a “counter-controlled” cloud chamber (where quality photographs are virtually ensured by using Geiger counters to trigger the chamber when a particle passes through it), and 3) for linking the positron discovery to the predictions of Paul Dirac’s quantum electrodynamical theory. 

However, I would emphasize another important but unheralded aspect of Blackett and Occhialini’s achievements.  By collecting a large number of tracks using their counter-controlled apparatus, they were able to demonstrate an absence of tracks that could be interpreted as low-energy protons.  If a positron resembles a proton, it will always look like one with high energy. But, because of atmospheric collisions, if one finds a particle in the cosmic radiation, one would expect it (pace Millikan’s band theory) to exhibit a broad energy spectrum.  Thus, Blackett and Occhialini were able to argue persuasively that positrons were not simply those particles that could not otherwise be interpreted as protons—in fact, there were probably few, if any, protons in the secondary cosmic radiation at all.

Now, Blackett and Occhialini’s conclusion that all positively charged particles in the cosmic radiation could be interpreted as positrons was certainly flawed, not least because they did not know that they were doubtless seeing a large number of muons (which were yet to be “discovered”).  However, their interpretation allowed measurements of cosmic ray energies to continue, now assuming that positively charged particles had electronic rather than protonic mass.³

Source: P. M. S. Blackett and R. B. Brode, “The Measurement of the Energy of Cosmic Rays II: The Curvature Measurements and the Energy Spectrum,” Proceedings of the Royal Society of London: Series A, Mathematical and Physical Sciences 154, no. 883 (1936): 573–87, on 584.

What is remarkable to me about AMS-02 is the sheer endurance of the cosmic ray energy spectrum as a fruitful problem in physics.

AMS-02 revises and refines results of other measurements of the prevalence of positrons in the primary cosmic radiation. Source: http://press.web.cern.ch/backgrounders/first-result-ams-experiment

From Anderson’s positron discovery up through the formulation of the Standard Model in the 1970s, particle physics saw a period of astonishing productivity and change.  Meanwhile, the study of cosmic rays by particle physicists was eclipsed by high-energy accelerators by the mid-1950s.  Cosmic ray studies then mainly became the province of terrestrial physics, and over the ensuing half-century the basic problem of detecting and measuring the cosmic ray energy spectrum did not undergo radical changes.  This, of course, isn’t to say that nothing has changed: experimenters, for instance, have gotten a lot better at telling protons from positrons—AMS-02 fails to make the distinction less than one time in a million.  But even with results corroborated across multiple, very precise detectors, the reason we don’t have high-energy results is because it continues to be difficult, after all this time, to tell protons and positrons apart at very high energies (although “very high” is a lot higher than it used to be). Thus we have to wait for more measurements to ensure that results are statistically reliable.

AMS-02 instrumentation. Source: http://www.ams02.org/what-is-ams/tecnology/

It would be wrong to overstate and over-romanticize historical symmetries and ironies.  Nevertheless, it is remarkable that 80 years after Robert Millikan mistakenly believed he would solve one of the fundamental physical questions of his time by analyzing the cosmic ray energy spectrum, and some 60 years after high-energy accelerators tore fundamental discoveries away from studies of the cosmic radiation,⁴ Samuel Ting, one of the great figures of high-energy experimental physics, has pushed a return to the cosmic ray energy spectrum as part of an effort to solve one of the fundamental physical questions of our time.

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¹See Robert H. Kargon, “Birth Cries of the Elements: Theory and Experiment along Millikan’s Route to Cosmic Rays,” in The Analytic Spirit: Essays in the History of Science in Honor of Henry Guerlac, edited by Harry Woolf (Ithaca: Cornell University Press, 1981).

²See Peter Galison, “The Discovery of the Muon and the Failed Revolution against Quantum Electrodynamics,” Centaurus 26 (1982): 262–316; and Michelangelo De Maria and Arturo Russo, “The Discovery of the Positron,” Rivista di Storia della Scienza 2 (1985): 237–86.

³One of the reasons why I emphasize in “Strategies of Detection” the inadequacy of Peter Galison’s claim that the single image, or “golden event” was integral to the epistemology of “image” tradition experimentation is precisely that it fails to capture these sorts of developments.  First, the inadequacies of cloud chamber images often made the aggregation of evidence the only reliable means of drawing certain kinds of conclusions from unreliable photographic interpretations; and, second, some important tasks, such as measuring the prevalence of positrons in the cosmic radiation or the energy spectrum of cosmic rays could never, even in principle, be accomplished on the basis of a single image.

⁴ In his Image and Logic, Peter Galison notes another irony in how Donald Glaser was originally trying to save small-scale cosmic ray physics by inventing the bubble chamber, but ended up augmenting the dominance of “big physics” instead.