An Interesting Discovery (or Not)

In writing up the bibliography for an upcoming paper on the background radiation environment measured by the MoEDAL Timepix detector array, I was going through the collaboration's bibliography - the list of papers that are typically cited in the papers that we publish. These include things like the MoEDAL Technical Design Report [1] and physics programme [2], but also key theory papers like Dirac's original prediction [3] (which you can read for free here) and Polyakov's [4] and 't Hooft's [5] GUT monopole papers. Julian Schwinger - who shared the 1965 Nobel Prize for Physics with Tomonaga and Feynman for his work on Quantum Electrodynamics (QED) - also has a paper in this list; in Magnetic charge and the charge quantization condition [6] he talks about dyons - particles that have both magnetic and electric charge. We are looking for these with MoEDAL too; these are one of the "Exotics" represented by the "E".

Anyway, in reading the paper to check exactly where dyons are mentioned, I noticed the following throwaway paragraph placed inconspicuously at the very end:

Added note: At long last there is experimental evidence for magnetic charge. P. B. Price, E. K. Shirk, W. Z. Osborne, and L. S. Pinsky [Phys. Rev. Lett. 35, 487 (1975)] have detected a very heavily ionizing particle that has all the characteristics of a particle with magnetic charge...
— J. Schwinger, Phys. Rev. D 12 3105 (1975)

Now, I wasn't aware that there was any experimental evidence for magnetic monopoles - MoEDAL would be a very different proposition if there had been, and I'm pretty sure I'd have remembered it from my undergraduate degree - so I thought I'd check out the cited paper. Lo and behold:

The paper itself can be found here [7]. In short, in the last flight of a balloon-borne stack of Cerenkov film, emulsion, and Lexan polycarbonate sheets, the authors spotted a very heavily-ionising track that they believed had all the characteristics of a magnetic monopole. The paper itself is a nice read; you can almost imagine the authors sitting around carefully preparing and sorting through the emulsion plates and plastic sheets, half-jokingly discussing what they’d wear on stage for the ceremony in Stockholm. As one would expect, the figures very much resemble the sort of thing we’re looking for with the MoEDAL Nuclear Track Detectors (NTDs) - etch-pit cones aligned along the path of the particle through the Lexan layers of the stack. We would certainly get very excited if we saw something similar in our layers of NTD plastic.

So what happened? Had they found experimental evidence for a magnetic monopole? Well, no. An alternative explanation for their signal would have been a very fast, very heavy ion fragment (Z>100ish) reaching the atmosphere from space. Price et al. claim to rule this out in the paper, but in the same journal a fellow balloon-stack experimentalist shows how [8], once you take into account saturation effects in the Lexan and the ambiguities introduced by only having one layer of emulsion (their lab always used two or more in their balloon stacks, apparently), a heavy ion fragment cannot be ruled out.

And so, I suppose, it wasn’t.

However, it wasn’t just the monopole discovery claim that had caught my eye. One of the authors in the citation was listed as L. S. Pinsky. As I read it, I thought, "could this be the L. S. Pinsky, friend of the Institute for Research in Schools and keynote speaker at the 2015 CERN@school Research Symposium? Could the world be so small that the supporting scientist for IRIS’s TimPix project was an author on a magnetic monopole discovery paper from back in 1975?"

Yes! Yes it was! And perhaps it shouldn’t be too surprising, really - after all, Professor Pinsky is one of the world’s leading experts on heavy ion fragments from space at the University of Houston (where he was in 1975!) and NASA’s Space Radiation Analysis Group at the Johnson Space Centre. And of course both they and MoEDAL are using the Medipix Collaborations’s Timepix detectors to study highly ionising particles on the International Space Station and at the Large Hadron Collider respectively - and both are offering projects for school students with us at Institute for Research in School’s CERN@school programme.

So at least I'd made one interesting discovery from going through the bibliography. And who knows - perhaps one day we’ll publish our own monopole discovery paper!

References

  1. The MoEDAL Collaboration, "Technical Design Report of the Moedal Experiment", CERN-LHCC-2009-006 (2009)
  2. The MoEDAL Collaboration, “The Physics Programme Of The MoEDAL Experiment At The LHC”, Int. J. Mod. Phys. A 29 1430050 (2014)
  3. P. A. M. Dirac, “Quantised Singularities in the Electromagnetic Field”, Proc. Roy. Soc. A 133 60 (1931)
  4. A. M. Polyakov, “Particle Spectrum in the Quantum Field Theory”, JETP Lett. 20 194 (1974)
  5. G. ’t Hooft, “Magnetic Monopoles in Unified Gauge Theories”, Nucl. Phys. B 79 27690486-6) (1974)
  6. J. Schwinger, "Magnetic charge and the charge quantization condition", Phys. Rev. D 12 3105 (1975)
  7. P. B. Price et al., "Evidence for Detection of a Moving Magnetic Monopole", Phys. Rev. Lett. 35 487 (1975)
  8. M. W. Friedlander, "Comments on the Reported Observation of a Monopole", Phys. Rev. Lett. 35 1167 (1975)