This past weekend in Cambridge, MA, was one of several town hall meetings being held around the country to get input from the community as NSAC (Nuclear Science Advisory Committee) undertakes the next “Long-Range Plan.” These plans are developed every seven years or so and set the priorities for nuclear physics research funding by government agencies such as the U.S. Department of Energy and the National Science Foundation. This weekend’s town hall was on Quantum Chromodynamics (QCD), and I had the privilege to be invited to talk about physics that would be made possible with an accelerator that could accelerate positrons.
Accelerating positrons isn’t hard. An existing electron accelerator, such as Jefferson Lab’s CEBAF, can do the job, provided the polarities of all the steering magnets are reversed. But it’s making the positrons in the first place that requires some investment. It’s not cheap (in the ball park of tens of millions of dollars), but it’s not nearly as expensive as, say, upgrading CEBAF’s terminal energy from 12 GeV to 24 GeV. And it’s no where near as expensive as building a new accelerator, such as the upcoming electron ion collider. As part of the Jefferson Lab Positron Working Group, I am advocating that a positron source be built at Jefferson Lab, so that positrons could be accelerated using th CEBAF accelerator, and experiments could be conducted with all of the amazing detectors and equipment that are already part of the laboratory’s infrastructure.
There are many great experiments one can do with a positron beam, and among them, the application I find to be most critical is determining how much two-photon exchange occurs in electron scattering. Electron scattering is a great probe for learning about how quarks are distributed within protons, but in analyzing such measurements, we assume that the electron exchanges one and only one virtual photon with the quark that it collides with. It can exchange more than one photon (with each additional photon being significantly less likely), but calculating the probability of a second exchanged photon is not straightforward, and we typically assume that it has a negligible impact on the results. This may not have gotten us into trouble everywhere, but it likely has gotten us into trouble in our efforts to measure the proton’s charge distribution (its so-called “electromagnetic form factors.”)
Positrons can help clear up this mess because two-photon exchange has the opposite effect on positron scattering as it does on electron scattering. If we can measure a difference between, say, a positron-proton scattering reaction, and an electron-proton scattering reaction, then it tells us exactly what the effects of two-photon exchange are. This could help clear up the proton’s form factors, and tell us what to watch out for as we embark on measuring challenging new reactions like deeply virtual compton scattering or deeply virtual meson production.
Positrons are great for answering lots of other questions too, such as helping to map out the protons 3D structure (via its Generalized Parton Distributions), probing for “strange quarks” in the proton, or even searching for light dark matter. I think the benefits are really clear, and cost is not exorbitant, and I hope that a positron source at Jefferson Lab is endorsed in the Long-Range Plan.
If you’d like to learn more, I highly recommend checking out the JLab Positron Working Group’s White Paper, published as a special topical issue of the European Physical Journal A.