In a new paper in Journal of Physics G, I argue that three recent experiments measuring the positron-proton to electron-proton elastic cross section ratio can make no definitive statements about proton form factor discrepancy because even the size of the discrepancy is not well-constrained. It is widely believed that the discrepancy between polarized and unpolarized measurements of the proton’s form factors is caused by the effects of two-photon exchange (TPE). Three recent experiments—one at the VEPP-3 Storage Ring in Novosibirsk, one using the CLAS detector at Jefferson Lab, and finally the OLYMPUS Experiment at DESY in Hamburg—looked for two-photon exchange by looking for any difference between the positron-proton and electron-proton cross sections. The measurements found only modest differences, and the results have been variously interpreted as supporting or contradicting the TPE hypothesis. Using a phenomenological approach, I estimate the e+p/e–p ratio one would need to resolve the discrepancy, as well as, for the first time, an estimate of the uncertainty. I find wide variation depending on what one assumes about unpolarized form factor measurements. The TPE hypothesis is neither confirmed nor denied, and future measurements at higher momentum transfer are needed.
I was going to make a snarky comment about measuring in haste and repenting in leisure, but then the anti-spam phrase that it prompted me with was “rue using”.