Elastic electron-proton scattering (e−p) and the spectroscopy of hydrogen atoms are the two traditional methods used to determine the proton charge radius (r p). About a decade ago, a new method using muonic hydrogen (µH) atoms 1 found a significant discrepancy with the compilation of all previous results 2 , creating the "proton radius puzzle". Despite intensive worldwide experimental and theoretical efforts, the "puzzle" remains unresolved. In fact, a new discrepancy was reported between the two most recent spectroscopic measurements on ordinary hydrogen 3, 4. Here, we report on the PRad experiment, the first high-precision e − p experiment since the emergence of the "puzzle". For the first time, a magnetic-spectrometerfree method was employed along with a windowless hydrogen gas target, which overcame several limitations of previous e − p experiments and reached unprecedented small angles.

No abstract

We propose † to measure the photo-production cross section of J/ψ near threshold, in search of the recently observed LHCb hidden-charm resonances P c (4380) and P c (4450) consistent with 'pentaquarks'. The observation of these resonances in photo-production will provide strong evidence of the true resonance nature of the LHCb states, distinguishing them from kinematic enhancements. A bremsstrahlung photon beam produced with an 11 GeV electron beam at CEBAF covers the energy range of J/ψ production from the threshold photo-production energy of 8.2 GeV, to an energy beyond the presumed P c (4450) resonance. The experiment will be carried out in Hall C at Jefferson Lab, using a 50 µA electron beam incident on a 9% copper radiator. The resulting photon beam passes through a 15 cm liquid hydrogen target, producing J/ψ mesons through a diffractive process in the t-channel, or through a resonant process in the s-and u-channel. The decay e + e − pair of the J/ψ will be detected in coincidence using the two highmomentum spectrometers of Hall C. The spectrometer settings have been optimized to distinguish the resonant s-and u-channel production from the diffractive t-channel J/ψ production. The s-and u-channel production of the charmed 5-quark resonance dominates the t-distribution at large t. The momentum and angular resolution of the spectrometers is sufficient to observe a clear resonance enhancement in the total cross section and t-distribution. We request a total of 11 days of beam time with 9 days to carry the main experiment and 2 days to acquire the needed t-channel elastic J/ψ production data for a calibration measurement. This calibration measurement in itself will greatly enhance our knowledge of t-channel elastic J/ψ production near threshold. † This document is an updated version of the original proposal PR12-16-007, which was approved with an 'A' rating and a 'high-impact' label by the Jefferson Lab PAC 44 in July 2016. The experiment was awarded 11 days of beam time.