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.

We present the results of our final analysis of the full data set of g p 1 (Q 2 ), the spin structure function of the proton, collected using CLAS at Jefferson Laboratory in [2000][2001] We compare our final results with various theoretical models and expectations, as well as with parametrizations of the world data. Our data, with their precision and dense kinematic coverage, are able to constrain fits of polarized parton distributions, test pQCD predictions for quark polarizations at large x, offer a better understanding of quark-hadron duality, and provide more precise values of higher twist matrix elements in the framework of the operator product expansion.

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Background: The response of hadrons, the bound states of the strong force (QCD), to external probes can be described in two different, complementary frameworks: As direct interactions with their fundamental constituents, quarks and gluons, or alternatively as elastic or inelastic coherent scattering that leaves the hadrons in their ground state or in one of their excited (resonance) states. The former picture emerges most clearly in hard processes with high momentum transfer, where the hadron response can be described by the perturbative expansion of QCD, while at lower energy and momentum transfers, the resonant excitations of the hadrons dominate the cross section. The overlap region between these two pictures, where both yield similar predictions, is referred to as quark-hadron duality and has been extensively studied in reactions involving unpolarized hadrons. Some limited information on this phenomenon also exists for polarized protons, deuterons and 3 He nuclei.Purpose: In this paper, we present for the first time comprehensive and detailed results on the correspondence between the extrapolated deep inelastic structure function g1 of both the proton and the neutron with the same quantity measured in the nucleon resonance region.Method: We use a QCD parameterization of the world data on DIS spin structure functions, extrapolated into the nucleon resonance region and averaged over various intervals in the scaling variable x. We compare the results with the large data set collected in the quark-hadron transition region by the CLAS collaboration, averaged over the same intervals. We present this comparison as a function of the momentum transfer Q 2 .Results: We find that, depending on the averaging interval and the minimum momentum transfer chosen, a clear transition to quark-hadron duality can be observed in both nucleon species. Furthermore, we show, for the first time, the scaling behavior of g1 measured in the resonance region at sufficiently high momentum transfer.Conclusions: Our results can be used to quantify the deviations from the applicability of pQCD for data taken at moderate energies, and help with extraction of quark distribution functions from such data.

Upcoming spin structure experiments in Hall B at Jefferson Lab will employ a new dynamically polarized target inside the CLAS12 detector system. Protons and deuterons in irradiated NH 3 and ND 3 will be polarized at 1 K using the 5 T field of the CLAS12 solenoidal magnet. For optimum polarization, the field uniformity requirements are around 100 ppm over the volume of the target sample. I will discuss methods to improve field uniformity utilizing thin superconducting shim coils integrated within the 1 K refrigerator. I will also demonstrate that this method to adjust the 5 T field also enables the simultaneous opposite polarization of two adjacent target cells.