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Elastic electron–proton scattering (e–p) and the spectroscopy of hydrogen atoms are the two methods traditionally used to determine the proton charge radius, r p . In 2010, a new method using muonic hydrogen atoms 1 found a substantial discrepancy compared with previous results 2 , which became known as the ‘proton radius puzzle’. Despite experimental and theoretical efforts, the puzzle remains unresolved. In fact, there is a discrepancy between the two most recent spectroscopic measurements conducted on ordinary hydrogen 3 , 4 . Here we report on the proton charge radius experiment at Jefferson Laboratory (PRad), a high-precision e–p experiment that was established after the discrepancy was identified. We used a magnetic-spectrometer-free method along with a windowless hydrogen gas target, which overcame several limitations of previous e–p experiments and enabled measurements at very small forward-scattering angles. Our result, r p  = 0.831 ± 0.007 stat  ± 0.012 syst  femtometres, is smaller than the most recent high-precision e–p measurement 5 and 2.7 standard deviations smaller than the average of all e–p experimental results 6 . The smaller r p we have now measured supports the value found by two previous muonic hydrogen experiments 1 , 7 . In addition, our finding agrees with the revised value (announced in 2019) for the Rydberg constant 8 —one of the most accurately evaluated fundamental constants in physics. A magnetic-spectrometer-free method for electron–proton scattering data reveals a proton charge radius 2.7 standard deviations smaller than the currently accepted value from electron–proton scattering, yet consistent with other recent experiments.

Elastic electron-proton scattering (e-p) and the spectroscopy of hydrogen atoms are the two methods traditionally used to determine the proton charge radius, r.sub.p. In 2010, a new method using muonic hydrogen atoms.sup.1 found a substantial discrepancy compared with previous results.sup.2, which became known as the 'proton radius puzzle'. Despite experimental and theoretical efforts, the puzzle remains unresolved. In fact, there is a discrepancy between the two most recent spectroscopic measurements conducted on ordinary hydrogen.sup.3,4. Here we report on the proton charge radius experiment at Jefferson Laboratory (PRad), a high-precision e-p experiment that was established after the discrepancy was identified. We used a magnetic-spectrometer-free method along with a windowless hydrogen gas target, which overcame several limitations of previous e-p experiments and enabled measurements at very small forward-scattering angles. Our result, r.sub.p = 0.831 [plus or minus] 0.007.sub.stat [plus or minus] 0.012.sub.syst femtometres, is smaller than the most recent high-precision e-p measurement.sup.5 and 2.7 standard deviations smaller than the average of all e-p experimental results.sup.6. The smaller r.sub.p we have now measured supports the value found by two previous muonic hydrogen experiments.sup.1,7. In addition, our finding agrees with the revised value (announced in 2019) for the Rydberg constant.sup.8--one of the most accurately evaluated fundamental constants in physics.

Elastic electron-proton scattering (e-p) and the spectroscopy of hydrogen atoms are the two methods traditionally used to determine the proton charge radius, r.sub.p. In 2010, a new method using muonic hydrogen atoms.sup.1 found a substantial discrepancy compared with previous results.sup.2, which became known as the 'proton radius puzzle'. Despite experimental and theoretical efforts, the puzzle remains unresolved. In fact, there is a discrepancy between the two most recent spectroscopic measurements conducted on ordinary hydrogen.sup.3,4. Here we report on the proton charge radius experiment at Jefferson Laboratory (PRad), a high-precision e-p experiment that was established after the discrepancy was identified. We used a magnetic-spectrometer-free method along with a windowless hydrogen gas target, which overcame several limitations of previous e-p experiments and enabled measurements at very small forward-scattering angles. Our result, r.sub.p = 0.831 [plus or minus] 0.007.sub.stat [plus or minus] 0.012.sub.syst femtometres, is smaller than the most recent high-precision e-p measurement.sup.5 and 2.7 standard deviations smaller than the average of all e-p experimental results.sup.6. The smaller r.sub.p we have now measured supports the value found by two previous muonic hydrogen experiments.sup.1,7. In addition, our finding agrees with the revised value (announced in 2019) for the Rydberg constant.sup.8--one of the most accurately evaluated fundamental constants in physics.

Elastic electron-proton scattering (e-p) and the spectroscopy of hydrogen atoms are the two methods traditionally used to determine the proton charge radius, r.sub.p. In 2010, a new method using muonic hydrogen atoms.sup.1 found a substantial discrepancy compared with previous results.sup.2, which became known as the 'proton radius puzzle'. Despite experimental and theoretical efforts, the puzzle remains unresolved. In fact, there is a discrepancy between the two most recent spectroscopic measurements conducted on ordinary hydrogen.sup.3,4. Here we report on the proton charge radius experiment at Jefferson Laboratory (PRad), a high-precision e-p experiment that was established after the discrepancy was identified. We used a magnetic-spectrometer-free method along with a windowless hydrogen gas target, which overcame several limitations of previous e-p experiments and enabled measurements at very small forward-scattering angles. Our result, r.sub.p = 0.831 [plus or minus] 0.007.sub.stat [plus or minus] 0.012.sub.syst femtometres, is smaller than the most recent high-precision e-p measurement.sup.5 and 2.7 standard deviations smaller than the average of all e-p experimental results.sup.6. The smaller r.sub.p we have now measured supports the value found by two previous muonic hydrogen experiments.sup.1,7. In addition, our finding agrees with the revised value (announced in 2019) for the Rydberg constant.sup.8--one of the most accurately evaluated fundamental constants in physics.

Elastic electron–proton scattering (e–p) and the spectroscopy of hydrogen atoms are the two methods traditionally used to determine the proton charge radius, rp. In 2010, a new method using muonic hydrogen atoms found a substantial discrepancy compared with previous results, which became known as the ‘proton radius puzzle’. Despite experimental and theoretical efforts, the puzzle remains unresolved. In fact, there is a discrepancy between the two most recent spectroscopic measurements conducted on ordinary hydrogen. Here we report on the proton charge radius experiment at Jefferson Laboratory (PRad), a high-precision e–p experiment that was established after the discrepancy was identified. We used a magnetic-spectrometer-free method along with a windowless hydrogen gas target, which overcame several limitations of previous e–p experiments and enabled measurements at very small forward-scattering angles. Our result, rp = 0.831 ± 0.007stat ± 0.012syst femtometres, is smaller than the most recent high-precision e–p measurement and 2.7 standard deviations smaller than the average of all e–p experimental results. Here, the smaller rp we have now measured supports the value found by two previous muonic hydrogen experiments. In addition, our finding agrees with the revised value (announced in 2019) for the Rydberg constant—one of the most accurately evaluated fundamental constants in physics.

The atomic nucleus is composed of two different kinds of fermions: protons and neutrons. If the protons and neutrons did not interact, the Pauli exclusion principle would force the majority of fermions (usually neutrons) to have a higher average momentum. Our high-energy electron-scattering measurements using (12)C, (27)Al, (56)Fe, and (208)Pb targets show that even in heavy, neutron-rich nuclei, short-range interactions between the fermions form correlated high-momentum neutron-proton pairs. Thus, in neutron-rich nuclei, protons have a greater probability than neutrons to have momentum greater than the Fermi momentum. This finding has implications ranging from nuclear few-body systems to neutron stars and may also be observable experimentally in two-spin-state, ultracold atomic gas systems.

We present the first measurement of di-hadron angular correlations in electron-nucleus scattering. The data were taken with the CLAS detector and a 5.0 GeV electron beam incident on deuterium, carbon, iron, and lead targets. Relative to deuterium, the nuclear yields of charged-pion pairs show a strong suppression for azimuthally opposite pairs, no suppression for azimuthally nearby pairs, and an enhancement of pairs with large invariant mass. These effects grow with increased nuclear size. The data are qualitatively described by the GiBUU model, which suggests that hadrons form near the nuclear surface and undergo multiple-scattering in nuclei. These results show that angular correlation studies can open a new way to elucidate how hadrons form and interact inside nuclei

We report on a windowless, high-density, gas flow target at Jefferson Lab that was used to measure \\(r_p\\), the root-mean-square charge radius of the proton. To our knowledge, this is the first such system used in a fixed-target experiment at a (non-storage ring) electron accelerator. The target achieved its design goal of an areal density of 2\\(\\times\\)10\\(^{18}\\) atoms/cm\\(^2\\), with the gas uniformly distributed over the 4 cm length of the cell and less than 1% residual gas outside the cell. This design eliminated scattering from the end caps of the target cell, a problem endemic to previous measurements of the proton charge radius in electron scattering experiments, and permitted a precise, model-independent extraction of \\(r_p\\) by reaching unprecedentedly low values of \\(Q^2\\), the square of the electron's transfer of four-momentum to the proton.

Background: Energetic quarks in nuclear DIS propagate through the nuclear medium. Processes that are believed to occur inside nuclei include quark energy loss through medium-stimulated gluon bremsstrahlung and intra-nuclear interactions of forming hadrons. More data are required to gain a more complete understanding of these effects. Purpose: To test the theoretical models of parton transport and hadron formation, we compared their predictions for the nuclear and kinematic dependence of pion production in nuclei. Methods: We have measured charged-pion production in semi-inclusive DIS off D, C, Fe, and Pb using the CLAS detector and the CEBAF 5.014 GeV electron beam. We report results on the nuclear-to-deuterium multiplicity ratio for \\(\\pi^{+}\\) and \\(\\pi^{-}\\) as a function of energy transfer, four-momentum transfer, and pion energy fraction or transverse momentum - the first three-dimensional study of its kind. Results: The \\(\\pi^{+}\\) multiplicity ratio is found to depend strongly on the pion fractional energy \\(z\\), and reaches minimum values of \\(0.67\\pm0.03\\), \\(0.43\\pm0.02\\), and \\(0.27\\pm0.01\\) for the C, Fe, and Pb targets, respectively. The \\(z\\) dependences of the multiplicity ratios for \\(\\pi^{+}\\) and \\(\\pi^{-}\\) are equal within uncertainties for C and Fe targets but show differences at the level of 10\\(\\%\\) for the Pb-target data. The results are qualitatively described by the GiBUU transport model, as well as with a model based on hadron absorption, but are in tension with calculations based on nuclear fragmentation functions. Conclusions: These precise results will strongly constrain the kinematic and flavor dependence of nuclear effects in hadron production, probing an unexplored kinematic region. They will help to reveal how the nucleus reacts to a fast quark, thereby shedding light on its color structure, transport properties, and on the mechanisms of the hadronization process.

Measuring the spin structure of protons and neutrons tests our understanding of how they arise from quarks and gluons, the fundamental building blocks of nuclear matter. At long distances the coupling constant of the strong interaction becomes large, requiring non-perturbative methods to calculate quantum chromodynamics processes, such as lattice gauge theory or effective field theories. Here we report proton spin structure measurements from scattering a polarized electron beam off polarized protons. The spin-dependent cross-sections were measured at large distances, corresponding to the region of low momentum transfer squared between 0.012 and 1.0 GeV\\(^2\\). This kinematic range provides unique tests of chiral effective field theory predictions. Our results show that a complete description of the nucleon spin remains elusive, and call for further theoretical works, e.g. in lattice quantum chromodynamics. Finally, our data extrapolated to the photon point agree with the Gerasimov-Drell-Hearn sum rule, a fundamental prediction of quantum field theory that relates the anomalous magnetic moment of the proton to its integrated spin-dependent cross-sections.