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The proton is one of the main building blocks of all visible matter in the Universe 1 . Among its intrinsic properties are its electric charge, mass and spin 2 . These properties emerge from the complex dynamics of its fundamental constituents—quarks and gluons—described by the theory of quantum chromodynamics 3 – 5 . The electric charge and spin of protons, which are shared among the quarks, have been investigated previously using electron scattering 2 . An example is the highly precise measurement of the electric charge radius of the proton 6 . By contrast, little is known about the inner mass density of the proton, which is dominated by the energy carried by gluons. Gluons are hard to access using electron scattering because they do not carry an electromagnetic charge. Here we investigated the gravitational density of gluons using a small colour dipole, through the threshold photoproduction of the J / ψ particle. We determined the gluonic gravitational form factors of the proton 7 , 8  from our measurement. We used a variety of models 9 – 11 and determined, in all cases, a mass radius that is notably smaller than the electric charge radius. In some, but not all cases, depending on the model, the determined radius agrees well with first-principle predictions from lattice quantum chromodynamics 12 . This work paves the way for a deeper understanding of the salient role of gluons in providing gravitational mass to visible matter. The gluonic gravitational form factor of the proton was determined using various models, and these analyses showed that the mass radius of the proton was smaller than the electric charge radius.

The neutron is a cornerstone in our depiction of the visible universe. Despite the neutron zero-net electric charge, the asymmetric distribution of the positively- (up) and negatively-charged (down) quarks, a result of the complex quark-gluon dynamics, lead to a negative value for its squared charge radius, ⟨ r n 2 ⟩ . The precise measurement of the neutron’s charge radius thus emerges as an essential part of unraveling its structure. Here we report on a ⟨ r n 2 ⟩ measurement, based on the extraction of the neutron electric form factor, G E n , at low four-momentum transfer squared ( Q 2 ) by exploiting the long known connection between the N  → Δ quadrupole transitions and the neutron electric form factor. Our result, ⟨ r n 2 ⟩ = − 0.110 ± 0.008 ( fm 2 ) , addresses long standing unresolved discrepancies in the ⟨ r n 2 ⟩ determination. The dynamics of the strong nuclear force can be viewed through the precise picture of the neutron’s constituent distributions that result into the non-zero ⟨ r n 2 ⟩ value. The charge radius of nucleons provides information about their structure. Here the authors present a method, based values of neutron electric form factors, to determine the charge radius of the neutron and provide information on improving the uncertainty of neutron charge radius measurements

The visible world is founded on the proton, the only composite building block of matter that is stable in nature. Consequently, understanding the formation of matter relies on explaining the dynamics and the properties of the proton’s bound state. A fundamental property of the proton involves the response of the system to an external electromagnetic field. It is characterized by the electromagnetic polarizabilities 1 that describe how easily the charge and magnetization distributions inside the system are distorted by the electromagnetic field. Moreover, the generalized polarizabilities 2 map out the resulting deformation of the densities in a proton subject to an electromagnetic field. They disclose essential information about the underlying system dynamics and provide a key for decoding the proton structure in terms of the theory of the strong interaction that binds its elementary quark and gluon constituents. Of particular interest is a puzzle in the electric generalized polarizability of the proton that remains unresolved for two decades 2 . Here we report measurements of the proton’s electromagnetic generalized polarizabilities at low four-momentum transfer squared. We show evidence of an anomaly to the behaviour of the proton’s electric generalized polarizability that contradicts the predictions of nuclear theory and derive its signature in the spatial distribution of the induced polarization in the proton. The reported measurements suggest the presence of a new, not-yet-understood dynamical mechanism in the proton and present notable challenges to the nuclear theory. Measurements of the proton’s electromagnetic structure show enhancement of its electric generalized polarizability compared with theoretical expectations, confirming the presence of a new dynamical mechanism not accounted for by current theories.

We have determined the proton and the neutron charge radii from a global analysis of the proton and the neutron elastic form factors, after first performing a flavor decomposition of these form factors under charge symmetry in the light cone frame formulation. We then extracted the transverse mean-square radii of the flavor dependent quark distributions. In turn, these are related in a model-independent way to the proton and neutron charge radii but allow us to take into account motion effects of the recoiling nucleon for data at finite but high momentum transfer. In the proton case we find ⟨ r p ⟩ = 0.852 ± 0 . 002 ( stat . ) ± 0 . 009 ( syst . ) ( fm ) , consistent with the proton charge radius obtained from muonic hydrogen spectroscopy [ 1 , 2 ]. The current method improves on the precision of the ⟨ r p ⟩ extraction based on the form factor measurements. Furthermore, we find no discrepancy in the ⟨ r p ⟩ determination among the different electron scattering measurements, all of which, utilizing the current method of extraction, result in a value that is consistent with the smallest ⟨ r p ⟩ extraction from the electron scattering measurements [ 3 ]. Concerning the neutron case, past results relied solely on the neutron-electron scattering length measurements, which suffer from an underestimation of underlying systematic uncertainties inherent to the extraction technique. Utilizing the present method we have performed the first extraction of the neutron charge radius based on nucleon form factor data, and we find ⟨ r n 2 ⟩ = - 0.122 ± 0 . 004 ( stat . ) ± 0 . 010 ( syst . ) ( fm 2 ) .

. We report on new measurements of the electric Generalized Polarizability (GP) of the proton α E in a kinematic region where a puzzling dependence on momentum transfer has been observed, and we have found that α E = ( 5 . 3 ± 0 . 6 s t a t ± 1 . 3 s y s ) 10 - 4 fm 3 at Q 2 = 0 . 20 (GeV/ c ) 2 . The new measurements, when considered along with the rest of the world data, suggest that α E can be described by either a local plateau or by an enhancement in the region Q 2 = 0 . 20 (GeV/ c ) 2 to 0.33 (GeV/ c ) 2 . The experiment also provides the first measurement of the Coulomb quadrupole amplitude in the N → Δ transition through the exploration of the p ( e , e p ) γ reaction. The new measurement gives C M R = ( - 4 . 4 ± 0 . 8 s t a t ± 0 . 6 s y s ) % at Q 2 = 0 . 20 (GeV/ c ) 2 and is consistent with the results from the pion electroproduction world data. It has been obtained using a completely different extraction method, and therefore represents a strong validation test of the world data model uncertainties.

We report new pion electroproduction measurements in the Δ ( 1232 ) resonance, utilizing the SHMS - HMS magnetic spectrometers of Hall C at Jefferson Lab. The data focus on a region that exhibits a strong and rapidly changing interplay of the mesonic cloud and quark-gluon dynamics in the nucleon. The results are in reasonable agreement with models that employ pion cloud effects and chiral effective field theory calculations, but at the same time they suggest that an improvement is required to the theoretical calculations and provide valuable input that will allow their refinements. The data illustrate the potential of the magnetic spectrometers setup in Hall C towards the study the Δ ( 1232 ) resonance. These first reported results will be followed by a series of measurements in Hall C, that will expand the studies of the Δ ( 1232 ) resonance offering a high precision insight within a wide kinematic range from low to high momentum transfers.

Neutrinos exist in one of three types or ‘flavours’—electron, muon and tau neutrinos—and oscillate from one flavour to another when propagating through space. This phenomena is one of the few that cannot be described using the standard model of particle physics (reviewed in ref.  1 ), and so its experimental study can provide new insight into the nature of our Universe (reviewed in ref.  2 ). Neutrinos oscillate as a function of their propagation distance ( L ) divided by their energy ( E ). Therefore, experiments extract oscillation parameters by measuring their energy distribution at different locations. As accelerator-based oscillation experiments cannot directly measure E , the interpretation of these experiments relies heavily on phenomenological models of neutrino–nucleus interactions to infer E . Here we exploit the similarity of electron–nucleus and neutrino–nucleus interactions, and use electron scattering data with known beam energies to test energy reconstruction methods and interaction models. We find that even in simple interactions where no pions are detected, only a small fraction of events reconstruct to the correct incident energy. More importantly, widely used interaction models reproduce the reconstructed energy distribution only qualitatively and the quality of the reproduction varies strongly with beam energy. This shows both the need and the pathway to improve current models to meet the requirements of next-generation, high-precision experiments such as Hyper-Kamiokande (Japan) 3 and DUNE (USA) 4 . Electron scattering measurements are shown to reproduce only qualitatively state-of-the-art lepton–nucleus energy reconstruction models, indicating that improvements to these particle-interaction models are required to ensure the accuracy of future high-precision neutrino oscillation experiments.

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 GeV2. 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, for example, 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.Measurements of the proton’s spin structure in experiments scattering a polarized electron beam off polarized protons in regions of low momentum transfer squared test predictions from chiral effective field theory of the strong interaction.

Positron beams, both polarized and unpolarized, are identified as important ingredients for the experimental programs at the next generation of lepton accelerators. In the context of the hadronic physics program at Jefferson Lab (JLab), positron beams are complementary, even essential, tools for a precise understanding of the electromagnetic structure of nucleons and nuclei, in both the elastic and deep-inelastic regimes. For instance, elastic scattering of polarized and unpolarized electrons and positrons from the nucleon enables a model independent determination of its electromagnetic form factors. Also, the deeply-virtual scattering of polarized and unpolarized electrons and positrons allows unambiguous separation of the different contributions to the cross section of the lepto-production of photons and of lepton-pairs, enabling an accurate determination of the nucleons and nuclei generalized parton distributions, and providing an access to the gravitational form factors of the energy-momentum tensor. Furthermore, positron beams offer the possibility of alternative tests of the Standard Model of particle physics through the search of a dark photon, the precise measurement of electroweak couplings, and the investigation of charged lepton flavor violation. This document discusses the perspectives of an experimental program with high duty-cycle positron beams at JLab.