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The atomic nucleus is made of protons and neutrons (nucleons), which are themselves composed of quarks and gluons. Understanding how the quark–gluon structure of a nucleon bound in an atomic nucleus is modified by the surrounding nucleons is an outstanding challenge. Although evidence for such modification—known as the EMC effect—was first observed over 35 years ago, there is still no generally accepted explanation for its cause 1 – 3 . Recent observations suggest that the EMC effect is related to close-proximity short-range correlated (SRC) nucleon pairs in nuclei 4 , 5 . Here we report simultaneous, high-precision measurements of the EMC effect and SRC abundances. We show that EMC data can be explained by a universal modification of the structure of nucleons in neutron–proton SRC pairs and present a data-driven extraction of the corresponding universal modification function. This implies that in heavier nuclei with many more neutrons than protons, each proton is more likely than each neutron to belong to an SRC pair and hence to have distorted quark structure. This universal modification function will be useful for determining the structure of the free neutron and thereby testing quantum chromodynamics symmetry-breaking mechanisms and may help to discriminate between nuclear physics effects and beyond-the-standard-model effects in neutrino experiments. Simultaneous high-precision measurements of the EMC effect and short-range correlated abundances for several nuclei reveal a universal modification of the structure of nucleons in short-range correlated neutron–proton pairs.

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.

The double-spin-polarization observable $\\mathbb{E}$ for $\\vec{\\gamma}\\vec{p}\\to p\\pi^0$ has been measured with the CEBAF Large Acceptance Spectrometer (CLAS) at photon beam energies $E_\\gamma$ from 0.367 to $2.173~\\mathrm{GeV}$ (corresponding to center-of-mass energies from 1.240 to $2.200~\\mathrm{GeV}$) for pion center-of-mass angles, $\\cos\\theta_{\\pi^0}^{c.m.}$, between -0.86 and 0.82. These new CLAS measurements cover a broader energy range and have smaller uncertainties compared to previous CBELSA data and provide an important independent check on systematics. These measurements are compared to predictions as well as new global fits from The George Washington University, Mainz, and Bonn-Gatchina groups. Their inclusion in multipole analyses will refine our understanding of the single-pion production contribution to the Gerasimov-Drell-Hearn sum rule and improve the determination of resonance properties.

We report the first measurements of deep inelastic scattering spin-dependent azimuthal asymmetries in back-to-back dihadron electroproduction, where two hadrons are produced in opposite hemispheres along the z-axis in the center-of-mass frame, with the first hadron produced in the current-fragmentation region and the second in the target-fragmentation region. The data were taken with longitudinally polarized electron beams of 10.2 and 10.6 GeV incident on an unpolarized liquid-hydrogen target using the CLAS12 spectrometer at Jefferson Lab. Observed non-zero \\(\\sin\\Delta\\phi\\) modulations in \\(ep \\rightarrow e'p\\pi^+X\\) events, where \\(\\Delta\\phi\\) is the difference of the azimuthal angles of the proton and pion in the virtual photon and target nucleon center-of-mass frame, indicate that correlations between the spin and transverse momenta of hadrons produced in the target- and current-fragmentation regions may be significant. The measured beam-spin asymmetries provide a first access in dihadron production to a previously unobserved leading-twist spin- and transverse-momentum-dependent fracture function. The fracture functions describe the hadronization of the target remnant after the hard scattering of a virtual photon off a quark in the target particle and provide a new avenue for studying nucleonic structure and hadronization.

The cross section for coherent \\(\\omega\\)-meson photoproduction off the deuteron has been measured for the first time as a function of the momentum transfer \\(t = (P_{\\gamma}-P_{\\omega})^2\\) and photon energy \\(E_{\\gamma}\\) using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cross sections are measured in the energy range \\(1.4 < E_{\\gamma} < 3.4\\) GeV. A model based on \\(\\omega-N\\) rescattering is consistent with the data at low and intermediate momentum transfer, \\(|t|\\). For \\(2.8 < E_{\\gamma} < 3.4\\) GeV, the total cross-section of \\(\\omega-N\\) scattering, based on fits within the framework of the Vector Meson Dominance model, is in the range of 30-40 mb.

The unpolarized and polarized Beam Char\\-ge Asymmetries (BCAs) of the \\(\\vv{e}^{\\pm}p \\to e^{\\pm}p \\gamma\\) process off unpolarized hydrogen are discussed. The measurement of BCAs with the CLAS12 spectrometer at the Thomas Jefferson National Accelerator Facility, using polarized positron and electron beams at 10.6 GeV is investigated. This experimental configuration allows to measure azimuthal and \\(t\\)-dependences of the unpolarized and polarized BCAs over a large \\((x_B,Q^2)\\) phase space, providing a direct access to the real part of the Compton Form Factor (CFF) \\({\\mathcal H}\\). Additionally, these measurements confront the Bethe-Heitler dominance hypothesis and eventual effects beyond leading twist. The impact of potential positron beam data on the determination of CFFs is also investigated within a local fitting approach of experimental observables. Positron data are shown to strongly reduce correlations between CFFs and consequently improve significantly the determination of \\(\\Re {\\rm e} [\\mathcal{H}]\\).

The strong nuclear interaction between nucleons (protons and neutrons) is the effective force that holds the atomic nucleus together. This force stems from fundamental interactions between quarks and gluons (the constituents of nucleons) that are described by the equations of Quantum Chromodynamics (QCD). However, as these equations cannot be solved directly, physicists resort to describing nuclear interactions using effective models that are well constrained at typical inter-nucleon distances in nuclei but not at shorter distances. This limits our ability to describe high-density nuclear matter such as in the cores of neutron stars. Here we use high-energy electron scattering measurements that isolate nucleon pairs in short-distance, high-momentum configurations thereby accessing a kinematical regime that has not been previously explored by experiments, corresponding to relative momenta above 400 MeV/c. As the relative momentum between two nucleons increases and their separation thereby decreases, we observe a transition from a spin-dependent tensor-force to a predominantly spin-independent scalar-force. These results demonstrate the power of using such measurements to study the nuclear interaction at short-distances and also support the use of point-like nucleons with two- and three-body effective interactions to describe nuclear systems up to densities several times higher than the central density of atomic nuclei.

The parameterization of the nucleon structure through Generalized Parton Distributions (GPDs) shed a new light on the nucleon internal dynamics. For its direct interpretation, Deeply Virtual Compton Scattering (DVCS) is the golden channel for GPDs investigation. The DVCS process interferes with the Bethe-Heitler (BH) mechanism to constitute the leading order amplitude of the \\(eN \\to eN\\gamma\\) process. The study of the \\(ep\\gamma\\) reaction with polarized positron and electron beams gives a complete set of unique observables to unravel the different contributions to the \\(ep \\gamma\\) cross section. This separates the different reaction amplitudes, providing a direct access to their real and imaginary parts which procures crucial constraints on the model dependences and associated systematic uncertainties on GPDs extraction. The real part of the BH-DVCS interference amplitude is particularly sensitive to the \\(D\\)-term which parameterizes the Gravitational Form Factors of the nucleon. The separation of the imaginary parts of the interference and DVCS amplitudes provides insights on possible higher-twist effects. We propose to measure the unpolarized and polarized Beam Charge Asymmetries (BCAs) of the \\(\\vec{e}^{\\pm}p \\to e^{\\pm}p \\gamma\\) process on an unpolarized hydrogen target with {\\tt CLAS12}, using polarized positron and electron beams at 10.6 GeV. The azimuthal and \\(t\\)-dependences of the unpolarized and polarized BCAs will be measured over a large \\((x_B,Q^2)\\) phase space using a 100 day run with a luminosity of 0.66\\(\\times 10^{35}\\)cm\\(^{-2}\\cdot\\)s\\(^{-1}\\).

The three-dimensional picture of quarks and gluons in the nucleon is set to be revealed through deeply virtual Compton scattering (DVCS). With the absence of a free neutron target, the deuterium target represents the simplest nucleus to be used to probe the internal 3D partonic structure of the neutron. We propose here to measure the beam spin asymmetry (BSA) in incoherent neutron DVCS together with the approved E12-06-113 experiment (BONuS12) within the run group F, using the same beam time, simply with addition of beam polarization. The DVCS BSA on the quasi-free neutron will be measured in a wide range of kinematics by tagging the scattered electron and the real photon final state with the spectator proton. We will also measure BSA with all final state particles detected including the struck neutron. The proposed measurements is complementary to the approved CLAS12 experiment E12-11-003, which will also measure the quasi-free neutron DVCS by detecting the scattered neutron, but not the spectator proton. Indeed, besides providing more data for neutron DVCS, this experiment will allow a comparison of the measurement of the BSA of neutron DVCS from the approved E12-11-003 with the measurements using the two methods proposed herein. This comparison will help to understand the impact of nuclear effects, such as the final state interactions (FSI) and Fermi motion on the measurement of the neutron DVCS.

The quark structure of the \\(f_2(1270)\\) meson has, for many years, been assumed to be a pure quark-antiquark (\\(q\\bar{q}\\)) resonance with quantum numbers \\(J^{PC} = 2^{++}\\). Recently, it was proposed that the \\(f_2(1270)\\) is a molecular state made from the attractive interaction of two \\(\\rho\\)-mesons. Such a state would be expected to decay strongly to final states with charged pions, due to the dominant decay \\(\\rho \\to \\pi^+ \\pi^-\\), whereas decay to two neutral pions would likely be suppressed. Here, we measure for the first time the reaction \\(\\gamma p \\to \\pi^0 \\pi^0 p\\), using the CLAS detector at Jefferson Lab for incident beam energies between 3.6-5.4~GeV. Differential cross sections, \\(d\\sigma / dt\\), for \\(f_2(1270)\\) photoproduction are extracted with good precision, due to low backgrounds, and are compared with theoretical calculations.