Modified structure of protons and neutrons in correlated pairs

Nucleon-nucleon correlations inside atomic nuclei: synergies, observations and theoretical models

While the main features of atomic nuclei are well described by nuclear mean-field models, there is a large and growing body of evidence which indicates an important additional role played by spatially-correlated nucleon-nucleon structures. The role of nucleonic structures was first suggested by Heidmann in 1950 to explain the pick-up reactions of energetic nucleons. Since then, a steady flux of new experimental evidence has confirmed the presence of similar structures inside atomic nuclei, dominated by correlations between pairs of nucleons. The role of these internal nucleon-nucleon correlations has been established using various energetic probes like photons, pions, leptons and hadrons. These correlated structures are essential for understanding the interaction of particles with nuclei and their presence provides an explanation of many specific nuclear phenomena, including backscattered protons, copious deuteron production, sub-threshold particle production, neutrino interactions with nuclei and the European Muon Collaboration effect. On the theoretical side, these measurements have stimulated a large number of phenomenological models specifically devised to address these enigmatic observations. While reviews exist for specific interactions, there is currently no published commentary which systematically encompasses the wide range of experimental signatures and theoretical frameworks developed thus far. The present review draws together the synergies between a wide range of different experimental and theoretical studies, summarizes progress in this area and highlights outstanding issues for further study.

First Measurement of Λ Electroproduction off Nuclei in the Current and Target Fragmentation Regions

We report results of Λ hyperon production in semi-inclusive deep-inelastic scattering off deuterium, carbon, iron, and lead targets obtained with the CLAS detector and the Continuous Electron Beam Accelerator Facility 5.014 GeV electron beam. These results represent the first measurements of the Λ multiplicity ratio and transverse momentum broadening as a function of the energy fraction (z) in the current and target fragmentation regions. The multiplicity ratio exhibits a strong suppression at high z and an enhancement at low z. The measured transverse momentum broadening is an order of magnitude greater than that seen for light mesons. This indicates that the propagating entity interacts very strongly with the nuclear medium, which suggests that propagation of diquark configurations in the nuclear medium takes place at least part of the time, even at high z. The trends of these results are qualitatively described by the Giessen Boltzmann-Uehling-Uhlenbeck transport model, particularly for the multiplicity ratios. These observations will potentially open a new era of studies of the structure of the nucleon as well as of strange baryons.

New Structure in the Deuteron

We demonstrate that a paradigm shift from considering the deuteron as a system of a bound proton and neutron to that of a pseudovector system in which we observe a proton and neutron results in the possibility of probing a new "incomplete" P-statelike structure on the light front (LF). This occurs at extremely large internal momenta, which can be achieved in a high energy transfer electrodisintegration of the deuteron. Investigating the deuteron on the light front, where the vacuum fluctuations are suppressed, we found that this new structure, together with the conventional S and D states, is leading order in transferred energy of the reaction and thus not suppressed on the light front. The incompleteness of the observed P state results in a violation of the angular condition that can happen only if the deuteron contains non-nucleonic structures, such as Δ Δ, N^{*}N or hidden color components. We demonstrate that experimentally verifiable signatures of incomplete P states are angular anisotropy of the light front momentum distribution of the nucleon in the deuteron, as well as an enhancement of the tensor polarization strength beyond the S- and D-wave predictions at large internal momenta in the deuteron.

Revealing the short-range structure of the mirror nuclei 3H and 3He

When protons and neutrons (nucleons) are bound into atomic nuclei, they are close enough to feel significant attraction, or repulsion, from the strong, short-distance part of the nucleon-nucleon interaction. These strong interactions lead to hard collisions between nucleons, generating pairs of highly energetic nucleons referred to as short-range correlations (SRCs). SRCs are an important but relatively poorly understood part of nuclear structure^1-3, and mapping out the strength and the isospin structure (neutron-proton (np) versus proton-proton (pp) pairs) of these virtual excitations is thus critical input for modelling a range of nuclear, particle and astrophysics measurements^3-5. Two-nucleon knockout or 'triple coincidence' reactions have been used to measure the relative contribution of np-SRCs and pp-SRCs by knocking out a proton from the SRC and detecting its partner nucleon (proton or neutron). These measurements^6-8 have shown that SRCs are almost exclusively np pairs, but they had limited statistics and required large model-dependent final-state interaction corrections. Here we report on measurements using inclusive scattering from the mirror nuclei hydrogen-3 and helium-3 to extract the np/pp ratio of SRCs in systems with a mass number of three. We obtain a measure of the np/pp SRC ratio that is an order of magnitude more precise than previous experiments, and find a marked deviation from the near-total np dominance observed in heavy nuclei. This result implies an unexpected structure in the high-momentum wavefunction for hydrogen-3 and helium-3. Understanding these results will improve our understanding of the short-range part of the nucleon-nucleon interaction.

Lattice QCD Constraints on the Parton Distribution Functions of ^{3}He

The fraction of the longitudinal momentum of ^{3}He that is carried by the isovector combination of u and d quarks is determined using lattice QCD for the first time. The ratio of this combination to that in the constituent nucleons is found to be consistent with unity at the few-percent level from calculations with quark masses corresponding to m_{π}∼800  MeV. With a naive extrapolation to the physical quark masses, this constraint is consistent with, and more precise than, determinations from global nuclear parton distribution function fits through the nnnpdf framework. It is thus concretely demonstrated that lattice QCD calculations of light nuclei have imminent potential to enable more precise determinations of the u and d parton distributions in light nuclei and to reveal the QCD origins of the EMC effect.

Do Short-Range Correlations Cause the Nuclear EMC Effect in the Deuteron?

The relative contributions to the valence nuclear European Muon Collaboration (EMC) effect in the deuteron arising from nucleon off-shell effects and Fermi motion are examined in models which include nuclear binding and off-shell effects. Contrary to expectations, the effect of Fermi motion overwhelms the off-shell effects for nucleons in short-range correlations (SRCs), calling into question the hypothesized causal connection between SRCs and the EMC effect.

Neutron Valence Structure from Nuclear Deep Inelastic Scattering

Mechanisms of spin-flavor SU(6) symmetry breaking in quantum chromodynamics (QCD) are studied via an extraction of the free neutron structure function from a global analysis of deep inelastic scattering (DIS) data on the proton and on nuclei from A=2 (deuterium) to 208 (lead). Modification of the structure function of nucleons bound in atomic nuclei (known as the EMC effect) are consistently accounted for within the framework of a universal modification of nucleons in short-range correlated (SRC) pairs. Our extracted neutron-to-proton structure function ratio F_{2}^{n}/F_{2}^{p} becomes constant for x_{B}≥0.6, equaling 0.47±0.04 as x_{B}→1, in agreement with theoretical predictions of perturbative QCD and the Dyson-Schwinger equation, and in disagreement with predictions of the scalar diquark dominance model. We also predict F_{2}^{^{3}He}/F_{2}^{^{3}H}, recently measured, as yet unpublished, by the MARATHON Collaboration, the nuclear correction function that is needed to extract F_{2}^{n}/F_{2}^{p} from F_{2}^{^{3}He}/F_{2}^{^{3}H}, and the theoretical uncertainty associated with this extraction.

Searching for Flavor Dependence in Nuclear Quark Behavior

The observed correlation between the EMC effect and the contribution of short-range correlations (SRCs) in nuclei suggests that the modification of the quark distributions of bound protons and neutrons might occur within SRCs. This raises the possibility that the EMC effect may have an isospin dependence arising from the np dominance of SRCs. We discuss previous attempts to test this possibility and perform a new analysis of existing data. We find no experimental support for the observation of an isospin dependence of the EMC effect.

Direct Observation of Proton-Neutron Short-Range Correlation Dominance in Heavy Nuclei

We measured the triple coincidence A(e,e^{'}np) and A(e,e^{'}pp) reactions on carbon, aluminum, iron, and lead targets at Q^{2}>1.5  (GeV/c)^{2}, x_{B}>1.1 and missing momentum >400  MeV/c. This was the first direct measurement of both proton-proton (pp) and neutron-proton (np) short-range correlated (SRC) pair knockout from heavy asymmetric nuclei. For all measured nuclei, the average proton-proton (pp) to neutron-proton (np) reduced cross-section ratio is about 6%, in agreement with previous indirect measurements. Correcting for single-charge exchange effects decreased the SRC pairs ratio to ∼3%, which is lower than previous results. Comparisons to theoretical generalized contact formalism (GCF) cross-section calculations show good agreement using both phenomenological and chiral nucleon-nucleon potentials, favoring a lower pp to np pair ratio. The ability of the GCF calculation to describe the experimental data using either phenomenological or chiral potentials suggests possible reduction of scale and scheme dependence in cross-section ratios. Our results also support the high-resolution description of high-momentum states being predominantly due to nucleons in SRC pairs.