The atomic nucleus is one of the densest and most complex quantum-mechanical systems in nature. Nuclei account for nearly all the mass of the visible Universe. The properties of individual nucleons ...(protons and neutrons) in nuclei can be probed by scattering a high-energy particle from the nucleus and detecting this particle after it scatters, often also detecting an additional knocked-out proton. Analysis of electron- and proton-scattering experiments suggests that some nucleons in nuclei form close-proximity neutron-proton pairs
with high nucleon momentum, greater than the nuclear Fermi momentum. However, how excess neutrons in neutron-rich nuclei form such close-proximity pairs remains unclear. Here we measure protons and, for the first time, neutrons knocked out of medium-to-heavy nuclei by high-energy electrons and show that the fraction of high-momentum protons increases markedly with the neutron excess in the nucleus, whereas the fraction of high-momentum neutrons decreases slightly. This effect is surprising because in the classical nuclear shell model, protons and neutrons obey Fermi statistics, have little correlation and mostly fill independent energy shells. These high-momentum nucleons in neutron-rich nuclei are important for understanding nuclear parton distribution functions (the partial momentum distribution of the constituents of the nucleon) and changes in the quark distributions of nucleons bound in nuclei (the EMC effect)
. They are also relevant for the interpretation of neutrino-oscillation measurements
and understanding of neutron-rich systems such as neutron stars
.
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KISLJ, NUK, SBMB, UL, UM, UPUK
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.
We report the first measurement of the \eep three-body breakup reaction cross sections in helium-3 (\(^3\)He) and tritium (\(^3\)H) at large momentum transfer (\(\langle Q^2 \rangle \approx 1.9\) ...(GeV/c)\(^2\)) and \(x_B>1\) kinematics, where the cross section should be sensitive to quasielastic (QE) scattering from single nucleons. The data cover missing momenta \(40 \le p_{miss} \le 500\) MeV/c that, in the QE limit with no rescattering, equals the initial momentum of the probed nucleon. The measured cross sections are compared with state-of-the-art ab-initio calculations. Overall good agreement, within \(\pm20\%\), is observed between data and calculations for the full \(p_{miss}\) range for \(^3\)H and for \(100 \le p_{miss} \le 350\) MeV/c for \(^3\)He. Including the effects of rescattering of the outgoing nucleon improves agreement with the data at \(p_{miss} > 250\) MeV/c and suggests contributions from charge-exchange (SCX) rescattering. The isoscalar sum of \(^3\)He plus \(^3\)H, which is largely insensitive to SCX, is described by calculations to within the accuracy of the data over the entire \(p_{miss}\) range. This validates current models of the ground state of the three-nucleon system up to very high initial nucleon momenta of \(500\) MeV/c.
Inclusive electron scattering at carefully chosen kinematics can isolate scattering from short-range correlations (SRCs), produced through hard, short-distance interactions of nucleons in the ...nucleus. Because the two-nucleon (2N) SRCs arise from the same N-N interaction in all nuclei, the cross section in the SRC-dominated regime is identical up to an overall scaling factor, and the A/2H cross section ratio is constant in this region. This scaling behavior has been used to identify SRC dominance and to map out the contribution of SRCs for a wide range of nuclei. We examine this scaling behavior at lower momentum transfers using new data on \(^2\)H, \(^3\)H, and \(^3\)He which show that the scaling region is larger than in heavy nuclei. Based on the improved scaling, especially for \(^3\)H/\(^3\)He, we examine the ratios at kinematics where three-nucleon SRCs may play an important role. The data for the largest initial nucleon momenta are consistent with isolation of scattering from 3N-SRCs, and suggest that the very-highest momentum nucleons in \(^3\)He have a nearly isospin-independent momentum configuration, or a small enhancement of the proton distribution.
When protons and neutrons (nucleons) are bound into atomic nuclei, they are close enough together 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 and mapping out the strength and isospin structure (neutron-proton vs proton-proton pairs) of these virtual excitations is thus critical input for modeling a range of nuclear, particle, and astrophysics measurements. Hitherto measurements used two-nucleon knockout or ``triple-coincidence'' reactions to measure the relative contribution of np- and pp-SRCs by knocking out a proton from the SRC and detecting its partner nucleon (proton or neutron). These measurementsshow that SRCs are almost exclusively np pairs, but had limited statistics and required large model-dependent final-state interaction (FSI) corrections. We report on the first measurement using inclusive scattering from the mirror nuclei \(^3\)H and \(^3\)He to extract the np/pp ratio of SRCs in the A=3 system. We obtain a measure of the np/pp SRC ratio that is an order of magnitude more precise than previous experiments, and find a dramatic deviation from the near-total np dominance observed in heavy nuclei. This result implies an unexpected structure in the high-momentum wavefunction for \(^3\)He and \(^3\)H. Understanding these results will improve our understanding of the short-range part of the N-N interaction.
The ratio of the nucleon \(F_2\) structure functions, \(F_2^n/F_2^p\), is determined by the MARATHON experiment from measurements of deep inelastic scattering of electrons from \(^3\)H and \(^3\)He ...nuclei. The experiment was performed in the Hall A Facility of Jefferson Lab and used two high resolution spectrometers for electron detection, and a cryogenic target system which included a low-activity tritium cell. The data analysis used a novel technique exploiting the mirror symmetry of the two nuclei, which essentially eliminates many theoretical uncertainties in the extraction of the ratio. The results, which cover the Bjorken scaling variable range \(0.19 < x < 0.83\), represent a significant improvement compared to previous SLAC and Jefferson Lab measurements for the ratio. They are compared to recent theoretical calculations and empirical determinations of the \(F_2^n/F_2^p\) ratio.
We report the first measurement of the \((e,e'p)\) reaction cross-section ratios for Helium-3 (\(^3\)He), Tritium (\(^3\)H), and Deuterium (\(d\)). The measurement covered a missing momentum range of ...\(40 \le p_{miss} \le 550\) MeV\(/c\), at large momentum transfer (\(\langle Q^2 \rangle \approx 1.9\) (GeV\(/c\))\(^2\)) and \(x_B>1\), which minimized contributions from non quasi-elastic (QE) reaction mechanisms. The data is compared with plane-wave impulse approximation (PWIA) calculations using realistic spectral functions and momentum distributions. The measured and PWIA-calculated cross-section ratios for \(^3\)He\(/d\) and \(^3\)H\(/d\) extend to just above the typical nucleon Fermi-momentum (\(k_F \approx 250\) MeV\(/c\)) and differ from each other by \(\sim 20\%\), while for \(^3\)He/\(^3\)H they agree within the measurement accuracy of about 3\%. At momenta above \(k_F\), the measured \(^3\)He/\(^3\)H ratios differ from the calculation by \(20\% - 50\%\). Final state interaction (FSI) calculations using the generalized Eikonal Approximation indicate that FSI should change the \(^3\)He/\(^3\)H cross-section ratio for this measurement by less than 5\%. If these calculations are correct, then the differences at large missing momenta between the \(^3\)He/\(^3\)H experimental and calculated ratios could be due to the underlying \(NN\) interaction, and thus could provide new constraints on the previously loosely-constrained short-distance parts of the \(NN\) interaction.
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