Individual Nucleons Research Articles

Modification of Quark-Gluon Distributions in Nuclei by Correlated Nucleon Pairs.

We extend the QCD Parton Model analysis using a factorized nuclear structure model incorporating individual nucleons and pairs of correlated nucleons. Our analysis of high-energy data from lepton deep-inelastic scattering, Drell-Yan, and W and Z boson production simultaneously extracts the universal effective distribution of quarks and gluons inside correlated nucleon pairs, and their nucleus-specific fractions. Such successful extraction of these universal distributions marks a significant advance in our understanding of nuclear structure properties connecting nucleon- and parton-level quantities.

Generation of Fragment Angular Momentum in Nuclear Fission

As a fissioning nucleus approaches scission, the angular-momentum bearing modes in the evolving dinuclear complex may be agitated by multiple transfers of individual nucleons. It is discussed how this mechanism populates the various rotational modes at different rates and leads to fragment angular momenta that are preferentially perpendicular to the fission axis but mutually largely uncorrelated. Using the fission simulation code FREYA, it is demonstrated how a measurement of the angular distribution of photons from identified collective transitions in the product nuclei can provide quantitative information on the relative importance of the twisting mode in fission.

The neutron and proton mass radii from the vector meson photoproduction data on the deuterium target

In this study, we try to extract the mass radii of the neutron and the proton from the differential cross section data of near-threshold $$\omega $$ and $$\phi $$ photoproductions on deuterium target, which is often approximated as a quasi-free neutron plus a quasi-free proton. The incoherent data of $$\omega $$ and $$\phi $$ photoproductions are provided by CBELSA/TAPS collaboration and LEPS collaboration respectively, where the deuteron is disintegrated in the experiments to measure the properties of individual nucleons. Under the VMD model and the assumption of dipole gravitational form factor, we determined the loosely bound neutron and proton mass radii to be $$0.795\pm 0.092\mathrm (stat.)\pm 0.073\mathrm (syst.)$$ fm and $$0.744\pm 0.029\mathrm (stat.)\pm 0.042\mathrm (syst.)$$ fm respectively from the near-threshold data of $$\gamma d \rightarrow \omega n (p)$$ and $$\gamma d \rightarrow \omega p (n)$$ , for the first time. With the near-threshold and incoherent $$\phi $$ photoproduction data of $$\gamma d \rightarrow \phi p n$$ , we determined the average mass radius of the bound nucleon (neutron or proton) inside the deuteron to be $$0.755\pm 0.039\mathrm (stat.)\pm 0.039\mathrm (syst.)$$ fm, for the first time. For a comparison study, we also extracted the mass radius of the free proton from the $$\omega $$ photoproduction on the hydrogen target by CBELSA/TAPS collaboration. Based on our analysis results under the assumptions of VMD model and a low energy QCD theorem, we find that the neutron mass radius is consistent with the proton mass radius within the current statistical uncertainties, and that the nuclear modification on the nucleon mass radius is small inside the deuteron.

Outer crust of a cold, nonaccreting neutron star within the quark-meson-coupling model

The outer crust properties of cold nonaccreting neutron stars are studied within the framework of the quark-meson coupling (QMC) model, which includes the effects of modifications of the quark structure inside individual nucleons when they are within a high-density nuclear medium. With a unique set of five well-constrained adjustable parameters, which have a clear physical basis, the QMC model gives predictions for the ground state observables of even-even nuclei which agree with experiment as well as traditional models. Furthermore, it gives improved theoretical values for nuclei thought to play a role in the outer crusts of neutron stars but for which experimental data are not available. Using the latest experimental data tables wherever possible but otherwise the predictions from the QMC model, we construct an equation of state for the outer crust, which is then used within stellar model calculations to obtain an equilibrium sequence of crustal layers, each characterized by a particular neutron-rich nucleus. Various properties of the layers are calculated for a range of neutron-star masses and comparisons are made with alternative equations of state from the literature. This leads to the conclusion that the QMC model successfully predicts the outer crust properties and is fully comparable with the more traditional mass models, which all depend on a larger number of parameters.

Spinodal instability at the onset of collective expansion in nuclear collisions

Using transport theory to model central Au + Au collisions in the energy region of 20–110 MeV/u, at impact parameters b≤5 fm, we predict a measurable impact of the spinodal instability as the collective expansion sets in with energy. Two transport models are employed, the pBUU model, solving a Boltzmann-Uehling-Uhlenbeck equation, and the Brownian Motion (BM) model, solving a set of Langevin equations to describe the motion of individual nucleons in a noisy nuclear medium. We find without ambiguity, for the first time, that a combination of delayed equilibration, onset of collective expansion and the spinodal instability produces a pair of transient ring structures, made of the projectile and target remnants, with spectator nucleons predicted to end in the entities reminiscent of stones in jewelry, on the rings. The ring structures, calculated in the configuration space and mapped onto the velocity space, could be detected in experimental collective flow data.

Nucleon localization function in rotating nuclei

Background: An electron localization function was originally introduced to visualize bond structures in molecules. It became a useful tool to describe electron configurations in atoms, molecules and solids. In nuclear physics, a nucleon localization function (NLF) has been used to characterize clusters in light nuclei, fragment formation in fission and pasta phases in the inner crust of neutron stars. Purpose: We use the NLF to study the nuclear response to fast rotation. Methods: We generalize the NLF to the case of nuclear rotation. The extended expressions involve both time-even and time-odd local densities. Since current density and density gradient contribute to the NLF primarily at the surface, we propose a simpler spatial measure given by the kinetic-energy density. Illustrative calculations for the superdeformed yrast band of $^{152}$Dy were carried out by using the cranked Skyrme-Hartree-Fock method. We also employed the cranked harmonic-oscillator model to gain insights into patterns revealed by the NLF at high angular momentum. Results: In a deformed rotating nucleus, several NLFs can be introduced, depending on the definition of the spin-quantization axis and self-consistent symmetries of the system. The oscillating pattern of the NLF can be explained by a constructive interference between the kinetic-energy and particle densities. The nodal pattern seen in the NLF along the major axis of a rotating nucleus comes from single-particle orbits with large aligned angular momentum. The variation of the NLF along the minor axis is traced back to deformation-aligned orbits. Conclusions: The NLF allows a simple interpretation of the shell structure evolution in the rotating nucleus in terms of the angular-momentum alignment of individual nucleons. We expect that the NLF will be useful for the characterization of other collective modes and time-dependent processes.

How to produce antinuclei from dark matter

We show how to produce antideuteron, antihelium, and other antinuclei in large fractions from the decays of a new particle ϕ that carries baryon number. Close to threshold, the production of nuclear bound states is preferred over the decay into individual nucleons, effectively decoupling antinuclei and antiproton fluxes and allowing the former to dominate, in clear contrast to antimatter production via coalescence. ϕ can either form dark matter itself or be produced by it, and can give rise to a potentially testable amount of antinuclei.

Coupled K+Λ and K0Λ photoproduction off the nucleon: Consequences from the recent CLAS and MAMI data and the N(1680)P11 narrow state

The new $\gamma n\to K^0\Lambda$ data obtained from the CLAS and MAMI collaborations are analyzed by employing an effective Lagrangian method. The constructed model can describe all available experimental data in both $\gamma p \to K^+\Lambda$ and $\gamma n\to K^0\Lambda$ channels, simultaneously. The background part of the model is built from the appropriate intermediate states involving the nucleon, kaon, and hyperon exchanges, whereas the resonance part is constructed from the consistent interaction Lagrangians and propagators. To check the performance of the model a detailed comparison between the calculated observables and experimental data in both isospin channels is presented, from which a nice agreement can be observed. The discrepancy between the CLAS and MAMI data in the $\gamma n\to K^0\Lambda$ channel is analyzed by utilizing three different models; M1, M2, and M3 that fit the CLAS, MAMI, and both CLAS and MAMI data sets, respectively. The effect of this discrepancy is studied by investigating the significance of individual nucleon resonances and the predicted beam-target helicity asymmetry $E$ that has been measured by the CLAS collaboration recently. It is found that the $N(1720)P_{13}$, $N(1900)P_{13}$, and $N(2060)D_{15}$ resonances are significant for improving the agreement between model calculation and data. This result is relatively stable to the choice of the model. The helicity asymmetry $E$ can be better explained by the models M1 and M3. Finally, the effect of the $N(1680)P_{11}$ narrow resonance on the cross section of both isospin channels is explored. It is found that the effect is more sensitive in the $\gamma n\to K^0\Lambda$ channel. In this case the model M3, that fits both CLAS and MAMI data, yields a more realistic effect.

Deep inelastic scattering on a nucleus using holography

We consider deep inelastic scattering (DIS) on a nucleus described using a density expansion. In leading order, the scattering is dominated by the incoherent scattering on individual nucleons distributed using the Thomas-Fermi approximation. We use the holographic structure functions for DIS scattering on single nucleons to make a non-perturbative estimate of the nuclear structure function in leading order in the density. Our results are compared to the data in the large-x regime.

Exploring new small system geometries in heavy ion collisions

Relativistic heavy ion collisions produce nuclei-sized droplets of quark-gluon plasma whose expansion is well described by viscous hydrodynamic calculations. Over the past half decade, this formalism was also found to apply to smaller droplets closer to the size of individual nucleons, as produced in $p$$+$$p$ and $p$$+$$A$ collisions. The hydrodynamic paradigm was further tested with a variety of collision species, including $p$$+$Au, $d$$+$Au, and $^{3}$He$+$Au producing droplets with different geometries. Nevertheless, questions remain regarding the importance of pre-hydrodynamic evolution and the exact medium properties during the hydrodynamic evolution phase, as well as the applicability of alternative theories that argue the agreement with hydrodynamics is accidental. In this work we explore options for new collision geometries including $p$$+$O and O$+$O proposed for running at the Large Hadron Collider, as well as, $^{4}$He$+$Au, C$+$Au, O$+$Au, and $^{7,9}$Be$+$Au at the Relativistic Heavy Ion Collider.

Probing high-momentum protons and neutrons in neutron-rich nuclei

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 pairs1-12 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)1,13,14. They are also relevant for the interpretation of neutrino-oscillation measurements15 and understanding of neutron-rich systems such as neutron stars3,16.

First lattice QCD study of the gluonic structure of light nuclei

The role of gluons in the structure of the nucleon and light nuclei is investigated using lattice quantum chromodynamics (QCD) calculations. The first moment of the unpolarised gluon distribution is studied in nuclei up to atomic number $A=3$ at quark masses corresponding to pion masses of $m_\pi\sim 450$ and $806$ MeV. Nuclear modification of this quantity defines a gluonic analogue of the EMC effect and is constrained to be less than $\sim 10$% in these nuclei. This is consistent with expectations from phenomenological quark distributions and the momentum sum rule. In the deuteron, the combination of gluon distributions corresponding to the $b_1$ structure function is found to have a small first moment compared with the corresponding momentum fraction. The first moment of the gluon transversity structure function is also investigated in the spin-1 deuteron, where a non-zero signal is observed at $m_\pi \sim 806$ MeV. This is the first indication of gluon contributions to nuclear structure that can not be associated with an individual nucleon.

New Measurement of the Direct 3α Decay from the ^{12}C Hoyle State.

Excited states in certain atomic nuclei possess an unusual structure, where the dominant degrees of freedom are those of α clusters rather than individual nucleons. It has been proposed that the diffuse 3α system of the ^{12}C Hoyle state may behave like a Bose-Einstein condensate, where the α clusters maintain their bosonic identities. By measuring the decay of the Hoyle state into three α particles, we obtained an upper limit for the rare direct 3α decay branch of 0.047%. This value is now at a level comparable with theoretical predictions and could be a sensitive probe of the structure of this state.

Erratum: Gluonic transversity from lattice QCD [Phys. Rev. D 94 , 014507 (2016)

We present an exploratory study of the gluonic structure of the $\phi$ meson using lattice QCD (LQCD). This includes the first investigation of gluonic transversity via the leading moment of the twist-two double-helicity-flip gluonic structure function $\Delta(x,Q^2)$. This structure function only exists for targets of spin $J\ge1$ and does not mix with quark distributions at leading twist, thereby providing a particularly clean probe of gluonic degrees of freedom. We also explore the gluonic analogue of the Soffer bound which relates the helicity flip and non-flip gluonic distributions, finding it to be saturated at the level of 80%. This work sets the stage for more complex LQCD studies of gluonic structure in the nucleon and in light nuclei where $\Delta(x,Q^2)$ is an 'exotic glue' observable probing gluons in a nucleus not associated with individual nucleons.

Structural and decay properties of Z = 132, 138 superheavy nuclei

In this paper, we analyze the structural properties of Z = 132 and Z = 138 superheavy nuclei within the ambit of axially deformed relativistic mean-field framework with NL3 * parametrization and calculate the total binding energies, radii, quadrupole deformation parameter, separation energies, density distributions. We also investigate the phenomenon of shape coexistence by performing the calculations for prolate, oblate and spherical configurations. For clear presentation of nucleon distributions, the two-dimensional contour representation of individual nucleon density and total matter density has been made. Further, a competition between possible decay modes such as $\alpha$ -decay, $\beta$ -decay and spontaneous fission of the isotopic chain of superheavy nuclei with Z = 132 within the range $ 312 \le A \le 392$ and $318 \le A \le 398$ for Z = 138 is systematically analyzed within self-consistent relativistic mean-field model. From our analysis, we inferred that the $\alpha$ -decay and spontaneous fission are the principal modes of decay in majority of the isotopes of superheavy nuclei under investigation apart from $\beta$ -decay as dominant mode of decay in 318-322138 isotopes.

Gluonic transversity from lattice QCD

We present an exploratory study of the gluonic structure of the $\ensuremath{\phi}$ meson using lattice QCD (LQCD). This includes the first investigation of gluonic transversity via the leading moment of the twist-2 double-helicity-flip gluonic structure function $\mathrm{\ensuremath{\Delta}}(x,{Q}^{2})$. This structure function only exists for targets of spin $J\ensuremath{\ge}1$ and does not mix with quark distributions at leading twist, thereby providing a particularly clean probe of gluonic degrees of freedom. We also explore the gluonic analogue of the Soffer bound which relates the helicity flip and nonflip gluonic distributions, finding it to be saturated at the level of 80%. This work sets the stage for more complex LQCD studies of gluonic structure in the nucleon and in light nuclei where $\mathrm{\ensuremath{\Delta}}(x,{Q}^{2})$ is an ``exotic glue'' observable probing gluons in a nucleus not associated with individual nucleons.

Cluster dynamics and symmetries in light nuclei

Many light nuclei display behaviour that indicates that, rather than behaving as an A -body system of individual nucleons, the degrees of freedom are those of clusters. The appearance of α -particle clustering is most widespread. In the present proceedings the symmetries and dynamics of the nuclei 8 Be, 12 C and 16 O are examined together with some recent experimental measurements.

Effect of breakup and transfer on complete and incomplete fusion in6Li+209Bi reaction in multi-body classical molecular dynamics calculation

The effect of breakup and transfer in 6 Li+209 Bi reaction is studied in a multi-body classical molecular dynamics approach in which the weakly-bound projectile 6 Li is constructed as a 2-body cluster of 4 He and 2 H in a configuration corresponding to the observed breakup energy. This 3-body system with their individual nucleon configuration in their ground state is dynamically evolved with given initial conditions using Classical Rigid Body Dynamics (CRBD) approach up to distances close to the barrier when the rigid-body constraint on the target, inter-fragment distance, and 2 H itself are relaxed, allowing for possible breakup of 2 H which may result in incomplete fusion following the transfer of the n or p . Relative probabilities of the possible events such as scattering with and without breakup, DCF, SCF, ICF(x ) where x may be 4 He, 2 H, 4 He+n , 4 He+p , n , p are calculated. Comparison of the calculated event-probabilities, complete, and incomplete fusion cross sections with the calculation in which 2 H is kept rigid demonstrates the effect of the transfer reactions on complete and incomplete fusion in the 4-body reaction. Events ICF(4 He+n ) corresponding to nstripping followed by breakup of the resultant 5 Li to 4 He+p are found to contribute significantly in the fusion process in agreement with a recent experimental observation of direct reaction processes in breakup of weakly-bound projectiles.

Enhancing the interaction between nuclear experiment and theory through information and statistics

This Focus Issue draws from a range of topics within nuclear physics, from studies of individual nucleons to the heaviest of nuclei. The unifying theme, however, is to illustrate the extent to which uncertainty is a key quantity, and to showcase applications of the latest computational methodologies. It is our assertion that a paradigm shift is needed in nuclear physics to enhance the coupling between theory and experiment, and we hope that this collection of articles is a good start.

Classical molecular dynamics simulation of weakly-bound projectile heavy-ion reactions

A 3-body classical molecular dynamics approach for heavy-ion reactions involving weakly bound projectiles is developed. In this approach a weakly bound projectile is constructed as a two-body cluster of the constituent tightly bound nuclei in a configuration corresponding to the observed breakup energy. This 3-body system with their individual nucleon configuration in their ground state is dynamically evolved for given initial conditions using the three-stage classical molecular dynamics approach (3S-CMD). Various levels of rigidbody constraints on the projectile constituents and the target are considered at appropriate stages. This 3-dimensional approach explicitly takes into account not only the long range Coulomb reorientation of the deformed collision partner but internal excitations and breakup probabilities at distances close to the barrier also. Dynamical simulations of ^6Li+^209Bi show all the possible reaction mechanism like complete fusion, incomplete fusion, scattering and breakup scattering. Complete fusion cross sections of ^6Li+^209Bi and ^7Li+^209Bi reactions are calculated in this approach with systematic relaxations of the rigid-body constraints on one or more constituent nuclei.