Search Results (11)

We investigate whether short-range nucleon–nucleon correlations (NN-SRC) and cluster configurations in nuclei can be explored by studying spectator neutrons produced in high-energy nucleus–nucleus collisions. In particular, we propose to measure the multiplicity distributions of forward spectator neutrons in symmetric ¹²C–¹²C and ⁴⁰Ca–⁴⁰Ca collisions at $\sqrt{s_{\mathrm{NN}}}=11$ GeV with the Spin Physics Detector (SPD) at the NICA facility. To assess this method, we simulate the production of spectator nucleons in these reactions using the Abrasion–Ablation Monte Carlo for Colliders model with MST clustering (AAMCC-MST). Short-range nucleon–nucleon correlations inside ¹²C and ⁴⁰Ca are implemented via a Monte Carlo rejection sampling procedure. Our results indicate that spectator production exhibits only a weak dependence on the specific features of NN-SRC. We also observe that including $\alpha$-cluster configurations in ¹²C leads to a reduction of the average multiplicity of spectator neutrons as a function of collision centrality. Full article

This study investigates the masses and quadrupole deformations of even-Z nuclei within the range $8⩽Z⩽104$ using the triaxial relativistic Hartree–Bogoliubov model (TRHB) with the PC-PK1 density functional. For odd-mass nuclei, the global minima were determined using the automatic blocking method and their dynamical correlation energies (DCEs) were approximated using the average values of neighboring even–even nuclei calculated from a microscopic, five-dimensional, collective Hamiltonian (5DCH). The mean-field results underestimate the binding energies of most open-shell nuclei, with an initial root–mean–square (rms) deviation of 2.56 MeV for 1223 even-Z nuclei. Incorporating DCEs significantly reduces this deviation to 1.36 MeV. Additionally, the descriptions of two-neutron and one-neutron separation energies are improved, with rms deviations decreasing to 0.75 MeV and 0.65 MeV, respectively. Further refinement through accounting for odd–even differences in DCEs reduces the rms deviations for binding energies and one-neutron separation energies to 1.30 MeV and 0.63 MeV, respectively. Regarding the quadrupole deformations, TRHB calculations reveal spherical shapes near shell and subshell closures, well-deformed shapes at the mid-shell, and rapid shape transitions in medium- and heavy-mass regions. Oblate shapes dominate in regions $(Z,N)\sim (14,14),(34,36)$, and $(40,60)$, and the neutron-deficient Pb region, with notable odd–even shape staggering attributed to the blocking effect of the odd nucleon. Triaxial shapes are favored in the mass regions $(Z,N)\sim (60,76)$ and $(76,116)$. Full article

Two extensions of the variational method with explicit energy functionals (EEFs) with respect to the spin-orbit force were performed. In this method, the energy per nucleon of nuclear matter is explicitly expressed as a functional of various two-body distribution functions, starting from realistic nuclear forces. The energy was then minimized by solving the Euler–Lagrange equation for the distribution functions derived from the EEF. In the first extension, an EEF of symmetric nuclear matter at zero temperature was constructed using the two-body central, tensor, and spin-orbit nuclear forces. The energy per nucleon calculated using the Argonne v8’ two-body nuclear potential was found to be lower than those calculated using other many-body methods, implying that the energy contribution caused by the spin-orbit correlation, whose relative orbital angular momentum operator acts on other correlations, is necessary. In a subsequent extension, the EEF of neutron matter at zero temperature, including the spin-orbit force, was extended to neutron matter at finite temperatures using the method by Schmidt and Pandharipande. The thermodynamic quantities of neutron matter calculated using the Argonne v8’ nuclear potential were found to be reasonable and self-consistent. Full article

A survey of the calculations of the isovector axial vector form factor of the nucleon using lattice QCD is presented. Attention is paid to statistical and systematic uncertainties, in particular those due to excited state contributions. Based on a comparison of results from various collaborations, a case is made that lattice results are consistent within 10%. A similar level of uncertainty is in the axial charge $g_A^{u-d}$, the mean squared axial charge radius $⟨r_A^2⟩$, the induced pseudoscalar charge $g_P^{\ast }$, and the pion–nucleon coupling $g_{\pi NN}$. Even with the current methodology, a significant reduction in errors is expected over the next few years with higher statistics data on more ensembles closer to the physical point. Lattice QCD results for the form factor $G_A\left(Q^2\right)$ are compatible with those obtained from the recent MINER$\nu$A experiment but lie 2–3$\sigma$ higher than the phenomenological extraction from the old $\nu$–deuterium bubble chamber scattering data for $Q^2>0.3$ GeV². Current data show that the dipole ansatz does not have enough parameters to fit the form factor over the range $0\le Q^2\le 1$ GeV², whereas even a $z^2$ truncation of the z expansion or a low order Padé are sufficient. Looking ahead, lattice QCD calculations will provide increasingly precise results over the range $0\le Q^2\le 1$ GeV², and MINER$\nu$A-like experiments will extend the range to $Q^2\sim 2$ GeV² or higher. Nevertheless, improvements in lattice methods to (i) further control excited state contributions and (ii) extend the range of $Q^2$ are needed. Full article

We introduce a method for consistently incorporating meson-exchange currents (MEC) within the superscaling analysis with relativistic effective mass, featuring a new scaling variable, ${\psi }^{*}$, and single-nucleon cross-sections derived from the relativistic mean field (RMF) model of nuclear matter. The single-nucleon prefactor is obtained from the 1p1h matrix element of the one-body current, combined with the two-body current, averaged over a momentum distribution of Fermi kind. The approach is applied to selected quasielastic cross-sectional data on ${}^{12}$C. The results reveal a departure from scaling behavior, yet, intriguingly, the data collapse into a discernible band that is parametrized using a simple function of ${\psi }^{*}$. This calculation, as developed, is not intended to provide pinpoint precision in extracting nuclear responses. Instead, it offers a global description of the quasielastic data with a considerable level of uncertainty. However, this approach effectively captures the overall trends of the quasielastic data beyond the Fermi gas model with a minimal number of parameters. The model incorporates partially transverse enhancement of the response, as embedded within the relativistic mean field framework. However, it does not account for enhancements attributed to the combined effects of tensor correlations and MEC, given that the initial RMF model lacks these correlations. A potential avenue for improvement involves starting with a correlated Fermi gas model to incorporate additional enhancements into single-nucleon responses. This study serves as a practical demonstration of implementing such corrections. Full article

This review covers several recent developments in the physics of dense QCD with an emphasis on the impact of multiple phase transitions on astrophysical manifestations of compact stars. To motivate the multi-phase modeling of dense QCD and delineate the perspectives, we start with a discussion of the structure of its phase diagram and the arrangement of possible color-superconducting and other phases. It is conjectured that pair-correlated quark matter in $\beta$-equilibrium is within the same universality class as spin-imbalanced cold atoms and the isospin asymmetrical nucleonic matter. This then implies the emergence of phases with broken space symmetries and tri-critical (Lifshitz) points. The beyond-mean-field structure of the quark propagator and its non-trivial implications are discussed in the cases of two- and three-flavor quark matter within the Eliashberg theory, which takes into account the frequency dependence (retardation) of the gap function. We then construct an equation of state (EoS) that extends the two-phase EoS of dense quark matter within the constant speed of sound parameterization by adding a conformal fluid with a speed of sound $c_{\mathrm{conf}.}=1/\sqrt{3}$ at densities $\ge 10n_{\mathrm{sat}}$, where $n_{\mathrm{sat}}$ is the saturation density. With this input, we construct static, spherically symmetrical compact hybrid stars in the mass–radius diagram, recover such features as the twins and triplets, and show that the transition to conformal fluid leads to the spiraling-in of the tracks in this diagram. Stars on the spirals are classically unstable with respect to the radial oscillations but can be stabilized if the conversion timescale between quark and nucleonic phases at their interface is larger than the oscillation period. Finally, we review the impact of a transition from high-temperature gapped to low-temperature gapless two-flavor phase on the thermal evolution of hybrid stars. Full article

Nuclear super-allowed $\beta$ decay has been used to obtain tight limits on the value of the CKM matrix element $V_{ud}$ that is important for unitarity tests and, therefore, for tests of the standard model. Current requirements on precision are so intense that effects formerly thought too small to matter are now considered relevant. This article is a brief review of personal efforts to include the effects of strong interactions on Fermi $\beta$ decay. First, I examine the role of isospin violation in the decay of the neutron. The size of the necessary correction depends upon detailed strong-interaction dynamics. The isospin violating parts of the nucleon wave function, important at the low energy of $\beta$ decay, can be constrained by data taken at much higher energies, via measurements, for example, of $ed\to e^{\prime }{\pi }^{\pm }+\mathrm{X}$ reactions at Jefferson Laboratory. The next point of focus is on the role of nuclear short-ranged correlations, which affect the value of the correction needed to account for isospin violation in extracting the value of $V_{ud}$. The net result is that effects previously considered as irrelevant are now considered relevant for both neutron and nuclear $\beta$ decay. Full article

The midrapidity transverse momentum distributions of the charged pions, kaons, protons, and antiprotons, measured by ALICE Collaboration at ten centrality classes of Pb + Pb collisions at $\sqrt{s_{nn}}$ = 5.02 TeV in the Large Hadron Collider (LHC, CERN, Switzerland), are successfully analyzed using combined minimum χ² fits with a thermodynamically non-consistent, as well as thermodynamically consistent, Tsallis function with transverse flow. The extracted non-extensivity parameter q decreases systematically for all considered particle species with increasing Pb + Pb collision centrality, suggesting an increase in the degree of system thermalization with an increase in collision centrality. The results for q suggest quite a large degree of thermalization of quark–gluon plasma (QGP) created in central Pb + Pb collisions at $\sqrt{s_{nn} }$= 5.02 TeV with the average number of participant nucleons $⟨N_{part}⟩$ > 160. The obtained significantly different growth rates of transverse flow velocity, $⟨{\beta }_T⟩$, in regions $⟨N_{part}⟩$ < 71 ± 7 and $⟨N_{part}⟩$ > 71 ± 7 with the temperature parameter T₀ remaining constant within uncertainties in region $⟨N_{part}⟩$ > 71 ± 7 probably indicates that $⟨N_{part}⟩$ ≈ 71 ± 7 (corresponding to $⟨d{N}_{ch}/d\eta ⟩$ ≈ 251 ± 20) is a threshold border value for a crossover transition from a dense hadronic state to the QGP phase (or mixed phase of QGP and hadrons) in Pb + Pb collisions at $\sqrt{s_{nn}}$ = 5.02 TeV. The threshold border value for transverse flow velocity $⟨{\beta }_T⟩$ ≈ 0.46 ± 0.03 (corresponding to $⟨N_{part}⟩$ ≈ 71 ± 7), estimated by us in Pb + Pb collisions at $\sqrt{s_{nn}}$ = 5.02 TeV, agrees well with the corresponding border value $⟨{\beta }_T⟩$ ≈ 0.44 ± 0.02, recently obtained in Xe + Xe collisions at $\sqrt{s_{nn} }$= 5.44 TeV, and with almost constant $⟨{\beta }_T⟩$ values extracted earlier in the Beam Energy Scan (BES) program of the Relativistic Heavy-Ion Collider (RHIC, Brookhaven, GA, USA) in central Au + Au collisions in the $\sqrt{s_{nn} }$= 7.7 − 39 GeV energy range, where the threshold for QGP production is achieved. The correlations between extracted T₀ and $⟨{\beta }_T⟩$ parameters are found to be greatly different in regions $⟨{\beta }_T⟩$ < 0.46 and $⟨{\beta }_T⟩$ > 0.46, which further supports our result obtained for the threshold border value in Pb + Pb collisions at $\sqrt{s_{nn}}$= 5.02 TeV. Full article

The effect of isospin-dependent nuclear forces on the inner crust of neutron stars is modeled within the framework of Quantum Molecular Dynamics (QMD). To successfully control the density dependence of the symmetry energy of neutron-star matter below nuclear saturation density, a mixed vector-isovector potential is introduced. This approach is inspired by the baryon density and isospin density-dependent repulsive Skyrme force of asymmetric nuclear matter. In isospin-asymmetric nuclear matter, the system shows nucleation, as nucleons are arranged into shapes resembling nuclear pasta. The dependence of clusterization in the system on the isospin properties is also explored by calculating two-point correlation functions. We show that, as compared to previous results that did not involve such mixed interaction terms, the energy symmetry slope L is successfully controlled by varying the corresponding coupling strength. Nevertheless, the effect of changing the slope of the nuclear symmetry energy L on the crust-core transition density does not seem significant. To the knowledge of the authors, this is the first implementation of such a coupling in a QMD model for isospin asymmetric matter, which is relevant to the inner crust of neutron and proto-neutron stars. Full article

In the current paper, we argue that the ground state of a hadron contains a significant perturbative quantum chromodynamics (pQCD) core as the result of color gauge invariance and the values of chiral and gluon vacuum condensates. The evaluation within the method of dispersion sum rules (DSR) of the vacuum matrix elements of the correlator of local currents with the proper quantum numbers leads to the value of the radius of the pQCD core of a nucleon of about 0.4–0.5 fm. The selection of the initial and final states allows to select processes in which the pQCD core of the projectile gives the dominant contribution to the process. It is explained that the transparency of nuclear matter for the propagation of a spatially small and color-neutral wave packet of quarks and gluons—a color transparency (CT) phenomenon—for a group of hard processes off nuclear targets can be derived in the form of the QCD factorization theorem accounting for the color screening phenomenon. Based on the success of the method of DSR, we argue that a pQCD core in a hadron wave function is surrounded by the layer consisting of quarks interacting with quark and gluon condensates. As a result, in the quasi-elastic processes $e+A\to e^{\prime }+N+{(A-1)}^{\ast }$, the quasi-Feynman mechanism could be dominating in a wide range of the momentum transfer squared, $Q^2$. In this scenario, a virtual photon is absorbed by a single quark, which carries a large fraction of the momentum of the nucleon and dominates in a wide range of $Q^2$. CT should reveal itself in these processes at extremely large $Q^2$ as the consequence of the presence of the Sudakov form factors, which squeeze a nucleon. Full article

Relativistic mean-field models are successfully used for the description of finite nuclei and nuclear matter. Approaches with density-dependent meson-nucleon couplings assume specific functional forms and a dependence on vector densities in most cases. In this work, parametrizations with a larger sample of functions and dependencies on vector and scalar densities are investigated. They are obtained from fitting properties of finite nuclei. The quality of the description of nuclei and the obtained equations of state of symmetric nuclear matter and neutron matter below saturation are very similar. However, characteristic nuclear matter parameters, the equations of state and the symmetry energy at suprasaturation densities show some correlations with the choice of the density dependence and functional form of the couplings. Conditions are identified that can lead to problems for some of the parametrizations. Full article