We investigate a reaction model that describes a fast removal of the $\alpha$ particle from the ${}^8\mathrm{He}$ nucleus with eventual emission of four neutrons. The obtained four neutron energy distribution allows one to explain the sharp low energy peak observed by studying the missing mass spectra of four neutrons in Duer et al. [Nature (London) 606, 678 (2022)], as a consequence of dineutron-dineutron correlations. The phenomenon of the emergence of a sharp low-energy peak in the four-neutron energy distribution should be more general and is expected in the decay of other systems containing a four-neutron halo.

The ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ reaction is critical for AGB star nucleosynthesis due to its connection to the abundances of several key isotopes, such as ${}^{21}\mathrm{Ne}$ and ${}^{22}\mathrm{Ne}$. However, the ambiguous resonance energy and spin-parity of the dominant 470 keV resonance leads to substantial uncertainty in the ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ reaction rate for the temperature of interest. We have measured the resonance energies and strengths of the low-energy resonances in ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ at the Jinping Underground Nuclear Astrophysics experimental facility (JUNA) with improved precision. The key 470 keV resonance energy has been measured to be $E_{\alpha }=474.0\pm 1.1\mathrm{keV}$, with such high precision achieved for the first time. The spin-parity of this resonance state is determined to be $1^{-}$, removing discrepancies in the resonance strengths in earlier studies. The results significantly improve the precision of the ${}^{18}\mathrm{O}(\alpha ,\gamma ){}^{22}\mathrm{Ne}$ reaction rates by up to about 10 times compared with the previous data at typical AGB temperatures of 0.1–0.3 GK. We demonstrate that such improvement leads to precise ${}^{21}\mathrm{Ne}$ abundance predictions, with an impact on probing the origin of meteoritic stardust SiC grains from AGB stars.

We report the beam energy and collision centrality dependence of fifth and sixth order cumulants ($C_5$, $C_6$) and factorial cumulants (${\kappa }_5$, ${\kappa }_6$) of net-proton and proton number distributions, from center-of-mass energy ($\sqrt{s_{NN}}$) 3 GeV to 200 GeV $\mathrm{Au}+\mathrm{Au}$ collisions at RHIC. Cumulant ratios of net-proton (taken as proxy for net-baryon) distributions generally follow the hierarchy expected from QCD thermodynamics, except for the case of collisions at 3 GeV. The measured values of $C_6/C_2$ for 0%–40% centrality collisions show progressively negative trend with decreasing energy, while it is positive for the lowest energy studied. These observed negative signs are consistent with QCD calculations (for baryon chemical potential, ${\mu }_B\le 110\mathrm{MeV}$) which contains the crossover transition range. In addition, for energies above 7.7 GeV, the measured proton ${\kappa }_n$, within uncertainties, does not support the two-component ($\mathrm{Poisson}+\mathrm{binomial}$) shape of proton number distributions that would be expected from a first-order phase transition. Taken in combination, the hyperorder proton number fluctuations suggest that the structure of QCD matter at high baryon density, ${\mu }_B\sim 750\mathrm{MeV}$ at $\sqrt{s_{NN}}=3\mathrm{GeV}$ is starkly different from those at vanishing ${\mu }_B\sim 24\mathrm{MeV}$ at $\sqrt{s_{NN}}=200\mathrm{GeV}$ and higher collision energies.

We calculate the equation of state of asymmetric nuclear matter at finite temperature based on chiral effective field theory interactions to next-to-next-to-next-to-leading order. Our results assess the theoretical uncertainties from the many-body calculation and the chiral expansion. Using a Gaussian process emulator for the free energy, we derive the thermodynamic properties of matter through consistent derivatives and use the Gaussian process to access arbitrary proton fraction and temperature. This enables a first nonparametric calculation of the equation of state in beta equilibrium, and of the speed of sound and the symmetry energy at finite temperature. Moreover, our results show that the thermal part of the pressure decreases with increasing densities.

Shapes and shape evolution in the mass-130 region, including the Te, Xe, and Ba isotopes, have long been a focus of discussion in nuclear physics. This mass region consists of complex many-body systems that can behave in astonishingly simple and regular ways, as classified in the Casten symmetry triangle. By applying the shell model Hamiltonian proposed recently, we carry out calculations using the Hartree-Fock-Bogolyubov plus generator coordinate method, in the large model space containing the $(1g_{9/2},1g_{7/2},2d_{5/2},2d_{3/2},3s_{1/2},1h_{11/2},2f_{7/2})$ orbits. Based on good reproduction of the experimentally known energy levels, spectroscopic quadrupole moments, and $E2$ transition probabilities, we identify the quasi-SU(3) couplings across the $N=50$ and 82 shell gaps, which play a role in driving shape evolution and phase transition discussed in the extended Casten triangle. Specifically, we demonstrate that the quasi-SU(3) coupling mechanism in the proton partner orbits ($1g_{9/2}$, $2d_{5/2}$) tends to drive the system to be more $\gamma$ soft, and that in the neutron partner orbits ($1h_{11/2}$, $2f_{7/2}$) are responsible for the oblate-to-prolate shape phase transition. With an emphasis on discussing spectroscopic quadrupole moments, our Letter uncovers hidden symmetries from the vast shell-model configurations and adds microscopical insights into the empirical symmetry triangle.

Many transport coefficients of the quark-gluon plasma and nuclear structure functions can be written as gauge invariant correlation functions of non-Abelian field strengths dressed with Wilson lines. We discuss the applicability of axial gauge $n·A=0$ to calculate them. In particular, we address issues that appear when one attempts to trivialize the Wilson lines in the correlation functions by gauge fixing. We find it is always impossible to completely remove the gauge fields $n·A$ in Wilson lines that extend to infinity in the $n$ direction by means of gauge transformations. We show how the obstruction appears in an explicit example of a perturbative calculation, and we also explain it more generally from the perspective of the path integral that defines the theory. Our results explain why the two correlators that define the heavy quark and quarkonium transport coefficients, which are seemingly equal in axial gauge, are actually different physical quantities of the quark-gluon plasma and have different values. Furthermore, our findings provide insights into the difference between two inequivalent gluon parton distribution functions.

The diffusion wake accompanying the jet-induced Mach cone provides a unique probe of the properties of quark-gluon plasma in high-energy heavy-ion collisions. It can be characterized by a depletion of soft hadrons in the opposite direction of the propagating jet. We explore the 3D structure of the diffusion wake induced by $\gamma$-triggered jets in $\mathrm{Pb}+\mathrm{Pb}$ collisions at the LHC energy within the coupled linear Boltzmann transport and hydro model. We identify a valley structure caused by the diffusion wake on top of a ridge from the initial multiple parton interaction (MPI) in jet-hadron correlation as a function of rapidity and azimuthal angle. This leads to a double-peak structure in the rapidity distribution of soft hadrons in the opposite direction of the jets as an unambiguous signal of the diffusion wake. Using a two-Gaussian fit, we extract the diffusion wake and MPI contributions to the double peak. The diffusion wake valley is found to deepen with the jet energy loss as characterized by the $\gamma$-jet asymmetry. Its sensitivity to the equation of state and shear viscosity is also studied.

The neutron skin of atomic nuclei impacts the structure of neutron-rich nuclei, the equation of state of nucleonic matter, and the size of neutron stars. Here we predict the neutron skin of selected light- and medium-mass nuclei using coupled-cluster theory and the auxiliary field diffusion Monte Carlo method with two- and three-nucleon forces from chiral effective field theory. We find a linear correlation between the neutron skin and the isospin asymmetry in agreement with the liquid-drop model and compare with data. We also extract the linear relationship that describes the difference between neutron and proton radii of mirror nuclei and quantify the effect of charge symmetry breaking terms in the nuclear Hamiltonian. Our results for the mirror-difference charge radii and binding energies per nucleon agree with existing data.

We report the first measurements of deep inelastic scattering spin-dependent azimuthal asymmetries in back-to-back dihadron electroproduction in the deep inelastic scattering process. In this reaction, two hadrons are produced in opposite hemispheres along the $z$ axis in the virtual photon-target nucleon 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 nonzero $\sin \mathrm{\Delta }\varphi$ modulations in $ep\to e^{\prime }p{\pi }^{+}X$ events, where $\mathrm{\Delta }\varphi$ 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 unexplored 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 atomic masses of ${}^{55}\mathrm{Sc}$, ${}^{56,58}\mathrm{Ti}$, and ${}^{56-59}\mathrm{V}$ have been determined using the high-precision multireflection time-of-flight technique. The radioisotopes have been produced at RIKEN’s Radioactive Isotope Beam Factory (RIBF) and delivered to the novel designed gas cell and multireflection system, which has been recently commissioned downstream of the ZeroDegree spectrometer following the BigRIPS separator. For ${}^{56,58}\mathrm{Ti}$ and ${}^{56-59}\mathrm{V}$, the mass uncertainties have been reduced down to the order of 10 keV, shedding new light on the $N=34$ shell effect in Ti and V isotopes by the first high-precision mass measurements of the critical species ${}^{58}\mathrm{Ti}$ and ${}^{59}\mathrm{V}$. With the new precision achieved, we reveal the nonexistence of the $N=34$ empirical two-neutron shell gaps for Ti and V, and the enhanced energy gap above the occupied $\nu p_{3/2}$ orbit is identified as a feature unique to Ca. We perform new Monte Carlo shell model calculations including the $\nu d_{5/2}$ and $\nu g_{9/2}$ orbits and compare the results with conventional shell model calculations, which exclude the $\nu g_{9/2}$ and the $\nu d_{5/2}$ orbits. The comparison indicates that the shell gap reduction in Ti is related to a partial occupation of the higher orbitals for the outer two valence neutrons at $N=34$.

The one-neutron knockout from ${}^{52}\mathrm{Ca}$ in inverse kinematics onto a proton target was performed at $\sim 230\mathrm{MeV}$/nucleon combined with prompt $\gamma$ spectroscopy. Exclusive quasifree scattering cross sections to bound states in ${}^{51}\mathrm{Ca}$ and the momentum distributions corresponding to the removal of $1f_{7/2}$ and $2p_{3/2}$ neutrons were measured. The cross sections, interpreted within the distorted-wave impulse approximation reaction framework, are consistent with a shell closure at the neutron number $N=32$, found as strong as at $N=28$ and $N=34$ in Ca isotopes from the same observables. The analysis of the momentum distributions leads to a difference of the root-mean-square radii of the neutron $1f_{7/2}$ and $2p_{3/2}$ orbitals of 0.61(23) fm, in agreement with the modified-shell-model prediction of 0.7 fm suggesting that the large root-mean-square radius of the $2p_{3/2}$ orbital in neutron-rich Ca isotopes is responsible for the unexpected linear increase of the charge radius with the neutron number.

We discuss an interpretation that a peak in the sound velocity in neutron star matter, as suggested by the observational data, signifies strongly coupled conformal matter. The normalized trace anomaly is a dimensionless measure of conformality leading to the derivative and the nonderivative contributions to the sound velocity. We find that the peak in the sound velocity is attributed to the derivative contribution from the trace anomaly that steeply approaches the conformal limit. Smooth continuity to the behavior of high-density QCD implies that the matter part of the trace anomaly may be positive definite. We discuss a possible implication of the positivity condition of the trace anomaly on the $M\text{−}R$ relation of the neutron stars.

We present the first full $(3+1)\mathrm{D}$ dynamical simulations of ultraperipheral $\mathrm{Pb}+\mathrm{Pb}$ collisions at the Large Hadron Collider. Extrapolating from $p+\mathrm{Pb}$ collisions, we explore whether a quasireal photon ${\gamma }^{*}$ interacting with the lead nucleus in an ultraperipheral collision can create a many-body system exhibiting fluid behavior. Assuming strong final-state interactions, we provide model results for charged hadron multiplicity, identified particle mean transverse momenta, and charged hadron anisotropic flow coefficients, and compare them with experimental data from the ALICE and ATLAS Collaborations. The elliptic flow hierarchy between $p+\mathrm{Pb}$ and ${\gamma }^{*}+\mathrm{Pb}$ collisions is dominated by the difference in longitudinal flow decorrelations and reproduces the experimental data well. We have demonstrated that our theoretical framework provides a quantitative tool to study particle production and collectivity for all system sizes, ranging from central heavy-ion collisions to small asymmetric collision systems at the Relativistic Heavy-Ion Collider and the Large Hadron Collider and even at the future Electron-Ion Collider.

Elliptic flow measurements from two-, four-, and six-particle correlations are used to investigate flow fluctuations in collisions of $\mathrm{U}+\mathrm{U}$ at $\sqrt{s_{\mathrm{NN}}}=193\mathrm{GeV}$, $\mathrm{Cu}+\mathrm{Au}$ at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$ and $\mathrm{Au}+\mathrm{Au}$ spanning the range $\sqrt{s_{\mathrm{NN}}}=11.5–200\mathrm{GeV}$. The measurements show a strong dependence of the flow fluctuations on collision centrality, a modest dependence on system size, and very little if any, dependence on particle species and beam energy. The results, when compared to similar LHC measurements, viscous hydrodynamic calculations, and trento model eccentricities, indicate that initial-state-driven fluctuations predominate the flow fluctuations generated in the collisions studied.

While the phenomenon of gluon nuclear shadowing at small $x$ has been getting confirmation in QCD analyses of various LHC measurements involving heavy nuclei, it has not been possible so far to establish experimentally the number of target nucleons responsible for nuclear shadowing in a given process. To address this issue, we study coherent $J/\psi$ electroproduction on ${}^4\mathrm{He}$ and ${}^3\mathrm{He}$ in the kinematics of a future electron-ion collider and show that this process has the power to disentangle the contributions of the interaction with a specific number of nucleons $k$, in particular, with two nucleons at the momentum transfer $t\ne 0$. We predict a dramatic shift of the $t$ dependence of the differential cross section toward smaller values of $|t|$ due to a nontrivial correlation between $x$ and $k$. This calculation, which makes use for the first time of realistic wave functions, provides a stringent test of models of nuclear shadowing and a novel probe of the 3D imaging of gluons in light nuclei. In addition, thanks to this analysis, unique information on the real part of the corresponding scattering amplitude could be accessed.

A novel pathway for the formation of multiparticle-multihole excited states in rare isotopes is reported from highly energy- and momentum-dissipative inelastic-scattering events measured in reactions of an intermediate-energy beam of ${}^{38}\mathrm{Ca}$ on a Be target. The negative-parity, complex-structure final states in ${}^{38}\mathrm{Ca}$ are observed following the in-beam $\gamma$-ray spectroscopy of events in the ${}^9\mathrm{Be}({}^{38}\mathrm{Ca},{}^{38}\mathrm{Ca}+\gamma )X$ reaction in which the scattered projectile loses longitudinal momentum of order $\mathrm{\Delta }{p}_{||}=700\mathrm{MeV}/\mathrm{c}$. The characteristics of the observed final states are discussed and found to be consistent with the formation of excited states involving the rearrangement of multiple nucleons in a single, highly energetic projectile-target collision. Unlike the far-less-dissipative, surface-grazing reactions usually exploited for the in-beam $\gamma$-ray spectroscopy of rare isotopes, these more energetic collisions appear to offer a practical pathway to nuclear-structure studies of more complex multiparticle configurations in rare isotopes—final states conventionally thought to be out of reach with high-luminosity fast-beam-induced reactions.

The recent experimental determination of the parity violating asymmetry $A_{\mathrm{PV}}$ in ${}^{48}\mathrm{Ca}$ and ${}^{208}\mathrm{Pb}$ at Jefferson Lab is important for our understanding on how neutrons and protons arrange themselves inside the atomic nucleus. To better understand the impact of these measurements, we present a rigorous theoretical investigation of $A_{\mathrm{PV}}$ in ${}^{48}\mathrm{Ca}$ and ${}^{208}\mathrm{Pb}$ and assess the associated uncertainties. We complement our study by inspecting the static electric dipole polarizability in these nuclei. The analysis is carried out within nuclear energy density functional theory with quantified input. We conclude that the simultaneous accurate description of $A_{\mathrm{PV}}$ in ${}^{48}\mathrm{Ca}$ and ${}^{208}\mathrm{Pb}$ cannot be achieved by our models that accommodate a pool of global nuclear properties, such as masses and charge radii, throughout the nuclear chart, and describe—within one standard deviation—the experimental dipole polarizabilities ${\alpha }_D$ in these nuclei.