We demonstrate, within symmetry unrestricted time-dependent density functional theory, the existence of new effects in low-energy nuclear reactions which originate from superfluidity. The dynamics of the pairing field induces solitonic excitations in the colliding nuclear systems, leading to qualitative changes in the reaction dynamics. The solitonic excitation prevents collective energy dissipation and effectively suppresses the fusion cross section. We demonstrate how the variations of the total kinetic energy of the fragments can be traced back to the energy stored in the superfluid junction of colliding nuclei. Both contact time and scattering angle in noncentral collisions are significantly affected. The modification of the fusion cross section and possibilities for its experimental detection are discussed.

We report the first result for the electron-antineutrino angular correlation ($a$ coefficient) in free neutron $\beta$ decay from the aCORN experiment. aCORN uses a novel method in which the $a$ coefficient is proportional to an asymmetry in proton time of flight for events where the $\beta$ electron and recoil proton are detected in delayed coincidence. Data are presented from a 15 month run at the NIST Center for Neutron Research. We obtained $a=-0.1090\pm 0.0030(\mathrm{stat})\pm 0.0028(\mathrm{sys})$, the most precise measurement of the neutron $a$ coefficient reported to date.

The search for a resonant four-neutron system has been revived thanks to the recent experimental hints reported in [1]. The existence of such a system would deeply impact our understanding of nuclear matter and requires a critical investigation. In this work, we study the existence of a four-neutron resonance in the quasistationary formalism using ab initio techniques with various two-body chiral interactions. We employ no-core Gamow shell model and density matrix renormalization group method, both supplemented by the use of natural orbitals and a new identification technique for broad resonances. We demonstrate that while the energy of the four-neutron system may be compatible with the experimental value, its width must be larger than the reported upper limit, supporting the interpretation of the experimental observation as a reaction process too short to form a nucleus.

New measurements of the beam normal single spin asymmetry in the electron elastic and quasielastic scattering on the proton and deuteron, respectively, at large backward angles and at $⟨Q^2⟩=0.22(\mathrm{GeV}/c{)}^2$ and $⟨Q^2⟩=0.35(\mathrm{GeV}/c{)}^2$ are reported. The experimentally observed asymmetries are compared with the theoretical calculation of Pasquini and Vanderhaeghen [Phys. Rev. C 70, 045206 (2004).]. The agreement of the measurements with the theoretical calculations shows a dominance of the inelastic intermediate excited states of the nucleon, $\pi N$ and the $\mathrm{\Delta }$ resonance. The measurements explore a new, important parameter region of the exchanged virtual photon virtualities.

The first elastic electron scattering has been successfully performed at the self-confining radioactive-isotope ion target (SCRIT) facility, the world’s first electron scattering facility for SCRIT technique achieved high luminosity (over ${10}^{27}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, sufficient for determining the nuclear shape) with only ${10}^8$ target ions. While ${}^{132}\mathrm{Xe}$ used in this time as a target is a stable isotope, the charge density distribution was first extracted from the momentum transfer distributions of the scattered electrons by comparing the results with those calculated by a phase shift calculation.

The electric dipole strength distribution in ${}^{48}\mathrm{Ca}$ between 5 and 25 MeV has been determined at RCNP, Osaka from proton inelastic scattering experiments at forward angles. Combined with photoabsorption data at higher excitation energy, this enables the first extraction of the electric dipole polarizability ${\alpha }_{\mathit{D}}({}^{48}\mathrm{Ca})=2.07(22){\mathrm{fm}}^3$. Remarkably, the dipole response of ${}^{48}\mathrm{Ca}$ is found to be very similar to that of ${}^{40}\mathrm{Ca}$, consistent with a small neutron skin in ${}^{48}\mathrm{Ca}$. The experimental results are in good agreement with ab initio calculations based on chiral effective field theory interactions and with state-of-the-art density-functional calculations, implying a neutron skin in ${}^{48}\mathrm{Ca}$ of 0.14–0.20 fm.

The level structure of the neutron-rich ${}^{77}\mathrm{Cu}$ nucleus is investigated through $\beta$-delayed $\gamma$-ray spectroscopy at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. Ions of ${}^{77}\mathrm{Ni}$ are produced by in-flight fission, separated and identified in the BigRIPS fragment separator, and implanted in the WAS3ABi silicon detector array, surrounded by Ge cluster detectors of the EURICA array. A large number of excited states in ${}^{77}\mathrm{Cu}$ are identified for the first time by correlating $\gamma$ rays with the $\beta$ decay of ${}^{77}\mathrm{Ni}$, and a level scheme is constructed by utilizing their coincidence relationships. The good agreement between large-scale Monte Carlo shell model calculations and experimental results allows for the evaluation of the single-particle structure near ${}^{78}\mathrm{Ni}$ and suggests a single-particle nature for both the $5/2_1^{-}$ and $3/2_1^{-}$ states in ${}^{77}\mathrm{Cu}$, leading to doubly magic ${}^{78}\mathrm{Ni}$.

We report on the first $\gamma$-ray spectroscopy of low-lying states in neutron-rich ${}^{98,100}\mathrm{Kr}$ isotopes obtained from ${}^{99,101}\mathrm{Rb}(p,2p)$ reactions at $\sim 220\mathrm{MeV}/\mathrm{nucleon}$. A reduction of the $2_1^{+}$ state energies beyond $N=60$ demonstrates a significant increase of deformation, shifted in neutron number compared to the sharper transition observed in strontium and zirconium isotopes. State-of-the-art beyond-mean-field calculations using the Gogny D1S interaction predict level energies in good agreement with experimental results. The identification of a low-lying ($0_2^{+}$, $2_2^{+}$) state in ${}^{98}\mathrm{Kr}$ provides the first experimental evidence of a competing configuration at low energy in neutron-rich krypton isotopes consistent with the oblate-prolate shape coexistence picture suggested by theory.

Recent ab initio lattice studies have found that the interactions between alpha particles (${}^4\mathrm{He}$ nuclei) are sensitive to seemingly minor details of the nucleon-nucleon force, such as interaction locality. In order to uncover the essential physics of this puzzling phenomenon without unnecessary complications, we study a simple model involving two-component fermions in one spatial dimension. We probe the interaction between two bound dimers for several different particle-particle interactions and measure an effective potential between the dimers using external point potentials which act as numerical tweezers. We find that the strength and range of the local part of the particle-particle interactions play a dominant role in shaping the interactions between the dimers and can even determine the overall sign of the effective potential.

We present quantum Monte Carlo calculations of few-neutron systems confined in external potentials based on local chiral interactions at next-to-next-to-leading order in chiral effective field theory. The energy and radial densities for these systems are calculated in different external Woods-Saxon potentials. We assume that their extrapolation to zero external-potential depth provides a quantitative estimate of three- and four-neutron resonances. The validity of this assumption is demonstrated by benchmarking with an exact diagonalization in the two-body case. We find that the extrapolated trineutron resonance, as well as the energy for shallow well depths, is lower than the tetraneutron resonance energy. This suggests that a three-neutron resonance exists below a four-neutron resonance in nature and is potentially measurable. To confirm that the relative ordering of three- and four-neutron resonances is not an artifact of the external confinement, we test that the odd-even staggering in the helium isotopic chain is reproduced within this approach. Finally, we discuss similarities between our results and ultracold Fermi gases.

We present the first azimuthally differential measurements of the pion source size relative to the second harmonic event plane in Pb-Pb collisions at a center-of-mass energy per nucleon-nucleon pair of $\sqrt{s_{NN}}=2.76\mathrm{TeV}$. The measurements have been performed in the centrality range 0%–50% and for pion pair transverse momenta $0.2<k_T<0.7\mathrm{GeV}/c$. We find that the $R_{\mathrm{side}}$ and $R_{\mathrm{out}}$ radii, which characterize the pion source size in the directions perpendicular and parallel to the pion transverse momentum, oscillate out of phase, similar to what was observed at the Relativistic Heavy Ion Collider. The final-state source eccentricity, estimated via $R_{\mathrm{side}}$ oscillations, is found to be significantly smaller than the initial-state source eccentricity, but remains positive—indicating that even after a stronger expansion in the in-plane direction, the pion source at the freeze-out is still elongated in the out-of-plane direction. The $3+1$D hydrodynamic calculations are in qualitative agreement with observed centrality and transverse momentum $R_{\mathrm{side}}$ oscillations, but systematically underestimate the oscillation magnitude.

Fast-neutron-induced fission of ${}^{238}\mathrm{U}$ at an energy just above the fission threshold is studied with a novel technique which involves the coupling of a high-efficiency $\gamma$-ray spectrometer (MINIBALL) to an inverse-kinematics neutron source (LICORNE) to extract charge yields of fission fragments via $\gamma \text{−}\gamma$ coincidence spectroscopy. Experimental data and fission models are compared and found to be in reasonable agreement for many nuclei; however, significant discrepancies of up to 600% are observed, particularly for isotopes of Sn and Mo. This indicates that these models significantly overestimate the standard 1 fission mode and suggests that spherical shell effects in the nascent fission fragments are less important for low-energy fast-neutron-induced fission than for thermal neutron-induced fission. This has consequences for understanding and modeling the fission process, for experimental nuclear structure studies of the most neutron-rich nuclei, for future energy applications (e.g., Generation IV reactors which use fast-neutron spectra), and for the reactor antineutrino anomaly.

We report the first measurement of the elliptic anisotropy ($v_2$) of the charm meson $D^0$ at midrapidity ($|y|<1$) in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The measurement was conducted by the STAR experiment at RHIC utilizing a new high-resolution silicon tracker. The measured $D^0$ $v_2$ in 0%–80% centrality $\mathrm{Au}+\mathrm{Au}$ collisions can be described by a viscous hydrodynamic calculation for a transverse momentum ($p_T$) of less than $4\mathrm{GeV}/c$. The $D^0$ $v_2$ as a function of transverse kinetic energy ($m_T-m_0$, where $m_T=\sqrt{p_T^2+m_0^2}$) is consistent with that of light mesons in 10%–40% centrality $\mathrm{Au}+\mathrm{Au}$ collisions. These results suggest that charm quarks have achieved local thermal equilibrium with the medium created in such collisions. Several theoretical models, with the temperature-dependent, dimensionless charm spatial diffusion coefficient ($2\pi T{D}_s$) in the range of $\sim 2–12$, are able to simultaneously reproduce our $D^0$ $v_2$ result and our previously published results for the $D^0$ nuclear modification factor.

The $E2/M1$ multipole mixing ratio ${\delta }_{1\to 2}$ of the $1_{\mathrm{sc}}^{+}\to 2_1^{+}$ $\gamma$-ray decay in ${}^{156}\mathrm{Gd}$ and hence the isovector $E2$ transition rate of the scissors mode of a well-deformed rotational nucleus has been measured for the first time. It has been obtained from the angular distribution of an artificial quasimonochromatic linearly polarized $\gamma$-ray beam of energy 3.07(6) MeV scattered inelastically off an isotopically highly enriched ${}^{156}\mathrm{Gd}$ target. The data yield first direct support for the deformation dependence of effective proton and neutron quadrupole boson charges in the framework of algebraic nuclear models. First evidence for a low-lying $J^{\pi }=2^{+}$ member of the rotational band of states on top of the $1^{+}$ band head is obtained, too, indicating a significant signature splitting in the $K=1$ scissors mode rotational band.

The reduced magnetic dipole and electric quadrupole transition probabilities for the radiative decay of the ${}^{229}\mathrm{Th}7.8\mathrm{eV}$ isomer to the ground state are predicted within a detailed nuclear-structure model approach. We show that the presence and decay of this isomer can only be accounted for by the Coriolis mixing emerging from a remarkably fine interplay between the coherent quadrupole-octupole motion of the nuclear core and the single-nucleon motion within a reflection-asymmetric deformed potential. We find that the magnetic dipole transition probability which determines the radiative lifetime of the isomer is considerably smaller than presently estimated. The so-far disregarded electric quadrupole component may have non-negligible contributions to the internal conversion channel. These findings support new directions in the experimental search of the ${}^{229}\mathrm{Th}$ transition frequency for the development of a future nuclear frequency standard.

Excited states in the nucleus ${}^{133}\mathrm{Sn}$, with one neutron outside the double magic ${}^{132}\mathrm{Sn}$ core, were populated following one-neutron knockout from a ${}^{134}\mathrm{Sn}$ beam on a carbon target at relativistic energies at the Radioactive Isotope Beam Factory at RIKEN. Besides the $\gamma$ rays emitted in the decay of the known neutron single-particle states in ${}^{133}\mathrm{Sn}$ additional $\gamma$ strength in the energy range 3.5–5.5 MeV was observed for the first time. Since the neutron-separation energy of ${}^{133}\mathrm{Sn}$ is low, $S_n=2.402(4)\mathrm{MeV}$, this observation provides direct evidence for the radiative decay of neutron-unbound states in this nucleus. The ability of electromagnetic decay to compete successfully with neutron emission at energies as high as 3 MeV above threshold is attributed to a mismatch between the wave functions of the initial and final states in the latter case. These findings suggest that in the region southeast of ${}^{132}\mathrm{Sn}$ nuclear structure effects may play a significant role in the neutron versus $\gamma$ competition in the decay of unbound states. As a consequence, the common neglect of such effects in the evaluation of the neutron-emission probabilities in calculations of global $\beta$-decay properties for astrophysical simulations may have to be reconsidered.

We present, for the first time, simultaneous determination of shear viscosity ($\eta$) and entropy density ($s$) and thus, $\eta /s$ for equilibrated nuclear systems from $A\sim 30$ to $A\sim 208$ at different temperatures. At finite temperature, $\eta$ is estimated by utilizing the $\gamma$ decay of the isovector giant dipole resonance populated via fusion evaporation reaction, while $s$ is evaluated from the nuclear level density parameter ($a$) and nuclear temperature ($T$), determined precisely by the simultaneous measurements of the evaporated neutron energy spectra and the compound nuclear angular momenta. The transport parameter $\eta$ and the thermodynamic parameter $s$ both increase with temperature, resulting in a mild decrease of $\eta /s$ with temperature. The extracted $\eta /s$ is also found to be independent of the neutron-proton asymmetry at a given temperature. Interestingly, the measured $\eta /s$ values are comparable to that of the high-temperature quark-gluon plasma, pointing towards the fact that strong fluidity may be the universal feature of the strong interaction of many-body quantum systems.

An extension of the ideal hadron resonance gas (HRG) model is constructed which includes the attractive and repulsive van der Waals (VDW) interactions between baryons. This VDW-HRG model yields the nuclear liquid-gas transition at low temperatures and high baryon densities. The VDW parameters $a$ and $b$ are fixed by the ground state properties of nuclear matter, and the temperature dependence of various thermodynamic observables at zero chemical potential are calculated within the VDW-HRG model. Compared to the ideal HRG model, the inclusion of VDW interactions between baryons leads to a qualitatively different behavior of second and higher moments of fluctuations of conserved charges, in particular in the so-called crossover region $T\sim 140–190\mathrm{MeV}$. For many observables this behavior resembles closely the results obtained from lattice QCD simulations. This hadronic model also predicts nontrivial behavior of net-baryon fluctuations in the region of phase diagram probed by heavy-ion collision experiments. These results imply that VDW interactions play a crucial role in the thermodynamics of hadron gas. Thus, the commonly performed comparisons of the ideal HRG model with the lattice and heavy-ion data may lead to misconceptions and misleading conclusions.