We present a nucleus-dependent valence-space approach for calculating ground and excited states of nuclei, which generalizes the shell-model in-medium similarity renormalization group to an ensemble reference with fractionally filled orbitals. Because the ensemble is used only as a reference, and not to represent physical states, no symmetry restoration is required. This allows us to capture three-nucleon ($3N$) forces among valence nucleons with a valence-space Hamiltonian specifically targeted to each nucleus of interest. Predicted ground-state energies from carbon through nickel agree with results of other large-space ab initio methods, generally to the 1% level. In addition, we show that this new approach is required in order to obtain convergence for nuclei in the upper $p$ and $sd$ shells. Finally, we address the $1^{+}/3^{+}$ inversion problem in ${}^{22}\mathrm{Na}$ and ${}^{46}\mathrm{V}$. This approach extends the reach of ab initio nuclear structure calculations to essentially all light- and medium-mass nuclei.

The Porter-Thomas (PT) distribution of resonance widths is one of the oldest and simplest applications of statistical ideas in nuclear physics. Previous experimental data confirmed it quite well, but recent and more careful investigations show clear deviations from this distribution. To explain these discrepancies, Volya, Weidenmüller, and Zelevinsky [Phys. Rev. Lett. 115, 052501 (2015)] argued that to get a realistic model of nuclear resonances is not enough to consider one of the standard random matrix ensembles which leads immediately to the PT distribution, but it is necessary to add a rank-one interaction which couples resonances to decay channels. The purpose of this Letter is to solve this model analytically and to find explicitly the modifications of the PT distribution due to such an interaction. Resulting formulas are simple, in good agreement with numerics, and could explain experimental results.

The nuclear level density (NLD) and radiative strength function (RSF) are simultaneously described within a microscopic approach, which takes into account the thermal effects of the exact pairing as well as the giant resonances within the phonon-damping model. The good agreement between the results of calculations and experimental data extracted by the Oslo group for ${}^{170,171,172}\mathrm{Yb}$ isotopes shows the importance of exact thermal pairing in the description of NLD at low and intermediate excitation energies. It also invalidates the assumption based on the Brink-Axel hypothesis in the description of the RSF.

We present the first measurement of charge-dependent directed flow in $\mathrm{Cu}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The results are presented as a function of the particle transverse momentum and pseudorapidity for different centralities. A finite difference between the directed flow of positive and negative charged particles is observed that qualitatively agrees with the expectations from the effects of the initial strong electric field between two colliding ions with different nuclear charges. The measured difference in directed flow is much smaller than that obtained from the parton-hadron-string-dynamics model, which suggests that most of the electric charges, i.e., quarks and antiquarks, have not yet been created during the lifetime of the strong electric field, which is of the order of, or less than, $1\mathrm{fm}/c$.

High-resolution experiments have recently lead to a complete identification (energy, spin, and parity) of 151 nuclear levels up to an excitation energy of $E_x=6.20\mathrm{MeV}$ in ${}^{208}\mathrm{Pb}$ [Heusler et al., Phys. Rev. C 93, 054321 (2016)]. We present a thorough study of the fluctuation properties in the energy spectra of the unprecedented set of nuclear bound states. In a first approach, we group states with the same spin and parity into 14 subspectra, analyze standard statistical measures for short- and long-range correlations, i.e., the nearest-neighbor spacing distribution, the number variance ${\mathrm{\Sigma }}^2$, the Dyson-Mehta ${\mathrm{\Delta }}_3$ statistics, and the novel distribution of the ratios of consecutive spacings of adjacent energy levels in each energy sequence, and then compute their ensemble average. Their comparison with a random matrix ensemble which interpolates between Poisson statistics expected for regular systems and the Gaussian orthogonal ensemble (GOE) predicted for chaotic systems shows that the data are well described by the GOE. In a second approach, following an idea of Rosenzweig and Porter [Phys. Rev. 120, 1698 (1960)], we consider the complete spectrum composed of the independent subspectra. We analyze their fluctuation properties using the method of Bayesian inference involving a quantitative measure, called the chaoticity parameter $f$, which also interpolates between Poisson ($f=0$) and GOE statistics ($f=1$). It turns out to be $f\approx 0.9$. This is so far the closest agreement with a GOE observed in the spectra of bound states in a nucleus. The same analysis is also performed with spectra computed on the basis of shell model calculations with different interactions (surface-delta interaction, Kuo-Brown, Michigan-three-Yukawa). While the simple surface-delta interaction exhibits features typical for nuclear many-body systems with regular dynamics, the other, more realistic interactions yield chaoticity parameters $f$ close to the experimental values.

This Letter reports a study of the highly debated ${}^{10}\mathrm{Li}$ structure through the $d({}^9\mathrm{Li},p){}^{10}\mathrm{Li}$ one-neutron transfer reaction at 100 MeV. The ${}^{10}\mathrm{Li}$ energy spectrum is measured up to 4.6 MeV and angular distributions corresponding to different excitation energy regions are reported for the first time. The comparison between data and theoretical predictions, including pairing correlation effects, shows the existence of a $p_{1/2}$ resonance at $0.45\pm 0.03\mathrm{MeV}$ excitation energy, while no evidence for a significant $s$-wave contribution close to the threshold energy is observed. Moreover, two high-lying structures are populated at 1.5 and 2.9 MeV. The corresponding angular distributions suggest a significant $s_{1/2}$ partial-wave contribution for the 1.5 MeV structure and a mixing of configurations at higher energy, with the $d_{5/2}$ partial-wave contributing the most to the cross section.

Large-scale shell-model calculations predict that the region of deformation which comprises the heaviest chromium and iron isotopes at and beyond $N=40$ will merge with a new one at $N=50$ in an astonishing parallel to the $N=20$ and $N=28$ case in the neon and magnesium isotopes. We propose a valence space including the full $pf$ shell for the protons and the full $sdg$ shell for the neutrons, which represents a comeback of the the harmonic oscillator shells in the very neutron- rich regime. The onset of deformation is understood in the framework of the algebraic SU(3)-like structures linked to quadrupole dominance. Our calculations preserve the doubly magic nature of the ground state of ${}^{78}\mathrm{Ni}$, which, however, exhibits a well-deformed prolate band at low excitation energy, providing a striking example of shape coexistence far from stability. This new IOI adds to the four well-documented ones at $N=8$, 20, 28, and 40.

Bunched-beam collinear laser spectroscopy is performed on neutron deficient ${}^{52,53}\mathrm{Fe}$ prepared through in-flight separation followed by a gas stopping. This novel scheme is a major step to reach nuclides far from the stability line in laser spectroscopy. Differential mean-square charge radii $\delta ⟨r^2⟩$ of ${}^{52,53}\mathrm{Fe}$ are determined relative to stable ${}^{56}\mathrm{Fe}$ as $\delta ⟨r^2{⟩}^{56,52}=-0.034(13){\mathrm{fm}}^2$ and $\delta ⟨r^2{⟩}^{56,53}=-0.218(13){\mathrm{fm}}^2$, respectively, from the isotope shift of atomic hyperfine structures. The multiconfiguration Dirac-Fock method is used to calculate atomic factors to deduce $\delta ⟨r^2⟩$. The values of $\delta ⟨r^2⟩$ exhibit a minimum at the $N=28$ neutron shell closure. The nuclear density functional theory with Fayans and Skyrme energy density functionals is used to interpret the data. The trend of $\delta ⟨r^2⟩$ along the Fe isotopic chain results from an interplay between single-particle shell structure, pairing, and polarization effects and provides important data for understanding the intricate trend in the $\delta ⟨r^2⟩$ of closed-shell Ca isotopes.

The weakly bound exotic ${}^{11}\mathrm{Be}$ nucleus, famous for its ground-state parity inversion and distinct $n+{}^{10}\mathrm{Be}$ halo structure, is investigated from first principles using chiral two- and three-nucleon forces. An explicit treatment of continuum effects is found to be indispensable. We study the sensitivity of the ${}^{11}\mathrm{Be}$ spectrum to the details of the three-nucleon force and demonstrate that only certain chiral interactions are capable of reproducing the parity inversion. With such interactions, the extremely large $E1$ transition between the bound states is reproduced. We compare our photodisintegration calculations to conflicting experimental data and predict a distinct dip around the $3/2_1^{-}$ resonance energy. Finally, we predict low-lying $3/2^{+}$ and $9/2^{+}$ resonances that are not or not sufficiently measured in experiments.

The nature of the interaction between superfluid vortices and the neutron star crust, conjectured by Anderson and Itoh in 1975 to be at the heart vortex creep and the cause of glitches, has been a long-standing question in astrophysics. Using a qualitatively new approach, we follow the dynamics as superfluid vortices move in response to the presence of “nuclei” (nuclear defects in the crust). The resulting motion is perpendicular to the force, similar to the motion of a spinning top when pushed. We show that nuclei repel vortices in the neutron star crust, and characterize the force per unit length of the vortex line as a function of the vortex element to the nucleus separation.

The ${}^{54}\mathrm{Fe}$ nucleus was populated from a ${}^{56}\mathrm{Fe}$ beam impinging on a Be target with an energy of $E/A=500\mathrm{MeV}$. The internal decay via $\gamma$-ray emission of the ${10}^{+}$ metastable state was observed. As the structure of this isomeric state has to involve at least four unpaired nucleons, it cannot be populated in a simple two-neutron removal reaction from the ${}^{56}\mathrm{Fe}$ ground state. The isomeric state was produced in the low-momentum (-energy) tail of the parallel momentum (energy) distribution of ${}^{54}\mathrm{Fe}$, suggesting that it was populated via the decay of the ${\mathrm{\Delta }}^0$ resonance into a proton. This process allows the population of four-nucleon states, such as the observed isomer. Therefore, it is concluded that the observation of this ${10}^{+}$ metastable state in ${}^{54}\mathrm{Fe}$ is a consequence of the quark structure of the nucleons.

The Borromean ${}^6\mathrm{He}$ nucleus is an exotic system characterized by two halo neutrons orbiting around a compact ${}^4\mathrm{He}$ (or $\alpha$) core, in which the binary subsystems are unbound. The simultaneous reproduction of its small binding energy and extended matter and point-proton radii has been a challenge for ab initio theoretical calculations based on traditional bound-state methods. Using soft nucleon-nucleon interactions based on chiral effective field theory potentials, we show that supplementing the model space with ${}^4\mathrm{He}+n+n$ cluster degrees of freedom largely solves this issue. We analyze the role played by $\alpha$ clustering and many-body correlations, and study the dependence of the energy spectrum on the resolution scale of the interaction.

Excitation spectra of ${}^{11}\mathrm{C}$ are measured in the ${}^{12}\mathrm{C}(p,d)$ reaction near the ${\eta }^{\prime }$ emission threshold. A proton beam extracted from the synchrotron SIS-18 at GSI with an incident energy of 2.5 GeV impinges on a carbon target. The momenta of deuterons emitted at 0° are precisely measured with the fragment separator (FRS) operated as a spectrometer. In contrast to theoretical predictions on the possible existence of deeply bound ${\eta }^{\prime }$-mesic states in carbon nuclei, no distinct structures are observed associated with the formation of bound states. The spectra are analyzed to set stringent constraints on the formation cross section and on the hitherto barely known ${\eta }^{\prime }$-nucleus interaction.

The phases and properties of matter under global rotation have attracted much interest recently. In this Letter we investigate the pairing phenomena in a system of fermions under the presence of rotation. We find that there is a generic suppression effect on pairing states with zero angular momentum. We demonstrate this effect with the chiral condensation and the color superconductivity in hot dense QCD matter as explicit examples. In the case of chiral condensation, a new phase diagram in the temperature-rotation parameter space is found, with a nontrivial critical point.

We report the first measurement of the flux-averaged cross section for charged current coherent ${\pi }^{+}$ production on carbon for neutrino energies less than 1.5 GeV, and with a restriction on the final state phase space volume in the T2K near detector, ND280. Comparisons are made with predictions from the Rein-Sehgal coherent production model and the model by Alvarez-Ruso et al., the latter representing the first implementation of an instance of the new class of microscopic coherent models in a neutrino interaction Monte Carlo event generator. We observe a clear event excess above background, disagreeing with the null results reported by K2K and SciBooNE in a similar neutrino energy region. The measured flux-averaged cross sections are below those predicted by both the Rein-Sehgal and Alvarez-Ruso et al. models.

Fermions become polarized in a vortical fluid due to spin-vorticity coupling, and the polarization density is proportional to the local fluid vorticity. The radial expansion converts spatial vortical structures in the transverse plane to spin correlations in the azimuthal angle of final $\mathrm{\Lambda }$ hyperons’ transverse momentum in high-energy heavy-ion collisions. Using a $(3+1)\mathrm{D}$ viscous hydrodynamic model with fluctuating initial conditions from a multiphase transport (AMPT) model, we reveal two vortical structures that are common in many fluid dynamic systems: a right-handed toroidal structure around each beam direction for transverse vorticity and pairing of longitudinal vortices with opposite signs in the transverse plane. The calculated azimuthal correlation of the transverse spin is shown to have a cosine form plus an offset due to the toroidal structure of the transverse vorticity around the beam direction and the global spin polarization. The longitudinal spin correlation in the azimuthal angle shows an oscillatory structure due to multiple vorticity pairs in the transverse plane. Mechanisms of these vortical structures, physical implications of hyperon spin correlations, dependence on colliding energy, rapidity, centrality, and sensitivity to the shear viscosity are also investigated.