The first ($v_1^{\mathrm{fluc}}$), second ($v_2$), and third ($v_3$) harmonic coefficients of the azimuthal particle distribution at midrapidity are extracted for charged hadrons and studied as a function of transverse momentum ($p_T$) and mean charged particle multiplicity density $⟨N_{\mathrm{ch}}⟩$ in $U+U$ ($\sqrt{s_{NN}}=193\mathrm{GeV}$), $\mathrm{Au}+\mathrm{Au}$, $\mathrm{Cu}+\text{Au}$, $\mathrm{Cu}+\mathrm{Cu}$, $d+\mathrm{Au}$, and $p+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ with the STAR detector. For the same $⟨N_{\mathrm{ch}}⟩$, the $v_1^{\mathrm{fluc}}$ and $v_3$ coefficients are observed to be independent of the collision system, while $v_2$ exhibits such a scaling only when normalized by the initial-state eccentricity (${ϵ}_2$). The data also show that $\ln (v_2/{ϵ}_2)$ scales linearly with $⟨N_{\mathrm{ch}}{⟩}^{-1/3}$. These measurements provide insight into initial-geometry fluctuations and the role of viscous hydrodynamic attenuation on $v_n$ from small to large collision systems.

A recent laser spectroscopy experiment [J. Thielking , Nature, (London) 556, 321 (2018)] has determined for the first time the magnetic dipole moment of the 7.8 eV isomeric state ${}^{229m}\mathrm{Th}$. The measured value differs by a factor of approximately 5 from previous nuclear theory predictions based on the Nilsson model, raising questions about our understanding of the underlying nuclear structure. Here, we present a new theoretical prediction based on a nuclear model with coupled collective quadrupole-octupole and single-particle motions. Our calculations yield an isomer magnetic dipole moment of ${\mu }_{\mathrm{IS}}=-0.35{\mu }_N$ in surprisingly good agreement with the experimentally determined value of $-0.37(6){\mu }_N$, while overestimating the ground state dipole moment by a factor 1.4. The model provides further information on the role and strength of the Coriolis mixing and the most probable value of the gyromagnetic ratio $g_R$ and its consequences for the transition probability $B(M1)$. The key role of the magnetic moment values as constraints in the determination of the isomer decay rates is discussed.

Fifty-five inclusive single nucleon-removal cross sections from medium mass neutron-rich nuclei impinging on a hydrogen target at $\sim 250\mathrm{MeV}/\mathrm{nucleon}$ are measured at the RIKEN Radioactive Isotope Beam Factory. Systematically higher cross sections are found for proton removal from nuclei with an even number of protons as compared to odd-proton number projectiles for a given neutron separation energy. Neutron removal cross sections display no even-odd splitting, contrary to nuclear cascade model predictions. Both effects are understood through simple considerations of neutron separation energies and bound state level densities originating in pairing correlations in the daughter nuclei. These conclusions are supported by comparison with semimicroscopic model predictions, highlighting the enhanced role of low-lying level densities in nucleon-removal cross sections from loosely bound nuclei.

We derive the rotation-vibration spectrum of a $3\alpha +1$ neutron(proton) configuration with triangular $D_{3h}$ symmetry by exploiting the properties of the double group $D_{3h}^{\prime }$ and show evidence for this symmetry to occur in the rotation-vibration spectra of ${}^{13}\mathrm{C}$. Our results, based on purely symmetry considerations, provide benchmarks for microscopic calculations of the cluster structure of light nuclei.

First measurements of double-polarization observables in $\omega$ photoproduction off the proton are presented using transverse target polarization and data from the CEBAF Large Acceptance Spectrometer (CLAS) FROST experiment at Jefferson Lab. The beam-target asymmetry $F$ has been measured using circularly polarized, tagged photons in the energy range 1200–2700 MeV, and the beam-target asymmetries $H$ and $P$ have been measured using linearly polarized, tagged photons in the energy range 1200–2000 MeV. These measurements significantly increase the database on polarization observables. The results are included in two partial-wave analyses and reveal significant contributions from several nucleon ($N^{*}$) resonances. In particular, contributions from new $N^{*}$ resonances listed in the Review of Particle Properties are observed, which aid in reaching the goal of mapping out the nucleon resonance spectrum.

We carry out nonperturbative calculations of the single-particle excitation spectrum in strongly interacting neutron matter. These are microscopic quantum Monte Carlo computations of many-neutron energies at different densities as well as several distinct excited states. As input, we employ both phenomenological and chiral two- and three-nucleon interactions. We use the single-particle spectrum to extract the effective mass in neutron matter. With a view to systematizing the error involved in this extraction, we carefully assess the impact of finite-size effects on the quasiparticle dispersion relation. We find an effective-mass ratio that drops from 1 as the density is increased. We conclude by connecting our results with the physics of ultracold gases as well as with energy-density functional theories of nuclei and neutron-star matter.

We study chemical equilibration in out-of-equilibrium quark-gluon plasma using the first principles method of QCD effective kinetic theory, accurate at weak coupling. In longitudinally expanding systems—relevant for relativistic nuclear collisions—we find that for realistic couplings chemical equilibration takes place after hydrodynamization, but well before local thermalization. We estimate that hadronic collisions with final state multiplicities $d{N}_{\mathrm{ch}}/d\eta \gtrsim {10}^2$ live long enough to reach approximate chemical equilibrium, which is consistent with the saturation of strangeness enhancement observed in proton-proton, proton-nucleus, and nucleus-nucleus collisions.

We investigate the $P_T$-broadening effects in dilepton production through photon-photon scattering in heavy ion collisions. The QED multiple interaction effect with the medium is found to be consistent with the recent observation of a low transverse momentum lepton pair from the ATLAS Collaboration at the LHC. We further comment on the magnetic broadening effect and point out a number of ways to disentangle these two mechanisms. In particular, the rapidity dependence of the $P_T$-broadening effect provides a unique probe to the magnetic effect.

The structure of the extremely proton-rich nucleus ${{}_8{}^{11}\mathrm{O}}_3$, the mirror of the two-neutron halo nucleus ${{}_3{}^{11}\mathrm{Li}}_8$, has been studied experimentally for the first time. Following two-neutron knockout reactions with a ${}^{13}\mathrm{O}$ beam, the ${}^{11}\mathrm{O}$ decay products were detected after two-proton emission and used to construct an invariant-mass spectrum. A broad peak of width $\sim 3.4\mathrm{MeV}$ was observed. Within the Gamow coupled-channel approach, it was concluded that this peak is a multiplet with contributions from the four lowest ${}^{11}\mathrm{O}$ resonant states: $J^{\pi }=3/2_1^{-}$, $3/2_2^{-}$, $5/2_1^{+}$, and $5/2_2^{+}$. The widths and configurations of these states show strong, nonmonotonic dependencies on the depth of the $p$-${}^9\mathrm{C}$ potential. This unusual behavior is due to the presence of a broad threshold resonant state in ${}^{10}\mathrm{N}$, which is an analog of the virtual state in ${}^{10}\mathrm{Li}$ in the presence of the Coulomb potential. After optimizing the model to the data, only a moderate isospin asymmetry between ground states of ${}^{11}\mathrm{O}$ and ${}^{11}\mathrm{Li}$ was found.

Prescaling is a far-from-equilibrium phenomenon which describes the rapid establishment of a universal scaling form of distributions much before the universal values of their scaling exponents are realized. We consider the example of the spatiotemporal evolution of the quark-gluon plasma explored in heavy-ion collisions at sufficiently high energies. Solving QCD kinetic theory with elastic and inelastic processes, we demonstrate that the gluon and quark distributions very quickly adapt a self-similar scaling form, which is independent of initial condition details and system parameters. The dynamics in the prescaling regime is then fully encoded in a few time-dependent scaling exponents, whose slow evolution gives rise to far-from-equilibrium hydrodynamic behavior.

We consider quarkyonic matter to naturally explain the observed properties of neutron stars. We argue that such matter might exist at densities close to that of nuclear matter, and at the onset, the pressure and the sound velocity in quarkyonic matter increase rapidly. In the limit of large number of quark colors $N_c$, this transition is characterized by a discontinuous change in pressure as a function of baryon number density. We make a simple model of quarkyonic matter and show that generically the sound velocity is a nonmonotonic function of density—it reaches a maximum at relatively low density, decreases, and then increases again to its asymptotic value of $1/\sqrt{3}$.

Above-barrier complete fusion involving nuclides with low binding energy is typically suppressed by 30%. The mechanism that causes this suppression, and produces the associated incomplete fusion products, is controversial. We have developed a new experimental approach to investigate the mechanisms that produce incomplete fusion products, combining singles and coincidence measurements of light fragments and heavy residues in ${}^7\mathrm{Li}+{}^{209}\mathrm{Bi}$ reactions. For polonium isotopes, the dominant incomplete fusion product, only a small fraction can be explained by projectile breakup followed by capture: the dominant mechanism is triton cluster transfer. Suppression of complete fusion is therefore primarily a consequence of clustering in weakly bound nuclei rather than their breakup prior to reaching the fusion barrier. This implies that suppression of complete fusion will occur in reactions of nuclides where strong clustering is present.

We report the first measurement of low-energy proton-capture cross sections of ${}^{124}\mathrm{Xe}$ in a heavy-ion storage ring. ${{}^{124}\mathrm{Xe}}^{54+}$ ions of five different beam energies between 5.5 and 8 AMeV were stored to collide with a windowless hydrogen target. The ${}^{125}\mathrm{Cs}$ reaction products were directly detected. The interaction energies are located on the high energy tail of the Gamow window for hot, explosive scenarios such as supernovae and x-ray binaries. The results serve as an important test of predicted astrophysical reaction rates in this mass range. Good agreement in the prediction of the astrophysically important proton width at low energy is found, with only a 30% difference between measurement and theory. Larger deviations are found above the neutron emission threshold, where also neutron and $\gamma$ widths significantly impact the cross sections. The newly established experimental method is a very powerful tool to investigate nuclear reactions on rare ion beams at low center-of-mass energies.

Starting with the solution to the Bethe-Salpeter equation for the pion, in a beyond rainbow-ladder truncation to QCD’s Dyson-Schwinger equations, we determine the pion’s $l_z=0$ and $|l_z|=1$ leading Fock-state light-front wave functions (LFWFs) [labeled by ${\psi }_{l_z}(x,{\mathit{k}}_T^2)$]. The leading-twist time-reversal even transverse momentum dependent parton distribution function (TMD) of the pion is then directly obtained using these LFWFs. A key characteristic of the LFWFs, which is driven by dynamical chiral symmetry breaking, is that at typical hadronic scales they are broad functions in the light-cone momentum fraction $x$. The LFWFs have a nontrivial $(x,{\mathit{k}}_T^2)$ dependence and in general do not factorize into separate functions of each variable. For ${\mathit{k}}_T^2\lesssim 1{\mathrm{GeV}}^2$ the ${\mathit{k}}_T^2$ dependence of the LFWFs is well described by a Gaussian; however for ${\mathit{k}}_T^2\gtrsim 10{\mathrm{GeV}}^2$ these LFWFs behave as ${\psi }_0\propto x(1-x)/{\mathit{k}}_T^2$ and ${\psi }_1\propto x(1-x)/{\mathit{k}}_T^4$, and therefore exhibit the power-law behavior predicted by perturbative QCD. The pion’s TMD naturally inherits many features from the LFWFs. The TMD evolution of our result is studied using both the $b^{*}$ and $\zeta$ prescriptions which allows a qualitative comparison with Drell-Yan data.

Angular correlations between heavy-flavor decay electrons and charged particles at midrapidity ($|\eta |<0.8$) are measured in $p$-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$. The analysis is carried out for the 0%–20% (high) and 60%–100% (low) multiplicity ranges. The jet contribution in the correlation distribution from high-multiplicity events is removed by subtracting the distribution from low-multiplicity events. An azimuthal modulation remains after removing the jet contribution, similar to previous observations in two-particle angular correlation measurements for light-flavor hadrons. A Fourier decomposition of the modulation results in a positive second-order coefficient ($v_2$) for heavy-flavor decay electrons in the transverse momentum interval $1.5<p_{\mathrm{T}}<4\mathrm{GeV}/c$ in high-multiplicity events, with a significance larger than $5\sigma$. The results are compared with those of charged particles at midrapidity and those of inclusive muons at forward rapidity. The $v_2$ measurement of open heavy-flavor particles at midrapidity in small collision systems could provide crucial information to help interpret the anisotropies observed in such systems.

The physical properties of neutrons emitted from neutron-induced fission are fundamental to our understanding of nuclear fission. However, while state-of-the-art fission models still incorporate isotropic fission neutron spectra, it is believed that the preequilibrium prefission component of these spectra is strongly anisotropic. The lack of experimental guidance on this feature has not motivated incorporation of anisotropic neutron spectra in fission models, though any significant anisotropy would impact descriptions of a fissioning system. In the present work, an excess of counts at high energies in the fission neutron spectrum of ${}^{239}\mathrm{Pu}$ is clearly observed and identified as an excess of the preequilibrium prefission distribution above the postfission neutron spectrum. This excess is separated from the underlying postfission neutron spectrum, and its angular distribution is determined as a function in incident neutron energy and outgoing neutron detection angle. Comparison with neutron scattering models provides the first experimental evidence that the preequilibrium angular distribution is uncorrelated with the fission axis. The results presented here also impact the interpretation of several influential prompt fission neutron spectrum measurements.

We report the first observation of the double strange baryon $\mathrm{\Xi }(1620{)}^0$ in its decay to ${\mathrm{\Xi }}^{-}{\pi }^{+}$ via ${\mathrm{\Xi }}_c^{+}\to {\mathrm{\Xi }}^{-}{\pi }^{+}{\pi }^{+}$ decays based on a $980{\mathrm{fb}}^{-1}$ data sample collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The mass and width are measured to be $1610.4\pm 6.0(\text{stat}{)}_{-4.2}^{+6.1}(\text{syst})\mathrm{MeV}/c^2$ and $59.9\pm 4.8(\text{stat}{)}_{-7.1}^{+2.8}(\text{syst})\mathrm{MeV}$, respectively. We obtain $4.0\sigma$ evidence of the $\mathrm{\Xi }(1690{)}^0$ with the same data sample. These results shed light on the structure of hyperon resonances with strangeness $S=-2$.

The first $\gamma$-ray spectroscopy of ${}^{52}\mathrm{Ar}$, with the neutron number $N=34$, was measured using the ${}^{53}\mathrm{K}(p,2p)$ one-proton removal reaction at $\sim 210\mathrm{MeV}/\mathrm{u}$ at the RIBF facility. The $2_1^{+}$ excitation energy is found at 1656(18) keV, the highest among the Ar isotopes with $N>20$. This result is the first experimental signature of the persistence of the $N=34$ subshell closure beyond ${}^{54}\mathrm{Ca}$, i.e., below the magic proton number $Z=20$. Shell-model calculations with phenomenological and chiral-effective-field-theory interactions both reproduce the measured $2_1^{+}$ systematics of neutron-rich Ar isotopes, and support a $N=34$ subshell closure in ${}^{52}\mathrm{Ar}$.