The observation of beam spin asymmetries in two-pion production in semi-inclusive deep inelastic scattering off an unpolarized proton target is reported. The data presented here were taken in the fall of 2018 with the CLAS12 spectrometer using a 10.6 GeV longitudinally spin-polarized electron beam delivered by CEBAF at JLab. The measured asymmetries provide the first opportunity to extract the parton distribution function $e(x)$, which provides information about the interaction between gluons and quarks, in a collinear framework that offers cleaner access than previous measurements. The asymmetries also constitute the first ever signal sensitive to the helicity-dependent two-pion fragmentation function $G_1^{\perp }$. A clear sign change is observed around the $\rho$ mass that appears in model calculations and is indicative of the dependence of the produced pions on the helicity of the fragmenting quark.

The discrepancy between observations from $\gamma$-ray astronomy of the ${}^{60}\mathrm{Fe}/{}^{26}\mathrm{Al}$ $\gamma$-ray flux ratio and recent calculations is an unresolved puzzle in nuclear astrophysics. The stellar $\beta$-decay rate of ${}^{59}\mathrm{Fe}$ is one of the major nuclear uncertainties impeding us from a precise prediction. The important Gamow-Teller strengths from the low-lying states in ${}^{59}\mathrm{Fe}$ to the ${}^{59}\mathrm{Co}$ ground state are measured for the first time using the exclusive measurement of the ${}^{59}\mathrm{Co}(t,{}^3\mathrm{He}+\gamma ){}^{59}\mathrm{Fe}$ charge-exchange reaction. The new stellar decay rate of ${}^{59}\mathrm{Fe}$ is a factor of $3.5\pm 1.1$ larger than the currently adopted rate at $T=1.2\mathrm{GK}$. Stellar evolution calculations show that the ${}^{60}\mathrm{Fe}$ production yield of an 18 solar mass star is decreased significantly by 40% when using the new rate. Our result eliminates one of the major nuclear uncertainties in the predicted yield of ${}^{60}\mathrm{Fe}$ and alleviates the existing discrepancy of the ${}^{60}\mathrm{Fe}/{}^{26}\mathrm{Al}$ ratio.

Three-body decay is a rare decay mode observed in a handful of unbound rare isotopes. The angular and energy correlations between emitted nucleons are of particular interest, as they provide invaluable information on the interplay between structure and reaction aspects of the nuclear open quantum system. To study the mechanism of two-nucleon emission, we developed a time-dependent approach that allows us to probe emitted nucleons at long times and large distances. We successfully benchmarked the new method against the Green’s function approach and applied it to low-energy two-proton and two-neutron decays. In particular, we studied the interplay between initial-state nucleon-nucleon correlations and final-state interaction. We demonstrated that the time evolution of the two-nucleon wave function is strongly impacted by the diproton/dineutron dynamics and that the correlations between emitted nucleons provide invaluable information on the dinucleon structure in the initial state.

The intrinsic spins and their correlations are the least understood characteristics of fission fragments from both theoretical and experimental points of view. In many nuclear reactions, the emerging fragments are typically excited and acquire an intrinsic excitation energy and an intrinsic spin depending on the type of the reactions and interaction mechanism. Both the intrinsic excitation energies and the fragments’ intrinsic spins and parities are controlled by the interaction mechanism and conservations laws, which lead to their correlations and determines the character of their deexcitation mechanism. We outline here a framework for the theoretical extraction of the intrinsic spin distributions of the fragments and their correlations within the fully microscopic real-time density-functional theory formalism and illustrate it on the example of induced fission of ${}^{236}\mathrm{U}$ and ${}^{240}\mathrm{Pu}$, using two nuclear energy density functionals. These fission fragment intrinsic spin distributions display new qualitative features previously not discussed in literature. Within this fully microscopic framework, we extract for the first time the intrinsic spin distributions of fission fragments of ${}^{236}\mathrm{U}$ and ${}^{240}\mathrm{Pu}$ as well as the correlations of their intrinsic spins, which have been debated in literature for more than six decades with no definite conclusions so far.

Isotopic distributions of fragments from fission of the neutron-deficient ${}^{178}\mathrm{Hg}$ nuclide are reported. This experimental observable is obtained for the first time in the region around lead using an innovative approach based on inverse kinematics and the coincidence between the large acceptance magnetic spectrometer $\mathrm{VAMOS}++$ and a new detection arm close to the target. The average fragment $N/Z$ ratio and prompt neutron $M_n$ multiplicity are derived and compared with current knowledge from actinide fission. A striking consistency emerges, revealing the unexpected dominant role of the proton subsystem with atomic number between the $Z=28$ and 50 magic numbers. The origin of nuclear charge polarization in fission and fragment deformation at scission are discussed.

We compute continuum and infinite volume limit extrapolations of the structure factors of neutron matter at finite temperature and density. Using a lattice formulation of leading-order pionless effective field theory, we compute the momentum dependence of the structure factors at finite temperature and at densities beyond the reach of the virial expansion. The Tan contact parameter is computed and the result agrees with the high momentum tail of the vector structure factor. All errors, statistical and systematic, are controlled for. This calculation is a first step towards a model-independent understanding of the linear response of neutron matter at finite temperature.

A ${}^{13}\mathrm{F}$ resonance was observed following a charge-exchange reaction between a fast ${}^{13}\mathrm{O}$ beam and a ${}^9\mathrm{Be}$ target. The resonance was found in the invariant-mass distribution of $3p+{}^{10}\mathrm{C}$ events and probably corresponds to a $5/2^{+}$ excited state. The ground state was also expected to be populated, but was not resolved from the background. The observed level decays via initial proton emissions to both the ground and first $2^{+}$ state of ${}^{12}\mathrm{O}$, which subsequently undergo $2p$ decay. In addition, there may also be a significant proton decay branch to the second $2^{+}$ level in ${}^{12}\mathrm{O}$. The wave function associated with the observed level may be collectivized due to coupling to the continuum as is it located just above the threshold for proton decay to the $2_2^{+}$ state of ${}^{12}\mathrm{O}$.

The first measurement of longitudinal decorrelations of harmonic flow amplitudes $v_n$ for $n=2–4$ in $\mathrm{Xe}+\mathrm{Xe}$ collisions at $\sqrt{s_{NN}}=5.44\mathrm{TeV}$ is obtained using $3\mu {\mathrm{b}}^{-1}$ of data with the ATLAS detector at the LHC. The decorrelation signal for $v_3$ and $v_4$ is found to be nearly independent of collision centrality and transverse momentum ($p_T$) requirements on final-state particles, but for $v_2$ a strong centrality and $p_T$ dependence is seen. When compared with the results from $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$, the longitudinal decorrelation signal in midcentral $\mathrm{Xe}+\mathrm{Xe}$ collisions is found to be larger for $v_2$, but smaller for $v_3$. Current hydrodynamic models reproduce the ratios of the $v_n$ measured in $\mathrm{Xe}+\mathrm{Xe}$ collisions to those in $\mathrm{Pb}+\mathrm{Pb}$ collisions but fail to describe the magnitudes and trends of the ratios of longitudinal flow decorrelations between $\mathrm{Xe}+\mathrm{Xe}$ and $\mathrm{Pb}+\mathrm{Pb}$. The results on the system-size dependence provide new insights and an important lever arm to separate effects of the longitudinal structure of the initial state from other early and late time effects in heavy-ion collisions.

A direct measurement of the decay width of the excited $0_1^{+}$ state of ${}^6\mathrm{Li}$ using the relative self-absorption technique is reported. Our value of ${\mathrm{\Gamma }}_{\gamma ,0_1^{+}\to 1_1^{+}}=8.17(14{)}_{\mathrm{stat}.}(11{)}_{\mathrm{syst}\mathrm{.}}\mathrm{eV}$ provides sufficiently low experimental uncertainties to test modern theories of nuclear forces. The corresponding transition rate is compared to the results of ab initio calculations based on chiral effective field theory that take into account contributions to the magnetic dipole operator beyond leading order. This enables a precision test of the impact of two-body currents that enter at next-to-leading order.

By modeling the transition paths of the nuclear $\gamma$-decay cascade using a scale-free random network, we uncover a universal power-law distribution of $\gamma$-ray intensity ${\rho }_I(I)\propto I^{-2}$, with $I$ the $\gamma$-ray intensity of each transition. This property is consistently observed for all datasets with a sufficient number of $\gamma$-ray intensity entries in the National Nuclear Data Center database, regardless of the reaction type or nuclei involved. In addition, we perform numerical simulations that support the model’s predictions of level population density.

Nonmonotonic variation with collision energy ($\sqrt{s_{\mathrm{NN}}}$) of the moments of the net-baryon number distribution in heavy-ion collisions, related to the correlation length and the susceptibilities of the system, is suggested as a signature for the quantum chromodynamics critical point. We report the first evidence of a nonmonotonic variation in the kurtosis times variance of the net-proton number (proxy for net-baryon number) distribution as a function of $\sqrt{s_{\mathrm{NN}}}$ with $3.1\sigma$ significance for head-on (central) gold-on-gold ($\mathrm{Au}+\mathrm{Au}$) collisions measured solenoidal tracker at Relativistic Heavy Ion Collider. Data in noncentral $\mathrm{Au}+\mathrm{Au}$ collisions and models of heavy-ion collisions without a critical point show a monotonic variation as a function of $\sqrt{s_{\mathrm{NN}}}$.

The pion-nucleon coupling constants determine the strength of the long-range nuclear forces and play a fundamental part in our understanding of nuclear physics. While the charged- and neutral-pion couplings to protons and neutrons are expected to be very similar, owing to the approximate isospin symmetry of the strong interaction, the different masses of the up and down quarks and electromagnetic effects may result in their slightly different values. Despite previous attempts to extract these coupling constants from different systems, our knowledge of their values is still deficient. In this Letter, we present a precision determination of these fundamental observables with fully controlled uncertainties from neutron-proton and proton-proton scattering data using chiral effective field theory. To achieve this goal, we use a novel methodology based on the Bayesian approach and perform, for the first time, a full-fledged partial-wave analysis of nucleon-nucleon scattering up to the pion production threshold in the framework of chiral effective field theory, including a complete treatment of isospin-breaking effects and our own determination of mutually consistent data. The resulting values of the pion-nucleon coupling constants are accurate at the percent level and show no significant charge dependence. These results mark an important step toward developing a precision theory of nuclear forces and structure.

Quasielastic ${}^{12}\mathrm{C}(e,e^{\prime }p)$ scattering was measured at spacelike 4-momentum transfer squared $Q^2=8$, 9.4, 11.4, and $14.2(\mathrm{GeV}/c{)}^2$, the highest ever achieved to date. Nuclear transparency for this reaction was extracted by comparing the measured yield to that expected from a plane-wave impulse approximation calculation without any final state interactions. The measured transparency was consistent with no $Q^2$ dependence, up to proton momenta of $8.5\mathrm{GeV}/c$, ruling out the quantum chromodynamics effect of color transparency at the measured $Q^2$ scales in exclusive $(e,e^{\prime }p)$ reactions. These results impose strict constraints on models of color transparency for protons.

A kinematically complete quasifree $(p,pn)$ experiment in inverse kinematics was performed to study the structure of the Borromean nucleus ${}^{17}\mathrm{B}$, which had long been considered to have a neutron halo. By analyzing the momentum distributions and exclusive cross sections, we obtained the spectroscopic factors for $1s_{1/2}$ and $0d_{5/2}$ orbitals, and a surprisingly small percentage of 9(2)% was determined for $1s_{1/2}$. Our finding of such a small $1s_{1/2}$ component and the halo features reported in prior experiments can be explained by the deformed relativistic Hartree-Bogoliubov theory in continuum, revealing a definite but not dominant neutron halo in ${}^{17}\mathrm{B}$. The present work gives the smallest $s$- or $p$-orbital component among known nuclei exhibiting halo features and implies that the dominant occupation of $s$ or $p$ orbitals is not a prerequisite for the occurrence of a neutron halo.

The yield of charged particles opposite to a $Z$ boson with large transverse momentum ($p_T$) is measured in $260{\mathrm{pb}}^{-1}$ of $pp$ and $1.7{\mathrm{nb}}^{-1}$ of $\mathrm{Pb}+\mathrm{Pb}$ collision data at 5.02 TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The $Z$ boson tag is used to select hard-scattered partons with specific kinematics, and to observe how their showers are modified as they propagate through the quark-gluon plasma created in $\mathrm{Pb}+\mathrm{Pb}$ collisions. Compared with $pp$ collisions, charged-particle yields in $\mathrm{Pb}+\mathrm{Pb}$ collisions show significant modifications as a function of charged-particle $p_T$ in a way that depends on event centrality and $Z$ boson $p_T$. The data are compared with a variety of theoretical calculations and provide new information about the medium-induced energy loss of partons in a $p_T$ regime difficult to measure through other channels.

We calculate basis-space converged neutrinoless $\beta \beta$-decay nuclear matrix elements for the lightest candidates: ${}^{48}\mathrm{Ca}$, ${}^{76}\mathrm{Ge}$, and ${}^{82}\mathrm{Se}$. Starting from initial two- and three-nucleon forces, we apply the ab initio in-medium similarity renormalization group to construct valence-space Hamiltonians and consistently transformed $\beta \beta$-decay operators. We find that the tensor component is non-negligible in ${}^{76}\mathrm{Ge}$ and ${}^{82}\mathrm{Se}$, and the resulting nuclear matrix elements are overall 25%–45% smaller than those obtained from the phenomenological shell model. While a final matrix element with uncertainties still requires substantial developments, this work nevertheless opens a path toward a true first-principles calculation of neutrinoless $\beta \beta$ decay in all nuclei relevant for ongoing large-scale searches.

Proton capture on the excited isomeric state of ${}^{26}\mathrm{Al}$ strongly influences the abundance of ${}^{26}\mathrm{Mg}$ ejected in explosive astronomical events and, as such, plays a critical role in determining the initial content of radiogenic ${}^{26}\mathrm{Al}$ in presolar grains. This reaction also affects the temperature range for thermal equilibrium between the ground and isomeric levels. We present a novel technique, which exploits the isospin symmetry of the nuclear force, to address the long-standing challenge of determining proton-capture rates on excited nuclear levels. Such a technique has in-built tests that strongly support its veracity and, for the first time, we have experimentally constrained the strengths of resonances that dominate the astrophysical ${}^{26m}\mathrm{Al}(p,\gamma ){}^{27}\mathrm{Si}$ reaction. These constraints demonstrate that the rate is at least a factor $\sim 8$ lower than previously expected, indicating an increase in the stellar production of ${}^{26}\mathrm{Mg}$ and a possible need to reinvestigate sensitivity studies involving the thermal equilibration of ${}^{26}\mathrm{Al}$.