EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless $\beta \beta$ decay of ${}^{136}\mathrm{Xe}$ to the ground state of ${}^{136}\mathrm{Ba}$. We report here on a search for the two-neutrino $\beta \beta$ decay of ${}^{136}\mathrm{Xe}$ to the first $0^{+}$ excited state, $0_1^{+}$, of ${}^{136}\mathrm{Ba}$ based on a $100 \mathrm{kg}\mathrm{yr}$ exposure of ${}^{136}\mathrm{Xe}$. Using a specialized analysis employing a machine learning algorithm, we obtain a 90% CL half-life sensitivity of $1.7\times {10}^{24}$ yr. We find no statistically significant evidence for the $2\nu \beta \beta$ decay to the excited state resulting in a lower limit of $T_{1/2}^{2\nu } (0^{+}\to 0_1^{+}) > 6.9 \times {10}^{23}$ yr at 90% CL. This observed limit is consistent with the estimated half-life of $2.5\times {10}^{25}$ yr.

The transport properties of neutron star matter play an important role in many astrophysical processes. We report the results of a calculation of the shear viscosity and thermal conductivity coefficients of the hard-sphere fermion system of degeneracy $\nu =2$, that can be regarded as a model of pure neutron matter. Our approach is based on the effective interaction obtained from the formalism of correlated basis functions and the cluster expansion technique. The resulting transport coefficients show a strong sensitivity to the quasiparticle effective mass, reflecting the effect of second-order contributions to the self-energy that are not taken into account in nuclear matter studies available in the literature.

We calculate reduced $B\left(E2\right)$ electromagnetic transition strengths for light nuclei of mass numbers $B=8,12,16,20,24$, and 32 within the Skyrme model. We find that the predicted transition strengths are of the correct order of magnitude and the computed intrinsic quadrupole moments match the experimentally observed effective nuclear shapes. For the Hoyle state we predict a large $B\left(E2\right)\uparrow$ value of $0.0521e^2{\text{b}}^2$. For oxygen-16, we can obtain a quantitative understanding of the ground state rotational band and the rotational excitations of the second spin-0 state, $0_2^{+}$.

We have measured the $K$-shell and total internal-conversion coefficients, ${\alpha }_K$ and ${\alpha }_T$, for the 150.8-keV $E3$ transition in ${}^{111}\text{Cd}$ to be 1.449(18) and 2.217(26), respectively. The ${\alpha }_K$ result agrees well with Dirac-Fock calculations in which the effect of the $K$-shell atomic vacancy is accounted for; it extends our precision tests of ${\alpha }_K$ calculations to $Z=48$, the lowest $Z$ yet measured. However, the result for ${\alpha }_T$ disagrees by about two standard deviations from the calculated ${\alpha }_T$ value, whether or not the atomic vacancy is included.

A comprehensive formalism to calculate fission cross sections based on the extension of the optical model for fission is presented. It can be used for description of nuclear reactions on actinides featuring multi-humped fission barriers with partial absorption in the wells and direct transmission through discrete and continuum fission channels. The formalism describes the gross fluctuations observed in the fission probability due to vibrational resonances, and can be easily implemented in existing statistical reaction model codes. The extended optical model for fission is applied for neutron induced fission cross-section calculations on ${}^{234,235,238}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$ targets. A triple-humped fission barrier is used for ${}^{234,235}\mathrm{U}(n,f)$, while a double-humped fission barrier is used for ${}^{238}\mathrm{U}(n,f)$ and ${}^{239}\mathrm{Pu}(n,f)$ reactions as predicted by theoretical barrier calculations. The impact of partial damping of class-II/III states, and of direct transmission through discrete and continuum fission channels, is shown to be critical for a proper description of the measured fission cross sections for ${}^{234,235,238}\mathrm{U}(n,f)$ reactions. The ${}^{239}\mathrm{Pu}(n,f)$ reaction can be calculated in the complete damping approximation. Calculated cross sections for ${}^{235,238}\mathrm{U}(n,f)$ and ${}^{239}\mathrm{Pu}(n,f)$ reactions agree within 3% with the corresponding cross sections derived within the Neutron Standards least-squares fit of available experimental data. The extended optical model for fission can be used for both theoretical fission studies and nuclear data evaluation.

Background: Direct evidence of the $\alpha \text{-cluster}$ manifestation in bound states has not been obtained yet, although a number of experimental studies were carried out to extract the information of the clustering. In particular in conventional analyses of $\alpha \text{-transfer}$ reactions, there exist a few significant problems on reaction models, which are insufficient to qualitatively discuss the cluster structure.

Purpose: We aim to verify the manifestation of the $\alpha \text{-cluster}$ structure from observables. As the first application, we plan to extract the spatial information of the cluster structure of the ${}^{20}\mathrm{Ne}$ nucleus in its ground state through the cross section of the $\alpha \text{-transfer}$ reaction ${}^{16}\mathrm{O}({}^6\mathrm{Li},d){}^{20}\mathrm{Ne}$.

Methods: For the analysis of the transfer reaction, we work with the coupled-channel Born approximation (CCBA) approach, in which the breakup effect of ${}^6\mathrm{Li}$ is explicitly taken into account by means of the continuum-discretized coupled-channel method based on the three-body $\alpha +d+{}^{16}\mathrm{O}$ model. The two methods are adopted to calculate the overlap function between ${}^{20}\mathrm{Ne}$ and $\alpha +{}^{16}\mathrm{O}$; one is the microscopic cluster model (MCM) with the generator coordinate method, and the other is the phenomenological two-body potential model (PM).

Results: We show that the CCBA calculation with the MCM wave function gives a significant improvement of the theoretical result on the angular distribution of the transfer cross section, which is consistent with the experimental data. Employing the PM, it is discussed which region of the cluster wave function is probed on the transfer cross section.

Conclusions: It is found that the surface region of the cluster wave function is sensitive to the cross section. The present work is situated as the first step in obtaining important information to systematically investigate the cluster structure.

To investigate the two-body dissipation effects on the synthesis of superheavy elements, we calculate low-energy collisions of the $N=50$ isotones $({}^{82}\mathrm{Ge},{}^{84}\mathrm{Se},{}^{86}\mathrm{Kr}$, and ${}^{88}\mathrm{Sr})$ on ${}^{208}\mathrm{Pb}$ using the time-dependent density-matrix theory (TDDM). TDDM is an extension of the time-dependent Hartree-Fock theory and can determine the time evolution of one- and two-body density matrices. Thus TDDM describes both one- and two-body dissipation of collective energies. It is shown that two-body dissipation may increase fusion cross sections and enhance the synthesis of superheavy elements.

Background: Exclusive two-nucleon knockout after electroexcitation of nuclei $[A(e,e^{\prime }NN)$ in brief] is considered to be a primary source of information about short-range correlations (SRCs) in nuclei. For a proper interpretation of the data, final-state interactions (FSIs) need to be theoretically controlled.

Purpose: Our goal is to quantify the role of FSI effects in exclusive $A(e,e^{\prime }pN)$ reactions for four target nuclei representative of the whole mass region. Our focus is on processes that are SRC driven. We investigate the role of FSIs for two characteristic detector setups corresponding to “small” and “large” coverage of the available phase space.

Method: Use is made of a factorized expression for the $A(e,e^{\prime }pN)$ cross section that is proportional to the two-body center-of-mass (c.m.) momentum distribution of close-proximity pairs. The $A(e,e^{\prime }pp)$ and $A(e,e^{\prime }pn)$ reactions for the target nuclei ${}^{12}\mathrm{C},{}^{27}\mathrm{Al},{}^{56}\mathrm{Fe}$, and ${}^{208}\mathrm{Pb}$ are investigated. The elastic attenuation mechanisms in the FSIs are included using the relativistic multiple-scattering Glauber approximation (RMSGA). Single-charge exchange (SCX) reactions are also included. We introduce the nuclear transparency $T_A^{pN}$, defined as the ratio of exclusive $(e,e^{\prime }pN)$ cross sections on nuclei to those on “free” nucleon pairs, as a measure for the aggregated effect of FSIs in $pN$ knockout reactions from nucleus $A$. A toy model is introduced in order to gain a better understanding of the $A$ dependence of $T_A^{pN}$.

Results: The transparency $T_A^{pN}$ drops from $0.2–0.3$ for ${}^{12}\mathrm{C}$ to $0.04–0.07$ for ${}^{208}\mathrm{Pb}$. For all considered kinematics, the mass dependence of $T_A^{pN}$ can be captured by the power law $T_A^{pN}\propto A^{-\lambda }$ with $0.4\lesssim \lambda \lesssim 0.5$. Apart from an overall reduction factor, we find that FSIs only modestly affect the distinct features of SRC-driven $A(e,e^{\prime }pN)$ which are dictated by the c.m. distribution of close-proximity pairs.

Conclusion: The SCX mechanisms represent a relatively small (order of a few percent) contribution of SRC-driven $A(e,e^{\prime }pN)$ processes. The mass dependence of FSI effects in exclusive $A(e,e^{\prime }pN)$ can be captured in a robust power law and is in agreement with the predictions obtained in a toy model.

The PHENIX Collaboration at the Relativistic Heavy Ion Collider has measured open heavy flavor production in minimum bias $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200$ GeV via the yields of electrons from semileptonic decays of charm and bottom hadrons. Previous heavy flavor electron measurements indicated substantial modification in the momentum distribution of the parent heavy quarks owing to the quark-gluon plasma created in these collisions. For the first time, using the PHENIX silicon vertex detector to measure precision displaced tracking, the relative contributions from charm and bottom hadrons to these electrons as a function of transverse momentum are measured in $\mathrm{Au}+\mathrm{Au}$ collisions. We compare the fraction of electrons from bottom hadrons to previously published results extracted from electron-hadron correlations in $p+p$ collisions at $\sqrt{s_{NN}}=200$ GeV and find the fractions to be similar within the large uncertainties on both measurements for $p_T>4\text{GeV}/c$. We use the bottom electron fractions in $\mathrm{Au}+\mathrm{Au}$ and $p+p$ along with the previously measured heavy flavor electron $R_{AA}$ to calculate the $R_{AA}$ for electrons from charm and bottom hadron decays separately. We find that electrons from bottom hadron decays are less suppressed than those from charm for the region $3<p_T<4\text{GeV}/c$.

We study the structure of vorticity and hydrodynamic helicity fields in peripheral heavy-ion collisions using the kinetic quark-gluon string model. The angular momentum conservation within this model holds with a good accuracy. We observe the formation of specific toroidal structures of vorticity field (vortex sheets). Their existence is mirrored in the polarization of hyperons of the percent order.

The $\gamma$-ray strength function and level density in the quasi-continuum of ${}^{151,153}\mathrm{Sm}$ have been measured using bismuth germanate shielded Ge clover detectors of the STARLiTeR system. The Compton shields allow an extraction of the $\gamma$ strength down to unprecedentedly low $\gamma$ energies of $\approx 500$ keV. For the first time an enhanced low-energy $\gamma$-ray strength has been observed in the rare-earth region. In addition, for the first time both the upbend and the well-known scissors resonance have been observed simultaneously for the same nucleus. Hauser-Feshbach calculations show that this strength enhancement at low $\gamma$ energies could have an impact of 2–3 orders of magnitude on the ($n,\gamma$) reaction rates for $r$-process nucleosynthesis.

Background: Heavy-ion fusion reactions at energies near the Coulomb barrier are influenced by couplings between the relative motion and nuclear intrinsic degrees of freedom of the colliding nuclei. The time-dependent Hartree-Fock (TDHF) theory, incorporating the couplings at the mean-field level, as well as the coupled-channels (CC) method are standard approaches to describe low energy nuclear reactions.

Purpose: To investigate the effect of couplings to inelastic and transfer channels on the fusion cross sections for the reactions ${}^{40}\mathrm{Ca}+{}^{58}\mathrm{Ni}$ and ${}^{40}\mathrm{Ca}+{}^{64}\mathrm{Ni}$.

Methods: Fusion cross sections around and below the Coulomb barrier have been obtained from CC calculations, using the bare nucleus-nucleus potential calculated with the frozen Hartree-Fock method and coupling parameters taken from known nuclear structure data. The fusion thresholds and neutron transfer probabilities have been calculated with the TDHF method.

Results: For ${}^{40}\mathrm{Ca}+{}^{58}\mathrm{Ni}$, the TDHF fusion threshold is in agreement with the most probable barrier obtained in the CC calculations including the couplings to the low-lying octupole $3_1^{-}$ state for ${}^{40}\mathrm{Ca}$ and to the low-lying quadrupole $2_1^{+}$ state for ${}^{58}\mathrm{Ni}$. This indicates that the octupole and quadrupole states are the dominant excitations while neutron transfer is shown to be weak. For ${}^{40}\mathrm{Ca}+{}^{64}\mathrm{Ni}$, the TDHF barrier is lower than predicted by the CC calculations including the same inelastic couplings as those for ${}^{40}\mathrm{Ca}+{}^{58}\mathrm{Ni}$. TDHF calculations show large neutron transfer probabilities in ${}^{40}\mathrm{Ca}+{}^{64}\mathrm{Ni}$ which could result in a lowering of the fusion threshold.

Conclusions: Inelastic channels play an important role in ${}^{40}\mathrm{Ca}+{}^{58}\mathrm{Ni}$ and ${}^{40}\mathrm{Ca}+{}^{64}\mathrm{Ni}$ reactions. The role of neutron transfer channels has been highlighted in ${}^{40}\mathrm{Ca}+{}^{64}\mathrm{Ni}$.

We discuss the approximate inclusion of three-nucleon $\left(3N\right)$ interactions into ab initio nuclear structure calculations using a multireference formulation of normal ordering and Wick's theorem. Following the successful application of single-reference normal ordering for the study of ground states of closed-shell nuclei, e.g., in coupled-cluster theory, multireference normal ordering opens a path to open-shell nuclei and excited states. Based on different multideterminantal reference states we benchmark the truncation of the normal-ordered Hamiltonian at the two-body level in no-core shell-model calculations for $p$-shell nuclei, including ${}^6\mathrm{Li},{}^{12}\mathrm{C}$, and ${}^{10}\mathrm{B}$. We find that this multireference normal-ordered two-body approximation is able to capture the effects of the $3N$ interaction with sufficient accuracy, both for ground-state and excitation energies, at the computational cost of a two-body Hamiltonian. It is robust with respect to the choice of reference states and has a multitude of applications in ab initio nuclear structure calculations of open-shell nuclei and their excitations as well as in nuclear reaction studies.

The lowest quadrupole $\gamma$-vibrational $K^{\pi }=2^{+}$ states in axially deformed rare-earth (Nd, Sm, Gd, Dy, Er, Yb, Hf, W) and actinide (U) nuclei are systematically investigated within the separable random-phase-approximation (SRPA) based on the Skyrme functional. The energies $E_{\gamma }$ and reduced transition probabilities $B\left(E2\right)$ of $2_{\gamma }^{+}$ states are calculated with the Skyrme forces SV-bas and ${\mathrm{SkM}}^{*}$. The energies of two-quasiparticle configurations forming the SRPA basis are corrected by using the pairing blocking effect. This results in a systematic downshift of $E_{\gamma }$ by 0.3–0.5 MeV and thus in a better agreement with the experiment, especially in Sm, Gd, Dy, Hf, and W regions. For other isotopic chains, a noticeable overestimation of $E_{\gamma }$ and too weak collectivity of $2_{\gamma }^{+}$ states still persist. It is shown that domains of nuclei with low and high $2_{\gamma }^{+}$ collectivity are related to the structure of the lowest two-quasiparticle states and conservation of the Nilsson selection rules. The description of $2_{\gamma }^{+}$ states with SV-bas and ${\mathrm{SkM}}^{*}$ is similar in light rare-earth nuclei but deviates in heavier nuclei. However SV-bas much better reproduces the quadrupole deformation and energy of the isoscalar giant quadrupole resonance. The accuracy of SRPA is justified by comparison with exact RPA. The calculations suggest that a further development of the self-consistent calculation schemes is needed for a systematic satisfactory description of the $2_{\gamma }^{+}$ states.

Odd-even double mass differences (DMDs) of magic nuclei are found within an approach starting from the free $NN$ interaction, accounting for particle-phonon coupling (PC) effects. We consider three PC effects: the phonon-induced effective interaction, the renormalization of the “ends” due to the pole PC contribution to the nucleon mass operator, and the change of the single-particle energies. The perturbation theory in $g_L^2$, where $g_L$ is the vertex of the creation of the $L$-multipole phonon, is used for PC calculations. PC corrections to single-particle energies are found with an approximate accounting for the tadpole diagram. Results for magic ${}^{40,48}\mathrm{Ca}, {}^{56,78}\mathrm{Ni}, {}^{100,132}\mathrm{Sn}$, and ${}^{208}\mathrm{Pb}$ nuclei are presented. For the lighter part of this set of nuclei, from ${}^{40}\mathrm{Ca}$ to ${}^{56}\mathrm{Ni}$, the cases divide approximately in half, between those where the PC corrections to DMD values are in good agreement with the data and the ones with the opposite result. In the major part of the cases of worsening description of DMD, a poor applicability of the perturbation theory for the induced interaction is the most probable reason of the phenomenon. For intermediate nuclei, ${}^{78}\mathrm{Ni}$ and ${}^{100}\mathrm{Sn}$, there are no sufficiently accurate data on masses of nuclei necessary for finding DMD values. Finally, for heavier nuclei, ${}^{132}\mathrm{Sn}$ and ${}^{208}\mathrm{Pb}$, PC corrections always result in better agreement with experiment.

Background: One important ingredient for many applications of nuclear physics to astrophysics, nuclear energy, and stockpile stewardship are cross sections for reactions of neutrons with rare isotopes. Since direct measurements are often not feasible, indirect methods, e.g., $(d,p)$ reactions, should be used. Those $(d,p)$ reactions may be viewed as three-body reactions and described with Faddeev techniques.

Purpose: Faddeev equations in momentum space have a long tradition of utilizing separable interactions in order to arrive at sets of coupled integral equations in one variable. Optical potentials representing the effective interactions in the neutron (proton) nucleus subsystem are usually non-Hermitian as well as energy dependent. Potential matrix elements as well as transition matrix elements calculated with them must fulfill the reciprocity theorem. The purpose of this paper is to introduce a separable, energy-dependent representation of complex, energy-dependent optical potentials that fulfill reciprocity exactly.

Methods: Momentum space Lippmann-Schwinger integral equations are solved with standard techniques to obtain the form factors for the separable representation.

Results: Starting from a separable, energy-independent representation of global optical potentials based on a generalization of the Ernst-Shakin-Thaler (EST) scheme, a further generalization is needed to take into account the energy dependence. Applications to $n+{}^{48}\mathrm{Ca},n+{}^{208}\mathrm{Pb}$, and $p+{}^{208}\mathrm{Pb}$ are investigated for energies from 0 to 50 MeV with special emphasis on fulfilling reciprocity.

Conclusions: We find that the energy-dependent separable representation of complex, energy-dependent phenomenological optical potentials fulfills reciprocity exactly. In addition, taking into account the explicit energy dependence slightly improves the description of the $S$ matrix elements.

The cross section of the neutron capture reaction ${}^{176}\mathrm{Lu}\left(n,\gamma \right)$ has been measured for a wide incident neutron energy range with the Detector for Advanced Neutron Capture Experiments at the Los Alamos Neutron Science Center. The thermal neutron capture cross section was determined to be $\left(1912\pm 132\right)$ b for one of the Lu natural isotopes, ${}^{176}\mathrm{Lu}$. The resonance part was measured and compared to the Mughabghab's atlas using the $R$-matrix code, sammy. At higher neutron energies the measured cross sections are compared to ENDF/B-VII.1, JEFF-3.2, and BRC evaluated nuclear data. The Maxwellian averaged cross sections in a stellar plasma for thermal energies between 5 keV and 100 keV were extracted using these data.

This work investigates the neutron-induced fission of ${}^{234}\mathrm{U}$ and the fission-fragment properties for neutron energies between $E_{\mathrm{n}}=0.2$ and 5.0 MeV with a special highlight on the prominent vibrational resonance at $E_{\mathrm{n}}=0.77$ MeV. Angular, energy, and mass distributions were determined based on the double-energy technique by means of a twin Frisch-grid ionization chamber. The experimental data are parametrized in terms of fission modes based on the multimodal random neck-rupture model. The main results are a verified strong angular anisotropy and fluctuations in the energy release as a function of incident-neutron energy.

The invariant yields, $dN/dy$, for $J/\psi$ production at forward rapidity $(1.2<|y|<2.2)$ in $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{{}_{NN}}}=193\mathrm{GeV}$ have been measured as a function of collision centrality. The invariant yields and nuclear-modification factor $R_{AA}$ are presented and compared with those from $\mathrm{Au}+\mathrm{Au}$ collisions in the same rapidity range. Additionally, the direct ratio of the invariant yields from $\mathrm{U}+\mathrm{U}$ and $\mathrm{Au}+\mathrm{Au}$ collisions within the same centrality class is presented, and used to investigate the role of $c\overline{c}$ coalescence. Two different parametrizations of the deformed Woods-Saxon distribution were used in Glauber calculations to determine the values of the number of nucleon-nucleon collisions in each centrality class, $N_{\mathrm{coll}}$, and these were found to give significantly different $N_{\mathrm{coll}}$ values. Results using $N_{\mathrm{coll}}$ values from both deformed Woods-Saxon distributions are presented. The measured ratios show that the $J/\psi$ suppression, relative to binary collision scaling, is similar in $\mathrm{U}+\mathrm{U}$ and $\mathrm{Au}+\mathrm{Au}$ for peripheral and midcentral collisions, but that $J/\psi$ show less suppression for the most central $\mathrm{U}+\mathrm{U}$ collisions. The results are consistent with a picture in which, for central collisions, increase in the $J/\psi$ yield due to $c\overline{c}$ coalescence becomes more important than the decrease in yield due to increased energy density. For midcentral collisions, the conclusions about the balance between $c\overline{c}$ coalescence and suppression depend on which deformed Woods-Saxon distribution is used to determine $N_{\mathrm{coll}}$.

${}^{26}\mathrm{Al}$ remains an intriguing target for observational $\gamma$-ray astronomy, thanks to its characteristic decay. The ${}^{25}\mathrm{Al}\left(p,\gamma \right){}^{26}\mathrm{Si}$ reaction is the crucial link in a sequence that bypasses the production of the observable ${}^{26}\mathrm{Al}^g$ in favor of the short-lived isomeric state; determining its astrophysical reaction rate across a range of stellar environments has been the focus of many studies. A reanalysis of previous work, utilizing recent improvements in excitation energies and ground state masses, is presented to reduce the ambiguities in the literature and provide focus to future measurements.