In the rapid shape change from spherical to deformed nuclei in the $Z=40$ Zr isotopic chain, recent work has identified shape coexistence in ${}^{96}\mathrm{Zr}$. Between ${}^{96}\mathrm{Zr}$ and the strongly deformed ${}^{100}\mathrm{Zr}, {}^{98}\mathrm{Zr}$ is expected to also exhibit coexistence of nuclear shapes. The degree of mixing between different configurations is mainly determined by the nucleon-nucleon interactions. For nuclear model predictions, experimental constraints are needed, but they are barely available for ${}^{98}\mathrm{Zr}$. To study low-lying transitions in ${}^{98}\mathrm{Zr}$, a Coulomb excitation experiment was conducted at the Argonne Tandem-Linac Accelerator System (ATLAS) facility using a ${}^{98}\mathrm{Zr}$ beam extracted from the Californium Rare Ion Breeder Upgrade (CARIBU) ion source and Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) for $\gamma$-ray spectroscopy coupled to the compact heavy ion counter (CHICO2) for ion detection. This paper reports on the first decisive deduction of the $B(E2;2_1^{+}\to 0_1^{+})$ transition strength in ${}^{98}\mathrm{Zr}$ and on its interpretation.

A key question concerning the spherical-vibrator attributes of states in cadmium isotopes is addressed by means of a boson Hamiltonian encompassing U(5) partial dynamical symmetry. The U(5) symmetry is preserved in a segment of the spectrum and is broken in particular nonyrast states, and the resulting mixing with the intruder states is small. The vibrational character is thus maintained in the majority of low-lying normal states as observed in ${}^{110}\mathrm{Cd}$.

We study systems of bosons and fermions in finite periodic boxes and show how the existence and properties of few-body resonances can be extracted from studying the volume dependence of the calculated energy spectra. We use and briefly review a plane-wave-based discrete variable representation, which allows a convenient implementation of periodic boundary conditions. With these calculations we establish that avoided level crossings occur in the spectra of up to four particles and can be linked to the existence of multibody resonances. To benchmark our method we use two-body calculations, where resonance properties can be determined with other methods, as well as a three-boson model interaction known to generate a three-boson resonance state. Finding good agreement for these cases, we then predict three-body and four-body resonances for models using a shifted Gaussian potential. Our results establish few-body finite-volume calculations as a new tool to study few-body resonances. In particular, the approach can be used to study few-neutron systems, where such states have been conjectured to exist.

Background: The mass, or binding energy, is the basis property of the atomic nucleus. It determines its stability and reaction and decay rates. Quantifying the nuclear binding is important for understanding the origin of elements in the universe. The astrophysical processes responsible for the nucleosynthesis in stars often take place far from the valley of stability, where experimental masses are not known. In such cases, missing nuclear information must be provided by theoretical predictions using extreme extrapolations. To take full advantage of the information contained in mass model residuals, i.e., deviations between experimental and calculated masses, one can utilize Bayesian machine-learning techniques to improve predictions.

Purpose: To improve the quality of model-based predictions of nuclear properties of rare isotopes far from stability, we consider the information contained in the residuals in the regions where the experimental information exist. As a case in point, we discuss two-neutron separation energies $S_{2n}$ of even-even nuclei. Through this observable, we assess the predictive power of global mass models towards more unstable neutron-rich nuclei and provide uncertainty quantification of predictions.

Methods: We consider 10 global models based on nuclear density functional theory with realistic energy density functionals as well as two more phenomenological mass models. The emulators of $S_{2n}$ residuals and credibility intervals (Bayesian confidence intervals) defining theoretical error bars are constructed using Bayesian Gaussian processes and Bayesian neural networks. We consider a large training dataset pertaining to nuclei whose masses were measured before 2003. For the testing datasets, we considered those exotic nuclei whose masses have been determined after 2003. By establishing statistical methodology and parameters, we carried out extrapolations toward the $2n$ dripline.

Results: While both Gaussian processes and Bayesian neural networks reduce the root-mean-square (rms) deviation from experiment significantly, GP offers a better and much more stable performance. The increase in the predictive power of microscopic models aided by the statistical treatment is quite astonishing: The resulting rms deviations from experiment on the testing dataset are similar to those of more phenomenological models. We found that Bayesian neural networks results are prone to instabilities caused by the large number of parameters in this method. Moreover, since the classical sigmoid activation function used in this approach has linear tails that do not vanish, it is poorly suited for a bounded extrapolation. The empirical coverage probability curves we obtain match very well the reference values, in a slightly conservative way in most cases, which is highly desirable to ensure honesty of uncertainty quantification. The estimated credibility intervals on predictions make it possible to evaluate predictive power of individual models and also make quantified predictions using groups of models.

Conclusions: The proposed robust statistical approach to extrapolation of nuclear model results can be useful for assessing the impact of current and future experiments in the context of model developments. The new Bayesian capability to evaluate residuals is also expected to impact research in the domains where experiments are currently impossible, for instance, in simulations of the astrophysical $r$ process.

A hybrid (hydrodynamics + hadronic transport) theoretical framework is assembled to model the bulk dynamics of relativistic heavy-ion collisions at energies accessible in the beam energy scan program at the Relativistic Heavy-Ion Collider and the NA61/SHINE experiment at CERN. The system's energy-momentum tensor and net-baryon current are evolved according to relativistic hydrodynamics with finite shear viscosity and nonzero net-baryon diffusion. Our hydrodynamic description is matched to a hadronic transport model in the dilute region. With this fully integrated theoretical framework, we present a pilot study of the hadronic chemistry, particle spectra, and anisotropic flow. Phenomenological effects of a nonzero net-baryon current and its diffusion on hadronic observables are presented for the first time. The importance of the hadronic transport phase is also investigated.

The dependence of the nuclear level density on intrinsic deformation is an important input to dynamical nuclear processes such as fission. The auxiliary-field Monte Carlo (AFMC) method is a powerful method for computing state densities. However, the statistical distribution of intrinsic shapes is not readily accessible due to the formulation of the AFMC method in a spherical configuration-interaction shell-model approach. Instead, the theory of deformation has largely relied on a mean-field approximation which breaks rotational symmetry. We show here how the distributions of the intrinsic quadrupole deformation parameters can be calculated within the AFMC method and present results for a chain of even-mass samarium nuclei (${}^{148}\mathrm{Sm}, {}^{150}\mathrm{Sm}, {}^{152}\mathrm{Sm}, {}^{154}\mathrm{Sm}$) which includes spherical, transitional, and strongly deformed isotopes. The method relies on a Landau-like expansion of the Helmholtz free energy in invariant polynomials of the quadrupole tensor. We find that an expansion to fourth order provides an excellent description of the AFMC results.

The precision of the current generation of cosmic-ray (CR) experiments, such as AMS-02, PAMELA, CALET, and ISS-CREAM, is now reaching $\approx 1–3\%$ in a wide range in energy per nucleon from GeV/nucleon to multi-TeV/nucleon. Their correct interpretation could potentially lead to discoveries of new physics and subtle effects that were unthinkable just a decade ago. However, a major obstacle in doing so is the current uncertainty in the isotopic production cross sections that can be as high as 20–50% or even larger in some cases. While there is a recently reached consensus in the astrophysics community that new measurements of cross sections are desirable, no attempt to evaluate the importance of particular reaction channels and their required precision has been made yet. It is, however, clear that it is a huge work that requires an incremental approach. The goal of this study is to provide the ranking of the isotopic cross sections contributing to the production of the most astrophysically important CR Li, Be, B, C, and N species. In this paper, we (i) rank the reaction channels by their importance for a production of a particular isotope, (ii) provide comparisons plots between the models and data used, and (iii) evaluate a generic beam time necessary to reach a 3% precision in the production cross sections pertinent to the AMS-02 experiment. This first road map may become a starting point in the planning of new measurement campaigns that could be carried out in several nuclear and/or particle physics facilities around the world. A comprehensive evaluation of other isotopes $Z\le 30$ will be a subject of follow-up studies.

In proton-nucleus and nucleus-nucleus collision experiments, one determines the centrality of a collision according to the multiplicity or energy deposited in a detector. This serves as a proxy for the true collision centrality, as defined by the impact parameter. We show that the probability distribution of impact parameter in a given bin of experiment-defined centrality can be reconstructed without assuming any specific model for the collision dynamics, in both proton-nucleus and nucleus-nucleus systems. The reconstruction is reliable up to about 10% centrality, and is more accurate for nucleus-nucleus collisions. We perform an application of our procedure to experimental data from all the CERN Large Hadron Collider collaborations, from which we extract, in Pb+Pb and $p$+Pb collisions, the corresponding distributions of impact parameter.

The structure of even-even ${}^{108–116}\mathrm{Cd}$ isotopes is investigated based on the self-consistent mean-field approach. By mapping the quadrupole-$(\beta ,\gamma )$ deformation energy surface, obtained from the constrained self-consistent mean-field calculations with a choice of the Skyrme force and pairing property, onto the Hamiltonian of the interacting boson model with configuration mixing, the strength parameters of the Hamiltonian are determined. The low-lying excitation spectra and electric quadrupole and monopole transition rates for the considered Cd nuclei are computed by the resultant Hamiltonian, and are compared in detail with the experimental data. Our semimicroscopic prediction identifies several intruder states as suggested empirically, and overall, provides a reasonable qualitative description of the experimental energy levels and transition rates.

Background: Many nuclear-structure features have been observed in actinides in recent decades. In particular, the octupole degree of freedom has been discussed lately after the successful measurement of the $B\left(E3;0_1^{+}\to 3_1^{-}\right)$ reduced transition strength in ${}^{224}\mathrm{Ra}$. Recent results stemming from $\gamma$-spectroscopy experiments and high-resolution $(p,t)$ experiments suggested that strong octupole correlations might be observed for some positive-parity states of actinide nuclei.

Purpose: This work completes a series of $(p,t)$ experiments on actinide nuclei by adding the data on ${}^{240}\mathrm{Pu}$. The $(p,t)$ experiments allow us to study low-spin states up to $J^{\pi }=6^{+}$. Besides two-nucleon transfer cross sections, spin and parity can be assigned to excited states by measuring angular distributions, and several rotational bands are recognized based on these assignments.

Methods: A high-resolution (p,t) experiment at $E_p=24$ MeV was performed to populate low-spin states in the actinide nucleus ${}^{240}\mathrm{Pu}$. The Q3D magnetic spectrograph of the Maier-Leibnitz Laboratory (MLL) in Munich (Germany) was used to identify the ejected tritons via $dE/E$ particle identification with its focal-plane detection system. Angular distributions were measured at nine different Q3D angles to assign spin and parity to the excited states based on a comparison with coupled-channel distorted-wave Born approximation calculations.

Results: In total, 209 states have been excited in ${}^{240}\mathrm{Pu}$ up to an excitation energy of 3 MeV. Many previously known states have also been observed and their spin-parity assignments were confirmed. However, many of the populated states have been seen for the first time, e.g., 15 new and firmly assigned $J^{\pi }=0^{+}$ states. In addition, all low-spin one-octupole phonon excitations, i.e., $K^{\pi }=0^{-},1^{-},2^{-},3^{-}$, could be observed and a new candidate for the $K=3$ projection is proposed. Furthermore, the double-octupole or $\alpha$-cluster structure of the $0_2^{+}$ state in ${}^{240}\mathrm{Pu}$ has been studied in more detail. It is shown that the $0_2^{+}$ state in ${}^{230}\mathrm{Th}$ has a distinctly different structure. In addition, strongly excited $1^{-}$ states have been observed at 1.5 and 1.8 MeV in ${}^{240}\mathrm{Pu}$. The present study suggests that similar states might be observed in ${}^{230}\mathrm{Th}$.

Conclusions: At least two different and distinct structures for $J^{\pi }=0^{+}$ states are present in the actinides. These are pairing states and states with enhanced octupole correlations. We have shown that it is crucial to consider negative-parity single-particle states being admixed to some $K^{\pi }=0_2^{+}$ rotational bands to understand the $\alpha$-decay hindrance factors and enhanced $E1$-decay rates. Based on our analysis, we have identified the double-octupole or $\alpha$-cluster $K^{\pi }=0^{+}$ candidates from ${}^{224}\mathrm{Ra}$ to ${}^{240}\mathrm{Pu}$.

The electromagnetic responses obtained from Green's function Monte Carlo (GFMC) calculations are based on realistic treatments of nuclear interactions and currents. The main limitations of this method comes from its nonrelativistic nature and its computational cost, the latter hampering the direct evaluation of the inclusive cross sections as measured by experiments. We extend the applicability of GFMC in the quasielastic region to intermediate momentum transfers by performing the calculations in a reference frame that minimizes nucleon momenta. Additional relativistic effects in the kinematics are accounted for employing the two-fragment model. In addition, we developed a novel algorithm, based on the concept of first-kind scaling, to compute the inclusive electromagnetic cross section of ${}^4\mathrm{He}$ through an accurate and reliable interpolation of the response functions. A very good agreement is obtained between theoretical and experimental cross sections for a variety of kinematical setups. This offers a promising prospect for the data analysis of neutrino-oscillation experiments that requires an accurate description of nuclear dynamics in which relativistic effects are fully accounted for.

The first numerical solution of the five-body Faddeev-Yakubovsky equations is presented. Modern realistic nucleon-nucleon Hamiltonians have been tested when describing low-energy elastic neutron scattering on the ${}^4\mathrm{He}$. Results obtained are compared with those available in the literature and based on the solution of the Schrödinger equation.

Intense electromagnetic fields are created in the quark-gluon plasma by the external ultrarelativistic valence charges. The time evolution and the strength of this field are strongly affected by the electrical conductivity of the plasma. Yet, it has recently been observed that the effect of the magnetic field on the plasma flow is small. We compute the effect of plasma flow on magnetic field and demonstrate that it is less than 10%. These observations indicate that the plasma hydrodynamics and the dynamics of electromagnetic field decouple. Thus, it is a very good approximation, on the one hand, to study QGP in the background electromagnetic field generated by external sources and, on the other hand, to investigate the dynamics of magnetic field in the background plasma. We also argue that the wake induced by the magnetic field in plasma is negligible.

We report the first measurement of the fusion excitation functions for ${}^{39,47}\mathrm{K}+{}^{28}\mathrm{Si}$ at near-barrier energies. Evaporation residues resulting from the fusion process were identified by direct measurement of their energy and time of flight with high geometric efficiency. At the lowest incident energy, the cross section measured for the neutron-rich ${}^{47}\mathrm{K}$-induced reaction is $\approx 6$ times larger than that of the $\beta$-stable system. This experimental approach, both in measurement and in analysis, demonstrates how to efficiently measure fusion with low-intensity re-accelerated radioactive beams, establishing the framework for future studies.

Background: The neutron $\beta$-decay asymmetry parameter $A_0$ defines the angular correlation between the spin of the neutron and the momentum of the emitted electron. Values for $A_0$ permit an extraction of the ratio of the weak axial-vector to vector coupling constants, $\lambda \equiv g_A/g_V$, which under assumption of the conserved vector current hypothesis ($g_V=1$) determines $g_A$. Precise values for $g_A$ are important as a benchmark for lattice QCD calculations and as a test of the standard model.

Purpose: The UCNA experiment, carried out at the Ultracold Neutron (UCN) source at the Los Alamos Neutron Science Center, was the first measurement of any neutron $\beta$-decay angular correlation performed with UCN. This article reports the most precise result for $A_0$ obtained to date from the UCNA experiment, as a result of higher statistics and reduced key systematic uncertainties, including from the neutron polarization and the characterization of the electron detector response.

Methods: UCN produced via the downscattering of moderated spallation neutrons in a solid deuterium crystal were polarized via transport through a 7 T polarizing magnet and a spin flipper, which permitted selection of either spin state. The UCN were then contained within a 3-m long cylindrical decay volume, situated along the central axis of a superconducting 1 T solenoidal spectrometer. With the neutron spins then oriented parallel or anti-parallel to the solenoidal field, an asymmetry in the numbers of emitted decay electrons detected in two electron detector packages located on both ends of the spectrometer permitted an extraction of $A_0$.

Results: The UCNA experiment reports a new 0.67% precision result for $A_0$ of $A_0=-0.12054{\left(44\right)}_{\mathrm{stat}}{\left(68\right)}_{\mathrm{syst}}$, which yields $\lambda =g_A/g_V=-1.2783\left(22\right)$. Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces $A_0=-0.12015{\left(34\right)}_{\mathrm{stat}}{\left(63\right)}_{\mathrm{syst}}$ and $\lambda =g_A/g_V=-1.2772\left(20\right)$.

Conclusions: This new result for $A_0$ and $g_A/g_V$ from the UCNA experiment has provided confirmation of the shift in values for $g_A/g_V$ that has emerged in the published results from more recent experiments, which are in striking disagreement with the results from older experiments. Individual systematic corrections to the asymmetries in older experiments (published prior to 2002) were $>10$%, whereas those in the more recent ones (published after 2002) have been of the scale of $<2$%. The impact of these older results on the global average will be minimized should future measurements of $A_0$ reach the 0.1% level of precision with central values near the most recent results.

We explore possible observable signatures of $\alpha$ clustering of light nuclei in ultrarelativistic nuclear collisions involving ${}^{7,9}\mathrm{Be}$, ${}^{12}\mathrm{C}$, and ${}^{16}\mathrm{O}$. The clustering leads to specific spatial correlations of the nucleon distributions in the ground state, which are manifest in the earliest stage of the ultrahigh energy reaction. The formed initial state of the fireball is sensitive to these correlations, and the effect influences, after the collective evolution of the system, the hadron production in the final stage. Specifically, we study effects on the harmonic flow in collisions of light clustered nuclei with a heavy target (${}^{208}\mathrm{Pb}$), showing that measures of the elliptic flow are sensitive to clusterization in ${}^{7,9}\mathrm{Be}$, whereas triangular flow is sensitive to clusterization in ${}^{12}\mathrm{C}$ and ${}^{16}\mathrm{O}$. Specific predictions are made for model collisions at energies available at the CERN Super Proton Synchrotron. In another exploratory development we also examine proton-beryllium collisions, where the $3/2^{-}$ ground state of ${}^{7,9}\mathrm{Be}$ nuclei is polarized by an external magnetic field. Clusterization leads to multiplicity distributions of participant nucleons which depend on the orientation of the polarization with respect to the collision axis, as well as on the magnetic number of the state. The obtained effects on multiplicities reach a factor of a few for collisions with a large number of participant nucleons.

We show that electron time-of-flight can be a useful tool in $\beta$-decay spectroscopy, even in the energy range up to 700 keV relevant for neutron $\beta$ decay. As a first application, we use electron time-of-flight measurements for the precise calibration of an electron detector.

Background: Classical novae are cataclysmic nuclear explosions occurring when a white dwarf in a binary system accretes hydrogen-rich material from its companion star. Novae are partially responsible for the galactic synthesis of a variety of nuclides up to the calcium ($A\sim 40$) region of the nuclear chart. Although the structure and dynamics of novae are thought to be relatively well understood, the predicted abundances of elements near the nucleosynthesis endpoint, in particular Ar and Ca, appear to sometimes be in disagreement with astronomical observations of the spectra of nova ejecta.

Purpose: One possible source of the discrepancies between model predictions and astronomical observations is nuclear reaction data. Most reaction rates near the nova endpoint are estimated only from statistical model calculations, which carry large uncertainties. For certain key reactions, these rate uncertainties translate into large uncertainties in nucleosynthesis predictions. In particular, the ${}^{38}\mathrm{K}\left(p,\gamma \right){}^{39}\mathrm{Ca}$ reaction has been identified as having a significant influence on Ar, K, and Ca production. In order to constrain the rate of this reaction, we have performed a direct measurement of the strengths of three candidate $\ell =0$ resonances within the Gamow window for nova burning, at $386\pm 10$ keV, $515\pm 10$ keV, and $689\pm 10$ keV.

Method: The experiment was performed in inverse kinematics using a beam of unstable ${}^{38}\mathrm{K}$ impinged on a windowless hydrogen gas target. The ${}^{39}\mathrm{Ca}$ recoils and prompt $\gamma$ rays from ${}^{38}\mathrm{K}\left(p,\gamma \right){}^{39}\mathrm{Ca}$ reactions were detected in coincidence using a recoil mass separator and a bismuth-germanate scintillator array, respectively.

Results: For the 689 keV resonance, we observed a clear recoil-$\gamma$ coincidence signal and extracted resonance strength and energy values of ${120}_{-30}^{+50}{(\mathrm{stat}.)}_{-60}^{+20}(\mathrm{sys}.)\mathrm{meV}$ and ${679}_{-1}^{+2}(\mathrm{stat}.)\pm 1(\mathrm{sys}.)\mathrm{keV}$, respectively. We also performed a singles analysis of the recoil data alone, extracting a resonance strength of $120\pm 20(\mathrm{stat}.)\pm 15(\mathrm{sys}.)$ meV, consistent with the coincidence result. For the 386 keV and 515 keV resonances, we extract $90\%$ confidence level upper limits of 2.54 meV and 18.4 meV, respectively.

Conclusions: We have established a new recommended ${}^{38}\mathrm{K}(p,\gamma ){}^{39}\mathrm{Ca}$ rate based on experimental information, which reduces overall uncertainties near the peak temperatures of nova burning by a factor of $\sim 250$. Using the rate obtained in this work in model calculations of the hottest oxygen-neon novae reduces overall uncertainties on Ar, K, and Ca synthesis to factors of 15 or less in all cases.

Bose-Einstein correlations between identified charged pions are measured for $p$+Pb collisions at $\sqrt{s_{\mathit{NN}}}=5.02\mathrm{TeV}$ using data recorded by the ATLAS detector at the CERN Large Hadron Collider corresponding to a total integrated luminosity of $28{\mathrm{nb}}^{-1}$. Pions are identified using ionization energy loss measured in the pixel detector. Two-particle correlation functions and the extracted source radii are presented as a function of collision centrality as well as the average transverse momentum ($k_{\mathrm{T}}$) and rapidity ($y_{\pi \pi }^{★}$) of the pair. Pairs are selected with a rapidity $-2<y_{\pi \pi }^{★}<1$ and with an average transverse momentum $0.1<k_{\mathrm{T}}<0.8\mathrm{GeV}$. The effect of jet fragmentation on the two-particle correlation function is studied, and a method using opposite-charge pair data to constrain its contributions to the measured correlations is described. The measured source sizes are substantially larger in more central collisions and are observed to decrease with increasing pair $k_{\mathrm{T}}$. A correlation of the radii with the local charged-particle density is demonstrated. The scaling of the extracted radii with the mean number of participating nucleons is also used to compare a selection of initial-geometry models. The cross term $R_{\mathrm{ol}}$ is measured as a function of rapidity, and a nonzero value is observed with $5.1\sigma$ combined significance for $-1<y_{\pi \pi }^{★}<1$ in the most central events.

We present the results of our final analysis of the full data set of $g_1^p\left(Q^2\right)$, the spin structure function of the proton, collected using CLAS at Jefferson Laboratory in 2000–2001. Polarized electrons with energies of 1.6, 2.5, 4.2, and 5.7 GeV were scattered from proton targets (${}^{15}\mathrm{NH}_3$ dynamically polarized along the beam direction) and detected with CLAS. From the measured double spin asymmetries, we extracted virtual photon asymmetries $A_1^p$ and $A_2^p$ and spin structure functions $g_1^p$ and $g_2^p$ over a wide kinematic range (0.05 ${\mathrm{GeV}}^2<Q^2<$ 5 ${\mathrm{GeV}}^2$ and 1.08 GeV $<W<$ 3 GeV) and calculated moments of $g_1^p$. We compare our final results with various theoretical models and expectations, as well as with parametrizations of the world data. Our data, with their precision and dense kinematic coverage, are able to constrain fits of polarized parton distributions, test pQCD predictions for quark polarizations at large $x$, offer a better understanding of quark-hadron duality, and provide more precise values of higher twist matrix elements in the framework of the operator product expansion.