We report on a precise measurement of the electron-antineutrino angular correlation ($a$ coefficient) in free neutron beta decay from the $a\mathrm{SPECT}$ experiment. The $a$ coefficient is inferred from the recoil energy spectrum of the protons which are detected in $4\pi$ by the $a\mathrm{SPECT}$ spectrometer using magnetic adiabatic collimation with an electrostatic filter. Data are presented from a 100-day run at the Institut Laue Langevin in 2013. The sources of systematic errors are considered and included in the final result. We obtain $a=-0.10430\left(84\right)$ which is the most precise measurement of the neutron $a$ coefficient to date. From this, the ratio of axial vector to vector coupling constants is derived giving $\left|\lambda \right|=1.2677\left(28\right)$.

Midrapidity production of ${\pi }^{\pm }, K^{\pm }$, and $\left(\overline{p}\right)p$ measured by the ALICE experiment at the CERN Large Hadron Collider, in Pb-Pb and inelastic $pp$ collisions at $\sqrt{s_{NN}}=5.02$ TeV, is presented. The invariant yields are measured over a wide transverse momentum ($p_{\mathrm{T}}$) range from hundreds of $\mathrm{MeV}/c$ up to 20 $\mathrm{GeV}/c$. The results in Pb-Pb collisions are presented as a function of the collision centrality, in the range $0–90\%$. The comparison of the $p_{\mathrm{T}}$-integrated particle ratios, i.e., proton-to-pion ($p/\pi$) and kaon-to-pion ($K/\pi$) ratios, with similar measurements in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV show no significant energy dependence. Blast-wave fits of the $p_{\mathrm{T}}$ spectra indicate that in the most central collisions radial flow is slightly larger at 5.02 TeV with respect to 2.76 TeV. Particle ratios ($p/\pi , K/\pi$) as a function of $p_{\mathrm{T}}$ show pronounced maxima at $p_{\mathrm{T}}\approx 3\mathrm{GeV}/c$ in central Pb-Pb collisions. At high $p_{\mathrm{T}}$, particle ratios at 5.02 TeV are similar to those measured in $pp$ collisions at the same energy and in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV. Using the pp reference spectra measured at the same collision energy of 5.02 TeV, the nuclear modification factors for the different particle species are derived. Within uncertainties, the nuclear modification factor is particle species independent for high $p_{\mathrm{T}}$ and compatible with measurements at $\sqrt{s_{NN}}=2.76$ TeV. The results are compared to state-of-the-art model calculations, which are found to describe the observed trends satisfactorily.

Background: Neutron-rich nuclei with mass number between 100 and 110 attract much attention, since several kinds of shapes, such as spherical, prolate, oblate, and triaxial shapes, are predicted. In particular, for neutron-rich Mo isotopes, different models predict different magnitudes and rigidity of triaxial deformation. Previous interpretations of experimental results based solely on low-lying $2_2^{+}$ states are insufficient to distinguish between the rigid triaxial shape, $\gamma$ vibration, or $\gamma$-soft rotor.

Purpose: The shape evolution of ${}^{106}\mathrm{Mo}$, ${}^{108}\mathrm{Mo}$, and ${}^{110}\mathrm{Mo}$ is investigated through their $2_1^{+}$-state lifetimes, decay-branching ratios of the $2_2^{+}$ state, and energies of the low-lying collective excited states with $K^{\pi }=0^{+},2^{+}$, and $4^{+}$.

Method: $\beta$-delayed $\gamma$-ray spectroscopy was employed for neutron-rich Nb and Zr isotopes produced at the RIKEN RI Beam Factory to populate excited states in ${}^{106}\mathrm{Mo}$, ${}^{108}\mathrm{Mo}$, and ${}^{110}\mathrm{Mo}$. The EUroball-RIKEN Cluster Array was used for high-resolution $\gamma$-ray detection and lifetimes of the $2_1^{+}$ states were determined using the UK fast-timing array of ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ detectors.

Results: New $\gamma$-ray transitions and levels are reported, including newly assigned $0_2^{+}$ states in ${}^{108,110}\mathrm{Mo}$. Quadrupole deformations were obtained for ${}^{106,108,110}\mathrm{Mo}$ from their $2_1^{+}$ energies and lifetimes. The $\beta$-delayed neutron-emission probabilities of ${}^{108}\mathrm{Nb}$ and ${}^{110}\mathrm{Nb}$ were determined by examining the $\gamma$ rays of their respective daughter decays.

Conclusions: The even-odd energy staggering in the $2_2^{+}$ band was compared with typical patterns of the $\gamma$-vibrational band, rigid triaxial rotor, and $\gamma$-soft rotor. The very small even-odd staggering of ${}^{106}\mathrm{Mo}$, ${}^{108}\mathrm{Mo}$, and ${}^{110}\mathrm{Mo}$ favors a $\gamma$-vibrational band assignment. The kinematic moment of inertia for the $2_2^{+}$ band showed a trend similar to the ground-state band, which is as expected for the $\gamma$-vibrational band. Beyond-mean-field calculations employing the constrained Hartree-Fock-Bogoliubov and local quasiparticle-random-phase approximation method using the $\mathrm{SLy}5+\mathrm{T}$ interaction reproduced the ground and $2_2^{+}$ bands in ${}^{106}\mathrm{Mo}$ and ${}^{108}\mathrm{Mo}$. The collective wave functions are consistent with the interpretation of the $2_2^{+}$ band as the $\gamma$-vibrational band of the prolate shape. However, the staggering pattern observed in ${}^{110}\mathrm{Mo}$ differs from the one suggested in the calculations which predict a $\gamma$-soft rotor. There was no experimental indication of the oblate shape or the $\gamma$-soft rotor predicted in these Mo isotopes.

Quantum many-body theory has witnessed tremendous progress in various fields, ranging from atomic and solid-state physics to quantum chemistry and nuclear structure. Due to the inherent computational burden linked to the ab initio treatment of microscopic fermionic systems, it is desirable to obtain accurate results through low-order perturbation theory. In atomic nuclei, however, effects such as strong short-range repulsion between nucleons can spoil the convergence of the expansion and make the reliability of perturbation theory unclear. Mathematicians have devised an extensive machinery to overcome the problem of divergent expansions by making use of so-called resummation methods. In large-scale many-body applications, such schemes are often of limited use since no a priori analytical knowledge of the expansion is available. We present here eigenvector continuation as an alternative resummation tool that is both efficient and reliable because it is based on robust and simple mathematical principles.

We report benchmark calculations of the energy per particle of pure neutron matter as a function of the baryon density using three independent many-body methods: Brueckner–Bethe–Goldstone, Fermi hypernetted chain/single-operator chain, and auxiliary-field diffusion Monte Carlo. Significant technical improvements are implemented in the latter two methods. The calculations are made for two distinct families of realistic coordinate-space nucleon-nucleon potentials fit to scattering data, including the standard Argonne $v_{18}$ interaction and two of its simplified versions, and four of the new Norfolk $\mathrm{\Delta }$-full chiral effective field theory potentials. Primarily because of the advancements in the auxiliary-field diffusion Monte Carlo, we observe good agreement among the three many-body techniques up to nuclear saturation density—the maximum difference in the energy per particle is within $1.5\mathrm{MeV}$ for all the potentials we consider. At higher densities, the divergences become more important, and are mainly driven by the Fermi hypernetted chain/single-operator calculations. We also study the connection between nucleon-nucleon scattering data and the energy per particle of pure neutron matter. Our results suggest that fitting to higher-energy nucleon-nucleon scattering helps reduce the spread of energies among the models.

The Ultracold Neutron Asymmetry (UCNA) experiment was designed to measure the $\beta$-decay asymmetry parameter, $A_0$, for free neutron decay. In the experiment, polarized ultracold neutrons are transported into a decay trap, and their $\beta$-decay electrons are detected with $\approx 4\pi$ acceptance into two detector packages which provide position and energy reconstruction. The experiment also has sensitivity to $b_n$, the Fierz interference term in the neutron $\beta$-decay rate. In this work, we determine $b_n$ from the energy dependence of $A_0$ using the data taken during the UCNA 2011–2013 run. In addition, we present the same type of analysis using the earlier 2010 $A$ dataset. Motivated by improved statistics and comparable systematic errors compared to the 2010 data-taking run, we present a new $b_n$ measurement using the weighted average of our asymmetry dataset fits, to obtain $b_n=0.066\pm 0.{041}_{\text{stat}}\pm 0.{024}_{\text{syst}}$ which corresponds to a limit of $-0.012<b_n<0.144$ at the 90% confidence level.

We present a microscopic calculation of $\alpha$-cluster formation in heavy nuclei ${}^{104}\mathrm{Te}$ $(\alpha +{}^{100}\mathrm{Sn})$, ${}^{212}\mathrm{Po}$ $(\alpha +{}^{208}\mathrm{Pb})$, and their neighbors ${}^{102}\mathrm{Sn},{}^{102}\mathrm{Te},{}^{210}\mathrm{Pb}$, and ${}^{210}\mathrm{Po}$ by using the quartetting wave function approach. To improve the local density approximation, the shell structure of the core nucleus is considered, and the center-of-mass (c.m.) effective potential for the quartet is obtained self-consistently from the shell model wave functions. The $\alpha$-cluster formation and decay probabilities are obtained by solving the bound-state of the c.m. motion of the quartet and the scattering state of the formed $\alpha$ cluster in the Gurvitz approach. Striking shell effects on the $\alpha$-cluster formation probabilities are analyzed for magic numbers 50, 82, and 126. The computed $\alpha$-decay half-lives of these special nuclei are compared with the newest experimental data.

The deuteron form factors are calculated using two model wave functions obtained from the 2007 covariant spectator theory high-precision fits to $np$ scattering data. Included in the calculation is a new class of isoscalar $np$ interaction currents which are automatically generated by the nuclear force model used in these fits. If the nuclear model WJC2 is used, a precision fit (${\chi }^2$/datum $\eqsim 1$) to the Sick global analysis (GA) of all $ed$ elastic scattering data can be obtained by adjusting the unknown off-shell nucleon form factors $F_3\left(Q^2\right)$ (discussed before) and $F_4\left(Q^2\right)$ (introduced in this paper) and predicting the high-$Q^2$ behavior of the neutron charge form factor $G_{En}\left(Q^2\right)$ well beyond the region where it has been measured directly. Relativistic corrections, isoscalar interaction currents, and off-shell effects are defined and discussed, and their sizes are displayed. A rationale for extending $ed$ elastic scattering measurements to higher $Q^2$ is presented.

Decay spectroscopy of ${}^{250}\mathrm{No}$ has been performed using digital electronics and pulse-shape analysis of the fast nuclear decays for the first time. Previous studies of ${}^{250}\mathrm{No}$ reported two distinct fission decay lifetimes, related to the direct fission of the ground state and to the decay of an isomeric state but without the possibility to determine if the isomeric state decayed directly via fission or via internal electromagnetic transitions to the ground state. The data obtained in the current experiment allowed the puzzle to finally be resolved, attributing the shorter half-life of $t_{1/2}=3.8\pm 0.3\mu \mathrm{s}$ to the ground state and the longer half-life $t_{1/2}=34.9_{-3.2}^{+3.9}\mu \mathrm{s}$ to the decay of an isomeric state. ${}^{250}\mathrm{No}$ becomes, thus, one of a very few examples of very heavy nuclei with an isomeric state living considerably longer than its ground state. This phenomenon has important consequences for the nuclear-structure models aiming to determine the borders of the island of stability of superheavy elements.

Excited states of the neutron-rich nucleus ${}^{148}\mathrm{Ce}$ have been populated by neutron-induced fission of a ${}^{235}\mathrm{U}$ target. Its electromagnetic decays were studied by means of $\gamma$-ray coincidence spectroscopy with fast-timing capabilities. Lifetimes of the $2_1^{+}$ and $4_1^{+}$ states of ${}^{148}\mathrm{Ce}$ were obtained and their $E2$ decay rates deduced. The $B_{4/2}=B(E2;4_1^{+}\to 2_1^{+})/B(E2;2_1^{+}\to 0_1^{+})$ ratio indicates that ${}^{148}\mathrm{Ce}$ is a transitional nucleus while the $N=88/90$ shape phase transition evolves into a gradual change of nuclear deformation for proton numbers $Z<60$.

The momentum correlation functions of baryon pairs, which reflects the baryon-baryon interaction at low energies, are investigated for multistrangeness pairs ($\mathrm{\Omega }\mathrm{\Omega }$ and $N\mathrm{\Omega }$) produced in relativistic heavy-ion collisions. We calculate the correlation functions based on an expanding source model constrained by single-particle distributions. The interaction potentials are taken from those obtained from recent lattice QCD calculations at nearly physical quark masses. Experimental measurements of these correlation functions for different system sizes will help to disentangle the strong interaction between baryons and to unravel the possible existence of strange dibaryons.

We report the first detection of the second-forbidden, nonunique, $2^{+}\to 0^{+}$, ground-state transition in the $\beta$ decay of ${}^{20}\mathrm{F}$. A low-energy, mass-separated ${}^{20}\mathrm{F}^{+}$ beam produced at the IGISOL facility in Jyväskylä, Finland, was implanted in a thin carbon foil and the $\beta$ spectrum measured using a magnetic transporter and a plastic-scintillator detector. The $\beta$-decay branching ratio inferred from the measurement is $b_{\beta }=[0.41\pm 0.08\left(\mathrm{stat}\right)\pm 0.07\left(\mathrm{sys}\right)]\times {10}^{-5}$ corresponding to $logft=10.89\left(11\right)$, making this one of the strongest second-forbidden, nonunique $\beta$ transitions ever measured. The experimental result is supported by shell-model calculations and has significant implications for the final evolution of stars that develop degenerate oxygen-neon cores. Using the new experimental data, we argue that the astrophysical electron-capture rate on ${}^{20}\mathrm{Ne}$ is now known to within better than 25% at the relevant temperatures and densities.

Background: Statistical model calculations have to be used for the determination of reaction rates in large-scale reaction networks for heavy-element nucleosynthesis. A basic ingredient of such a calculation is the $\alpha$-nucleus optical model potential. Several different parameter sets are available in literature, but their predictions of $\alpha$-induced reaction rates vary widely, sometimes even exceeding one order of magnitude.

Purpose: This paper presents the result of $\alpha$-induced reaction cross-section measurements on gold which could be carried out very close to the astrophysically relevant energy region. The new experimental data are used to test statistical model predictions and to constrain the $\alpha$-nucleus optical model potential.

Method: For the measurements, the activation technique was used. The cross section of the ($\alpha ,n$) and ($\alpha ,2n$) reactions was determined from $\gamma$-ray counting, while that of the radiative capture was determined via x-ray counting.

Results: The cross section of the reactions was measured below $E_{\alpha }=20.0$ MeV. In the case of the ${}^{197}\mathrm{Au}(\alpha ,2n){}^{199}\mathrm{Tl}$ reaction down to 17.5 MeV with 0.5-MeV steps, reaching closer to the reaction threshold than ever before. The cross section of ${}^{197}\mathrm{Au}(\alpha ,n){}^{200}\mathrm{Tl}$ and ${}^{197}\mathrm{Au}(\alpha ,\gamma ){}^{201}\mathrm{Tl}$ was measured down to $E_{\alpha }=13.6$ and 14.0 MeV, respectively, with 0.5-MeV steps above the ($\alpha ,2n$) reaction threshold and with 1.0-MeV steps below that.

Conclusions: The new data set is in agreement with the available values from the literature, but is more precise and extends toward lower energies. Cross sections two orders of magnitude lower than those in previous experiments which used $\gamma$-ray counting only were successfully measured, thus providing experimental data at lower energies than ever before. The new precision dataset allows us to find the best-fit $\alpha$-nucleus optical model potential and to predict cross sections in the Gamow window with smaller uncertainties.

Uncertainties in our knowledge of the properties of dense matter near and above nuclear saturation density are among the main sources of variations in multimessenger signatures predicted for core-collapse supernovae (CCSNe) and the properties of neutron stars (NSs). We construct 97 new finite-temperature equations of state (EOSs) of dense matter that obey current experimental, observational, and theoretical constraints and discuss how systematic variations in the EOS parameters affect the properties of cold nonrotating NSs and the core collapse of a $20\text{−}{M}_{\odot }$ progenitor star. The core collapse of the $20\text{−}{M}_{\odot }$ progenitor star is simulated in spherical symmetry using the general-relativistic radiation-hydrodynamics code GR1D where neutrino interactions are computed for each EOS using the NuLib library. We conclude that the effective mass of nucleons at densities above nuclear saturation density is the largest source of uncertainty in the CCSN neutrino signal and dynamics even though it plays a subdominant role in most properties of cold NS matter. Meanwhile, changes in other observables affect the properties of cold NSs, while having little effect in CCSNe. To strengthen our conclusions, we perform six octant three-dimensional CCSN simulations varying the effective mass of nucleons at nuclear saturation density. We conclude that neutrino heating and, thus, the likelihood of explosion is significantly increased for EOSs where the effective mass of nucleons at nuclear saturation density is large.

The mechanism that restores pseudospin symmetry (PSS) is investigated under the relativistic Hartree-Fock (RHF) approach, by focusing on the in-medium balance between nuclear attractive and repulsive interactions. It is illustrated that the modelings of both the equilibrium of nuclear dynamics and the in-medium effects can be essentially changed by the $\rho$-tensor coupling that plays the role almost fully via the Fock terms, from which the model discrepancy on the PSS restoration is verified. Specifically, the largely different density-dependent behaviors of the isoscalar coupling strengths $g_{\sigma }$ and $g_{\omega }$, deduced from the parametrization of the RHF Lagrangian PKA1, play an essential role in restoring the PSS of the high-$l^{\prime }$ pseudospin doublets around the Fermi levels. Qualitatively, guidance is provided for the modeling of both the equilibrium of nuclear dynamics and the in-medium effects via the PSS restoration.

Background: Elastic electron scattering has been used for decades to paint the most accurate picture of the proton distribution in atomic nuclei. This stands in stark contrast to the neutron distribution that is traditionally probed using hadronic reactions that are hindered by large uncertainties in the reaction mechanism. Spurred by new experimental developments, it is now possible to gain valuable insights into the neutron distribution using exclusively electroweak probes.

Purpose: We aim to assess the information content and complementarity of the following three electroweak experiments in constraining the neutron distribution of atomic nuclei: (a) parity-violating elastic electron scattering, (b) coherent elastic neutrino-nucleus scattering, and (c) elastic electron scattering of unstable nuclei.

Methods: Relativistic mean-field models informed by the properties of finite nuclei and neutron stars are used to compute ground state densities and form factors of a variety of nuclei. All the models follow the same fitting protocol, except for the assumed—and presently unknown—value of the neutron skin thickness of ${}^{208}\mathrm{Pb}$. This enables one to tune the density dependence of the symmetry energy without compromising the success in reproducing well known physical observables.

Results: We found that the ongoing PREX-II and upcoming CREX campaigns at Jefferson Lab will play a vital role in constraining the weak form factor of xenon and argon, liquid noble gases that are used for the detection of both neutrinos and dark matter particles.

Conclusions: Remarkable new advances in experimental physics have opened a new window into ground state densities of atomic nuclei using solely electroweak probes. The diversity and versatility of these experiments reveal powerful correlations that impose important nuclear structure constraints. In turn, these constraints provide quantitative theoretical uncertainties that are instrumental in searches for new physics and insights into the behavior of dense matter.

Background: Nuclear pasta, emerging due to the competition between the long-range Coulomb force and the short-range strong force, is believed to be present in astrophysical scenarios, such as neutron stars and core-collapse supernovae. Its structure can have a high impact, e.g., on neutrino transport or the tidal deformability of neutron stars.

Purpose: We study several possible pasta configurations, all of them minimal surface configurations, which are expected to appear in the mid-density regime of nuclear pasta, i.e., around 40% of the nuclear saturation density. In particular we are interested in the energy spectrum for different pasta configurations considered.

Method: Employing the density functional theory approach, we calculate the binding energy of the different configurations for three values of the proton content $X_P=1/10,1/3$, and $1/2$, by optimizing their periodic length. We study finite temperature effects and the impact of electron screening.

Results: Nuclear pasta lowers the energy significantly compared to uniform matter, especially for $X_P\ge 1/3$. However, the different configurations have very similar binding energies. For large proton content, $X_P\gtrsim 1/3$, the pasta configurations are very stable, for lower proton content temperatures of a few MeV are enough for the transition to uniform matter. Electron screening has a small influence on the binding energy of nuclear pasta, but increases its periodic length.

Conclusion: Nuclear pasta in the mid-density regime lowers the energy of the matter for all proton fractions under study. It can survive even large temperatures of several MeV. Since various configurations have very similar energy, it is to expect that many configurations can coexist simultaneously already at small temperatures.

The almost hermetic coverage of the CMS detector is used to measure the distribution of transverse energy, $E_{\mathrm{T}}$, over 13.2 units of pseudorapidity, $\eta$, for $p\text{Pb}$ collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{{}_{\mathrm{NN}}}}=5.02\text{TeV}$. The huge angular acceptance exploits the fact that the CASTOR calorimeter at $-6.6<\eta <-5.2$ is effectively present on both sides of the colliding system because of a switch in the proton-going and lead-going beam directions. This wide acceptance enables the study of correlations between well-separated angular regions and makes the measurement a particularly powerful test of event generators. For minimum bias $p\text{Pb}$ collisions the maximum value of $d{E}_{\mathrm{T}}/d\eta$ is $22\text{GeV}$, which implies an $E_{\mathrm{T}}$ per participant nucleon pair comparable to that of peripheral PbPb collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=2.76\text{TeV}$. The increase of $d{E}_{\mathrm{T}}/d\eta$ with centrality is much stronger for the lead-going side than for the proton-going side. The $\eta$ dependence of $d{E}_{\mathrm{T}}/d\eta$ is sensitive to the $\eta$ range in which the centrality variable is defined. Several modern generators are compared to these results but none is able to capture all aspects of the $\eta$ and centrality dependence of the data and the correlations observed between different $\eta$ regions.

We develop a theoretical framework to study the feed-down effect of higher-lying strange baryons on the spin polarization of the $\mathrm{\Lambda }$ hyperon. In this framework, we consider two-body decays through strong, electromagnetic, and weak processes and derive general formulas for the angular distribution and spin polarization of the daughter particle by adopting the helicity formalism. Using the realistic experimental data as input, we explore the feed-down contribution to the global and the local $\mathrm{\Lambda }$ polarizations and find that such a contribution suppresses the primordial $\mathrm{\Lambda }$ polarization, which is not strong enough to resolve the discrepancy between the current theoretical and the experimental results on the azimuthal-angle dependence of $\mathrm{\Lambda }$ polarization. Our paper may also be useful for the measurement of spin polarization of baryons heavier than $\mathrm{\Lambda }$ (e.g., ${\mathrm{\Xi }}^{-}$) in future experiments.

We show that experimental data on cumulants of anisotropic flow in heavy-ion collisions probe the non-Gaussian statistics of the energy-density field created right after the collision. We carry out a perturbative expansion of the initial anisotropies of the system in terms of its density fluctuations. We argue that the correlation between the magnitudes of elliptic flow and triangular flow, dubbed $sc(3,2)$, is generically of the same sign and order of magnitude as the kurtosis of triangular flow in a hydrodynamic picture. The experimental observation that these quantities are negative implies that the distribution of energy around a given point has positive skew.