We present an improved measurement of the carbon-nitrogen-oxygen (CNO) solar neutrino interaction rate at Earth obtained with the complete Borexino Phase-III dataset. The measured rate, $R_{\mathrm{CNO}}={6.7}_{-0.8}^{+2.0}\text{counts}/(\mathrm{day}\times 100\mathrm{tonnes})$, allows us to exclude the absence of the CNO signal with about $7\sigma$ C.L. The correspondent CNO neutrino flux is ${6.6}_{-0.9}^{+2.0}\times {10}^8{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, taking into account the neutrino flavor conversion. We use the new CNO measurement to evaluate the C and N abundances in the Sun with respect to the H abundance for the first time with solar neutrinos. Our result of $N_{\mathrm{CN}}=(5.78_{-1.00}^{+1.86})\times {10}^{-4}$ displays a $\sim 2\sigma$ tension with the “low-metallicity” spectroscopic photospheric measurements. Furthermore, our result used together with the ${}^7\mathrm{Be}$ and ${}^8\mathrm{B}$ solar neutrino fluxes, also measured by Borexino, permits us to disfavor at $3.1\sigma$ C.L. the “low-metallicity” standard solar model B16-AGSS09met as an alternative to the “high-metallicity” standard solar model B16-GS98.

$\beta$ decay of proton-rich nuclei plays an important role in exploring isospin mixing. The $\beta$ decay of ${}^{26}\mathrm{P}$ at the proton drip line is studied using double-sided silicon strip detectors operating in conjunction with high-purity germanium detectors. The $T=2$ isobaric analog state (IAS) at 13 055 keV and two new high-lying states at 13 380 and 11 912 keV in ${}^{26}\mathrm{Si}$ are unambiguously identified through $\beta$-delayed two-proton emission ($\beta 2p$). Angular correlations of two protons emitted from ${}^{26}\mathrm{Si}$ excited states populated by ${}^{26}\mathrm{P}$ $\beta$ decay are measured, which suggests that the two protons are emitted mainly sequentially. We report the first observation of a strongly isospin-mixed doublet that deexcites mainly via two-proton decay. The isospin mixing matrix element between the ${}^{26}\mathrm{Si}$ IAS and the nearby 13 380-keV state is determined to be 130(21) keV, and this result represents the strongest mixing, highest excitation energy, and largest level spacing of a doublet ever observed in $\beta$-decay experiments.

New half-lives for exotic isotopes approaching the neutron drip-line in the vicinity of $N\sim 28$ for $Z=12–15$ were measured at the Facility for Rare Isotope Beams (FRIB) with the FRIB decay station initiator. The first experimental results are compared to the latest quasiparticle random phase approximation and shell-model calculations. Overall, the measured half-lives are consistent with the available theoretical descriptions and suggest a well-developed region of deformation below ${}^{48}\mathrm{Ca}$ in the $N=28$ isotones. The erosion of the $Z=14$ subshell closure in Si is experimentally confirmed at $N=28$, and a reduction in the ${}^{38}\mathrm{Mg}$ half-life is observed as compared with its isotopic neighbors, which does not seem to be predicted well based on the decay energy and deformation trends. This highlights the need for both additional data in this very exotic region, and for more advanced theoretical efforts.

The new isotope ${}^{39}\mathrm{Na}$, the most neutron-rich sodium nucleus observed so far, was discovered at the RIKEN Nishina Center Radioactive Isotope Beam Factory using the projectile fragmentation of an intense ${}^{48}\mathrm{Ca}$ beam at $345\mathrm{MeV}/\text{nucleon}$ on a beryllium target. Projectile fragments were separated and identified in flight with the large-acceptance two-stage separator BigRIPS. Nine ${}^{39}\mathrm{Na}$ events have been unambiguously observed in this work and clearly establish the particle stability of ${}^{39}\mathrm{Na}$. Furthermore, the lack of observation of ${}^{35,36}\mathrm{Ne}$ isotopes in this experiment significantly improves the overall confidence that ${}^{34}\mathrm{Ne}$ is the neutron dripline nucleus of neon. These results provide new key information to understand nuclear binding and nuclear structure under extremely neutron-rich conditions. The newly established stability of ${}^{39}\mathrm{Na}$ has a significant impact on nuclear models and theories predicting the neutron dripline and also provides a key to understanding the nuclear shell property of ${}^{39}\mathrm{Na}$ at the neutron number $N=28$, which is normally a magic number.

We report a precise measurement of the parity-violating (PV) asymmetry $A_{\mathrm{PV}}$ in the elastic scattering of longitudinally polarized electrons from ${}^{48}\mathrm{Ca}$. We measure $A_{\mathrm{PV}}=2668\pm 106(\mathrm{stat})\pm 40(\mathrm{syst})$ parts per billion, leading to an extraction of the neutral weak form factor $F_W(q=0.8733{\mathrm{fm}}^{-1})=0.1304\pm 0.0052(\mathrm{stat})\pm 0.0020(\mathrm{syst})$ and the charge minus the weak form factor $F_{\mathrm{ch}}-F_W=0.0277\pm 0.0055$. The resulting neutron skin thickness $R_n-R_p=0.121\pm 0.026(\exp )\pm 0.024(\mathrm{model})\mathrm{fm}$ is relatively thin yet consistent with many model calculations. The combined CREX and PREX results will have implications for future energy density functional calculations and on the density dependence of the symmetry energy of nuclear matter.

For high-power heavy ion accelerators, the development of a suitable charge stripper, which can handle intense beams, is essential. This Letter describes the first experimental demonstration of a heavy ion liquid lithium charge stripper. A $10–20\mu \mathrm{m}$ thick liquid lithium jet flowing at $>50\mathrm{m}/\mathrm{s}$ was formed and confirmed stable when bombarded by various heavy ion beams, while increasing the charge state of the incoming beams to the desired charge state range. This demonstration proved the existing power limitation with the conventional strippers can be overcome by the liquid-metal stripper, opening completely new possibilities in high-power accelerator development.

We construct an effective field theory of a two-neutron halo nucleus in the limit where the two-neutron separation energy $B$ and the neutron-neutron two-body virtual energy ${\epsilon }_n$ are smaller than any other energy scale in the problem, but the scattering between the core and a single neutron is not fine-tuned, and the Efimov effect does not operate. The theory has one dimensionless coupling which formally runs to a Landau pole in the ultraviolet. We show that many properties of the system are universal in the double fine-tuning limit. The ratio of the mean-square matter radius and charge radius is found to be $⟨r_m^2⟩/⟨r_c^2⟩=Af({\epsilon }_n/B)$, where $A$ is the mass number of the core and $f$ is a function of the ratio ${\epsilon }_n/B$ which we find explicitly. In particular, when $B\gg {\epsilon }_n$, $⟨r_m^2⟩/⟨r_c^2⟩=\frac{2}{3}A$. The shape of the $E1$ dipole strength function also depends only on the ratio ${\epsilon }_n/B$ and is derived in explicit analytic form. We estimate that for the ${}^{22}\mathrm{C}$ nucleus higher-order corrections to our theory are of the order of 20% or less if the two-neutron separation energy is less than 100 keV and the $s$-wave scattering length between a neutron and a ${}^{20}\mathrm{C}$ nucleus is less than 2.8 fm.

We report precision determinations of the beam-normal single spin asymmetries ($A_n$) in the elastic scattering of 0.95 and 2.18 GeV electrons off ${}^{12}\mathrm{C}$, ${}^{40}\mathrm{Ca}$, ${}^{48}\mathrm{Ca}$, and ${}^{208}\mathrm{Pb}$ at very forward angles where the most detailed theoretical calculations have been performed. The first measurements of $A_n$ for ${}^{40}\mathrm{Ca}$ and ${}^{48}\mathrm{Ca}$ are found to be similar to that of ${}^{12}\mathrm{C}$, consistent with expectations and thus demonstrating the validity of theoretical calculations for nuclei with $\mathrm{Z}\le 20$. We also report $A_n$ for ${}^{208}\mathrm{Pb}$ at two new momentum transfers (${\mathrm{Q}}^2$) extending the previous measurement. Our new data confirm the surprising result previously reported, with all three data points showing significant disagreement with the results from the $Z\le 20$ nuclei. These data confirm our basic understanding of the underlying dynamics that govern $A_n$ for nuclei containing $\lesssim 50$ nucleons, but point to the need for further investigation to understand the unusual $A_n$ behavior discovered for scattering off ${}^{208}\mathrm{Pb}$.

Using the fusion-evaporation reaction ${}^{96}\mathrm{Ru}({}^{58}\mathrm{Ni},p4n){}^{149}\mathrm{Lu}$ and the MARA vacuum-mode recoil separator, a new proton-emitting isotope ${}^{149}\mathrm{Lu}$ has been identified. The measured decay $Q$ value of 1920(20) keV is the highest measured for a ground-state proton decay, and it naturally leads to the shortest directly measured half-life of ${450}_{-100}^{+170}\mathrm{ns}$ for a ground-state proton emitter. The decay rate is consistent with $l_p=5$ emission, suggesting a dominant $\pi h_{11/2}$ component for the wave function of the proton-emitting state. Through nonadiabatic quasiparticle calculations it was concluded that ${}^{149}\mathrm{Lu}$ is the most oblate deformed proton emitter observed to date.

${}^{18}\mathrm{Mg}$ was observed, for the first time, by the invariant-mass reconstruction of ${}^{14}\mathrm{O}+4p$ events. The ground-state decay energy and width are $E_T=4.865(34)\mathrm{MeV}$ and $\mathrm{\Gamma }=115(100)\mathrm{keV}$, respectively. The observed momentum correlations between the five particles are consistent with two sequential steps of prompt $2p$ decay passing through the ground state of ${}^{16}\mathrm{Ne}$. The invariant-mass spectrum also provides evidence for an excited state at an excitation energy of 1.84(14) MeV, which is likely the first excited $2^{+}$ state. As this energy exceeds that for the $2^{+}$ state in ${}^{20}\mathrm{Mg}$, this observation provides an argument for the demise of the $N=8$ shell closure in nuclei far from stability. However, in open systems this classical argument for shell strength is compromised by Thomas-Ehrman shifts.

Two long-standing puzzles in the decay of ${}^{185}\mathrm{Bi}$, the heaviest known proton-emitting nucleus are revisited. These are the nonobservation of the $9/2^{-}$ state, which is the ground state of all heavier odd-$A$ Bi isotopes, and the hindered nature of proton and $\alpha$ decays of its presumed $60\text{−}\mu \mathrm{s}$ $1/2^{+}$ ground state. The ${}^{185}\mathrm{Bi}$ nucleus has now been studied with the ${}^{95}\mathrm{Mo}({}^{93}\mathrm{Nb},3n)$ reaction in complementary experiments using the Fragment Mass Analyzer and Argonne Gas-Filled Analyzer at Argonne National Laboratory’s ATLAS facility. The experiments have established the existence of two states in ${}^{185}\mathrm{Bi}$; the short-lived $T_{1/2}={2.8}_{-1.0}^{+2.3}\mu \mathrm{s}$, proton- and $\alpha$-decaying ground state, and a $58(2)\text{−}\mu \mathrm{s}$ $\gamma$-decaying isomer, the half-life of which was previously attributed to the ground state. The reassignment of the ground-state lifetime results in a proton-decay spectroscopic factor close to unity and represents the only known example of a ground-state proton decay to a daughter nucleus (${}^{184}\mathrm{Pb}$) with a major shell closure. The data also demonstrate that the ordering of low- and high-spin states in ${}^{185}\mathrm{Bi}$ is reversed relative to the heavier odd-$A$ Bi isotopes, with the intruder-based $1/2^{+}$ configuration becoming the ground, similar to the lightest At nuclides.

This Letter presents the first experimental evidence of the attractive strong interaction between a proton and a $\varphi$ meson. The result is obtained from two-particle correlations of combined $p\text{−}\varphi \oplus \overline{p}\text{−}\varphi$ pairs measured in high-multiplicity $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$ by the ALICE Collaboration. The spin-averaged scattering length and effective range of the $p\text{−}\varphi$ interaction are extracted from the fully corrected correlation function employing the Lednický-Lyuboshits approach. In particular, the imaginary part of the scattering length vanishes within uncertainties, indicating that inelastic processes do not play a prominent role for the $p\text{−}\varphi$ interaction. These data demonstrate that the interaction is dominated by elastic $p\text{−}\varphi$ scattering. Furthermore, an analysis employing phenomenological Gaussian- and Yukawa-type potentials is conducted. Under the assumption of the latter, the $N\text{−}\varphi$ coupling constant is found to be $g_{N\text{−}\varphi }=0.14\pm 0.03(\mathrm{stat})\pm 0.02(\mathrm{syst})$. This work provides valuable experimental input to accomplish a self-consistent description of the $N\text{−}\varphi$ interaction, which is particularly relevant for the more fundamental studies on partial restoration of chiral symmetry in nuclear medium.

We report an improved measurement of the free neutron lifetime ${\tau }_n$ using the $\mathrm{UCN}\tau$ apparatus at the Los Alamos Neutron Science Center. We count a total of approximately $38\times {10}^6$ surviving ultracold neutrons (UCNs) after storing in $\mathrm{UCN}\tau$’s magnetogravitational trap over two data acquisition campaigns in 2017 and 2018. We extract ${\tau }_n$ from three blinded, independent analyses by both pairing long and short storage time runs to find a set of replicate ${\tau }_n$ measurements and by performing a global likelihood fit to all data while self-consistently incorporating the $\beta$-decay lifetime. Both techniques achieve consistent results and find a value ${\tau }_n=877.75\pm 0.28_{\mathrm{stat}}+0.22/-0.16_{\mathrm{syst}}\mathrm{s}$. With this sensitivity, neutron lifetime experiments now directly address the impact of recent refinements in our understanding of the standard model for neutron decay.

Fluorine is one of the most interesting elements in nuclear astrophysics, where the ${}^{19}\mathrm{F}(p,\alpha ){}^{16}\mathrm{O}$ reaction is of crucial importance for Galactic ${}^{19}\mathrm{F}$ abundances and CNO cycle loss in first generation Population III stars. As a day-one campaign at the Jinping Underground Nuclear Astrophysics experimental facility, we report direct measurements of the essential ${}^{19}\mathrm{F}(p,\alpha \gamma ){}^{16}\mathrm{O}$ reaction channel. The $\gamma$-ray yields were measured over $E_{\mathrm{c}.\mathrm{m}.}=72.4–344\mathrm{keV}$, covering the Gamow window; our energy of 72.4 keV is unprecedentedly low, reported here for the first time. The experiment was performed under the extremely low cosmic-ray-induced background environment of the China JinPing Underground Laboratory, one of the deepest underground laboratories in the world. The present low-energy $S$ factors deviate significantly from previous theoretical predictions, and the uncertainties are significantly reduced. The thermonuclear ${}^{19}\mathrm{F}(p,\alpha \gamma ){}^{16}\mathrm{O}$ reaction rate has been determined directly at the relevant astrophysical energies.

Using combined data from the Relativistic Heavy Ion and Large Hadron Colliders, we constrain the shear and bulk viscosities of quark-gluon plasma (QGP) at temperatures of $\sim 150–350\mathrm{MeV}$. We use Bayesian inference to translate experimental and theoretical uncertainties into probabilistic constraints for the viscosities. With Bayesian model averaging we propagate an estimate of the model uncertainty generated by the transition from hydrodynamics to hadron transport in the plasma’s final evolution stage, providing the most reliable phenomenological constraints to date on the QGP viscosities.

We report a precision measurement of the parity-violating asymmetry $A_{\mathrm{PV}}$ in the elastic scattering of longitudinally polarized electrons from ${}^{208}\mathrm{Pb}$. We measure $A_{\mathrm{PV}}=550\pm 16(\text{stat})\pm 8(\text{syst})$ parts per billion, leading to an extraction of the neutral weak form factor $F_W(Q^2=0.00616{\mathrm{GeV}}^2)=0.368\pm 0.013$. Combined with our previous measurement, the extracted neutron skin thickness is $R_n-R_p=0.283\pm 0.071\mathrm{fm}$. The result also yields the first significant direct measurement of the interior weak density of ${}^{208}\mathrm{Pb}$: ${\rho }_W^0=-0.0796\pm 0.0036(\text{exp})\pm 0.0013(\text{theo}){\mathrm{fm}}^{-3}$ leading to the interior baryon density ${\rho }_b^0=0.1480\pm 0.0036(\text{exp})\pm 0.0013(\text{theo}){\mathrm{fm}}^{-3}$. The measurement accurately constrains the density dependence of the symmetry energy of nuclear matter near saturation density, with implications for the size and composition of neutron stars.

Laboratory experiments sensitive to the equation of state of neutron rich matter in the vicinity of nuclear saturation density provide the first rung in a “density ladder” that connects terrestrial experiments to astronomical observations. In this context, the neutron skin thickness of ${}^{208}\mathrm{Pb}$ ($R_{\mathrm{skin}}^{208}$) provides a stringent laboratory constraint on the density dependence of the symmetry energy. In turn, an improved value of $R_{\mathrm{skin}}^{208}$ has been reported recently by the PREX collaboration. Exploiting the strong correlation between $R_{\mathrm{skin}}^{208}$ and the slope of the symmetry energy $L$ within a specific class of relativistic energy density functionals, we report a value of $L=(106\pm 37)\mathrm{MeV}$—which systematically overestimates current limits based on both theoretical approaches and experimental measurements. The impact of such a stiff symmetry energy on some critical neutron-star observables is also examined.

A new $\alpha$-emitting isotope ${}^{214}\mathrm{U}$, produced by the fusion-evaporation reaction ${}^{182}\mathrm{W}({}^{36}\mathrm{Ar},4\mathrm{n}){}^{214}\mathrm{U}$, was identified by employing the gas-filled recoil separator SHANS and the recoil-$\alpha$ correlation technique. More precise $\alpha$-decay properties of even-even nuclei ${}^{216,218}\mathrm{U}$ were also measured in the reactions of ${}^{40}\mathrm{Ar}$, ${}^{40}\mathrm{Ca}$ beams with ${}^{180,182,184}\mathrm{W}$ targets. By combining the experimental data, improved $\alpha$-decay reduced widths ${\delta }^2$ for the even-even Po-Pu nuclei in the vicinity of the magic neutron number $N=126$ are deduced. Their systematic trends are discussed in terms of the $N_p{N}_n$ scheme in order to study the influence of proton-neutron interaction on $\alpha$ decay in this region of nuclei. It is strikingly found that the reduced widths of ${}^{214,216}\mathrm{U}$ are significantly enhanced by a factor of two as compared with the $N_p{N}_n$ systematics for the $84\le Z\le 90$ and $N<126$ even-even nuclei. The abnormal enhancement is interpreted by the strong monopole interaction between the valence protons and neutrons occupying the $\pi 1f_{7/2}$ and $\nu 1f_{5/2}$ spin-orbit partner orbits, which is supported by the large-scale shell model calculation.

We studied the proton-rich $T_z=-1$ nucleus ${}^{70}\mathrm{Kr}$ through inelastic scattering at intermediate energies in order to extract the reduced transition probability, $B(E2;0^{+}\to 2^{+})$. Comparison with the other members of the $A=70$ isospin triplet, ${}^{70}\mathrm{Br}$ and ${}^{70}\mathrm{Se}$, studied in the same experiment, shows a $3\sigma$ deviation from the expected linearity of the electromagnetic matrix elements as a function of $T_z$. At present, no established nuclear structure theory can describe this observed deviation quantitatively. This is the first violation of isospin symmetry at this level observed in the transition matrix elements. A heuristic approach may explain the anomaly by a shape change between the mirror nuclei ${}^{70}\mathrm{Kr}$ and ${}^{70}\mathrm{Se}$ contrary to the model predictions.

In an emulsion-counter hybrid experiment performed at J-PARC, a ${\mathrm{\Xi }}^{-}$ absorption event was observed which decayed into twin single-$\mathrm{\Lambda }$ hypernuclei. Kinematic calculations enabled a unique identification of the reaction process as ${\mathrm{\Xi }}^{-}+{}^{14}\mathrm{N}\to {}_{\mathrm{\Lambda }}{}^{10}\mathrm{Be}+{}_{\mathrm{\Lambda }}{}^5\mathrm{He}$. For the binding energy of the ${\mathrm{\Xi }}^{-}$ hyperon in the ${\mathrm{\Xi }}^{-}-{}^{14}\mathrm{N}$ system a value of $1.27\pm 0.21\mathrm{MeV}$ was deduced. The energy level of ${\mathrm{\Xi }}^{-}$ is likely a nuclear $1p$ state which indicates a weak $\mathrm{\Xi }N–\mathrm{\Lambda }\mathrm{\Lambda }$ coupling.