Five previously unknown isotopes (${}^{182,183}\mathrm{Tm}$, ${}^{186,187}\mathrm{Yb}$, ${}^{190}\mathrm{Lu}$) were produced, separated, and identified for the first time at the Facility for Rare Isotope Beams (FRIB) using the Advanced Rare Isotope Separator (ARIS). The new isotopes were formed through the interaction of a ${}^{198}\mathrm{Pt}$ beam with a carbon target at an energy of $186\mathrm{MeV}/u$ and with a primary beam power of 1.5 kW. Event-by-event particle identification of $A$, $Z$, and $q$ for the reaction products was performed by combining measurements of the energy loss, time of flight, magnetic rigidity $B\rho$, and total kinetic energy. The ARIS separator has a novel two-stage design with high resolving power to strongly suppress contaminant beams. This successful new isotope search was performed less than one year after FRIB operations began and demonstrates the discovery potential of the facility which will ultimately provide 400 kW of primary beam power.

Using the fusion-evaporation reaction ${}^{106}\mathrm{Cd}({}^{58}\mathrm{Ni},4n){}^{160}\mathrm{Os}$ and the gas-filled recoil separator SHANS, two new isotopes ${}_{76}{}^{160}\mathrm{Os}$ and ${}_{74}{}^{156}\mathrm{W}$ have been identified. The $\alpha$ decay of ${}^{160}\mathrm{Os}$, measured with an $\alpha$-particle energy of 7080(26) keV and a half-life of ${201}_{-37}^{+58}\mathrm{\mu }\mathrm{s}$, is assigned to originate from the ground state. The daughter nucleus ${}^{156}\mathrm{W}$ is a ${\beta }^{+}$ emitter with a half-life of ${291}_{-61}^{+86}\mathrm{ms}$. The newly measured $\alpha$-decay data allow us to derive $\alpha$-decay reduced widths (${\delta }^2$) for the $N=84$ isotones up to osmium ($Z=76$), which are found to decrease with increasing atomic number above $Z=68$. The reduction of ${\delta }^2$ is interpreted as evidence for the strengthening of the $N=82$ shell closure toward the proton drip line, supported by the increase of the neutron-shell gaps predicted in theoretical models.

We report here the first observation of the $0_2^{+}$ state of ${}^8\mathrm{He}$, which has been predicted to feature the condensatelike $\alpha +{}^{2}n+{}^{2}n$ cluster structure. We show that this state is characterized by a spin parity of $0^{+}$, a large isoscalar monopole transition strength, and the emission of a strongly correlated neutron pair, in line with theoretical predictions. Our finding is further supported by the state-of-the-art microscopic $\alpha +4n$ model calculations. The present results may lead to new insights into clustering in neutron-rich nuclear systems and the pair correlation and condensation in quantum many-body systems under strong interactions.

The recent precise experimental determination of the monopole transition form factor from the ground state of ${}^4\mathrm{He}$ to its $0_2^{+}$ resonance via electron scattering has reinvigorated discussions about the nature of this first excited state of the $\alpha$ particle. The $0_2^{+}$ state has been traditionally interpreted in the literature as the isoscalar monopole resonance (breathing mode) or, alternatively, as a particle-hole shell-model excitation. To better understand the nature of this state, which lies only $\sim 410\mathrm{keV}$ above the proton emission threshold, we employ the coupled-channel representation of the no-core Gamow shell model. By considering the $[{}^3\mathrm{H}+p]$, $[{}^3\mathrm{He}+n]$, and $[{}^2\mathrm{H}+{}^2\mathrm{H}]$ reaction channels, we explain the excitation energy and monopole form factor of the $0_2^{+}$ state. We argue that the continuum coupling strongly impacts the nature of this state, which carries characteristics of the proton decay threshold.

Collinear laser spectroscopy was performed on the isomer of the aluminium isotope ${}^{26m}\mathrm{Al}$. The measured isotope shift to ${}^{27}\mathrm{Al}$ in the $3s^{2}3p{}^2{P}_{3/2}^{○}\to 3s^{2}4s{}^2{S}_{1/2}$ atomic transition enabled the first experimental determination of the nuclear charge radius of ${}^{26m}\mathrm{Al}$, resulting in $R_c=3.130(15)\mathrm{fm}$. This differs by 4.5 standard deviations from the extrapolated value used to calculate the isospin-symmetry breaking corrections in the superallowed $\beta$ decay of ${}^{26m}\mathrm{Al}$. Its corrected $\mathcal{F}t$ value, important for the estimation of $V_{ud}$ in the Cabibbo-Kobayashi-Maskawa matrix, is thus shifted by 1 standard deviation to 3071.4(1.0) s.

The cluster structure of the neutron-rich isotope ${}^{10}\mathrm{Be}$ has been probed via the $(p,p\alpha )$ reaction at $150\mathrm{MeV}/\mathrm{nucleon}$ in inverse kinematics and in quasifree conditions. The populated states of ${}^6\mathrm{He}$ residues were investigated through missing mass spectroscopy. The triple differential cross section for the ground-state transition was extracted for quasifree angle pairs $({\theta }_p,{\theta }_{\alpha })$ and compared to distorted-wave impulse approximation reaction calculations performed in a microscopic framework using successively the Tohsaki-Horiuchi-Schuck-Röpke product wave function and the wave function deduced from antisymmetrized molecular dynamics calculations. The remarkable agreement between calculated and measured cross sections in both shape and magnitude validates the molecular structure description of the ${}^{10}\mathrm{Be}$ ground-state, configured as an $\alpha \text{−}\alpha$ core with two valence neutrons occupying $\pi$-type molecular orbitals.

Bayesian methods are used to constrain the density dependence of the QCD equation of state (EOS) for dense nuclear matter using the data of mean transverse kinetic energy and elliptic flow of protons from heavy ion collisions (HICs), in the beam energy range $\sqrt{s_{\mathrm{NN}}}=2–10\mathrm{GeV}$. The analysis yields tight constraints on the density dependent EOS up to 4 times the nuclear saturation density. The extracted EOS yields good agreement with other observables measured in HIC experiments and constraints from astrophysical observations both of which were not used in the inference. The sensitivity of inference to the choice of observables is also discussed.

The changes in mean-squared charge radii of neutron-deficient gold nuclei have been determined using the in-source, resonance-ionization laser spectroscopy technique, at the ISOLDE facility (CERN). From these new data, nuclear deformations are inferred, revealing a competition between deformed and spherical configurations. The isotopes ${}^{180,181,182}\mathrm{Au}$ are observed to possess well-deformed ground states and, when moving to lighter masses, a sudden transition to near-spherical shapes is seen in the extremely neutron-deficient nuclides, ${}^{176,177,179}\mathrm{Au}$. A case of shape coexistence and shape staggering is identified in ${}^{178}\mathrm{Au}$ which has a ground and isomeric state with different deformations. These new data reveal a pattern in ground-state deformation unique to the gold isotopes, whereby, when moving from the heavy to light masses, a plateau of well-deformed isotopes exists around the neutron midshell, flanked by near-spherical shapes in the heavier and lighter isotopes—a trend hitherto unseen elsewhere in the nuclear chart. The experimental charge radii are compared to those from Hartree-Fock-Bogoliubov calculations using the D1M Gogny interaction and configuration mixing between states of different deformation. The calculations are constrained by the known spins, parities, and magnetic moments of the ground states in gold nuclei and show a good agreement with the experimental results.

The boundaries of the chart of nuclides contain exotic isotopes that possess extreme proton-to-neutron asymmetries. Here we report on strong evidence of ${}^9\mathrm{N}$, one of the most exotic proton-rich isotopes where more than one half of its constitute nucleons are unbound. With seven protons and two neutrons, this extremely proton-rich system would represent the first-known example of a ground-state five-proton emitter. The invariant-mass spectrum of its decay products can be fit with two peaks whose energies are consistent with the theoretical predictions of an open-quantum-system approach; however, we cannot rule out the possibility that only a single resonancelike peak is present in the spectrum.

We study whether in-medium showers of high-energy gluons can be treated as a sequence of individual splitting processes $g\to gg$, or whether there is significant quantum overlap between where one splitting ends and the next begins. Accounting for the Landau-Pomeranchuk-Migdal (LPM) effect, we calculate such overlap effects to leading order in high-energy ${\alpha }_{\mathrm{s}}(\mu )$ for the simplest theoretical situation. We investigate a measure of overlap effects that is independent of physics that can be absorbed into an effective value ${\widehat{q}}_{\mathrm{eff}}$ of the jet-quenching parameter $\widehat{q}$.

We present the measurement of the two-neutrino double-$\beta$ decay rate of ${}^{76}\mathrm{Ge}$ performed with the GERDA Phase II experiment. With a subset of the entire GERDA exposure, 11.8 kg yr, the half-life of the process has been determined: $T_{1/2}^{2\nu }=(2.022\pm 0.018_{\mathrm{stat}}\pm 0.038_{\mathrm{syst}})\times {10}^{21}\mathrm{yr}$. This is the most precise determination of the ${}^{76}\mathrm{Ge}$ two-neutrino double-$\beta$ decay half-life and one of the most precise measurements of a double-$\beta$ decay process. The relevant nuclear matrix element can be extracted: ${\mathcal{M}}_{\mathrm{eff}}^{2\nu }=(0.101\pm 0.001)$.

The absolute scale of the neutrino mass plays a critical role in physics at every scale, from the subatomic to the cosmological. Measurements of the tritium end-point spectrum have provided the most precise direct limit on the neutrino mass scale. In this Letter, we present advances by Project 8 to the cyclotron radiation emission spectroscopy (CRES) technique culminating in the first frequency-based neutrino mass limit. With only a ${\mathrm{cm}}^3$-scale physical detection volume, a limit of $m_{\beta }<155\mathrm{eV}/c^2$ ($152\mathrm{eV}/c^2$) is extracted from the background-free measurement of the continuous tritium beta spectrum in a Bayesian (frequentist) analysis. Using ${}^{83\mathrm{m}}\mathrm{Kr}$ calibration data, a resolution of $1.66\pm 0.19\mathrm{eV}$ (FWHM) is measured, the detector response model is validated, and the efficiency is characterized over the multi-keV tritium analysis window. These measurements establish the potential of CRES for a high-sensitivity next-generation direct neutrino mass experiment featuring low background and high resolution.

The most precise measurements to date of the ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ lifetime $\tau$ and $\mathrm{\Lambda }$ separation energy ${\mathrm{B}}_{\mathrm{\Lambda }}$ are obtained using the data sample of Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ collected by ALICE at the LHC. The ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ is reconstructed via its charged two-body mesonic decay channel (${}_{\mathrm{\Lambda }}{}^3\mathrm{H}\to {}^3\mathrm{He}+{\pi }^{-}$ and the charge-conjugate process). The measured values $\tau =[253\pm 11(\mathrm{stat})\pm 6(\mathrm{syst})]\mathrm{ps}$ and ${\mathrm{B}}_{\mathrm{\Lambda }}=[102\pm 63(\mathrm{stat})\pm 67(\mathrm{syst})]\mathrm{keV}$ are compatible with predictions from effective field theories and confirm that the ${}_{\mathrm{\Lambda }}{}^3\mathrm{H}$ structure is consistent with a weakly bound system.

We successfully performed electron scattering off unstable nuclei which were produced online from the photofission of uranium. The target ${}^{137}\mathrm{Cs}$ ions were trapped with a new target-forming technique that makes a high-density stationary target from a small number of ions by confining them in an electron storage ring. After developments of target generation and transportation systems and the beam stacking method to increase the ion beam intensity up to approximately $2\times {10}^7$ ions per pulse beam, an average luminosity of $0.9\times {10}^{26}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ was achieved for ${}^{137}\mathrm{Cs}$. The obtained angular distribution of elastically scattered electrons is consistent with a calculation. This success marks the realization of the anticipated femtoscope which clarifies the structures of exotic and short-lived unstable nuclei.

Potassium-40 is a widespread, naturally occurring isotope whose radioactivity impacts subatomic rare-event searches, nuclear structure theory, and estimated geological ages. A predicted electron-capture decay directly to the ground state of argon-40 has never been observed. The KDK (potassium decay) collaboration reports strong evidence of this rare decay mode. A blinded analysis reveals a nonzero ratio of intensities of ground-state electron-captures ($I_{{\mathrm{EC}}^0}$) over excited-state ones ($I_{{\mathrm{EC}}^{*}}$) of $I_{{\mathrm{EC}}^0}/I_{{\mathrm{EC}}^{*}}=0.0095\stackrel{\mathrm{stat}}{\pm }0.0022\stackrel{\mathrm{sys}}{\pm }0.0010$ (68% C.L.), with the null hypothesis rejected at $4\sigma$. In terms of branching ratio, this signal yields $I_{{\mathrm{EC}}^0}=0.098\%\stackrel{\mathrm{stat}}{\pm }0.023\%\stackrel{\mathrm{sys}}{\pm }0.010\%$, roughly half of the commonly used prediction, with consequences for various fields [L. Hariasz et al., companion paper, Phys. Rev. C 108, 014327 (2023)].

Excited-state spectroscopy from the first experiment at the Facility for Rare Isotope Beams (FRIB) is reported. A $24(2)\text{−}\mathrm{\mu }\mathrm{s}$ isomer was observed with the FRIB Decay Station initiator (FDSi) through a cascade of 224- and 401-keV $\gamma$ rays in coincidence with ${}^{32}\mathrm{Na}$ nuclei. This is the only known microsecond isomer ($1\mathrm{\mu }\mathrm{s}\le T_{1/2}<1\mathrm{ms}$) in the region. This nucleus is at the heart of the $N=20$ island of shape inversion and is at the crossroads of the spherical shell-model, deformed shell-model, and ab initio theories. It can be represented as the coupling of a proton hole and neutron particle to ${}^{32}\mathrm{Mg}$, ${}^{32}\mathrm{Mg}+{\pi }^{-1}+{\nu }^{+1}$. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of ${}^{32}\mathrm{Mg}$, where the onset of spherical-to-deformed shape inversion begins with a low-lying deformed $2^{+}$ state at 885 keV and a low-lying shape-coexisting $0_2^{+}$ state at 1058 keV. We suggest two possible explanations for the 625-keV isomer in ${}^{32}\mathrm{Na}$: a $6^{-}$ spherical shape isomer that decays by $E2$ or a $0^{+}$ deformed spin isomer that decays by $M2$. The present results and calculations are most consistent with the latter, indicating that the low-lying states are dominated by deformation.

We perform a systematic study of the $\alpha$-particle excitation from its ground state $0_1^{+}$ to the $0_2^{+}$ resonance. The so-called monopole transition form factor is investigated via an electron scattering experiment in a broad $Q^2$ range (from 0.5 to $5.0{\mathrm{fm}}^{-2}$). The precision of the new data dramatically supersedes that of older sets of data, each covering only a portion of the $Q^2$ range. The new data allow the determination of two coefficients in a low-momentum expansion, leading to a new puzzle. By confronting experiment to state-of-the-art theoretical calculations, we observe that modern nuclear forces, including those derived within chiral effective field theory that are well tested on a variety of observables, fail to reproduce the excitation of the $\alpha$ particle.

The new isotope ${}^{241}\mathrm{U}$ was synthesized and systematic atomic mass measurements of nineteen neutron-rich Pa-Pu isotopes were performed in the multinucleon transfer reactions of the ${}^{238}\mathrm{U}+{}^{198}\mathrm{Pt}$ system at the KISS facility. The present experimental results demonstrate the crucial role of the multinucleon transfer reactions for accessing unexplored neutron-rich actinide isotopes toward the $N=152$ shell gap in this region of nuclides.

The only proposed observation of a discrete, hexacontatetrapole ($E6$) transition in nature occurs from the $T_{1/2}=2.54(2)\text{−}\min$ decay of ${}^{53m}\mathrm{Fe}$. However, there are conflicting claims concerning its $\gamma$-decay branching ratio, and a rigorous interrogation of $\gamma$-ray sum contributions is lacking. Experiments performed at the Australian Heavy Ion Accelerator Facility were used to study the decay of ${}^{53m}\mathrm{Fe}$. For the first time, sum-coincidence contributions to the weak $E6$ and $M5$ decay branches have been firmly quantified using complementary experimental and computational methods. Agreement across the different approaches confirms the existence of the real $E6$ transition; the $M5$ branching ratio and transition rate have also been revised. Shell model calculations performed in the full $fp$ model space suggest that the effective proton charge for high-multipole, $E4$ and $E6$, transitions is quenched to approximately two-thirds of the collective $E2$ value. Correlations between nucleons may offer an explanation of this unexpected phenomenon, which is in stark contrast to the collective nature of lower-multipole, electric transitions observed in atomic nuclei.

The Majorana Demonstrator searched for neutrinoless double-$\beta$ decay ($0\nu \beta \beta$) of ${}^{76}\mathrm{Ge}$ using modular arrays of high-purity Ge detectors operated in vacuum cryostats in a low-background shield. The arrays operated with up to 40.4 kg of detectors (27.2 kg enriched to $\sim 88\%$ in ${}^{76}\mathrm{Ge}$). From these measurements, the Demonstrator has accumulated 64.5 kg yr of enriched active exposure. With a world-leading energy resolution of 2.52 keV FWHM at the 2039 keV $Q_{\beta \beta }$ (0.12%), we set a half-life limit of $0\nu \beta \beta$ in ${}^{76}\mathrm{Ge}$ at $T_{1/2}>8.3\times {10}^{25}\mathrm{yr}$ (90% C.L.). This provides a range of upper limits on $m_{\beta \beta }$ of (113–269) meV (90% C.L.), depending on the choice of nuclear matrix elements.