The GERDA experiment searches for the lepton-number-violating neutrinoless double-$\beta$ decay of ${}^{76}\mathrm{Ge}$ (${}^{76}\mathrm{Ge}\to {}^{76}\mathrm{Se}+2e^{-}$) operating bare Ge diodes with an enriched ${}^{76}\mathrm{Ge}$ fraction in liquid argon. The exposure for broad-energy germanium type (BEGe) detectors is increased threefold with respect to our previous data release. The BEGe detectors feature an excellent background suppression from the analysis of the time profile of the detector signals. In the analysis window a background level of $1.0_{-0.4}^{+0.6}\times {10}^{-3}\mathrm{counts}/(\mathrm{keV}\mathrm{kg}\mathrm{yr})$ has been achieved; if normalized to the energy resolution this is the lowest ever achieved in any $0\nu \beta \beta$ experiment. No signal is observed and a new 90% C.L. lower limit for the half-life of $8.0\times {10}^{25}\mathrm{yr}$ is placed when combining with our previous data. The expected median sensitivity assuming no signal is $5.8\times {10}^{25}\mathrm{yr}$.

A new method to tag the barium daughter in the double-beta decay of ${}^{136}\mathrm{Xe}$ is reported. Using the technique of single molecule fluorescent imaging (SMFI), individual barium dication (${\mathrm{Ba}}^{++}$) resolution at a transparent scanning surface is demonstrated. A single-step photobleach confirms the single ion interpretation. Individual ions are localized with superresolution ($\sim 2\mathrm{nm}$), and detected with a statistical significance of $12.9\sigma$ over backgrounds. This lays the foundation for a new and potentially background-free neutrinoless double-beta decay technology, based on SMFI coupled to high pressure xenon gas time projection chambers.

The main decay channels of the anomalous low-energy $3/2^{+}(7.8\pm 0.5\mathrm{eV})$ isomeric level of the ${}^{229}\mathrm{Th}$ nucleus, namely the $\gamma$ emission and internal conversion, inside a dielectric sphere, dielectric thin film, and conducting spherical microcavity are investigated theoretically, taking into account the effect of media interfaces. It is shown that (1) the $\gamma$ decay rate of the nuclear isomer inside a dielectric thin film and dielectric microsphere placed in a vacuum or in a metal cavity can decrease (increase) in dozen of times, (2) the $\gamma$ activity of the distributed source as a function of time can be nonexponential, and (3) the metal cavity, whose size is of the order of the radiation wavelength, does not affect the probability of the internal conversion in ${}^{229}\mathrm{Th}$, because the virtual photon attenuates at much shorter distances and the reflected wave is very weak.

Results from a search for neutrinoless double-beta decay ($0\nu \beta \beta$) of ${}^{136}\mathrm{Xe}$ are presented using the first year of data taken with the upgraded EXO-200 detector. Relative to previous searches by EXO-200, the energy resolution of the detector has been improved to $\sigma /E=1.23\%$, the electric field in the drift region has been raised by 50%, and a system to suppress radon in the volume between the cryostat and lead shielding has been implemented. In addition, analysis techniques that improve topological discrimination between $0\nu \beta \beta$ and background events have been developed. Incorporating these hardware and analysis improvements, the median 90% confidence level $0\nu \beta \beta$ half-life sensitivity after combining with the full data set acquired before the upgrade has increased twofold to $3.7\times {10}^{25}\mathrm{yr}$. No statistically significant evidence for $0\nu \beta \beta$ is observed, leading to a lower limit on the $0\nu \beta \beta$ half-life of $1.8\times {10}^{25}\mathrm{yr}$ at the 90% confidence level.

Muons can be created in nascent neutron stars (NSs) due to the high electron chemical potentials and the high temperatures. Because of their relatively lower abundance compared to electrons, their role has so far been ignored in numerical simulations of stellar core collapse and NS formation. However, the appearance of muons softens the NS equation of state, triggers faster NS contraction, and thus leads to higher luminosities and mean energies of the emitted neutrinos. This strengthens the postshock heating by neutrinos and can facilitate explosions by the neutrino-driven mechanism.

In-beam $\gamma$-ray spectroscopy of ${}^{79}\mathrm{Cu}$ is performed at the Radioactive Isotope Beam Factory of RIKEN. The nucleus of interest is produced through proton knockout from a ${}^{80}\mathrm{Zn}$ beam at $270\mathrm{MeV}/\mathrm{nucleon}$. The level scheme up to 4.6 MeV is established for the first time and the results are compared to Monte Carlo shell-model calculations. We do not observe significant knockout feeding to the excited states below 2.2 MeV, which indicates that the $Z=28$ gap at $N=50$ remains large. The results show that the ${}^{79}\mathrm{Cu}$ nucleus can be described in terms of a valence proton outside a ${}^{78}\mathrm{Ni}$ core, implying the magic character of the latter.

The masses of the neutron-rich copper isotopes ${}^{75–79}\mathrm{Cu}$ are determined using the precision mass spectrometer ISOLTRAP at the CERN-ISOLDE facility. The trend from the new data differs significantly from that of previous results, offering a first accurate view of the mass surface adjacent to the $Z=28$, $N=50$ nuclide ${}^{78}\mathrm{Ni}$ and supporting a doubly magic character. The new masses compare very well with large-scale shell-model calculations that predict shape coexistence in a doubly magic ${}^{78}\mathrm{Ni}$ and a new island of inversion for $Z<28$. A coherent picture of this important exotic region begins to emerge where excitations across $Z=28$ and $N=50$ form a delicate equilibrium with a spherical mean field.

The decay path of the Hoyle state in ${}^{12}\mathrm{C}$ ($E_x=7.654\mathrm{MeV}$) has been studied with the ${}^{14}\mathrm{N}(d,{\alpha }_2){}^{12}\mathrm{C}(7.654)$ reaction induced at 10.5 MeV. High resolution invariant mass spectroscopy techniques have allowed us to unambiguously disentangle direct and sequential decays of the state passing through the ground state of ${}^8\mathrm{Be}$. Thanks to the almost total absence of background and the attained resolution, a fully sequential decay contribution to the width of the state has been observed. The direct decay width is negligible, with an upper limit of 0.043% (95% C.L.). The precision of this result is about a factor 5 higher than previous studies. This has significant implications on nuclear structure, as it provides constraints to $3\alpha$ cluster model calculations, where higher precision limits are needed.

Excited states in certain atomic nuclei possess an unusual structure, where the dominant degrees of freedom are those of $\alpha$ clusters rather than individual nucleons. It has been proposed that the diffuse $3\alpha$ system of the ${}^{12}\mathrm{C}$ Hoyle state may behave like a Bose-Einstein condensate, where the $\alpha$ clusters maintain their bosonic identities. By measuring the decay of the Hoyle state into three $\alpha$ particles, we obtained an upper limit for the rare direct $3\alpha$ decay branch of 0.047%. This value is now at a level comparable with theoretical predictions and could be a sensitive probe of the structure of this state.

The production of jets in association with $Z$ bosons, reconstructed via the ${\mu }^{+}{\mu }^{-}$ and $e^{+}{e}^{-}$ decay channels, is studied in $pp$ and, for the first time, in Pb-Pb collisions. Both data samples were collected by the CMS experiment at the LHC, at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The Pb-Pb collisions were analyzed in the 0%–30% centrality range. The back-to-back azimuthal alignment was studied in both $pp$ and Pb-Pb collisions for $Z$ bosons with transverse momentum $p_{\mathrm{T}}^Z>60\mathrm{GeV}/c$ and a recoiling jet with $p_{\mathrm{T}}^{\text{jet}}>30\mathrm{GeV}/c$. The $p_{\mathrm{T}}$ imbalance $x_{jZ}=p_{\mathrm{T}}^{\text{jet}}/p_{\mathrm{T}}^Z$, as well as the average number of jet partners per $Z$, $R_{jZ}$, was studied in intervals of $p_{\mathrm{T}}^Z$. The $R_{jZ}$ is found to be smaller in Pb-Pb than in $pp$ collisions, which suggests that in Pb-Pb collisions a larger fraction of partons associated with the $Z$ bosons fall below the $30\mathrm{GeV}/c$ $p_{\mathrm{T}}^{\text{jet}}$ threshold because they lose energy.

We report the first dijet transverse momentum asymmetry measurements from $\mathrm{Au}+\mathrm{Au}$ and $pp$ collisions at RHIC. The two highest-energy back-to-back jets reconstructed from fragments with transverse momenta above $2\mathrm{GeV}/c$ display a significantly higher momentum imbalance in heavy-ion collisions than in the $pp$ reference. When reexamined with correlated soft particles included, we observe that these dijets then exhibit a unique new feature—momentum balance is restored to that observed in $pp$ for a jet resolution parameter of $R=0.4$, while rebalancing is not attained with a smaller value of $R=0.2$.

How does nature hold together protons and neutrons to form the wide variety of complex nuclei in the Universe? Describing many-nucleon systems from the fundamental theory of quantum chromodynamics has been the greatest challenge in answering this question. The chiral effective field theory description of the nuclear force now makes this possible but requires certain parameters that are not uniquely determined. Defining the nuclear force needs identification of observables sensitive to the different parametrizations. From a measurement of proton elastic scattering on ${}^{10}\mathrm{C}$ at TRIUMF and ab initio nuclear reaction calculations, we show that the shape and magnitude of the measured differential cross section is strongly sensitive to the nuclear force prescription.

We argue that many features of the structure of nuclei emerge from a strictly perturbative expansion around the unitarity limit, where the two-nucleon $S$ waves have bound states at zero energy. In this limit, the gross features of states in the nuclear chart are correlated to only one dimensionful parameter, which is related to the breaking of scale invariance to a discrete scaling symmetry and set by the triton binding energy. Observables are moved to their physical values by small perturbative corrections, much like in descriptions of the fine structure of atomic spectra. We provide evidence in favor of the conjecture that light, and possibly heavier, nuclei are bound weakly enough to be insensitive to the details of the interactions but strongly enough to be insensitive to the exact size of the two-nucleon system.

A search for shape isomers in the ${}^{66}\mathrm{Ni}$ nucleus was performed, following old suggestions of various mean–field models and recent ones, based on state-of-the-art Monte Carlo shell model (MCSM), all considering ${}^{66}\mathrm{Ni}$ as the lightest nuclear system with shape isomerism. By employing the two-neutron transfer reaction induced by an ${}^{18}\mathrm{O}$ beam on a ${}^{64}\mathrm{Ni}$ target, at the sub-Coulomb barrier energy of 39 MeV, all three lowest-excited $0^{+}$ states in ${}^{66}\mathrm{Ni}$ were populated and their $\gamma$ decay was observed by $\gamma$-coincidence technique. The $0^{+}$ states lifetimes were assessed with the plunger method, yielding for the $0_2^{+}$, $0_3^{+}$, and $0_4^{+}$ decay to the $2_1^{+}$ state the $B(E2)$ values of 4.3, 0.1, and 0.2 Weisskopf units (W.u.), respectively. MCSM calculations correctly predict the existence of all three excited $0^{+}$ states, pointing to the oblate, spherical, and prolate nature of the consecutive excitations. In addition, they account for the hindrance of the $E2$ decay from the prolate $0_4^{+}$ to the spherical $2_1^{+}$ state, although overestimating its value. This result makes ${}^{66}\mathrm{Ni}$ a unique nuclear system, apart from ${}^{236,238}U$, in which a retarded $\gamma$ transition from a $0^{+}$ deformed state to a spherical configuration is observed, resembling a shape-isomerlike behavior.

Charge-dependent azimuthal particle correlations with respect to the second-order event plane in $p\text{−}\mathrm{Pb}$ and PbPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV have been studied with the CMS experiment at the LHC. The measurement is performed with a three-particle correlation technique, using two particles with the same or opposite charge within the pseudorapidity range $|\eta |<2.4$, and a third particle measured in the hadron forward calorimeters ($4.4<|\eta |<5$). The observed differences between the same and opposite sign correlations, as functions of multiplicity and $\eta$ gap between the two charged particles, are of similar magnitude in $p\text{−}\mathrm{Pb}$ and PbPb collisions at the same multiplicities. These results pose a challenge for the interpretation of charge-dependent azimuthal correlations in heavy ion collisions in terms of the chiral magnetic effect.

We study neutron-proton equilibration in dynamically deformed atomic nuclei created in nuclear collisions. The two ends of the elongated nucleus are initially dissimilar in composition and equilibrate on a subzeptosecond time scale following first-order kinetics. We use angular momentum to relate the breakup orientation to the time scale of the breakup. The extracted rate constant is $3{\mathrm{zs}}^{-1}$, which corresponds to a mean equilibration time of 0.3 zs. This technique enables new insight into the nuclear equation of state that governs many nuclear and astrophysical phenomena leading to the origin of the chemical elements.

The first excited isomeric state of ${}^{229}\mathrm{Th}$ possesses the lowest energy among all known excited nuclear states. The expected energy is accessible with today’s laser technology and in principle allows for a direct optical laser excitation of the nucleus. The isomer decays via three channels to its ground state (internal conversion, $\gamma$ decay, and bound internal conversion), whose strengths depend on the charge state of ${}^{229m}\mathrm{Th}$. We report on the measurement of the internal-conversion decay half-life of neutral ${}^{229m}\mathrm{Th}$. A half-life of $7\pm 1\mu \mathrm{s}$ has been measured, which is in the range of theoretical predictions and, based on the theoretically expected lifetime of $\approx 10^4\mathrm{s}$ of the photonic decay channel, gives further support for an internal conversion coefficient of $\approx 10^9$, thus constraining the strength of a radiative branch in the presence of internal conversion.

The first measurement of the low-lying states of the neutron-rich ${}^{110}\mathrm{Zr}$ and ${}^{112}\mathrm{Mo}$ was performed via in-beam $\gamma$-ray spectroscopy after one proton removal on hydrogen at $\sim 200\mathrm{MeV}/\text{nucleon}$. The $2_1^{+}$ excitation energies were found at 185(11) keV in ${}^{110}\mathrm{Zr}$, and 235(7) keV in ${}^{112}\mathrm{Mo}$, while the $R_{42}=E(4_1^{+})/E(2_1^{+}$) ratios are 3.1(2), close to the rigid rotor value, and 2.7(1), respectively. These results are compared to modern energy density functional based configuration mixing models using Gogny and Skyrme effective interactions. We conclude that first levels of ${}^{110}\mathrm{Zr}$ exhibit a rotational behavior, in agreement with previous observations of lighter zirconium isotopes as well as with the most advanced Monte Carlo shell model predictions. The data, therefore, do not support a harmonic oscillator shell stabilization scenario at $Z=40$ and $N=70$. The present data also invalidate predictions for a tetrahedral ground state symmetry in ${}^{110}\mathrm{Zr}$.

Exploiting the universality between the QCD critical point and the three-dimensional Ising model, closed form expressions derived for nonequilibrium critical cumulants on the crossover side of the critical point reveal that they can differ in both magnitude and sign from equilibrium expectations. We demonstrate here that key elements of the Kibble-Zurek framework of nonequilibrium phase transitions can be employed to describe the dynamics of these critical cumulants. Our results suggest that observables sensitive to critical dynamics in heavy-ion collisions should be expressible as universal scaling functions, thereby providing powerful model-independent guidance in searches for the QCD critical point.

We report the measurements of correlations between event-by-event fluctuations of amplitudes of anisotropic flow harmonics in nucleus-nucleus collisions, obtained for the first time using a new analysis method based on multiparticle cumulants in mixed harmonics. This novel method is robust against systematic biases originating from nonflow effects and by construction any dependence on symmetry planes is eliminated. We demonstrate that correlations of flow harmonics exhibit a better sensitivity to medium properties than the individual flow harmonics. The new measurements are performed in Pb-Pb collisions at the center-of-mass energy per nucleon pair of $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ by the ALICE experiment at the Large Hadron Collider. The centrality dependence of correlation between event-by-event fluctuations of the elliptic $v_2$ and quadrangular $v_4$ flow harmonics, as well as of anticorrelation between $v_2$ and triangular $v_3$ flow harmonics are presented. The results cover two different regimes of the initial state configurations: geometry dominated (in midcentral collisions) and fluctuation dominated (in the most central collisions). Comparisons are made to predictions from Monte Carlo Glauber, viscous hydrodynamics, ampt, and hijing models. Together with the existing measurements of the individual flow harmonics the presented results provide further constraints on the initial conditions and the transport properties of the system produced in heavy-ion collisions.