Heavy nuclei exhibit a crossover from vibrational to rotational collectivity as the number of neutrons or protons increases from shell closure towards midshell, but the microscopic description of this crossover has been a major challenge. We apply the shell model Monte Carlo approach to families of even-even samarium and neodymium isotopes and identify a microscopic signature of the crossover from vibrational to rotational collectivity in the low-temperature behavior of $⟨{\mathbf{J}}^2{⟩}_T$, where $\mathbf{J}$ is the total spin and $T$ is the temperature. This signature agrees well with its values extracted from experimental data. We also calculate the state densities of these nuclei and find them to be in very good agreement with experimental data. Finally, we define a collective enhancement factor from the ratio of the total state density to the intrinsic state density as calculated in the finite-temperature Hartree-Fock-Bogoliubov approximation. The decay of this enhancement factor with excitation energy is found to correlate with the pairing and shape phase transitions in these nuclei.

We present an ab initio study of the isoscalar monopole excitations of ${}^4\mathrm{He}$ using different realistic nuclear interactions, including modern effective field theory potentials. In particular we concentrate on the transition form factor $F_{\mathcal{M}}$ to the narrow $0^{+}$ resonance close to threshold. $F_{\mathcal{M}}$ exhibits a strong potential model dependence, and can serve as a kind of prism to distinguish among different nuclear force models. Compared to the measurements obtained from inelastic electron scattering off ${}^4\mathrm{He}$, one finds that the state-of-the-art theoretical transition form factors are at variance with experimental data, especially in the case of effective field theory potentials. We discuss some possible reasons for such a discrepancy, which still remains a puzzle.

First measurements of the azimuthal anisotropy of neutral pions produced in Pb-Pb collisions at a center-of-mass energy of $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ are presented. The amplitudes of the second Fourier component ($v_2$) of the ${\pi }^0$ azimuthal distributions are extracted using an event-plane technique. The values of $v_2$ are studied as a function of the neutral pion transverse momentum ($p_T$) for different classes of collision centrality in the kinematic range $1.6<p_T<8.0\mathrm{GeV}/c$, within the pseudorapidity interval $|\eta |<0.8$. The CMS measurements of $v_2(p_T)$ are similar to previously reported ${\pi }^0$ azimuthal anisotropy results from $\sqrt{s_{NN}}=200\mathrm{GeV}$ Au-Au collisions at RHIC, despite a factor of $\sim 14$ increase in the center-of-mass energy. In the momentum range $2.5<p_T<5.0\mathrm{GeV}/c$, the neutral pion anisotropies are found to be smaller than those observed by CMS for inclusive charged particles.

Recent LHC data on $\mathrm{Pb}+\mathrm{Pb}$ reactions at ${\sqrt{s}}_{NN}=2.7\mathrm{TeV}$ suggests that the $p/\pi$ is incompatible with thermal models. We explore several hadron ratios ($K/\pi$, $p/\pi$, $\Lambda /\pi$, $\Xi /\pi$) within a hydrodynamic model with a hadronic after burner, namely the ultrarelativistic quantum molecular dynamics model 3.3, and show that the deviations can be understood as a final state effect. We propose the $p/\pi$ as an observable sensitive on whether final state interactions take place or not. The measured values of the hadron ratios do then allow us to gauge the transition energy density from hydrodynamics to the Boltzmann description. We find that the data can be explained with transition energy densities of $840\pm 150\mathrm{MeV}/{\mathrm{fm}}^3$.

Within a dynamical description of nuclear fragmentation, based on the liquid-gas phase transition scenario, we explore the relation between neutron-proton density fluctuations and nuclear symmetry energy. We show that, along the fragmentation path, isovector fluctuations follow the evolution of the local density and approach an equilibrium value connected to the local symmetry energy. Higher-density regions are characterized by smaller average asymmetry and narrower isotopic distributions. This dynamical analysis points out that fragment final state isospin fluctuations can probe the symmetry energy of the density domains from which fragments originate.

The charged-particle pseudorapidity density measured over four units of pseudorapidity in nonsingle-diffractive $p+\mathrm{Pb}$ collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ is presented. The average value at midrapidity is measured to be $16.81\pm 0.71(\mathrm{syst})$, which corresponds to $2.14\pm 0.17(\mathrm{syst})$ per participating nucleon, calculated with the Glauber model. This is 16% lower than in nonsingle-diffractive $pp$ collisions interpolated to the same collision energy and 84% higher than in $d+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=0.2\mathrm{TeV}$. The measured pseudorapidity density in $p+\mathrm{Pb}$ collisions is compared to model predictions and provides new constraints on the description of particle production in high-energy nuclear collisions.

The ${}^{18}\mathrm{F}(p,\alpha ){}^{15}\mathrm{O}$ reaction rate is crucial for constraining model predictions of the $\gamma$-ray observable radioisotope ${}^{18}\mathrm{F}$ produced in novae. The determination of this rate is challenging due to particular features of the level scheme of the compound nucleus, ${}^{19}\mathrm{Ne}$, which result in interference effects potentially playing a significant role. The dominant uncertainty in this rate arises from interference between $J^{\pi }=3/2^{+}$ states near the proton threshold ($S_p=6.411\mathrm{MeV}$) and a broad $J^{\pi }=3/2^{+}$ state at 665 keV above threshold. This unknown interference term results in up to a factor of 40 uncertainty in the astrophysical $S$-factor at nova temperatures. Here we report a new measurement of states in this energy region using the ${}^{19}\mathrm{F}({}^3\mathrm{He},t){}^{19}\mathrm{Ne}$ reaction. In stark contrast to previous assumptions we find at least 3 resonances between the proton threshold and $E_{cm}=50\mathrm{keV}$, all with different angular distributions. None of these are consistent with $J^{\pi }=3/2^{+}$ angular distributions. We find that the main uncertainty now arises from the unknown proton width of the 48 keV resonance, not from possible interference effects. Hydrodynamic nova model calculations performed indicate that this unknown width affects ${}^{18}\mathrm{F}$ production by at least a factor of two in the model considered.

We revisit the problem of the kink in the charge radius shift of neutron-rich even isotopes near the $N=126$ shell closure. We show that the ability of a Skyrme force to reproduce the isotope shift is determined by the occupation of the neutron $1i_{11/2}$ orbital beyond $N=126$ and the corresponding change it causes to deeply-bound protons orbitals with a principal quantum number of 1. Given the observed position of the single-particle energies, one must either ensure occupation is allowed through correlations, or not demand that the single-particle energies agree with experimental values at the mean-field level.

Neutron matter presents a unique system for chiral effective field theory because all many-body forces among neutrons are predicted to next-to-next-to-next-to-leading order (${\mathrm{N}}^3\mathrm{LO}$). We present the first complete ${\mathrm{N}}^3\mathrm{LO}$ calculation of the neutron matter energy. This includes the subleading three-nucleon forces for the first time and all leading four-nucleon forces. We find relatively large contributions from ${\mathrm{N}}^3\mathrm{LO}$ three-nucleon forces. Our results provide constraints for neutron-rich matter in astrophysics with controlled theoretical uncertainties.

The decay of ${}^{19}\mathrm{O}({\beta }^{-})$ and ${}^{19}\mathrm{Ne}({\beta }^{+})$ implanted in niobium in its superconducting and metallic phases was measured using purified radioactive beams produced by the SPIRAL GANIL facility. Half-lives and branching ratios measured in the two phases are consistent within a $1\sigma$ error bar. This measurement casts strong doubts on the predicted strong electron screening in a superconductor, the so-called superscreening. The measured difference in screening potential energy is 110(90) eV for ${}^{19}\mathrm{Ne}$ and 400(320) eV for ${}^{19}\mathrm{O}$. Precise determinations of the half-lives were obtained for ${}^{19}\mathrm{O}$, 26.476(9) s, and for ${}^{19}\mathrm{Ne}$, 17.254(5) s.

The neutron-rich unbound ${}^7\mathrm{He}$ nucleus has been the subject of many experimental investigations. While the ground-state $3/2^{-}$ resonance is well established, there is a controversy concerning the excited $1/2^{-}$ resonance reported in some experiments as low lying and narrow ($E_R\sim 1\mathrm{MeV}$, $\Gamma \le 1\mathrm{MeV}$) while in others as very broad and located at a higher energy. This issue cannot be addressed by ab initio theoretical calculations based on traditional bound-state methods. We introduce a new unified approach to nuclear bound and continuum states based on the coupling of the no-core shell model, a bound-state technique, with the no-core shell model combined with the resonating-group method, a nuclear scattering technique. Our calculations describe the ground-state resonance in agreement with experiment and, at the same time, predict a broad $1/2^{-}$ resonance above 2 MeV.

The $M1$ excitations in the nuclide ${}^{90}\mathrm{Zr}$ have been studied in a photon-scattering experiment with monoenergetic and linearly polarized beams from 7 to 11 MeV. More than 40 $J^{\pi }=1^{+}$ states have been identified from observed ground-state transitions, revealing the fine structure of the giant $M1$ resonance with a centroid energy of 9 MeV and a sum strength of 4.17(56) ${\mu }_N^2$. The result for the total $M1$ strength and its fragmentation are discussed in the framework of the three-phonon quasiparticle-phonon model.

Based on results from a measurement of weak decay branches observed following the ${\beta }^{-}$ decay of ${}^{94}\mathrm{Y}$ and on lifetime data from a study of ${}^{94}\mathrm{Zr}$ by inelastic neutron scattering, collective structure is deduced in the closed-subshell nucleus ${}^{94}\mathrm{Zr}$. These results establish shape coexistence in ${}^{94}\mathrm{Zr}$. The role of subshells for nuclear collectivity is suggested to be important.

The ATLAS experiment has observed 1995 $Z$ boson candidates in data corresponding to $0.15{\mathrm{nb}}^{-1}$ of integrated luminosity obtained in the 2011 LHC $\mathrm{Pb}+\mathrm{Pb}$ run at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$. The $Z$ bosons are reconstructed via dielectron and dimuon decay channels, with a background contamination of less than 3%. Results from the two channels are consistent and are combined. Within the statistical and systematic uncertainties, the per-event $Z$ boson yield is proportional to the number of binary collisions estimated by the Glauber model. The elliptic anisotropy of the azimuthal distribution of the $Z$ boson with respect to the event plane is found to be consistent with zero.

The ${}^{63}\mathrm{Ni}(n,\gamma )$ cross section has been measured for the first time at the neutron time-of-flight facility n_TOF at CERN from thermal neutron energies up to 200 keV. In total, capture kernels of 12 (new) resonances were determined. Maxwellian averaged cross sections were calculated for thermal energies from $kT=5–100\mathrm{keV}$ with uncertainties around 20%. Stellar model calculations for a $25M_{\odot }$ star show that the new data have a significant effect on the $s$-process production of ${}^{63}\mathrm{Cu}$, ${}^{64}\mathrm{Ni}$, and ${}^{64}\mathrm{Zn}$ in massive stars, allowing stronger constraints on the Cu yields from explosive nucleosynthesis in the subsequent supernova.

We present the first study of three-nucleon ($3N$) forces for proton-rich nuclei along the $N=8$ and $N=20$ isotones. Our results for the ground-state energies and proton separation energies are in very good agreement with experiment where available, and with the empirical isobaric multiplet mass equation. We predict the spectra for all $N=8$ and $N=20$ isotones to the proton dripline, which agree well with experiment for ${}^{18}\mathrm{Ne}$, ${}^{19}\mathrm{Na}$, ${}^{20}\mathrm{Mg}$ and ${}^{42}\mathrm{Ti}$. In all other cases, we provide first predictions based on nuclear forces. Our results are also very promising for studying isospin symmetry breaking in medium-mass nuclei based on chiral effective field theory.

We solve for the first time, the Faddeev equations for the bound state problem of the coupled $\Lambda \Lambda N-\Xi NN$ system to study whether or not a hypertriton with strangeness $-2$ may exist. We make use of the interactions obtained from a chiral quark model describing the low-energy observables of the two-baryon systems with strangeness 0, $-1$, and $-2$ and three-baryon systems with strangeness 0 and $-1$. The $\Lambda \Lambda N$ system alone is unbound. However, when the full coupling to $\Xi NN$ is considered, the strangeness $-2$ three-baryon system with quantum numbers $(I,J^P)=(\frac{1}{2},\frac{1}{2}\genfrac{}{}{0ex}{}{{+}^{}}{})$ becomes bound, with a binding energy of about 0.5 MeV. This result is compatible with the nonexistence of a stable ${}_{\Lambda }{}^3\mathrm{H}$ with isospin one.

The MuCap experiment at the Paul Scherrer Institute has measured the rate ${\Lambda }_S$ of muon capture from the singlet state of the muonic hydrogen atom to a precision of 1%. A muon beam was stopped in a time projection chamber filled with 10-bar, ultrapure hydrogen gas. Cylindrical wire chambers and a segmented scintillator barrel detected electrons from muon decay. ${\Lambda }_S$ is determined from the difference between the ${\mu }^{-}$ disappearance rate in hydrogen and the free muon decay rate. The result is based on the analysis of $1.2\times {10}^{10}$ ${\mu }^{-}$ decays, from which we extract the capture rate ${\Lambda }_S=(714.9\pm {5.4}_{\mathrm{stat}}\pm {5.1}_{\mathrm{syst}}){\mathrm{s}}^{-1}$ and derive the proton’s pseudoscalar coupling $g_P(q_0^2=-0.88m_{\mu }^2)=8.06\pm 0.55$.

Measurements of charge-dependent azimuthal correlations with the ALICE detector at the LHC are reported for Pb-Pb collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$. Two- and three-particle charge-dependent azimuthal correlations in the pseudorapidity range $|\eta |<0.8$ are presented as a function of the collision centrality, particle separation in pseudorapidity, and transverse momentum. A clear signal compatible with a charge-dependent separation relative to the reaction plane is observed, which shows little or no collision energy dependence when compared to measurements at RHIC energies. This provides a new insight for understanding the nature of the charge-dependent azimuthal correlations observed at RHIC and LHC energies.

Anisotropic flow coefficients $v_1–v_5$ in heavy ion collisions are computed by combining a classical Yang-Mills description of the early time Glasma flow with the subsequent relativistic viscous hydrodynamic evolution of matter through the quark-gluon plasma and hadron gas phases. The Glasma dynamics, as realized in the impact parameter dependent Glasma (IP-Glasma) model, takes into account event-by-event geometric fluctuations in nucleon positions and intrinsic subnucleon scale color charge fluctuations; the preequilibrium flow of matter is then matched to the music algorithm describing viscous hydrodynamic flow and particle production at freeze-out. The IP-$\mathrm{\text{Glasma}}+\mathrm{MUSIC}$ model describes well both transverse momentum dependent and integrated $v_n$ data measured at the Large Hadron Collider and the Relativistic Heavy Ion Collider. The model also reproduces the event-by-event distributions of $v_2$, $v_3$ and $v_4$ measured by the ATLAS Collaboration. The implications of our results for better understanding of the dynamics of the Glasma and for the extraction of transport properties of the quark-gluon plasma are outlined.