The $\beta$-decay half-lives of 110 neutron-rich isotopes of the elements from ${}_{37}\mathrm{Rb}$ to ${}_{50}\mathrm{Sn}$ were measured at the Radioactive Isotope Beam Factory. The 40 new half-lives follow robust systematics and highlight the persistence of shell effects. The new data have direct implications for $r$-process calculations and reinforce the notion that the second ($A\approx 130$) and the rare-earth-element ($A\approx 160$) abundance peaks may result from the freeze-out of an $(n,\gamma )\rightleftarrows (\gamma ,n)$ equilibrium. In such an equilibrium, the new half-lives are important factors determining the abundance of rare-earth elements, and allow for a more reliable discussion of the $r$ process universality. It is anticipated that universality may not extend to the elements Sn, Sb, I, and Cs, making the detection of these elements in metal-poor stars of the utmost importance to determine the exact conditions of individual $r$-process events.

We study the thermalization of gluons far from thermal equilibrium in relativistic kinetic theory. The initial distribution of gluons is assumed to resemble that in the early stage of ultrarelativistic heavy ion collisions. Only elastic scatterings in static, nonexpanding gluonic matter are considered. At first we show that the occurrence of condensation in the limit of vanishing particle mass requires a general constraint for the scattering matrix element. Then the thermalization of gluons with Bose-Einstein condensation is demonstrated in a transport calculation. We see a continuously increasing overpopulation of low energy gluons, followed by a decrease to the equilibrium distribution, when the condensation occurs. The times of the completion of the gluon condensation and of the entropy production are calculated. These times scale inversely with the energy density.

We apply principal component analysis to the study of event-by-event fluctuations in relativistic heavy-ion collisions. This method brings out all the information contained in two-particle correlations in a physically transparent way. We present a guide to the method, and apply it to multiplicity fluctuations and anisotropic flow, using ALICE data and simulated events. In particular, we study elliptic and triangular flow fluctuations as a function of transverse momentum and rapidity. This method reveals previously unknown subleading modes in both rapidity and transverse momentum for the momentum distribution as well as elliptic and triangular flows.

Nuclear $\beta$ decay in magic nuclei is investigated, taking into account the coupling between particles and collective vibrations, on top of self-consistent random phase approximation calculations based on Skyrme density functionals. The low-lying Gamow-Teller strength is shifted downwards and at times becomes fragmented; as a consequence, the $\beta$-decay half-lives are reduced due to the increase of the phase space available for the decay. In some cases, this leads to a very good agreement between theoretical and experimental lifetimes: this happens, in particular, in the case of the Skyrme force SkM* that can also reproduce the line shape of the high-energy Gamow-Teller resonance as was previously shown.

Statistical tools of uncertainty quantification can be used to assess the information content of measured observables with respect to present-day theoretical models, to estimate model errors and thereby improve predictive capability, to extrapolate beyond the regions reached by experiment, and to provide meaningful input to applications and planned measurements. To showcase new opportunities offered by such tools, we make a rigorous analysis of theoretical statistical uncertainties in nuclear density functional theory using Bayesian inference methods. By considering the recent mass measurements from the Canadian Penning Trap at Argonne National Laboratory, we demonstrate how the Bayesian analysis and a direct least-squares optimization, combined with high-performance computing, can be used to assess the information content of the new data with respect to a model based on the Skyrme energy density functional approach. Employing the posterior probability distribution computed with a Gaussian process emulator, we apply the Bayesian framework to propagate theoretical statistical uncertainties in predictions of nuclear masses, two-neutron dripline, and fission barriers. Overall, we find that the new mass measurements do not impose a constraint that is strong enough to lead to significant changes in the model parameters. The example discussed in this study sets the stage for quantifying and maximizing the impact of new measurements with respect to current modeling and guiding future experimental efforts, thus enhancing the experiment-theory cycle in the scientific method.

The spin polarizabilities of the nucleon describe how the spin of the nucleon responds to an incident polarized photon. The most model-independent way to extract the nucleon spin polarizabilities is through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the $\mathrm{\Delta }(1232)$ region using circularly polarized incident photons and a transversely polarized proton target at the Mainz Microtron. Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved. The analysis based on a dispersion model calculation yields ${\gamma }_{E1E1}=-3.5\pm 1.2$, ${\gamma }_{M1M1}=3.16\pm 0.85$, ${\gamma }_{E1M2}=-0.7\pm 1.2$, and ${\gamma }_{M1E2}=1.99\pm 0.29$, in units of $10^{-4}{\mathrm{fm}}^4$.

The quark gluon plasma produced in ultrarelativistic heavy-ion collisions exhibits remarkable features. It behaves like a nearly perfect liquid with a small shear viscosity to entropy density ratio and leads to the quenching of highly energetic particles. We show that both effects can be understood for the first time within one common framework. Employing the parton cascade Boltzmann approach to multiparton scatterings, the microscopic interactions and the space-time evolution of the quark gluon plasma are calculated by solving the relativistic Boltzmann equation. Based on cross sections obtained from perturbative QCD with explicitly taking the running coupling into account, we calculate the nuclear modification factor and elliptic flow in ultrarelativistic heavy-ion collisions. With only one single parameter associated with coherence effects of medium-induced gluon radiation, the experimental data of both observables can be understood on a microscopic level. Furthermore, we show that perturbative QCD interactions with a running coupling lead to a sufficiently small shear viscosity to entropy density ratio of the quark gluon plasma, which provides a microscopic explanation for the observations stated by hydrodynamic calculations.

The onset of hyperons in the core of neutron stars and the consequent softening of the equation of state have been questioned for a long time. Controversial theoretical predictions and recent astrophysical observations of neutron stars are the grounds for the so-called hyperon puzzle. We calculate the equation of state and the neutron star mass-radius relation of an infinite systems of neutrons and $\mathrm{\Lambda }$ particles by using the auxiliary field diffusion Monte Carlo algorithm. We find that the three-body hyperon-nucleon interaction plays a fundamental role in the softening of the equation of state and for the consequent reduction of the predicted maximum mass. We have considered two different models of three-body force that successfully describe the binding energy of medium mass hypernuclei. Our results indicate that they give dramatically different results on the maximum mass of neutron stars, not necessarily incompatible with the recent observation of very massive neutron stars. We conclude that stronger constraints on the hyperon-neutron force are necessary in order to properly assess the role of hyperons in neutron stars.

We analyze the power counting of two-body currents in nuclear effective field theories (EFTs). We find that the existence of nonperturbative physics at low energies, which is manifest in the existence of the deuteron and the ${}^1{S}_0$ $NN$ virtual bound state, combined with the appearance of singular potentials in versions of nuclear EFT that incorporate chiral symmetry, modifies the renormalization-group flow of the couplings associated with contact operators that involve nucleon-nucleon pairs and external fields. The order of these couplings is thereby enhanced with respect to the naive-dimensional-analysis estimate. Consequently, short-range currents enter at a lower order in the chiral EFT than has been appreciated up until now, and their impact on low-energy observables is concomitantly larger. We illustrate the changes in the power counting with a few low-energy processes involving external probes and few-nucleon systems, including electron-deuteron elastic scattering and radiative neutron capture by protons.

We argue that the proton multiplicities measured in Roman pot detectors at an electron ion collider can be used to determine centrality classes in incoherent diffractive scattering. Incoherent diffraction probes the fluctuations in the interaction strengths of multiparton Fock states in the nuclear wave functions. In particular, the saturation scale that characterizes this multiparton dynamics is significantly larger in central events relative to minimum bias events. As an application, we study the centrality dependence of incoherent diffractive vector meson production. We identify an observable which is simultaneously very sensitive to centrality triggered parton fluctuations and insensitive to details of the model.

A pair of transverse wobbling bands is observed in the nucleus ${}^{135}\mathrm{Pr}$. The wobbling is characterized by $\mathrm{\Delta }I=1$, $E2$ transitions between the bands, and a decrease in the wobbling energy confirms its transverse nature. Additionally, a transition from transverse wobbling to a three-quasiparticle band comprised of strong magnetic dipole transitions is observed. These observations conform well to results from calculations with the tilted axis cranking model and the quasiparticle rotor model.

We consider a nonperturbative approach to the thermal production of dileptons and photons at temperatures near the critical temperature in QCD. The suppression of colored excitations at low temperature is modeled by including a small value of the Polyakov loop, in a “semi”-quark-gluon plasma (QGP). Comparing the semi-QGP to the perturbative QGP, we find a mild enhancement of thermal dileptons. In contrast, to leading logarithmic order in weak coupling there are far fewer hard photons from the semi-QGP than the usual QGP. To illustrate the possible effects on photon and dilepton production in heavy-ion collisions, we integrate the rate with a simulation using ideal hydrodynamics. Dileptons uniformly exhibit a small flow, but the strong suppression of photons in the semi-QGP tends to weight the elliptical flow of photons to that generated in the hadronic phase.

Measurements of inclusive jet production are performed in $pp$ and $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}=2.76\mathrm{TeV}$ with the ATLAS detector at the LHC, corresponding to integrated luminosities of 4.0 and $0.14{\mathrm{nb}}^{-1}$, respectively. The jets are identified with the anti-$k_t$ algorithm with $R=0.4$, and the spectra are measured over the kinematic range of jet transverse momentum $32<p_{\mathrm{T}}<500\mathrm{GeV}$ and absolute rapidity $|y|<2.1$ and as a function of collision centrality. The nuclear modification factor $R_{\mathrm{AA}}$ is evaluated, and jets are found to be suppressed by approximately a factor of 2 in central collisions compared to $pp$ collisions. The $R_{\mathrm{AA}}$ shows a slight increase with $p_{\mathit{T}}$ and no significant variation with rapidity.

A precise measurement of the $g$ factor of the first-excited state in the self-conjugate ($N=Z$) nucleus ${}^{24}\mathrm{Mg}$ is performed by a new time-differential recoil-in-vacuum method based on the hyperfine field of hydrogenlike ions. Theory predicts that the $g$ factors of such states, in which protons and neutrons occupy the same orbits, should depart from 0.5 by a few percent due to configuration mixing and meson-exchange effects. The experimental result, $g=0.538\pm 0.013$, is in excellent agreement with recent shell-model calculations and shows a departure from 0.5 by almost 3 standard deviations, thus achieving, for the first time, the precision and accuracy needed to test theory. Proof of the new method opens the way for wide applications including measurements of the magnetism of excited states of exotic nuclei produced as radioactive beams.

Several experimental investigations have observed parity violation (PV) in nuclear systems—a consequence of the weak force between quarks. We apply the $1/N_c$ expansion of QCD to the $P$-violating $T$-conserving component of the nucleon-nucleon (NN) potential. We show there are two leading-order operators, both of which affect $\overrightarrow{p}p$ scattering at order $N_c$. We find an additional four operators at order $N_c^0{\sin }^2{\theta }_W$ and six at $\mathcal{O}(1/N_c)$. Pion exchange in the PV NN force is suppressed by $1/N_c$ and ${\sin }^2{\theta }_W$, providing a quantitative explanation for its nonobservation up to this time. The large-$N_c$ hierarchy of other PV NN force mechanisms is consistent with estimates of the couplings in phenomenological models. The PV observed in $\overrightarrow{p}p$ scattering data is compatible with natural values for the strong and weak coupling constants: there is no evidence of fine-tuning.

We show how nuclear effective field theory (EFT) and ab initio nuclear-structure methods can turn input from lattice quantum chromodynamics (LQCD) into predictions for the properties of nuclei. We argue that pionless EFT is the appropriate theory to describe the light nuclei obtained in LQCD simulations carried out at pion masses heavier than the physical pion mass. We solve the EFT using the effective-interaction hyperspherical harmonics and auxiliary-field diffusion Monte Carlo methods. Fitting the three leading-order EFT parameters to the deuteron, dineutron, and triton LQCD energies at $m_{\pi }\approx 800\mathrm{MeV}$, we reproduce the corresponding alpha-particle binding and predict the binding energies of mass-5 and mass-6 ground states.

We clarify the origin of the anomalously hindered $E2$ decay from the $4_1^{+}$ level in ${}^{44}\mathrm{S}$ by performing a novel many-body analysis in the shell model. Within a unified picture about the occurrence of isomerism in neutron-rich sulfur isotopes, the $4_1^{+}$ state is demonstrated to be a $K=4$ isomer dominated by the two-quasiparticle configuration $\nu {\mathrm{\Omega }}^{\pi }=1/2^{-}\otimes \nu {\mathrm{\Omega }}^{\pi }=7/2^{-}$. The $4_1^{+}$ state in ${}^{44}\mathrm{S}$ is a new type of high-$K$ isomer which has significant triaxiality.

We present results from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. We report the first mass measurements of ${}^{48}\mathrm{Ar}$ and ${}^{49}\mathrm{Ar}$ and find atomic mass excesses of $-22.28(31)\mathrm{MeV}$ and $-17.8(1.1)\mathrm{MeV}$, respectively. These masses provide strong evidence for the closed shell nature of neutron number $N=28$ in argon, which is therefore the lowest even-$Z$ element exhibiting the $N=28$ closed shell. The resulting trend in binding-energy differences, which probes the strength of the $N=28$ shell, compares favorably with shell-model calculations in the $sd$-$pf$ shell using SDPF-U and SDPF-MU Hamiltonians.