Asymptotic giant branch stars are responsible for the production of most of the heavy isotopes beyond Sr observed in the solar system. Among them, isotopes shielded from the $r$-process contribution by their stable isobars are defined as $s$-only nuclei. For a long time the abundance of ${}^{204}\mathrm{Pb}$, the heaviest $s$-only isotope, has been a topic of debate because state-of-the-art stellar models appeared to systematically underestimate its solar abundance. Besides the impact of uncertainties from stellar models and galactic chemical evolution simulations, this discrepancy was further obscured by rather divergent theoretical estimates for the neutron capture cross section of its radioactive precursor in the neutron-capture flow, ${}^{204}\mathrm{Tl}$ ($t_{1/2}=3.78\mathrm{yr}$), and by the lack of experimental data on this reaction. We present the first ever neutron capture measurement on ${}^{204}\mathrm{Tl}$, conducted at the CERN neutron time-of-flight facility n_TOF, employing a sample of only 9 mg of ${}^{204}\mathrm{Tl}$ produced at the Institute Laue Langevin high flux reactor. By complementing our new results with semiempirical calculations we obtained, at the $s$-process temperatures of $kT\approx 8\mathrm{keV}$ and $kT\approx 30\mathrm{keV}$, Maxwellian-averaged cross sections (MACS) of 580(168) mb and 260(90) mb, respectively. These figures are about 3% lower and 20% higher than the corresponding values widely used in astrophysical calculations, which were based only on theoretical calculations. By using the new ${}^{204}\mathrm{Tl}$ MACS, the uncertainty arising from the ${}^{204}\mathrm{Tl}(\mathrm{n},\gamma )$ cross section on the $s$-process abundance of ${}^{204}\mathrm{Pb}$ has been reduced from $\sim 30\%$ down to $+8\%/-6\%$, and the $s$-process calculations are in agreement with the latest solar system abundance of ${}^{204}\mathrm{Pb}$ reported by K. Lodders in 2021.

The CNO cycle is one of the most important nuclear energy sources in stars. At temperatures of hydrostatic H-burning ($20\mathrm{MK}<T<80\mathrm{MK}$) the ${}^{17}\mathrm{O}(p,\gamma ){}^{18}\mathrm{F}$ reaction rate is dominated by the poorly constrained 64.5 keV resonance. Here, we report on the first direct measurements of its resonance strength and of the direct capture contribution at 142 keV, performed with a new high sensitivity setup at LUNA. The present resonance strength of $\omega {\gamma }_{(p,\gamma )}^{\mathrm{bare}}=(30\pm 6_{\mathrm{stat}}\pm 2_{\mathrm{syst}})\mathrm{peV}$ is about a factor of 2 higher than the values in literature, leading to a ${\mathrm{\Gamma }}_p^{\mathrm{bare}}=(34\pm 7_{\mathrm{stat}}\pm 3_{\mathrm{syst}})\mathrm{neV}$, in agreement with the LUNA result from the $(p,\alpha )$ channel. Such agreement strengthens our understanding of the oxygen isotopic ratios measured in red giant stars and in O-rich presolar grains.

Precision spectroscopy of hyperfine splitting (HFS) is a crucial tool for investigating the structure of nuclei and testing quantum electrodynamics. However, accurate theoretical predictions are hindered by two-photon exchange (TPE) effects. We propose a novel formalism that accounts for nuclear excitations and recoil in TPE, providing a model-independent description of TPE effects on HFS in light ordinary and muonic atoms. Combining our formalism with pionless effective field theory at next-to-next-to-leading order, the predicted TPE effects on HFS are 41.7(4.4) kHz and 0.117(13) meV for the 1S state in deuterium and the 2S state in muonic deuterium. These results align within $1\sigma$ and $1.3\sigma$ from recent measurements and highlight the importance of nuclear structure effects on HFS and indicate the value of more precise measurements in future experiments.

We investigated decays of ${}^{51,52,53}\mathrm{K}$ at the ISOLDE Decay Station at CERN in order to understand the mechanism of the $\beta$-delayed neutron-emission ($\beta n$) process. The experiment quantified neutron and $\gamma$-ray emission paths for each precursor. We used this information to test the hypothesis, first formulated by Bohr in 1939, that neutrons in the $\beta n$ process originate from the structureless “compound nucleus.” The data are consistent with this postulate for most of the observed decay paths. The agreement, however, is surprising because the compound-nucleus stage should not be achieved in the studied $\beta$ decay due to insufficient excitation energy and level densities in the neutron emitter. In the ${}^{53}\mathrm{K}$ $\beta n$ decay, we found a preferential population of the first excited state in ${}^{52}\mathrm{Ca}$ that contradicted Bohr’s hypothesis. The latter was interpreted as evidence for direct neutron emission sensitive to the structure of the neutron-unbound state. We propose that the observed nonstatistical neutron emission proceeds through the coupling with nearby doorway states that have large neutron-emission probabilities. The appearance of “compound-nucleus” decay is caused by the aggregated small contributions of multiple doorway states at higher excitation energy.

We construct a kinetic model for matter-radiation interactions whose hydrodynamic gradient expansion can be computed analytically up to infinite order in derivatives, in the fully nonlinear regime, and for arbitrary flows. The frequency dependence of the opacity of matter is chosen to mimic the relaxation time of a self-interacting scalar field. In this way, the transient sector simulates that of a realistic quantum field theory. The gradient series is found to diverge for most flows, in agreement with previous findings. We identify, for the model at hand, the mechanism at the origin of the divergence, and we provide a successful regularization scheme. Additionally, we propose a universal qualitative framework for predicting the breakdown of the gradient expansion of an arbitrary microscopic system undergoing a given flow. This framework correctly recovers all previously known instances of gradient expansion divergence. As a new prediction, we show that the gradient expansion diverges when the energy-dependent mean free path is unbounded above.

We present simulations of stochastic fluid dynamics in the vicinity of a critical endpoint belonging to the universality class of the Ising model. This study is motivated by the challenge of modeling the dynamics of critical fluctuations near a conjectured critical endpoint in the phase diagram of quantum chromodynamics (QCD). We focus on the interaction of shear modes with a conserved scalar density, which is known as model $H$. We show that the observed dynamical scaling behavior depends on the correlation length and the shear viscosity of the fluid. As the correlation length is increased or the viscosity is decreased we observe a crossover from the dynamical exponent of critical diffusion, $z\simeq 4$, to the expected scaling exponent of model $H$, $z\simeq 3$. We use our method to investigate the time-dependent correlation function of non-Gaussian moments $M^n(t)$ of the order parameter. We find that the relaxation time depends in a nontrivial manner on the power $n$.

Nuclear hyperfine mixing (NHM) is a distinctive effect through which nuclear properties and dynamics can be efficiently controlled. Traditionally, the scientific consensus has predominantly associated NHM with its pronounced impact on ${}^{229}\mathrm{Th}$, owing to the existence of its low-lying isomeric state. However, in the present study, by developing a general theory for NHM, we predict a surprisingly substantial NHM effect in boronlike ${}^{205}\mathrm{Pb}$ ions (specifically, ${{}^{205}\mathrm{Pb}}^{77+}$), featuring a nuclear transition energy of 2.329 keV. The radiative lifetime of the ${}^{205}\mathrm{Pb}$ isomer is reduced by 4 orders of magnitude, from 15 min to 32 ms. This remarkable and unexpected NHM effect stems from a heretofore unexplored process: an opening of a magnetic dipole channel that is otherwise forbidden without NHM. ${}^{205}\mathrm{Pb}$ ions now emerge as an attractive alternative candidate for experimental validation and investigation of the NHM effect.

The production of $\mathrm{\Upsilon }(2S)$ and $\mathrm{\Upsilon }(3S)$ mesons in lead-lead (Pb-Pb) and proton-proton ($pp$) collisions is studied in their dimuon decay channel using the CMS detector at the LHC. The $\mathrm{\Upsilon }(3S)$ meson is observed for the first time in Pb-Pb collisions, with a significance above 5 standard deviations. The ratios of yields measured in Pb-Pb and $pp$ collisions are reported for both the $\mathrm{\Upsilon }(2S)$ and $\mathrm{\Upsilon }(3S)$ mesons, as functions of transverse momentum and Pb-Pb collision centrality. These ratios, when appropriately scaled, are significantly less than unity, indicating a suppression of $\mathrm{\Upsilon }$ yields in Pb-Pb collisions. This suppression increases from peripheral to central Pb-Pb collisions. Furthermore, the suppression is stronger for $\mathrm{\Upsilon }(3S)$ mesons compared to $\mathrm{\Upsilon }(2S)$ mesons, extending the pattern of sequential suppression of quarkonium states in nuclear collisions previously seen for the $J/\psi$, $\psi (2S)$, $\mathrm{\Upsilon }(1S)$, and $\mathrm{\Upsilon }(2S)$ mesons.

The rotational properties of the transfermium nuclei are investigated in the full deformation space by implementing a shell-model-like approach in the cranking covariant density functional theory on a three-dimensional lattice, where the pairing correlations, deformations, and moments of inertia are treated in a microscopic and self-consistent way. The kinematic and dynamic moments of inertia of the rotational bands observed in the transfermium nuclei ${}^{252}\mathrm{No}$, ${}^{254}\mathrm{No}$, ${}^{254}\mathrm{Rf}$, and ${}^{256}\mathrm{Rf}$ are well reproduced without any adjustable parameters using a well-determined universal density functional. It is found for the first time that the emergence of the octupole deformation should be responsible for the significantly different rotational behavior observed in ${}^{252}\mathrm{No}$ and ${}^{254}\mathrm{No}$. The present results provide a microscopic solution to the long-standing puzzle on the rotational behavior in No isotopes, and highlight the risk of investigating only the hexacontetrapole (${\beta }_{60}$) deformation effects in rotating transfermium nuclei without considering the octupole deformation.

The nuclear two-photon or double-gamma ($2\gamma$) decay is a second-order electromagnetic process whereby a nucleus in an excited state emits two gamma rays simultaneously. To be able to directly measure the $2\gamma$ decay rate in the low-energy regime below the electron-positron pair-creation threshold, we combined the isochronous mode of a storage ring with Schottky resonant cavities. The newly developed technique can be applied to isomers with excitation energies down to $\sim 100\mathrm{keV}$ and half-lives as short as $\sim 10\mathrm{ms}$. The half-life for the $2\gamma$ decay of the first-excited $0^{+}$ state in bare ${}^{72}\mathrm{Ge}$ ions was determined to be 23.9(6) ms, which strongly deviates from expectations.

The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum (high $p_{\mathrm{T}}$) hadron trigger in proton-proton and central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. A data-driven statistical method is used to mitigate the large uncorrelated background in central Pb-Pb collisions. Recoil jet distributions are reported for jet resolution parameter $R=0.2$, 0.4, and 0.5 in the range $7<p_{\mathrm{T},\text{jet}}<140\mathrm{GeV}/c$ and trigger-recoil jet azimuthal separation $\pi /2<\mathrm{\Delta }\phi <\pi$. The measurements exhibit a marked medium-induced jet yield enhancement at low $p_{\mathrm{T}}$ and at large azimuthal deviation from $\mathrm{\Delta }\phi \sim \pi$. The enhancement is characterized by its dependence on $\mathrm{\Delta }\phi$, which has a slope that differs from zero by $4.7\sigma$. Comparisons to model calculations incorporating different formulations of jet quenching are reported. These comparisons indicate that the observed yield enhancement arises from the response of the QGP medium to jet propagation.

To search for low-energy resonant structures in isospin $T=3/2$ three-body systems, we have performed the experiments ${}^3\mathrm{H}(t,{}^3\mathrm{He})3n$ and ${}^3\mathrm{He}({}^3\mathrm{He},t)3p$ at intermediate energies. For the $3n$ experiment, we have newly developed a thick Ti-${}^3\mathrm{H}$ target that has the largest tritium thickness among targets of this type ever made. The $3n$ experiment for the first time covered the momentum-transfer region as low as $15\mathrm{MeV}/c$, which provides ideal conditions for producing fragile systems. However, in the excitation-energy spectra we obtained, we did not observe any distinct peak structures. This is in sharp contrast to tetraneutron spectra. The distributions of the $3n$ and $3p$ spectra are found to be similar, except for the displacement in energy due to Coulomb repulsion. Comparisons with theoretical calculations suggest that three-body correlations exist in the $3n$ and $3p$ systems, although not enough to produce a resonant peak.

The hexadecapole deformation (${\beta }_4$) of the ${}^{238}\mathrm{U}$ nucleus has not been determined because its effect is overwhelmed by those from the nucleus’ large quadrupole deformation (${\beta }_2$) in nuclear electric transition measurements. In this Letter, we identify the nonlinear response of the hexadecapole anisotropy to ellipticity in relativistic $\mathrm{U}+\mathrm{U}$ collisions that is solely sensitive to ${\beta }_4$ and insensitive to ${\beta }_2$. We demonstrate this by state-of-the-art hydrodynamic calculations and discuss the prospects of discovering the ${\beta }_4$ of ${}^{238}\mathrm{U}$ in heavy-ion data at the Relativistic Heavy Ion Collider.

We demonstrate that astrophysical constraints on the dense-matter equation of state place an upper bound on the color-superconducting gap in dense matter above the transition from nuclear matter to quark matter. Pairing effects in the color-flavor locked quark matter phase increase the pressure at high density, and if this effect is sufficiently large then the requirements of causality and mechanical stability make it impossible to reach such a pressure in a way that is consistent with what is known at lower densities. The intermediate-density equation of state is inferred by considering extensions of chiral effective field theory to neutron star densities, and conditioning these using current astrophysical observations of neutron star radius, maximum mass, and tidal deformability (PSR $\mathrm{J}0348+0432$, PSR J1624-2230, PSR $\mathrm{J}0740+6620$, GW170817). At baryon number chemical potential $\mu =2.6\mathrm{GeV}$ we find a 95% upper limit on the color-flavor locked pairing gap $\mathrm{\Delta }$ of 457 MeV using overly conservative assumptions and 216 MeV with more reasonable assumptions. This constraint may be strengthened by future astrophysical measurements as well as by future advances in high-density QCD calculations.

In ultrarelativistic heavy-ion collisions, a plasma of deconfined quarks and gluons is formed within $1\mathrm{fm}/\mathrm{c}$ of the nuclei’s impact. The complex dynamics of the collision before $\approx 1\mathrm{fm}/\mathrm{c}$ is often described with parametric models, which affect the predictivity of calculations. In this work, we perform a systematic analysis of LHC measurements from Pb-Pb collisions, by combining an ab initio model of the early stage of the collisions with a hydrodynamic model of the plasma. We obtain state-of-the-art constraints on the shear and bulk viscosity of quark-gluon plasma. We mitigate the additional cost of the ab initio initial conditions by combining Bayesian model averaging with transfer learning, allowing us to account for important theoretical uncertainties in the hydrodynamics-to-hadron transition. We show that, despite the apparent strong constraints on the shear viscosity, metrics that balance the model’s predictivity with its degree of agreement with data do not prefer a temperature-dependent specific shear viscosity over a constant value. We validate the model by comparing with discriminating observables not used in the calibration, finding excellent agreement.

Just before a nucleus undergoes fission, a neck is formed between the emerging fission fragments. It is widely accepted that this neck undergoes a rather violent rupture, despite the absence of unambiguous experimental evidence. The main difficulty in addressing the neck rupture and saddle-to-scission stages of fission is that both are highly nonequilibrium processes. Here, we present the first fully microscopic characterization of the scission mechanism, along with the spectrum and the spatial distribution of scission neutrons (SNs), and some upper limit estimates for the emission of charged particles. The spectrum of SNs has a distinct angular distribution, with neutrons emitted in roughly equal numbers in the equatorial plane and along the fission axis. They carry an average energy around $3\pm 0.5\mathrm{MeV}$ for the fission of ${}^{236}\mathrm{U}$, ${}^{240}\mathrm{Pu}$, and ${}^{252}\mathrm{Cf}$, and a maximum of 16–18 MeV. We estimate a conservative lower bound of 9%–14% of the total emitted neutrons are produced at scission.

We investigate the effects of two-body currents on magnetic dipole moments of medium-mass and heavy nuclei using the valence-space in-medium similarity renormalization group with chiral effective field theory interactions and currents. Focusing on near doubly magic nuclei from oxygen to bismuth, we have found that the leading two-body currents globally improve the agreement with experimental magnetic moments. Moreover, our results show the importance of multishell effects for ${}^{41}\mathrm{Ca}$, which suggest that the $Z=N=20$ gap in ${}^{40}\mathrm{Ca}$ is not as robust as in ${}^{48}\mathrm{Ca}$. The increasing contribution of two-body currents in heavier systems is explained by the operator structure of the center-of-mass dependent Sachs term.

The magnetic dipole transition strength $B(M1)$ of ${}^{48}\mathrm{Ca}$ is dominated by a single resonant state at an excitation energy of 10.23 MeV. Experiments disagree about $B(M1)$ and this impacts our understanding of spin flips in nuclei. We performed ab initio computations based on chiral effective field theory and found that $B(M1:0^{+}\to 1^{+})$ lies in the range from 7.0 to $10.2{\mu }_N^2$. This is consistent with a $(\gamma ,n)$ experiment but larger than results from $(e,e^{\prime })$ and $(p,p^{\prime })$ scattering. Two-body currents yield no quenching of the $B(M1)$ strength and continuum effects reduce it by about 10%. For a validation of our approach, we computed magnetic moments in ${}^{47,49}\mathrm{Ca}$ and performed benchmark calculations in light nuclei.

Neutron stars contain neutron-rich matter with around 5% protons at nuclear saturation density. In this Letter, we consider equilibrium between bulk phases of matter based on asymmetric nuclear matter calculations using chiral effective field theory interactions rather than, as has been done in the past, by interpolation between the properties of symmetric nuclear matter and pure neutron matter. Neutron drip (coexistence of nuclear matter with pure neutrons) is well established, but from earlier work it is unclear whether proton drip (equilibrium between two phases, both of which contain protons and neutrons) is possible. We find that proton drip is a robust prediction of any physically reasonable equation of state, but that it occurs over a limited region of densities and proton fractions. An analytical model based on expanding the energy in powers of the proton density, rather than the neutron excess, is able to account for these features of the phase diagram.