In the hydrodynamic framework of heavy-ion collisions, elliptic flow $v_2$ is sensitive to the quadrupole deformation $\beta$ of the colliding ions. This enables one to test whether the established knowledge on the low-energy structure of nuclei is consistent with collider data from high-energy experiments. We derive a formula based on generic scaling laws of hydrodynamics to relate the difference in $v_2$ measured between collision systems that are close in size to the value of $\beta$ of the respective species. We validate our formula in simulations of ${}^{238}\mathrm{U}+{}^{238}\mathrm{U}$ and ${}^{197}\mathrm{Au}+{}^{197}\mathrm{Au}$ collisions at top Relativistic Heavy Ion Collider (RHIC) energy, and subsequently apply it to experimental data. Using the deformation of ${}^{238}\mathrm{U}$ from low-energy experiments, we find that RHIC $v_2$ data implies $0.16\lesssim |\beta |\lesssim 0.20$ for ${}^{197}\mathrm{Au}$ nuclei, i.e., significantly more deformed than reported in the literature, posing an interesting issue in nuclear phenomenology.

The microscopic structure of the low-energy electric dipole response, commonly denoted as pygmy dipole resonance (PDR), was studied for ${}^{120}\mathrm{Sn}$ in a ${}^{119}\mathrm{Sn}(d,p\gamma ){}^{120}\mathrm{Sn}$ experiment. Unprecedented access to the single-particle structure of excited $1^{-}$ states below and around the neutron-separation threshold was obtained by comparing experimental data to predictions from a novel theoretical approach. The novel approach combines detailed structure input from energy-density functional plus quasiparticle-phonon model theory with reaction theory to obtain a consistent description of both the structure and reaction aspects of the process. The presented results show that the understanding of one-particle–one-hole structures of the $1^{-}$ states in the PDR region is crucial to reliably predict properties of the PDR and its contribution to nucleosynthesis processes.

The parity-violating asymmetry $A_{\mathrm{PV}}$ in ${}^{208}\mathrm{Pb}$, recently measured by the PREX-2 Collaboration, is studied using modern relativistic (covariant) and nonrelativistic energy density functionals. We first assess the theoretical uncertainty on $A_{\mathrm{PV}}$ which is intrinsic to the adopted approach. To this end, we use quantified functionals that are able to accommodate our previous knowledge on nuclear observables such as binding energies, charge radii, and the dipole polarizability ${\alpha }_D$ of ${}^{208}\mathrm{Pb}$. We then add the quantified value of $A_{\mathrm{PV}}$ together with ${\alpha }_D$ to our calibration dataset to optimize new functionals. Based on these results, we predict a neutron skin thickness in ${}^{208}\mathrm{Pb}$ $r_{\mathrm{skin}}=0.19\pm 0.02\mathrm{fm}$ and the symmetry-energy slope $L=54\pm 8\mathrm{MeV}$. These values are consistent with other estimates based on astrophysical data and are significantly lower than those recently reported using a particular set of relativistic energy density functionals. We also make a prediction for the $A_{\mathrm{PV}}$ value in ${}^{48}\mathrm{Ca}$ that will be soon available from the CREX measurement.

Mass and angle distributions for the ${}^{52}\mathrm{Cr}+{}^{198}\mathrm{Pt}$ and ${}^{54}\mathrm{Cr}+{}^{196}\mathrm{Pt}$ reactions (both forming ${}^{250}\mathrm{No}$) were measured and subtracted, giving new information on fast quasifission mass evolution, and the first direct determination of the dependence of sticking times on angular momentum. TDHF calculations showed good agreement with average experimental values, but experimental mass distributions unexpectedly extended to symmetric splits while the peak yield remained close to the initial masses. This implies a strong role of fluctuations in mass division early in the collision, giving insights into the transition from fast energy dissipative deep-inelastic collisions to quasifission.

We investigate the angular momentum removal from fission fragments (FFs) through neutron and $\gamma$-ray emission, finding that about half the neutrons are emitted with angular momenta $\ge 1.5\hslash$ and that the change in angular momentum after the emission of neutrons and statistical $\gamma$ rays is significant, contradicting usual assumptions. Per fission event, in our simulations, the neutron and statistical $\gamma$-ray emissions change the spin of the fragment by $3.5-5\hslash$, with a large standard deviation comparable to the average value. Such wide angular momentum removal distributions can hide any underlying correlations in the fission fragment initial spin values. Within our model, we reproduce data on spin measurements from discrete transitions after neutron emissions, especially in the case of light FFs. The agreement further improves for the heavy fragments if one removes from the analysis the events that would produce isomeric states. Finally, we show that while in our model the initial FF spins do not follow a sawtoothlike behavior observed in recent measurements, the average FF spin computed after neutron and statistical $\gamma$ emissions exhibits a shape that resembles a sawtooth. This suggests that the average FF spin measured after statistical emissions is not necessarily connected with the scission mechanism as previously implied.

The prompt production of the charm baryon ${\mathrm{\Lambda }}_c^{+}$ and the ${\mathrm{\Lambda }}_c^{+}/D^0$ production ratios were measured at midrapidity with the ALICE detector in $pp$ and $p$-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. These new measurements show a clear decrease of the ${\mathrm{\Lambda }}_c^{+}/D^0$ ratio with increasing transverse momentum ($p_T$) in both collision systems in the range $2<p_T<12\mathrm{GeV}/c$, exhibiting similarities with the light-flavor baryon-to-meson ratios $p/\pi$ and $\mathrm{\Lambda }/K_S^0$. At low $p_T$, predictions that include additional color-reconnection mechanisms beyond the leading-color approximation, assume the existence of additional higher-mass charm-baryon states, or include hadronization via coalescence can describe the data, while predictions driven by charm-quark fragmentation processes measured in $e^{+}{e}^{-}$ and $e^{-}p$ collisions significantly underestimate the data. The results presented in this Letter provide significant evidence that the established assumption of universality (colliding-system independence) of parton-to-hadron fragmentation is not sufficient to describe charm-baryon production in hadronic collisions at LHC energies.

The changes in the mean-square charge radius (relative to ${}^{209}\mathrm{Bi}$), magnetic dipole, and electric quadrupole moments of ${}^{187,188,189,191}\mathrm{Bi}$ were measured using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). A large staggering in radii was found in ${{}^{187,188,189}\text{Bi}}^g$, manifested by a sharp radius increase for the ground state of ${}^{188}\mathrm{Bi}$ relative to the neighboring ${{}^{187,189}\text{Bi}}^g$. A large isomer shift was also observed for ${{}^{188}\mathrm{Bi}}^m$. Both effects happen at the same neutron number, $N=105$, where the shape staggering and a similar isomer shift were observed in the mercury isotopes. Experimental results are reproduced by mean-field calculations where the ground or isomeric states were identified by the blocked quasiparticle configuration compatible with the observed spin, parity, and magnetic moment.

The symmetry energy and its density dependence are crucial inputs for many nuclear physics and astrophysics applications, as they determine properties ranging from the neutron-skin thickness of nuclei to the crust thickness and the radius of neutron stars. Recently, PREX-II reported a value of $0.283\pm 0.071\mathrm{fm}$ for the neutron-skin thickness of ${}^{208}\mathrm{Pb}$, implying a slope parameter $L=106\pm 37\mathrm{MeV}$, larger than most ranges obtained from microscopic calculations and other nuclear experiments. We use a nonparametric equation of state representation based on Gaussian processes to constrain the symmetry energy $S_0$, $L$, and $R_{\mathrm{skin}}^{{}^{208}\mathrm{Pb}}$ directly from observations of neutron stars with minimal modeling assumptions. The resulting astrophysical constraints from heavy pulsar masses, LIGO/Virgo, and NICER clearly favor smaller values of the neutron skin and $L$, as well as negative symmetry incompressibilities. Combining astrophysical data with PREX-II and chiral effective field theory constraints yields $S_0=33.0_{-1.8}^{+2.0}\mathrm{MeV}$, $L=53_{-15}^{+14}\mathrm{MeV}$, and $R_{\mathrm{skin}}^{{}^{208}\mathrm{Pb}}=0.17_{-0.04}^{+0.04}\mathrm{fm}$.

The nuclear root-mean-square charge radius of ${}^{54}\mathrm{Ni}$ was determined with collinear laser spectroscopy to be $R({}^{54}\mathrm{Ni})=3.737(3)\mathrm{fm}$. In conjunction with the known radius of the mirror nucleus ${}^{54}\mathrm{Fe}$, the difference of the charge radii was extracted as $\mathrm{\Delta }{R}_{\mathrm{ch}}=0.049(4)\mathrm{fm}$. Based on the correlation between $\mathrm{\Delta }{R}_{\mathrm{ch}}$ and the slope of the symmetry energy at nuclear saturation density ($L$), we deduced $21\le L\le 88\mathrm{MeV}$. The present result is consistent with the $L$ from the binary neutron star merger GW170817, favoring a soft neutron matter EOS, and barely consistent with the PREX-2 result within $1\sigma$ error bands. Our result indicates the neutron-skin thickness of ${}^{48}\mathrm{Ca}$ as 0.15–0.21 fm.

We develop for the first time a microscopic global nucleon-nucleus optical potential with quantified uncertainties suitable for analyzing nuclear reaction experiments at next-generation rare-isotope beam facilities. Within the improved local density approximation and without any adjustable parameters, we begin by computing proton-nucleus and neutron-nucleus optical potentials from a set of five nuclear forces from chiral effective field theory for 1800 target nuclei in the mass range $12\le A\le 242$ for energies between $0\mathrm{MeV}<E\lesssim 150\mathrm{MeV}$. We then parameterize a global optical potential for each chiral force that depends smoothly on the projectile energy as well as the target nucleus mass number and isospin asymmetry. Uncertainty bands for elastic scattering observables are generated from a full covariance analysis of the parameters entering in the description of our global optical potential and benchmarked against existing experimental data for stable target nuclei. Since our approach is purely microscopic, we anticipate a similar quality of the model for nucleon scattering on unstable isotopes.

The validity of the Brink-Axel hypothesis, which is especially important for numerous astrophysical calculations, is addressed for ${}^{116,120,124}\mathrm{Sn}$ below the neutron separation energy by means of three independent experimental methods. The $\gamma$-ray strength functions (GSFs) extracted from primary $\gamma$-decay spectra following charged-particle reactions with the Oslo method and with the shape method demonstrate excellent agreement with those deduced from forward-angle inelastic proton scattering at relativistic beam energies. In addition, the GSFs are shown to be independent of excitation energies and spins of the initial and final states. The results provide a critical test of the generalized Brink-Axel hypothesis in heavy nuclei, demonstrating its applicability in the energy region of the pygmy dipole resonance.

The excited-state structure of atomic nuclei can modify nuclear processes in stellar environments. In this Letter, we study the influence of nuclear excitations on Urca cooling (repeated back-and-forth $\beta$ decay and electron capture in a pair of nuclear isotopes) in the crust and ocean of neutron stars. We provide for the first time an expression for Urca process neutrino luminosity which accounts for a thermal Boltzmann distribution of excited states in both members of an Urca pair. We use our new formula with state-of-the-art nuclear structure inputs to compute neutrino luminosities of candidate Urca cooling pairs. Our nuclear inputs consist of the latest experimental data supplemented with calculations using the projected shell model. We show that, in contrast to previous results that only consider the ground states of both nuclei in the pair, our calculated neutrino luminosities for different Urca pairs vary sensitively with the environment temperature and can be radically different from those obtained in the one-transition approximation. We find that nuclear excitations can lead to an enhancement in total Urca neutrino luminosities in the accreted neutron star crust by about 5 times as compared with the previous Urca results, which is expected to cause significant observational effects.

We report the first (in)elastic scattering measurement of ${}^{25}\mathrm{Al}+p$ with the capability to select and measure in a broad energy range the proton resonances in ${}^{26}\mathrm{Si}$ contributing to the ${}^{22}\mathrm{Mg}(\alpha ,p)$ reaction at type I x-ray burst energies. We measured spin-parities of four resonances above the $\alpha$ threshold of ${}^{26}\mathrm{Si}$ that are found to strongly impact the ${}^{22}\mathrm{Mg}(\alpha ,p)$ rate. The new rate advances a state-of-the-art model to remarkably reproduce light curves of the GS $1826–24$ clocked burster with mean deviation $<9\%$ and permits us to discover a strong correlation between the He abundance in the accreting envelope of the photospheric radius expansion burster and the dominance of ${}^{22}\mathrm{Mg}(\alpha ,p)$ branch.

We report measurements of the exclusive neutral pion electroproduction cross section off protons at large values of $x_B$ (0.36, 0.48, and 0.60) and $Q^2$ (3.1 to $8.4{\mathrm{GeV}}^2$) obtained from Jefferson Lab Hall A experiment E12-06-014. The corresponding structure functions $d{\sigma }_T/dt+\epsilon d{\sigma }_L/dt$, $d{\sigma }_{TT}/dt$, $d{\sigma }_{LT}/dt$, and $d{\sigma }_{L{T}^{\prime }}/dt$ are extracted as a function of the proton momentum transfer $t-t_{\min }$. The results suggest the amplitude for transversely polarized virtual photons continues to dominate the cross section throughout this kinematic range. The data are well described by calculations based on transversity generalized parton distributions coupled to a helicity flip distribution amplitude of the pion, thus providing a unique way to probe the structure of the nucleon.

Heavy-ion collisions at small beam energies have the potential to reveal the rich phase structure of QCD at low temperature and nonzero density. In this case spatially modulated regimes with a “moat” spectrum can arise, where the minimum of the energy is over a sphere at nonzero momentum. We show that if the matter created in heavy-ion collisions freezes out in such a regime, particle numbers and their correlations peak at nonzero, instead of zero, momentum. This effect is much more dramatic for multiparticle correlations than for single-particle spectra. Our results can serve as a first guideline for a systematic search of spatially modulated phases in heavy-ion collisions.

One of the main neutron sources for the astrophysical $s$ process is the reaction ${}^{13}\mathrm{C}(\alpha ,n){}^{16}\mathrm{O}$, taking place in thermally pulsing asymptotic giant branch stars at temperatures around 90 MK. To model the nucleosynthesis during this process the reaction cross section needs to be known in the 150–230 keV energy window (Gamow peak). At these sub-Coulomb energies, cross section direct measurements are severely affected by the low event rate, making us rely on input from indirect methods and extrapolations from higher-energy direct data. This leads to an uncertainty in the cross section at the relevant energies too high to reliably constrain the nuclear physics input to $s$-process calculations. We present the results from a new deep-underground measurement of ${}^{13}\mathrm{C}(\alpha ,n){}^{16}\mathrm{O}$, covering the energy range 230–300 keV, with drastically reduced uncertainties over previous measurements and for the first time providing data directly inside the $s$-process Gamow peak. Selected stellar models have been computed to estimate the impact of our revised reaction rate. For stars of nearly solar composition, we find sizeable variations of some isotopes, whose production is influenced by the activation of close-by branching points that are sensitive to the neutron density, in particular, the two radioactive nuclei ${}^{60}\mathrm{Fe}$ and ${}^{205}\mathrm{Pb}$, as well as ${}^{152}\mathrm{Gd}$.