A measurement of dielectron production in proton-proton (pp) collisions at $\sqrt{s}=13\mathrm{TeV}$, recorded with the ALICE detector at the CERN LHC, is presented in this Letter. The data set was recorded with a reduced magnetic solenoid field. This enables the investigation of a kinematic domain at low dielectron (ee) invariant mass $m_{\mathrm{ee}}$ and pair transverse momentum $p_{T,\mathrm{ee}}$ that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of $m_{\mathrm{ee}}$, $p_{T,\mathrm{ee}}$, and event multiplicity $\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\eta$. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured $\eta /{\pi }^0$ ratio in pp and proton-nucleus collisions, assuming that this ratio shows no strong dependence on collision energy at low transverse momentum. Comparison of the measured dielectron yield to the hadronic cocktail at $0.15<m_{\mathrm{ee}}<0.6\mathrm{GeV}/c^2$ and for $p_{T,\mathrm{ee}}<0.4\mathrm{GeV}/c$ indicates an enhancement of soft dielectrons, reminiscent of the “anomalous” soft-photon and soft-dilepton excess in hadron-hadron collisions reported by several experiments under different experimental conditions. The enhancement factor over the hadronic cocktail amounts to $1.61\pm 0.13(\mathrm{stat})\pm 0.17(\text{syst},\text{data})\pm 0.34(\text{syst},\text{cocktail})$ in the ALICE acceptance. Acceptance-corrected excess spectra in $m_{\mathrm{ee}}$ and $p_{T,\mathrm{ee}}$ are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach.

We present a method that may allow an estimate of the value of the speed of sound as well as its logarithmic derivative with respect to the baryon number density in matter created in heavy-ion collisions. To this end, we use well-known observables: cumulants of the baryon number distribution. In analyses aimed at uncovering the phase diagram of strongly interacting matter, cumulants gather considerable attention as their qualitative behavior along the explored range of collision energies is expected to aid in detecting the QCD critical point. We show that the cumulants may also reveal the behavior of the speed of sound in the temperature and baryon chemical potential plane. We demonstrate the applicability of such estimates within two models of nuclear matter and explore what might be understood from known experimental data.

A novel approach for isomer depletion in ion-atom collisions is proposed and considered theoretically. Analyses are performed for the depletion of the ${}^{93m}\mathrm{Mo}$ isomer for which an unexpectedly large probability was measured in the beam-based experiment of Chiara et al. [Nature (London) 554, 216 (2018)]. The subsequent attempt at a theoretical description based on state-of-the-art atomic theory did not reproduce the experimental result [Wu et al., Phys. Rev. Lett. 122, 212501 (2019)] and showed a dramatic disagreement with the experiment (by many orders of magnitude). This conflict calls for further research on the nature of isomer depletion mechanisms occurring in atomic processes. Here, we propose to consider the ${}^{93m}\mathrm{Mo}$ isomer depletion as the nuclear excitation by electron capture in resonant transfer process taking into account the momentum distribution of the target electrons. Although our results only slightly shift the upper theoretical limit for the total ${}^{93m}\mathrm{Mo}$ isomer depletion probability toward the experimental value, they show the importance of considering the Compton profile in the theoretical description, in particular for the $L$ shell, for which the depletion probability increases by many orders of magnitude.

The complexity of many-body quantum wave functions is a central aspect of several fields of physics and chemistry where nonperturbative interactions are prominent. Artificial neural networks (ANNs) have proven to be a flexible tool to approximate quantum many-body states in condensed matter and chemistry problems. In this work we introduce a neural-network quantum state ansatz to model the ground-state wave function of light nuclei, and approximately solve the nuclear many-body Schrödinger equation. Using efficient stochastic sampling and optimization schemes, our approach extends pioneering applications of ANNs in the field, which present exponentially scaling algorithmic complexity. We compute the binding energies and point-nucleon densities of $A\le 4$ nuclei as emerging from a leading-order pionless effective field theory Hamiltonian. We successfully benchmark the ANN wave function against more conventional parametrizations based on two- and three-body Jastrow functions, and virtually exact Green’s function Monte Carlo results.

A microscopic calculation of half-lives for both the $\alpha$ and $2\alpha$ decays of ${}^{212}\mathrm{Po}$ and ${}^{224}\mathrm{Ra}$ is performed, using a self-consistent framework based on energy density functionals. A relativistic density functional and a separable pairing interaction of finite range are used to compute axially symmetric deformation energy surfaces as functions of quadrupole, octupole, and hexadecapole collective coordinates. Dynamical least-action paths are determined, that trace the $\alpha$ and $2\alpha$ emission from the equilibrium deformation to the point of scission. The calculated half-lives for the $\alpha$ decay of ${}^{212}\mathrm{Po}$ and ${}^{224}\mathrm{Ra}$ are in good agreement with data. A new decay mode, the symmetric $2\alpha$ emission, is predicted with half-lives of the order of those observed for cluster emission.

Direct proton-knockout reactions of ${}^{55}\mathrm{Sc}$ at $\sim 220\mathrm{MeV}/\mathrm{nucleon}$ were studied at the RIKEN Radioactive Isotope Beam Factory. Populated states of ${}^{54}\mathrm{Ca}$ were investigated through $\gamma$-ray and invariant-mass spectroscopy. Level energies were calculated from the nuclear shell model employing a phenomenological internucleon interaction. Theoretical cross sections to states were calculated from distorted-wave impulse approximation estimates multiplied by the shell model spectroscopic factors, which describe the wave function overlap of the ${}^{55}\mathrm{Sc}$ ground state with states in ${}^{54}\mathrm{Ca}$. Despite the calculations showing a significant amplitude of excited neutron configurations in the ground-state of ${}^{55}\mathrm{Sc}$, valence proton removals populated predominantly the ground state of ${}^{54}\mathrm{Ca}$. This counterintuitive result is attributed to pairing effects leading to a dominance of the ground-state spectroscopic factor. Owing to the ubiquity of the pairing interaction, this argument should be generally applicable to direct knockout reactions from odd-even to even-even nuclei.

New constraints are found that must necessarily hold for Israel-Stewart-like theories of fluid dynamics to be causal far away from equilibrium. Conditions that are sufficient to ensure causality, local existence, and uniqueness of solutions in these theories are also presented. Our results hold in the full nonlinear regime, taking into account bulk and shear viscosities (at zero chemical potential), without any simplifying symmetry or near-equilibrium assumptions. Our findings provide fundamental constraints on the magnitude of viscous corrections in fluid dynamics far from equilibrium.

The understanding of heavy ion collisions and its quark-gluon plasma (QGP) formation requires a complicated interplay of rich physics in a wealth of experimental data. In this work we compare for identified particles the transverse momentum dependence of both the yields and the anisotropic flow coefficients for both PbPb and $p\mathrm{Pb}$ collisions. We do this in a global model fit including a free streaming prehydrodynamic phase with variable velocity $v_{\mathrm{fs}}$, thereby widening the scope of initial conditions. During the hydrodynamic phase we vary three second order transport coefficients. The free streaming velocity has a preference slightly below the speed of light. In this extended model the QGP bulk viscosity is small and even consistent with zero.

We demonstrate that oxygen-oxygen collisions at the LHC provide unprecedented sensitivity to parton energy loss in a system whose size is comparable to those created in very peripheral heavy-ion collisions. With leading and next-to-leading order calculations of nuclear modification factors, we show that the baseline in the absence of partonic rescattering is known with up to 2% theoretical accuracy in inclusive oxygen-oxygen collisions. Surprisingly, a $Z$-boson normalized nuclear modification factor does not lead to higher theoretical accuracy within current uncertainties of nuclear parton distribution functions. We study a broad range of parton energy loss models and we find that the expected signal of partonic rescattering can be disentangled from the baseline by measuring charged hadron spectra in the range $20\mathrm{GeV}<p_T<100\mathrm{GeV}$.

We present the first triaxial beyond-mean-field study of the excitation spectra of even-even superheavy nuclei. As representative examples, we have chosen the members of the $\alpha$-decay chains of ${}^{292}\mathrm{Lv}$ and ${}^{294}\mathrm{Og}$, the heaviest even-even nuclei which have been synthesized so far using ${}^{48}\mathrm{Ca}$-induced fusion-evaporation reactions. In our calculations, the effective finite-range density-dependent Gogny force is used and the angular-momentum and particle-number symmetries are restored. Configuration-mixing calculations are performed to determine ground- and excited-state deformations and to establish the collective band structures of these nuclei. Rapidly varying characteristics are predicted for the members of both decay chains, which are further accentuated when compared to the predictions of simple collective models. Based on the present calculations, the prospect of observing $\alpha$-decay fine structures in future experiments is discussed.

We use coupled-cluster theory and nuclear interactions from chiral effective field theory to compute the nuclear matrix element for the neutrinoless double-$\beta$ decay of ${}^{48}\mathrm{Ca}$. Benchmarks with the no-core shell model in several light nuclei inform us about the accuracy of our approach. For ${}^{48}\mathrm{Ca}$ we find a relatively small matrix element. We also compute the nuclear matrix element for the two-neutrino double-$\beta$ decay of ${}^{48}\mathrm{Ca}$ with a quenching factor deduced from two-body currents in recent ab initio calculation of the Ikeda sum rule in ${}^{48}\mathrm{Ca}$ [Gysbers et al., Nat. Phys. 15, 428 (2019)].

In two recent papers by Pore et al. and Khuyagbaatar et al., discovery of the new isotope ${}^{244}\mathrm{Md}$ was reported. The decay data, however, are conflicting. While Pore et al. report two isomeric states decaying by $\alpha$ emission with $E_{\alpha }(1)=8.66(2)\mathrm{MeV}$, $T_{1/2}(1)={0.4}_{-0.1}^{+0.4}\mathrm{s}$ and $E_{\alpha }(2)=8.31(2)\mathrm{MeV}$, $T_{1/2}(2)\approx 6\mathrm{s}$, Khuyagbaatar et al. [Phys. Rev. Lett. 125, 142504 (2020).] report only a single transition with a broad energy distribution of $E_{\alpha }=(8.73–8.86)\mathrm{MeV}$ and $T_{1/2}={0.30}_{-0.09}^{+0.19}\mathrm{s}$. The data published in Pore et al. are very similar to those published for ${}^{245m}\mathrm{Md}$ [$E_{\alpha }=8.64(2)$, 8.68(2) MeV, $T_{1/2}={0.35}_{-0.16}^{+0.23}\mathrm{s}$ [V. Ninov, F. P. Heßberger, S. Hofmann, H. Folger, G. Münzenberg, P. Armbruster, A. V. Yeremin, A. G. Popeko, M. Leino, and S. Saro, Z. Phys. A 356, 11 (1996).] ]. Therefore, we compare the data presented for ${}^{244}\mathrm{Md}$ in Pore et al. with those reported for ${}^{245}\mathrm{Md}$ in Ninov et al. and also in Khuyagbaatar et al. We conclude that the data presented in Pore et al. shall be attributed to ${}^{245}\mathrm{Md}$ with small contributions (one event each) from ${}^{245}\mathrm{Fm}$ and probably ${}^{246}\mathrm{Md}$.

In this Letter, we present a new expression for the overlaps of wave functions in Hartree-Fock-Bogoliubov based theories. Starting from the Pfaffian formula by Bertsch et al. [1], an exact and computationally stable formula for overlaps is derived. We illustrate the convenience of this new formulation with a numerical application in the context of the particle-number projection method. This new formula allows for substantially increased precision and versatility in chemical, atomic, and nuclear physics applications, particularly for methods dealing with superfluidity, symmetry restoration, and uses of nonorthogonal many-body basis states.

Global polarization of $\mathrm{\Xi }$ and $\mathrm{\Omega }$ hyperons has been measured for the first time in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The measurements of the ${\mathrm{\Xi }}^{-}$ and ${\overline{\mathrm{\Xi }}}^{+}$ hyperon polarization have been performed by two independent methods, via analysis of the angular distribution of the daughter particles in the parity violating weak decay $\mathrm{\Xi }\to \mathrm{\Lambda }+\pi$, as well as by measuring the polarization of the daughter $\mathrm{\Lambda }$ hyperon, polarized via polarization transfer from its parent. The polarization, obtained by combining the results from the two methods and averaged over ${\mathrm{\Xi }}^{-}$ and ${\overline{\mathrm{\Xi }}}^{+}$, is measured to be $⟨P_{\mathrm{\Xi }}⟩=0.47\pm 0.10(\mathrm{stat})\pm 0.23(\mathrm{syst})\%$ for the collision centrality 20%–80%. The $⟨P_{\mathrm{\Xi }}⟩$ is found to be slightly larger than the inclusive $\mathrm{\Lambda }$ polarization and in reasonable agreement with a multiphase transport model. The $⟨P_{\mathrm{\Xi }}⟩$ is found to follow the centrality dependence of the vorticity predicted in the model, increasing toward more peripheral collisions. The global polarization of $\mathrm{\Omega }$, $⟨P_{\mathrm{\Omega }}⟩=1.11\pm 0.87(\mathrm{stat})\pm 1.97(\mathrm{syst})\%$ was obtained by measuring the polarization of daughter $\mathrm{\Lambda }$ in the decay $\mathrm{\Omega }\to \mathrm{\Lambda }+K$, assuming the polarization transfer factor $C_{\mathrm{\Omega }\mathrm{\Lambda }}=1$.

We investigate the possibility that the low mass companion of the black hole in the source of GW190814 was a strange quark star. This possibility is viable within the so-called two-families scenario in which neutron stars and strange quark stars coexist. Strange quark stars can reach the mass range indicated by GW190814, $M\sim (2.5–2.67)M_{\odot }$ due to a large value of the adiabatic index, without the need for a velocity of sound close to the causal limit. Neutron stars (actually hyperonic stars in the two-families scenario) can instead fulfill the presently available astrophysical and nuclear physics constraints which require a softer equation of state. In this scheme it is possible to satisfy both the request of very large stellar masses and of small radii while using totally realistic and physically motivated equations of state. Moreover it is possible to get a radius for a $1.4M_{\odot }$ star of the order or less than 11 km, which is impossible if only one family of compact stars exists.

We report the first measurement of the average of the electron-proton and positron-proton elastic scattering cross sections. This lepton charge-averaged cross section is insensitive to the leading effects of hard two-photon exchange, giving more robust access to the proton’s electromagnetic form factors. The cross section was extracted from data taken by the OLYMPUS experiment at DESY, in which alternating stored electron and positron beams were scattered from a windowless gaseous hydrogen target. Elastic scattering events were identified from the coincident detection of the scattered lepton and recoil proton in a large-acceptance toroidal spectrometer. The luminosity was determined from the rates of Møller, Bhabha, and elastic scattering in forward electromagnetic calorimeters. The data provide some selectivity between existing form factor global fits and will provide valuable constraints to future fits.

Many neutron star properties, such as the proton fraction, reflect the symmetry energy contributions to the equation of state that dominate when neutron and proton densities differ strongly. To constrain these contributions at suprasaturation densities, we measure the spectra of charged pions produced by colliding rare isotope tin (Sn) beams with isotopically enriched Sn targets. Using ratios of the charged pion spectra measured at high transverse momenta, we deduce the slope of the symmetry energy to be $42<L<117\mathrm{MeV}$. This value is slightly lower but consistent with the $L$ values deduced from a recent measurement of the neutron skin thickness of ${}^{208}\mathrm{Pb}$.