We present the first ab initio lattice calculations of spin and density correlations in hot neutron matter using high-fidelity interactions at next-to-next-to-next-to-leading order in chiral effective field theory. These correlations have a large impact on neutrino heating and shock revival in core-collapse supernovae and are encapsulated in functions called structure factors. Unfortunately, calculations of structure factors using high-fidelity chiral interactions were well out of reach using existing computational methods. In this Letter, we solve the problem using a computational approach called the rank-one operator (RO) method. The RO method is a general technique with broad applications to simulations of fermionic many-body systems. It solves the problem of exponential scaling of computational effort when using perturbation theory for higher-body operators and higher-order corrections. Using the RO method, we compute the vector and axial static structure factors for hot neutron matter as a function of temperature and density. The ab initio lattice results are in good agreement with virial expansion calculations at low densities but are more reliable at higher densities. Random phase approximation codes used to estimate neutrino opacity in core-collapse supernovae simulations can now be calibrated with ab initio lattice calculations.

We propose the angular distribution of lepton pairs produced in ultrarelativistic heavy-ion collisions as a probe of thermalization of the quark-gluon plasma. We focus on dileptons with invariant masses large enough that they are produced through quark–antiquark annihilation in the early stages of the collision. The angular distribution of the lepton in the rest frame of the pair then reflects the angular distribution of quark momenta. At early times, the transverse pressure of the quark-gluon plasma is larger than its longitudinal pressure as a result of the fast longitudinal expansion, which results in an oblate lepton distribution. By contrast, direct (Drell-Yan) production by quarks and antiquarks from incoming nuclei, whose momenta are essentially longitudinal, results in a prolate distribution. As the invariant mass increases, Drell-Yan gradually becomes the dominant source of dilepton production, and the lepton distribution evolves from oblate to prolate. The invariant mass at which the transition occurs is highly sensitive to the equilibration time of the quark-gluon plasma or, equivalently, the shear viscosity over entropy ratio $\eta /s$ in the early stages of the collision.

The recently observed abnormal bifurcation of the double binding energy differences $\delta V_{pn}$ between the odd-odd and even-even nuclei along the $N=Z$ line from Ni to Rb has challenged the nuclear theories. To solve this problem, a shell-model-like approach based on the relativistic density functional theory is established, by treating simultaneously the neutron-neutron, proton-neutron, and proton-proton pairing correlations both microscopically and self-consistently. Without any ad hoc parameters, the calculated results well reproduce the observations, and the mechanism for this abnormal bifurcation is found to be due to the enhanced proton-neutron pairing correlations in the odd-odd $N=Z$ nuclei, compared with the even-even ones. The present results provide an excellent interpretation for the abnormal $\delta V_{pn}$ bifurcation, and provide a clear signal for the existence of the proton-neutron pairing correlations for nuclei close to the $N=Z$ line.

A general organizing principle is proposed that can be used to derive the equations of motion describing the near-equilibrium dynamics of causal and thermodynamically stable relativistic systems. The latter are found to display some new type of universal behavior near equilibrium that allows them to be grouped into universality classes defined by their degrees of freedom, information content, and conservation laws. The universality classes expose a number of surprising equivalences between different theories, shedding new light on the near-equilibrium behavior of relativistic systems.

$K^{+}{K}^{-}$ pairs may be produced in photonuclear collisions, either from the decays of photoproduced $\varphi (1020)$ mesons or directly as nonresonant $K^{+}{K}^{-}$ pairs. Measurements of $K^{+}{K}^{-}$ photoproduction probe the couplings between the $\varphi (1020)$ and charged kaons with photons and nuclear targets. The kaon-proton scattering occurs at energies far above those available elsewhere. We present the first measurement of coherent photoproduction of $K^{+}{K}^{-}$ pairs on lead ions in ultraperipheral collisions using the ALICE detector, including the first investigation of direct $K^{+}{K}^{-}$ production. There is significant $K^{+}{K}^{-}$ production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range $1.1<M_{KK}<1.4\mathrm{GeV}/c^2$ above the $\varphi (1020)$ resonance, for rapidity $|y_{KK}|<0.8$ and $p_{T,KK}<0.1\mathrm{GeV}/c$, the measured coherent photoproduction cross section is $\mathrm{d}\sigma /\mathrm{d}y=3.37\pm 0.61(\text{stat})\pm 0.15(\mathrm{syst})\mathrm{mb}$. The center-of-mass energy per nucleon of the photon-nucleus (Pb) system $W_{\gamma \mathrm{Pb},n}$ ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for $\varphi (1020)$ photoproduction alone. The mass spectrum is fit to a cocktail consisting of $\varphi (1020)$ decays, direct $K^{+}{K}^{-}$ photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct $K^{+}{K}^{-}$ photoproduction are presented.

We perform classical-statistical real-time lattice simulations to compute real-time spectral functions and momentum broadening of quarks in the presence of strongly populated non-Abelian gauge fields. Based on a novel methodology to extract the momentum broadening for relativistic quarks, we find that the momentum distribution of quarks exhibit interesting nonperturbative features as a function of time due to correlated momentum kicks it receives from the medium, eventually going over to a diffusive regime. We extract the momentum diffusion coefficient for a mass range describing charm and bottom quarks and find sizable discrepancies from the heavy-quark limit.

We show that the same QCD formalism that accounts for the suppression of high-$p_T$ hadron and jet spectra in heavy-ion collisions predicts medium-enhanced production of $c\overline{c}$ pairs in jets. We demonstrate that this phenomenon, which cannot be accessed by traditional jet-quenching observables, can be directly observed using $D^0{\overline{D}}^0$-tagged jets in nuclear collisions.

Angular correlations between heavy quarks provide a unique probe of the quark-gluon plasma created in ultrarelativistic heavy-ion collisions. Results are presented of a measurement of the azimuthal angle correlations between muons originating from semileptonic decays of heavy quarks produced in 5.02 TeV $\mathrm{Pb}+\mathrm{Pb}$ and $pp$ collisions at the LHC. The muons are measured with transverse momenta and pseudorapidities satisfying $p_{\mathrm{T}}^{\mu }>4\mathrm{GeV}$ and $|{\eta }^{\mu }|<2.4$, respectively. The distributions of azimuthal angle separation $\mathrm{\Delta }\varphi$ for muon pairs having pseudorapidity separation $|\mathrm{\Delta }\eta |>0.8$, are measured in different $\mathrm{Pb}+\mathrm{Pb}$ centrality intervals and compared to the same distribution measured in $pp$ collisions at the same center-of-mass energy. Results are presented separately for muon pairs with opposite-sign charges, same-sign charges, and all pairs. A clear peak is observed in all $\mathrm{\Delta }\varphi$ distributions at $\mathrm{\Delta }\varphi \sim \pi$, consistent with the parent heavy-quark pairs being produced via hard-scattering processes. The widths of that peak, characterized using Cauchy-Lorentz fits to the $\mathrm{\Delta }\varphi$ distributions, are found to not vary significantly as a function of $\mathrm{Pb}+\mathrm{Pb}$ collision centrality and are similar for $pp$ and $\mathrm{Pb}+\mathrm{Pb}$ collisions. This observation will provide important constraints on theoretical descriptions of heavy-quark interactions with the quark-gluon plasma.

The efficiency of the weak $s$ process in low-metallicity rotating massive stars depends strongly on the rates of the competing ${}^{17}\mathrm{O}(\alpha ,n){}^{20}\mathrm{Ne}$ and ${}^{17}\mathrm{O}(\alpha ,\gamma ){}^{21}\mathrm{Ne}$ reactions that determine the potency of the ${}^{16}\mathrm{O}$ neutron poison. Their reaction rates are poorly known in the astrophysical energy range of interest for core helium burning in massive stars because of the lack of spectroscopic information (partial widths, spin parities) for the relevant states in the compound nucleus ${}^{21}\mathrm{Ne}$. In this Letter, we report on the first experimental determination of the $\alpha$-particle spectroscopic factors and partial widths of these states using the ${}^{17}\mathrm{O}({}^7\mathrm{Li},t){}^{21}\mathrm{Ne}$ $\alpha$-transfer reaction. With these the ${}^{17}\mathrm{O}(\alpha ,n){}^{20}\mathrm{Ne}$ and ${}^{17}\mathrm{O}(\alpha ,\gamma ){}^{21}\mathrm{Ne}$ reaction rates were evaluated with uncertainties reduced by a factor more than 3 with respect to previous evaluations and the present ${}^{17}\mathrm{O}(\alpha ,n){}^{20}\mathrm{Ne}$ reaction rate is more than 20 times larger. The present $(\alpha ,n)/(\alpha ,\gamma )$ rate ratio favors neutron recycling and suggests an enhancement of the weak $s$ process in the Zr-Nd region by more than 1.5 dex in metal-poor rotating massive stars.

The observation of neutrinoless double-beta ($0\nu \beta \beta$) decay would offer proof of lepton number violation, demonstrating that neutrinos are Majorana particles, while also helping us understand why there is more matter than antimatter in the Universe. If the decay is driven by the exchange of the three known light neutrinos, a discovery would, in addition, link the observed decay rate to the neutrino mass scale through a theoretical quantity known as the nuclear matrix element (NME). Accurate values of the NMEs for all nuclei considered for use in $0\nu \beta \beta$ experiments are therefore crucial for designing and interpreting those experiments. Here, we report the first comprehensive ab initio uncertainty quantification of the $0\nu \beta \beta$-decay NME, in the key nucleus ${}^{76}\mathrm{Ge}$. Our method employs nuclear strong and weak interactions derived within chiral effective field theory and recently developed many-body emulators. Our result, with a conservative treatment of uncertainty, is an NME of ${2.60}_{-1.36}^{+1.28}$, which, together with the best-existing half-life sensitivity and phase-space factor, sets an upper limit for effective neutrino mass of ${187}_{-62}^{+205}\mathrm{meV}$. The result is important for designing next-generation germanium detectors aiming to cover the entire inverted hierarchy region of neutrino masses.

This Letter presents the measurement of near-side associated per-trigger yields, denoted ridge yields, from the analysis of angular correlations of charged hadrons in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$. Long-range ridge yields are extracted for pairs of charged particles with a pseudorapidity difference of $1.4<|\mathrm{\Delta }\eta |<1.8$ and a transverse momentum of $1<p_{\mathrm{T}}<2\mathrm{GeV}/c$, as a function of the charged-particle multiplicity measured at midrapidity. This Letter extends the measurements of the ridge yield to the low multiplicity region, where in hadronic collisions it is typically conjectured that a strongly interacting medium is unlikely to be formed. The precision of the new low multiplicity results allows for the first direct quantitative comparison with the results obtained in ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ collisions at $\sqrt{s}=91\mathrm{GeV}$ and $\sqrt{s}=183–209\mathrm{GeV}$, where initial-state effects such as preequilibrium dynamics and collision geometry are not expected to play a role. In the multiplicity range $8\lesssim ⟨N_{\mathrm{ch}}⟩\lesssim 24$ where the ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ results have good precision, the measured ridge yields in $pp$ collisions are substantially larger than the limits set in ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ annihilations. Consequently, the findings presented in this Letter suggest that the processes involved in ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ annihilations do not contribute significantly to the emergence of long-range correlations in $pp$ collisions.

Dileptons produced during heavy-ion collisions represent a unique probe of the QCD phase diagram, and convey information about the state of the strongly interacting system at the moment their preceding off-shell photon is created. In this study, we compute thermal dilepton yields from $\mathrm{Au}+\mathrm{Au}$ collisions performed at different beam energies, employing a ($3+1$)-dimensional dynamic framework combined with emission rates accurate at next-to-leading order in perturbation theory and which include baryon chemical potential dependencies. By comparing the effective temperature extracted from the thermal dilepton invariant mass spectrum with the average temperature of the fluid, we offer a robust quantitative validation of dileptons as an effective probe of the early quark-gluon plasma stage.

We show that linear superpositions of plane waves involving a single-valued, covariantly stable dispersion relation $\omega (k)$ always propagate outside the light cone unless $\omega (k)=a+bk$. This implies that there is no notion of causality for individual dispersion relations since no mathematical condition on the function $\omega (k)$ (such as the front velocity or the asymptotic group velocity conditions) can serve as a sufficient condition for subluminal propagation in dispersive media. Instead, causality can only emerge from a careful cancellation that occurs when one superimposes all the excitation branches of a physical model. This happens automatically in local theories of matter that are covariantly stable. Hence, we find that the need for nonhydrodynamic modes in relativistic fluid mechanics is analogous to the need for antiparticles in relativistic quantum mechanics.

The electromagnetic form factors of the proton and neutron encode information on the spatial structure of their charge and magnetization distributions. While measurements of the proton are relatively straightforward, the lack of a free neutron target makes measurements of the neutron’s electromagnetic structure more challenging and more sensitive to experimental or model-dependent uncertainties. Various experiments have attempted to extract the neutron form factors from scattering from the neutron in deuterium, with different techniques providing different, and sometimes large, systematic uncertainties. We present results from a novel measurement of the neutron magnetic form factor using quasielastic scattering from the mirror nuclei ${}^3\mathrm{H}$ and ${}^3\mathrm{He}$, where the nuclear effects are larger than for deuterium but expected to largely cancel in the cross-section ratios. We extracted values of the neutron magnetic form factor for low-to-modest momentum transfer, $0.6<Q^2<2.9{\mathrm{GeV}}^2$, where existing measurements give inconsistent results. The precision and $Q^2$ range of these data allow for a better understanding of the current world’s data and suggest a path toward further improvement of our overall understanding of the neutron’s magnetic form factor.

The nuclear charge radius of ${}^{32}\mathrm{Si}$ was determined using collinear laser spectroscopy. The experimental result was confronted with ab initio nuclear lattice effective field theory, valence-space in-medium similarity renormalization group, and mean field calculations, highlighting important achievements and challenges of modern many-body methods. The charge radius of ${}^{32}\mathrm{Si}$ completes the radii of the mirror pair ${}^{32}\mathrm{Ar}\text{−}{}^{32}\mathrm{Si}$, whose difference was correlated to the slope $L$ of the symmetry energy in the nuclear equation of state. Our result suggests $L\le 60\mathrm{MeV}$, which agrees with complementary observables.