In the standard model of particle physics, the weak interaction is described by vector and axial-vector couplings only. Nonzero scalar or tensor interactions would imply an additional contribution to the differential decay rate of the neutron, the Fierz interference term. We derive a limit on this hypothetical term from a measurement using spin-polarized neutrons. This method is statistically less sensitive than the determination from the spectral shape but features much cleaner systematics. We obtain a limit of $b=0.017(21)$ at 68.27% C.L., improving the previous best limit from neutron decay by a factor of four.

A beam-normal single-spin asymmetry generated in the scattering of transversely polarized electrons from unpolarized nucleons is an observable related to the imaginary part of the two-photon exchange process. We report a 2% precision measurement of the beam-normal single-spin asymmetry in elastic electron-proton scattering with a mean scattering angle of ${\theta }_{\mathrm{lab}}=7.9°$ and a mean energy of 1.149 GeV. The asymmetry result is $B_n=-5.194\pm 0.067(\mathrm{stat})\pm 0.082$ (syst) ppm. This is the most precise measurement of this quantity available to date and therefore provides a stringent test of two-photon exchange models at far-forward scattering angles (${\theta }_{\mathrm{lab}}\to 0$) where they should be most reliable.

A recently modified method to enable low-energy nuclear scattering results to be extracted from the discrete energy levels of the target-projectile clusters confined by harmonic potential traps is tested. We report encouraging results for neutron-$\alpha$ and neutron-${}^{24}\mathrm{O}$ elastic scattering from analyzing the trapped levels computed using two different ab initio nuclear structure methods. The $n\text{−}\alpha$ results have also been checked against a direct ab initio reaction calculation. The $n\text{−}{}^{24}\mathrm{O}$ results demonstrate the approach’s applicability for a large range of systems provided their spectra in traps can be computed by ab initio methods. A key ingredient is a rigorous understanding of the errors in the calculated energy levels caused by inevitable Hilbert-space truncations in the ab initio methods.

Nuclear fission plays an important role in fundamental and applied science, from astrophysics to nuclear engineering, yet it remains a major challenge to nuclear theory. Theoretical methods used so far to compute fission observables rely on symmetry-breaking schemes where basic information on the number of particles, angular momentum, and parity of the fissioning nucleus is lost. In this Letter, we analyze the impact of restoring broken symmetries in the benchmark case of ${}^{240}\mathrm{Pu}$.

Ab initio nuclear theory provides not only a microscopic framework for quantitative description of the nuclear many-body system, but also a foundation for deeper understanding of emergent collective correlations. A symplectic $\mathrm{Sp}(3,\mathbb{R})\supset \mathrm{U}(3)$ dynamical symmetry is identified in ab initio predictions, from a no-core configuration interaction approach, and found to provide a qualitative understanding of the spectrum of ${}^7\mathrm{Be}$. Low-lying states form an Elliott SU(3) spectrum, while an $\mathrm{Sp}(3,\mathbb{R})$ excitation gives rise to an excited rotational band with strong quadrupole connections to the ground state band.

The low-spin structure of the semimagic ${}^{64}\mathrm{Ni}$ nucleus has been considerably expanded: combining four experiments, several $0^{+}$ and $2^{+}$ excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate $0^{+}$ excitation is located at a surprisingly high energy (3463 keV), with a collective $2^{+}$ state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron $N=40$ subshell gap highlights the impact of the monopole interaction and its variation in strength with $N$.

New experimental data on the neutron single-particle character of the Pygmy Dipole Resonance (PDR) in ${}^{208}\mathrm{Pb}$ are presented. They were obtained from $(d,p)$ and resonant proton scattering experiments performed at the Q3D spectrograph of the Maier-Leibnitz Laboratory in Garching, Germany. The new data are compared to the large suite of complementary, experimental data available for ${}^{208}\mathrm{Pb}$ and establish $(d,p)$ as an additional, valuable, experimental probe to study the PDR and its collectivity. Besides the single-particle character of the states, different features of the strength distributions are discussed and compared to large-scale shell model (LSSM) and energy-density functional plus quasiparticle-phonon model theoretical approaches to elucidate the microscopic structure of the PDR in ${}^{208}\mathrm{Pb}$.

We present the first systematic nonlocal dispersive optical model analysis using both bound-state and scattering data of ${}^{16,18}\mathrm{O}$, ${}^{40,48}\mathrm{Ca}$, ${}^{58,64}\mathrm{Ni}$, ${}^{112,124}\mathrm{Sn}$, and ${}^{208}\mathrm{Pb}$. In all systems, roughly half the total nuclear binding energy is associated with the most-bound 10% of the total nucleon density. The extracted neutron skins reveal the interplay of asymmetry, Coulomb, and shell effects on the skin thickness. Our results indicate that simultaneous optical model fits of inelastic scattering and structural data on isotopic pairs are effective for constraining asymmetry-dependent nuclear structural quantities otherwise difficult to observe experimentally.

The size of a $\mathrm{\Delta }K=0$ $M1$ excitation strength has been determined for the first time in a predominantly axially deformed even-even nucleus. It has been obtained from the observation of a rare $K$-mixing situation between two close-lying $J^{\pi }=1^{+}$ states of the nucleus ${}^{164}\mathrm{Dy}$ with components characterized by intrinsic projection quantum numbers $K=0$ and $K=1$. Nuclear resonance fluorescence induced by quasimonochromatic linearly polarized $\gamma$-ray beams provided evidence for $K$ mixing of the $1^{+}$ states at 3159.1(3) and 3173.6(3) keV in excitation energy from their $\gamma$-decay branching ratios into the ground-state band. The $\mathrm{\Delta }K=0$ transition strength of $B(M1;0_1^{+}\to 1_{K=0}^{+})=0.008(1){\mu }_N^2$ was inferred from a mixing analysis of their $M1$ transition rates into the ground-state band. It is in agreement with predictions from the quasiparticle phonon nuclear model. This determination represents first experimental information on the $M1$ excitation strength of a nuclear quantum state with a negative $\mathcal{R}$-symmetry quantum number.

We investigate the role of partonic degrees of freedom in high-multiplicity $p\text{−}\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ carried out at the Large Hadron Collider (LHC) by studying the production and collective flow of identified hadrons at intermediate $p_T$ via the coalescence of soft partons from the viscous hydrodynamics ($\mathrm{VISH}2+1$) and hard partons from the energy loss model, linear Boltzmann transport (LBT). We find that combining these intermediate $p_T$ hadrons with the low $p_T$ hadrons from the hydrodynamically expanding fluid and high $p_T$ hadrons from the fragmentation of quenched jets, the resulting hydro-dynamics-coalescence-fragmentation model provides a nice description of measured $p_T$ spectra and differential elliptic flow $v_2(p_T)$ of pions, kaons, and protons over the $p_T$ range from 0 to 6 GeV. We further demonstrate the necessity of including the quark coalescence contribution to reproduce the experimentally observed approximate number of constituent quark scaling of hadron $v_2$ at intermediate $p_T$. Our results thus indicate the importance of partonic degrees of freedom and also hint at the possible formation of quark-gluon plasma in high-multiplicity $p\text{−}\mathrm{Pb}$ collisions at the LHC.

Precise antineutrino measurements are very sensitive to proper background characterization. We present an improved measurement of the ${}^{13}\mathrm{C}(\alpha ,n){}^{16}\mathrm{O}$ reaction cross section which constitutes significant background for large $\overline{\nu }$ detectors. We greatly improve the precision and accuracy by utilizing a setup that is sensitive to the neutron energies while making measurements of the excited state transitions via secondary $\gamma$-ray detection. Our results shows a 54% reduction in the background contributions from the ${}^{16}\mathrm{O}(3^{-},6.13\mathrm{MeV})$ state used in the KamLAND analysis.

Using a covariant and angular-momentum-conserved chiral transport model, which takes into account the spin-orbit interactions of chiral fermions in their scatterings via the side jumps, we study the quark spin polarization in quark matter. For a system of rotating and unpolarized massless quarks in an expanding box, we find that side jumps can dynamically polarize the quark spin and result in a final quark spin polarization consistent with that of thermally equilibrated massless quarks in a self-consistent vorticity field. For the quark matter produced in noncentral relativistic heavy ion collisions, we find that in the medium rest frame both the quark local spin polarizations in the direction perpendicular to the reaction plane and along the longitudinal beam direction show an azimuthal angle dependence in the transverse plane similar to those observed in experiments for the Lambda hyperon.

The low energy systems of three or four neutrons are treated within the adiabatic hyperspherical framework, yielding an understanding of the low energy quantum states in terms of an adiabatic potential energy curve. The dominant low energy potential curve for each system, computed here using widely accepted nucleon-nucleon interactions with and without the inclusion of a three-nucleon force, shows no sign of a low energy resonance. However, both systems exhibit a low energy enhancement of the density of states, or of the Wigner–Smith time delay, which derives from long-range universal physics analogous to the Efimov effect. That enhancement could be relevant to understanding the low energy excess of correlated four-neutron ejection events observed experimentally in a nuclear reaction by Kisamori et al. [Phys. Rev. Lett. 116, 052501 (2016)].

We derive the nucleon-nucleon isoscalar spin-orbit potential from the Skyrme model and find good agreement with the Paris potential. This solves a problem that has been open for more than 30 years and gives a new geometric understanding of the spin-orbit force. Our calculation is based on the dipole approximation to skyrmion dynamics and higher order perturbation theory.

We single out the role of fully coherent induced gluon radiation on light hadron production in $p$-A collisions. The effect has an interesting color structure, as the induced radiation depends on the global color charge of the partonic subprocess final state. Baseline predictions for light hadron nuclear suppression in $p$-Pb collisions at the LHC are provided, taking into account only the effect of fully coherent energy loss, which proves of the same order of magnitude as gluon shadowing or saturation. This underlines the need to include fully coherent energy loss in phenomenological studies of hadron production in $p$-A collisions.

We report a high-statistics measurement of the $L/K$ orbital electron capture ratio in ${}^7\mathrm{Be}$ embedded in cryogenic Ta. The thin Ta film formed part of a high-resolution superconducting tunnel junction radiation detector that was used to identify the signals from different decay channels. The measured $L/K$ capture ratio of 0.070(7) is significantly larger than the only previous measurement of this quantity and the theoretical predictions that include in-medium effects. This value is a uniquely sensitive probe of the 1s and 2s orbital overlaps with the nucleus and is of relevance to nuclear and atomic physics, as well as Li production in novae and other astrophysical scenarios. This is the first experiment that uses superconducting tunnel junctions for nuclear-recoil detection, opening a new experimental avenue for low-energy precision measurements with rare isotopes.

When Th nuclei are doped in ${\mathrm{CaF}}_2$ crystals, a set of electronic defect states appear in the crystal band gap which would otherwise provide complete transparency to vacuum-ultraviolet radiation. The coupling of these defect states to the 8 eV ${}^{229m}\mathrm{Th}$ nuclear isomer in the ${\mathrm{CaF}}_2$ crystal is investigated theoretically. We show that although previously viewed as a nuisance, the defect states provide a starting point for nuclear excitation via electronic bridge mechanisms involving stimulated emission or absorption using an optical laser. The rates of these processes are at least 2 orders of magnitude larger than direct photoexcitation of the isomeric state using available light sources. The nuclear isomer population can also undergo quenching when triggered by the reverse mechanism, leading to a fast and controlled decay via the electronic shell. These findings are relevant for a possible solid-state nuclear clock based on the ${}^{229m}\mathrm{Th}$ isomeric transition.

A new, very short-lived neutron-deficient isotope ${}^{222}\mathrm{Np}$ was produced in the complete-fusion reaction ${}^{187}\mathrm{Re}({}^{40}\mathrm{Ar},5\mathrm{n}){}^{222}\mathrm{Np}$, and observed at the gas-filled recoil separator SHANS. The new isotope ${}^{222}\mathrm{Np}$ was identified by employing a recoil-$\alpha$ correlation measurement, and six $\alpha$-decay chains were established for it. The decay properties of ${}^{222}\mathrm{Np}$ with $E_{\alpha }=10016(33)\mathrm{keV}$ and $T_{1/2}=380_{-110}^{+260}\mathrm{ns}$ were determined experimentally. The $\alpha$-decay systematics of Np isotopes is improved by adding the new data for ${}^{222}\mathrm{Np}$, which validates the $N=126$ shell effect in Np isotopes. The evolution of the $N=126$ shell closure is discussed in the neutron-deficient nuclei up to Np within the framework of $\alpha$-decay reduced width.

The first measurement at the LHC of charge-dependent directed flow ($v_1$) relative to the spectator plane is presented for Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. Results are reported for charged hadrons and ${\mathrm{D}}^0$ mesons for the transverse momentum intervals $p_{\mathrm{T}}>0.2\mathrm{GeV}/c$ and $3<p_{\mathrm{T}}<6\mathrm{GeV}/c$ in the 5%–40% and 10%–40% centrality classes, respectively. The difference between the positively and negatively charged hadron $v_1$ has a positive slope as a function of pseudorapidity $\eta$, $\mathrm{d}\mathrm{\Delta }{v}_1/\mathrm{d}\eta =[1.68\pm 0.49(\mathrm{stat})\pm 0.41(\mathrm{syst})]\times {10}^{-4}$. The same measurement for ${\mathrm{D}}^0$ and ${\overline{\mathrm{D}}}^0$ mesons yields a positive value $\mathrm{d}\mathrm{\Delta }{v}_1/\mathrm{d}\eta =[4.9\pm 1.7(\mathrm{stat})\pm 0.6(\mathrm{syst})]\times {10}^{-1}$, which is about 3 orders of magnitude larger than the one of the charged hadrons. These measurements can provide new insights into the effects of the strong electromagnetic field and the initial tilt of matter created in noncentral heavy ion collisions on the dynamics of light (u, d, and s) and heavy (c) quarks. The large difference between the observed $\mathrm{\Delta }{v}_1$ of charged hadrons and ${\mathrm{D}}^0$ mesons may reflect different sensitivity of the charm and light quarks to the early time dynamics of a heavy ion collision. These observations challenge some recent theoretical calculations, which predicted a negative and an order of magnitude smaller value of $\mathrm{d}\mathrm{\Delta }{v}_1/\mathrm{d}\eta$ for both light flavor and charmed hadrons.

The modification of the ground state properties of light atomic nuclei in the nuclear and stellar medium is addressed, using chemical equilibrium constants evaluated from a new analysis of the intermediate energy heavy-ion ($\mathrm{Xe}+\mathrm{Sn}$) collision data measured by the INDRA Collaboration. Three different reactions are considered, mainly differing by the isotopic content of the emission source. The thermodynamic conditions of the data samples are extracted from the measured multiplicities allowing for a parametrization of the in-medium modification, determined with the single hypothesis that the different nuclear species in a given sample correspond to a unique common value for the density of the expanding source. We show that this correction, which was not considered in previous analyses of chemical constants from heavy-ion collisions, is necessary, since the observables of the analyzed systems show strong deviations from the expected results for an ideal gas of free clusters. This dataset is further compared to a relativistic mean-field model, and seen to be reasonably compatible with a universal correction of the attractive $\sigma$-meson coupling.