We present a formula for the body-assisted van der Waals interaction potential between two atoms, one or both being prepared in an excited energy eigenstate. The presence of an arbitrary arrangement for a material environment is taken into account via the Green function. The resulting formula supports one of two conflicting findings recorded. The consistency of our formula is investigated by applying it for the case of two atoms in free space and comparing the resulting expression with the one found from the limiting Casimir-Polder potential between an excited atom and a small dielectric sphere.

When the plasma beta (ratio of thermal to magnetic pressure) in the core of a tokamak is raised to values of several percent, as required for a thermonuclear fusion reactor, continuous spectra of long-wavelength slow magnetosonic waves enter the frequency band occupied by continuous spectra of shear Alfvén waves. It is found that these two branches can couple strongly, so that Alfvén modes that are resonantly driven by suprathermal ions transfer some of their energy to sound waves. Since sound waves are heavily damped by thermal ion Landau resonances, these results reveal a new energy channel that contributes to the damping of Alfvénic instabilities and the noncollisional heating of bulk ions, with potentially important consequences for confinement and fusion performance.

High-intensity x-ray pulses from an x-ray free-electron laser are used to heat and probe a solid-density aluminum sample. The photon-energy-dependent transmission of the heating beam is studied through the use of a photodiode. Saturable absorption is observed, with the resulting transmission differing significantly from the cold case, in good agreement with atomic-kinetics simulations.

Molecular oxygen (${\mathrm{O}}_2$) is a paramagnetic linear molecule, yet how its electron spin affects the chemisorption probability remains unclear. We here present the first spin- and alignment-resolved ${\mathrm{O}}_2$ chemisorption experiment conducted with a single spin-rotational state-selected ${\mathrm{O}}_2$ beam and a magnetized Ni(111) surface. The results show that the ${\mathrm{O}}_2$ sticking probability is higher when its spin is oriented antiparallel to the majority spin direction of the Ni substrate. The spin dependence becomes more significant at low translational energy, and amounts to over 40% at thermal energy. This strong spin effect suggests that incident ${\mathrm{O}}_2$ molecules keep high spin polarizations even at the position of the dissociation barrier.

We show that, quite generically, a [111] slab of spin-orbit coupled pyrochlore lattice exhibits surface states whose constant energy curves take the shape of Fermi arcs, localized to different surfaces depending on their quasimomentum. Remarkably, these persist independently of the existence of Weyl points in the bulk. Considering interacting electrons in slabs of finite thickness, we find a plethora of known fractional Chern insulating phases, to which we add the discovery of a new higher Chern number state which is likely a generalization of the Moore-Read fermionic fractional quantum Hall state. By contrast, in the three-dimensional limit, we argue for the absence of gapped states of the flat surface band due to a topologically protected coupling of the surface to gapless states in the bulk. We comment on generalizations as well as experimental perspectives in thin slabs of pyrochlore iridates.

The precise determination of the Cabibbo-Kobayashi-Maskawa elements $V_{cb}$ and $V_{ub}$ is crucial for any new physics analysis in the flavor sector. Their values can be determined from several tree-level decays: $V_{cb}$ can be extracted from $B\to D\ell \nu$ and $B\to D^{*}\ell \nu$ while $V_{ub}$ can be obtained from $B\to \pi \ell \nu$, $B\to \rho \ell \nu$, and $B\to \tau \nu$. In addition, for both $V_{cb}$ and $V_{ub}$ an inclusive determination is available. There is a long lasting discrepancy between the inclusive and exclusive determinations which recently even increased for $V_{cb}$ above the $3\sigma$ level. In this Letter we study the possible effect of new physics on the inclusive and exclusive determination of $V_{cb}$ and $V_{ub}$ in a model independent way. We find that there is only one operator corresponding to a modified $W$ coupling which can achieve this. However, respecting $SU(2)$ gauge invariance at the high scale this would lead to very large violations of the $Z$ to $b\overline{b}$ coupling not compatible with experiment. Therefore, we conclude that a new physics explanation of the difference between the inclusive and exclusive determination of $V_{cb}$ and $V_{ub}$ is currently ruled out. Therefore, the discrepancies must be due to underestimated uncertainties in the theoretical and/or the experimental analysis.

The most compelling possibility for a new law of nature beyond the four fundamental forces comprising the standard model of high-energy physics is the discrepancy between measurements and calculations of the muon anomalous magnetic moment. Until now a key part of the calculation, the hadronic light-by-light contribution, has only been accessible from models of QCD, the quantum description of the strong force, whose accuracy at the required level may be questioned. A first principles calculation with systematically improvable errors is needed, along with the upcoming experiments, to decisively settle the matter. For the first time, the form factor that yields the light-by-light scattering contribution to the muon anomalous magnetic moment is computed in such a framework, lattice $\mathrm{QCD}+\mathrm{QED}$ and QED. A nonperturbative treatment of QED is used and checked against perturbation theory. The hadronic contribution is calculated for unphysical quark and muon masses, and only the diagram with a single quark loop is computed for which statistically significant signals are obtained. Initial results are promising, and the prospect for a complete calculation with physical masses and controlled errors is discussed.

The nuclear neutron-proton contact is introduced, generalizing Tan’s work, and evaluated from medium energy nuclear photodisintegration experiments. To this end we reformulate the quasideuteron model of nuclear photodisintegration and establish the bridge between the Levinger constant and the contact. Using experimental evaluations of Levinger’s constant, we extract the value of the neutron-proton contact in finite nuclei and in symmetric nuclear matter. Assuming isospin symmetry we propose to evaluate the neutron-neutron contact through the measurement of photonuclear spin correlated neutron-proton pairs.

Recent measurements of the positron energy spectrum obtained from inverse beta decay interactions of reactor electron antineutrinos show an excess in the 4 to 6 MeV region relative to current predictions. First-principles calculations of fission and beta decay processes within a typical pressurized water reactor core identify prominent fission daughter isotopes as a possible origin for this excess. These calculations also predict percent-level substructures in the antineutrino spectrum due to Coulomb effects in beta decay. Precise measurement of these substructures can elucidate the nuclear processes occurring within reactors. These substructures can be a systematic issue for measurements utilizing the detailed spectral shape.

By measuring the cyclotron frequency ratios of ${}^3{\mathrm{He}}^{+}$ to ${\mathrm{HD}}^{+}$ and ${\mathrm{T}}^{+}$ to ${\mathrm{HD}}^{+}$, and using ${\mathrm{HD}}^{+}$ as a mass reference, we obtain new atomic masses for ${}^3\mathrm{He}$ and T. Our results are $M[{}^3\mathrm{He}]=3.01602932243(19)\mathrm{u}$ and $M[\mathrm{T}]=3.01604928178(19)\mathrm{u}$, where the uncertainty includes an uncertainty of 0.12 nu in the mass reference. Allowing for cancellation of common systematic errors, we find the $Q$ value for tritium $\beta$ decay to be $(M[\mathrm{T}]-M[{}^3\mathrm{He}])c^2=18592.01(7)\mathrm{eV}$. This allows an improved test of systematics in measurements of tritium $\beta$ decay that set limits on neutrino mass.

We establish the emergence of chaotic motion in optomechanical systems. Chaos appears at negative detuning for experimentally accessible values of the pump power and other system parameters. We describe the sequence of period-doubling bifurcations that leads to chaos and state the experimentally observable signatures in the optical spectrum. In addition to the semiclassical dynamics, we analyze the possibility of chaotic motion in the quantum regime. We find that quantum mechanics protects the optomechanical system against irregular dynamics, such that simple periodic orbits reappear and replace the classically chaotic motion. In this way observation of the dynamical signatures makes it possible to pin down the crossover from quantum to classical mechanics.

Fast ignition (FI) is investigated via integrated particle-in-cell simulation including both generation and transport of fast electrons, where petawatt ignition lasers of 2 ps and compressed targets of a peak density of $300\mathrm{g}{\mathrm{cm}}^{-3}$ and areal density of $0.49\mathrm{g}{\mathrm{cm}}^{-2}$ at the core are taken. When a 20 MG static magnetic field is imposed across a conventional cone-free target, the energy coupling from the laser to the core is enhanced by sevenfold and reaches 14%. This value even exceeds that obtained using a cone-inserted target, suggesting that the magnetically assisted scheme may be a viable alternative for FI. With this scheme, it is demonstrated that two counterpropagating, 6 ps, 6 kJ lasers along the magnetic field transfer 12% of their energy to the core, which is then heated to 3 keV.

When a core level is excited by circularly polarized light, the angular momentum of light is transferred to the emitted photoelectron, which can be confirmed by the parallax shift of the forward focusing peak (FFP) direction in a stereograph of atomic arrangement. No angular momentum has been believed to be transferred to normal Auger electrons resulting from the decay process filling core hole after photoelectron ejection. We succeeded in detecting a non-negligible circular dichroism contrast in a normal Auger electron diffraction from a nonmagnetic Cu(001) surface far off from the absorption threshold. Moreover, we detected angular-momentum-polarized Cu $L_3{M}_{4,5}{M}_{4,5}$ Auger electrons at the $L_3$ absorption threshold, where the excited core electron is trapped at the conduction band. From the kinetic energy dependence of the Auger electron FFP parallax shift, we found that the angular momentum is transferred to the Auger electron most effectively in the case of the ${}^1{S}_0$ two-hole creation.

How to induce nanoscale directional motion via some intrinsic mechanisms pertaining to a nanosystem remains a challenge in nanotechnology. Here we show via molecular dynamics simulations that there exists a fundamental driving force for a nanoscale object to move from a region of lower stiffness toward one of higher stiffness on a substrate. Such nanoscale directional motion is induced by the difference in effective van der Waals potential energy due to the variation in stiffness of the substrate; i.e., all other conditions being equal, a nanoscale object on a stiffer substrate has lower van der Waals potential energy. This fundamental law of nanoscale directional motion could lead to promising routes for nanoscale actuation and energy conversion.