We propose a framework in which visible matter interacts with matter from a hidden sector through mass mixings of Stückelberg U(1) gauge fields. In contrast to other $Z^{\prime }$ mediation scenarios, our setup has the added appealing features that (i) the choice of $Z^{\prime }$’s can be significantly broadened without necessarily introducing unwanted exotic matter and (ii) there can be sizable tree-level interactions between the visible and hidden sectors. String theory embeddings of this scenario and their phenomenological features are briefly discussed.

Single inclusive hadron production at forward rapidity in high energy $p+A$ collisions is an important probe of the high gluon density regime of QCD and the associated small-$x$ formalism. We revisit an earlier one-loop calculation to illustrate the significance of the “rapidity factorization” approach in this regime. Such factorization separates the very small-$x$ unintegrated gluon density evolution and leads to a new correction term to the physical cross section at one-loop level. Importantly, this rapidity factorization formalism remedies the previous unphysical negative next-to-leading-order contribution to the cross section. It is much more stable with respect to “rapidity” variation when compared to the leading-order calculation and provides improved agreement between theory and experiment in the forward rapidity region.

The Sivers effect in single hadron semi-inclusive deep inelastic scattering on a transversely polarized nucleon describes the modulation of the cross section with the sine of the azimuthal angle between the produced hadron’s transverse momentum and the nucleon spin (${\mathit{P}}_h$ and ${\phi }_S$, respectively). This effect is attributed to the so-called Sivers parton distribution function of the nucleon. We employ a simple phenomenological parton model to derive the relevant cross section for two-hadron production in semi-inclusive deep inelastic scattering including the Sivers effect. We show that the Sivers effect can be observed in such a process as sine modulations involving the azimuthal angles ${\phi }_T$ and ${\phi }_R$ of both the total and the relative transverse momenta of the hadron pair. The existence of the modulation with respect to ${\phi }_R$ is new. Finally, we employ a modified version of the lepto Monte Carlo event generator that includes the Sivers effect to estimate the size of single spin asymmetries corresponding to these modulations. We show that $\sin ({\phi }_R-{\phi }_S)$ modulations can be significant, especially if we impose asymmetric cuts on the momenta of the hadrons in the pairs.

We propose an improved method for calculating electron inelastic mean free paths (IMFPs) in solids from experimental energy-loss functions based on the Mermin dielectric function. The “extended Mermin” method employs a nonlimited number of Mermin oscillators and allows negative oscillators to take into account not only electronic transitions, as is common in the traditional approaches, but also infrared transitions and inner shell electron excitations. The use of only Mermin oscillators naturally preserves two important sum rules when extending to infinite momentum transfer. Excellent agreement is found between calculated IMFPs for Cu and experimental measurements from elastic peak electron spectroscopy. Notably improved fits to the IMFPs derived from analyses of x-ray absorption fine structure measurements for Cu and Mo illustrate the importance of the contribution of infrared transitions in IMFP calculations at low energies.

We report on a dielectric target that concentrates Cherenkov radiation into a small spatial area. In contrast to traditional devices, this target can focus almost all of the radiation without using additional lenses or mirrors. We consider the case where radiation is produced by a point charge moving along the axis of a cylindrical channel inside an axially symmetrical target. The specific form of the target is determined using the laws of ray optics. The field is calculated using an aperture integration method that can determine the field near the focus. Typical field plots and the spatial distribution of the field outside the target are presented. We demonstrate that at terahertz frequencies, this concentrator can increase the field magnitude by up to at least 2 orders of magnitude relative to that on the surface of the target.

Proximity-induced superconductivity in a 3D topological insulator represents a new avenue for observing zero-energy Majorana fermions inside vortex cores. Relatively small gaps and low transition temperatures of conventional $s$-wave superconductors put hard constraints on these experiments. Significantly larger gaps and higher transition temperatures in cuprate superconductors might be an attractive alternative to considerably relax these constraints, but it is not clear whether the proximity effect would be effective in heterostructures involving cuprates and topological insulators. Here, we present angle-resolved photoemission studies of thin ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films grown in situ on optimally doped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ substrates that show the absence of proximity-induced gaps on the surfaces of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films as thin as a 1.5 quintuple layer. These results suggest that the superconducting proximity effect between a cuprate superconductor and a topological insulator is strongly suppressed, likely due to a very short coherence length along the $c$ axis, incompatible crystal and pairing symmetries at the interface, small size of the topological surface state’s Fermi surface, and adverse effects of a strong spin-orbit coupling in the topological material.

The magnetization dynamics of a wound ${[{\text{DyFe}}_2(20\AA )/{\mathrm{YFe}}_2(80\AA )]}_{\times 40}$ exchange spring multilayer have been explored in optical pump probe experiments. Ultrafast optical heating was used to modify the magnetic parameters of the multilayer, while the time resolved magneto-optical Kerr effect was used to probe its response. Although the probe signal is dominated by precession and winding of the exchange spring within the soft ${\mathrm{YFe}}_2$ layer, reorientation of the ${\mathrm{DyFe}}_2$ hard-layer magnetization is detected on time scales less than 100 ps. Micromagnetic simulations reproduce the main features of the experimental data and indicate a dramatic optically induced reduction of the hard-layer anisotropy. The results establish the feasibility of switching a spring system by means of parametric excitation.

The joint probability distribution of states of many degrees of freedom in biological systems, such as firing patterns in neural networks or antibody sequence compositions, often follows Zipf’s law, where a power law is observed on a rank-frequency plot. This behavior has been shown to imply that these systems reside near a unique critical point where the extensive parts of the entropy and energy are exactly equal. Here, we show analytically, and via numerical simulations, that Zipf-like probability distributions arise naturally if there is a fluctuating unobserved variable (or variables) that affects the system, such as a common input stimulus that causes individual neurons to fire at time-varying rates. In statistics and machine learning, these are called latent-variable or mixture models. We show that Zipf’s law arises generically for large systems, without fine-tuning parameters to a point. Our work gives insight into the ubiquity of Zipf’s law in a wide range of systems.

Using the transfer-matrix method, we numerically compute the precise position of the mobility edge of atoms exposed to a laser speckle potential and study its dependence versus the disorder strength and correlation function. Our results deviate significantly from previous theoretical estimates using an approximate, self-consistent approach of localization. In particular, we find that the position of the mobility edge in blue-detuned speckles is much lower than in the red-detuned counterpart, pointing out the crucial role played by the asymmetric on-site distribution of speckle patterns.

The Kennard-Stepanov relation describes a thermodynamic, Boltzmann-type scaling between the absorption and emission spectral profiles of an absorber, which applies in many liquid state dye solutions as well as in semiconductor systems. Here we examine absorption and emission spectra of rubidium atoms in a dense argon buffer gas environment. We demonstrate that the Kennard-Stepanov relation between absorption and emission spectra is well fulfilled in the collisionally broadened atomic gas system. Our experimental findings are supported by a simple theoretical model.

We report on measurements of second-order intensity correlations at the high-brilliance storage ring PETRA III using a prototype of the newly developed adaptive gain integrating pixel detector. The detector records individual synchrotron radiation pulses with an x-ray photon energy of 14.4 keV and repetition rate of about 5 MHz. The second-order intensity correlation function is measured simultaneously at different spatial separations, which allows us to determine the transverse coherence length at these x-ray energies. The measured values are in a good agreement with theoretical simulations based on the Gaussian Schell model.

We study the transfer of orbital angular momentum between vortex electron beams and chiral samples, such as staircase plasmonic nanostructures and biomolecules. Inelastic electron scattering from these samples produces large dichroism in the momentum-resolved electron energy-loss spectra. We illustrate this phenomenon with calculations for chiral and nonchiral clusters of silver spheres using both focused and extended electron beams, which exhibit $\sim 10\%$ difference between channels of opposite angular momentum. In addition to its fundamental interest, this remarkably high dichroism suggests a way of spatially resolving chiral optical excitations, including dark plasmons. We also predict a dichroic response when probing a chiral biomolecule, which suggests the use of these electron beams for resolving different enantiomers.

Parafermion zero energy modes are a vital element of fault-tolerant topological quantum computation. Although it is believed that such modes form on the border between topological and normal phases, this has been demonstrated only for $Z_2$ (Majorana) and $Z_3$ parafermions. I consider an integrable model of one-dimensional fermions where such a demonstration is possible for $Z_N$ parafermions with any $N$. The procedure is easily generalizable for more complicated symmetry groups.

The effect of electron confinement on the magnetocrystalline anisotropy of ultrathin bcc Fe films is explored by combining photoemission spectroscopy, x-ray magnetic circular dichroism, and magneto-optical Kerr effect measurements. Pronounced thickness-dependent variations in the magnetocrystalline anisotropy are ascribed to periodic changes in the density of states at the Fermi level, induced by quantization of $d_{xz}$, $d_{yz}$ out-of-plane orbitals. Our results reveal a direct correlation between quantum well states, the orbital magnetic moment, and the magnetocrystalline anisotropy.

We study the dissipation of small-scale adiabatic perturbations at early times when the Universe is hotter than $T\simeq 0.5\mathrm{keV}$. When the wavelength falls below the damping scale $k_D^{-1}$, the acoustic modes diffuse and thermalize, causing entropy production. Before neutrino decoupling, $k_D$ is primarily set by the neutrino shear viscosity, and we study the effect of acoustic damping on the relic neutrino number, primordial nucleosynthesis, dark-matter freeze-out, and baryogenesis. This sets a new limit on the amplitude of primordial fluctuations of ${\mathrm{\Delta }}_{\mathcal{R}}^2<0.007$ at $10^4{\mathrm{Mpc}}^{-1}\lesssim k\lesssim 10^5{\mathrm{Mpc}}^{-1}$ and a model-dependent limit of ${\mathrm{\Delta }}_{\mathcal{R}}^2\lesssim 0.3$ at $k\lesssim 10^{20–25}{\mathrm{Mpc}}^{-1}$.

We provide a simple but robust bound on the primordial curvature perturbation in the range $10^4{\mathrm{Mpc}}^{-1}<k<10^5{\mathrm{Mpc}}^{-1}$, which has not been constrained so far unlike low-wave-number modes. Perturbations on these scales dissipate the energy of their acoustic oscillations by the Silk damping after primordial nucleosynthesis but before the redshift $z\sim 2\times 10^6$ and reheat the photon bath without invoking cosmic microwave background distortions. This acoustic reheating results in the decrease of the baryon-photon ratio. By combining independent measurements probing the nucleosynthesis era and around the recombination epoch, we find an upper bound on the amplitude of the curvature perturbation over the above wave number range as ${\mathcal{P}}_{\zeta }<0.06$. Implications for supermassive black holes are also discussed.

We explore quark interactions mediated by new gauge bosons of masses in the 0.3–50 GeV range. A tight upper limit on the gauge coupling of light $Z^{\prime }$ bosons is imposed by the anomaly cancellation conditions in conjunction with collider bounds on new charged fermions. Limits from quarkonium decays are model dependent, while electroweak constraints are mild. We derive the limits for a $Z^{\prime }$ boson coupled to baryon number and then construct a $Z^{\prime }$ model with relaxed constraints, allowing quark couplings as large as 0.2 for a mass of a few GeV.