Results are reported from a search for the rare decays $B_s^0\to {\tau }^{\pm }{\mu }^{\mp }$ and $B^0\to {\tau }^{\pm }{\mu }^{\mp }$, where the $\tau$ lepton is reconstructed in the channel ${\tau }^{-}\to {\pi }^{-}{\pi }^{+}{\pi }^{-}{\nu }_{\tau }$. These processes are effectively forbidden in the standard model, but they can potentially occur at detectable rates in models of new physics that can induce lepton-flavor-violating decays. The search is based on a data sample corresponding to $3{\mathrm{fb}}^{-1}$ of proton-proton collisions recorded by the LHCb experiment in 2011 and 2012. The event yields observed in the signal regions for both processes are consistent with the expected standard model backgrounds. Because of the limited mass resolution arising from the undetected $\tau$ neutrino, the $B_s^0$ and $B^0$ signal regions are highly overlapping. Assuming no contribution from $B^0\to {\tau }^{\pm }{\mu }^{\mp }$, the upper limit $\mathcal{B}(B_s^0\to {\tau }^{\pm }{\mu }^{\mp })<4.2\times {10}^{-5}$ is obtained at 95% confidence level. If no contribution from $B_s^0\to {\tau }^{\pm }{\mu }^{\mp }$ is assumed, a limit of $\mathcal{B}(B^0\to {\tau }^{\pm }{\mu }^{\mp })<1.4\times {10}^{-5}$ is obtained at 95% confidence level. These results represent the first limit on $\mathcal{B}(B_s^0\to {\tau }^{\pm }{\mu }^{\mp })$ and the most stringent limit on $\mathcal{B}(B^0\to {\tau }^{\pm }{\mu }^{\mp })$.

We present results for the strange contribution to the electromagnetic form factors of the nucleon computed on the coordinated lattice simulation ensembles with $N_f=2+1$ flavors of $\mathcal{O}(a)$-improved Wilson fermions and an $\mathcal{O}(a)$-improved vector current. Several source-sink separations are investigated in order to estimate the excited-state contamination. We calculate the form factors on six ensembles with lattice spacings in the range of $a=0.049–0.086\mathrm{fm}$ and pion masses in the range of $m_{\pi }=200–360\mathrm{MeV}$, which allows for a controlled chiral and continuum extrapolation. In the computation of the quark-disconnected contributions, we employ hierarchical probing as a variance-reduction technique.

Measurements of two-particle angular correlations of charged particles emitted in hadronic $Z$ decays are presented. The archived $e^{+}{e}^{-}$ annihilation data at a center-of-mass energy of 91 GeV were collected with the ALEPH detector at LEP between 1992 and 1995. The correlation functions are measured over a broad range of pseudorapidity and full azimuth as a function of charged particle multiplicity. No significant long-range correlation is observed in either the lab coordinate analysis or the thrust coordinate analysis, where the latter is sensitive to a medium expanding transverse to the color string between the outgoing $q\overline{q}$ pair from $Z$ boson decays. The associated yield distributions in both analyses are in better agreement with the prediction from the pythia v6.1 event generator than from herwig v7.1.5. They provide new insights to showering and hadronization modeling. These results serve as an important reference to the observed long-range correlation in proton-proton, proton-nucleus, and nucleus-nucleus collisions.

We investigate the dipole-mediated transport of Rydberg impurities through an ultracold gas of atoms prepared in an auxiliary Rydberg state. In one experiment, we continuously probe the system by coupling the auxiliary Rydberg state to a rapidly decaying state that realizes a dissipative medium. In situ imaging of the impurities reveals diffusive spreading controlled by the intensity of the probe laser. By preparing the same density of hopping partners, but then switching off the dressing fields, the spreading is effectively frozen. This is consistent with numerical simulations, which indicate the coherently evolving system enters a nonergodic extended phase. This opens the way to study transport and localization phenomena in systems with long-range hopping and controllable dissipation.

Intense, mutually coherent beams of multiharmonic extreme ultraviolet light can now be created using seeded free-electron lasers, and the phase difference between harmonics can be tuned with attosecond accuracy. However, the absolute value of the phase is generally not determined. We present a method for determining precisely the absolute phase relationship of a fundamental wavelength and its second harmonic, as well as the amplitude ratio. Only a few easily calculated theoretical parameters are required in addition to the experimental data.

Previous research has attempted to minimize the influence of loss in reflection- and transmission-type acoustic metasurfaces. This Letter shows that, by treating the acoustic metasurface as a non-Hermitian system and by harnessing loss, unconventional wave behaviors that do not exist in lossless metasurfaces can be uncovered. Specifically, we theoretically and experimentally demonstrate a non-Hermitian acoustic metasurface mirror featuring extremely asymmetrical reflection at the exception point. As an example, the metasurface mirror is designed to have high-efficiency retroreflection when the wave comes from one side and near-perfect absorption when the wave comes from the opposite side. This work marries conventional gradient index metasurfaces with the exceptional point from non-Hermitian systems, and it paves the way for identifying new mechanisms and functionalities for wave manipulation.

Quasiparticle interference (QPI) provides a wealth of information relating to the electronic structure of a material. However, it is often assumed that this information is constrained to two-dimensional electronic states. We show that this is not necessarily the case. For FeSe, a system dominated by surface defects, we show that it is actually all electronic states with negligible group velocity in the $z$ axis that are contained within the experimental data. By using a three-dimensional tight-binding model of FeSe, fit to photoemission measurements, we directly reproduce the experimental QPI scattering dispersion, within a $T$-matrix formalism, by including both $k_z=0$ and $k_z=\pi$ electronic states. This result unifies both tunnelling based and photoemission based experiments on FeSe and highlights the importance of $k_z$ within surface sensitive measurements of QPI.

We experimentally analyze the intermittent nature of granular silo flow when the discharge is controlled by an extracting belt at the bottom. We discover the existence of four different scenarios. For low extraction rates, the system is characterized by an on-off intermittency. When the extraction rate is increased the structure functions of the grains velocity increments, calculated for different lag times, reveal the emergence of multifractal intermittency. Finally, for very high extraction rates that approach the purely gravitational discharge, we observe that the dynamics become dependent on the outlet size. For large orifices the behavior is monofractal, whereas for small ones, the fluctuations of the velocity increments deviate from Gaussianity even for very large time lags.

Given a quantum gate implementing a $d$-dimensional unitary operation $U_d$, without any specific description but $d$, and permitted to use $k$ times, we present a universal probabilistic heralded quantum circuit that implements the exact inverse $U_d^{-1}$, whose failure probability decays exponentially in $k$. The protocol employs an adaptive strategy, proven necessary for the exponential performance. It requires that $k\ge d-1$, proven necessary for the exact implementation of $U_d^{-1}$ with quantum circuits. Moreover, even when quantum circuits with indefinite causal order are allowed, $k\ge d-1$ uses are required. We then present a finite set of linear and positive semidefinite constraints characterizing universal unitary inversion protocols and formulate a convex optimization problem whose solution is the maximum success probability for given $k$ and $d$. The optimal values are computed using semidefinite programing solvers for $k\le 3$ when $d=2$ and $k\le 2$ for $d=3$. With this numerical approach we show for the first time that indefinite causal order circuits provide an advantage over causally ordered ones in a task involving multiple uses of the same unitary operation.

We demonstrate Floquet engineering in a basic yet scalable 2D architecture of individually trapped and controlled ions. Local parametric modulations of detuned trapping potentials steer the strength of long-range interion couplings and the related Peierls phase of the motional state. In our proof of principle, we initialize large coherent states and tune modulation parameters to control trajectories, directions, and interferences of the phonon flow. Our findings open a new pathway for future Floquet-based trapped-ion quantum simulators targeting correlated topological phenomena and dynamical gauge fields.

A quantum scar—an enhancement of a quantum probability density in the vicinity of a classical periodic orbit—is a fundamental phenomenon connecting quantum and classical mechanics. Here we demonstrate that some of the eigenstates of the perturbed two-dimensional anisotropic (elliptic) harmonic oscillator are strongly scarred by the Lissajous orbits of the unperturbed classical counterpart. In particular, we show that the occurrence and geometry of these quantum Lissajous scars are connected to the anisotropy of the harmonic confinement, but unlike the classical Lissajous orbits the scars survive under a small perturbation of the potential. This Lissajous scarring is caused by the combined effect of the quantum (near) degeneracies in the unperturbed system and the localized character of the perturbation. Furthermore, we discuss experimental schemes to observe this perturbation-induced scarring.

Shocks are abundant both in astrophysical and laboratory systems. While the electric fields generated at shock fronts have recently attracted great attention, the associated self-generated magnetic field is rarely studied, despite its ability to significantly affect the shock profile in the nonideal geometry where density and temperature gradients are not parallel. We report here the observation of a magnetic field at the front of a Mach $\sim 6$ shock propagating in a low-density helium gas system. Proton radiography from different projection angles not only confirms the magnetic field’s existence, but also provides a quantitative measurement of the field strength in the range $\sim 5$ to 7 T. X-ray spectrometry allowed inference of the density and temperature at the shock front, constraining the plasma conditions under which the magnetic and electric fields are generated. Simulations with the particle-in-cell code lsp attribute the self-generation of the magnetic field to the Biermann battery effect ($\nabla n_e\times \nabla T_e$).

The pseudogap, $d$-wave superconductivity and electron-boson coupling are three intertwined key ingredients in the phase diagram of the cuprates. ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ is a $5d$-electron counterpart of the cuprates in which both the pseudogap and a $d$-wave instability have been observed. Here, we report spectroscopic evidence for the presence of the third key player in electron-doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$: electron-boson coupling. A kink in nodal dispersion is observed with an energy scale of $\sim 50\mathrm{meV}$. The strength of the kink changes with doping, but the energy scale remains the same. These results provide the first noncuprate platform for exploring the relationship between the pseudogap, $d$-wave instability, and electron-boson coupling in doped Mott insulators.

Electronic screening can have direct consequences for structural arrangements on the nanoscale, such as on the periodic ordering of adatoms on a surface. So far, such ordering phenomena have been explained in terms of isotropic screening of free electronlike systems. Here, we directly illustrate the structural consequences of anisotropic screening, making use of a highly anisotropic two-dimensional electron gas (2DEG) near the surface of black phosphorous. The presence of the 2DEG and its filling is controlled by adsorbed potassium atoms, which simultaneously serve to probe the electronic ordering. Using scanning tunneling microscopy, we show that the anisotropic screening leads to the formation of potassium chains with a well-defined orientation and spacing. We quantify the mean interaction potential utilizing statistical methods and find that the dimensionality and anisotropy of the screening is consistent with the presence of a band bending-induced 2DEG near the surface. The electronic dispersion of the 2DEG inferred by electronic ordering is consistent with that measured by angle-resolved photoemission spectroscopy.

Although correlated electronic-structure calculations explain very well the normal state of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, its superconducting symmetry is still unknown. Here we construct the spin and charge fluctuation pairing interactions based on its correlated normal state. Correlations significantly reduce ferromagnetic in favor of antiferromagnetic fluctuations and increase interorbital pairing. From the normal-state Eliashberg equations, we find spin-singlet $d$-wave pairing close to magnetic instabilities. Away from these instabilities, where charge fluctuations increase, we find two time-reversal symmetry-breaking spin triplets: an odd-frequency $s$ wave, and a doubly degenerate interorbital pairing between $d_{xy}$ and ($d_{yz},d_{xz}$).

While the usual approach to tailor the behavior of condensed matter and nanosized systems is the choice of material or finite-size or interfacial effects, topology alone may be the key. In the context of the motion of magnetic domain walls (DWs), known to suffer from dynamic instabilities with low mobilities, we report unprecedented velocities $>600\mathrm{m}/\mathrm{s}$ for DWs driven by spin-transfer torques in cylindrical nanowires made of a standard ferromagnetic material. The reason is the robust stabilization of a DW type with a specific topology by the Œrsted field associated with the current. This opens the route to the realization of predicted new physics, such as the strong coupling of DWs with spin waves above $>600\mathrm{m}/\mathrm{s}$.