Recent investigations have suggested that the six-quark combination $uuddss$ could be a deeply bound state ($S$) that has eluded detection so far, and a potential dark matter candidate. We report the first search for a stable, doubly strange six-quark state in $\mathrm{\Upsilon }\to S\overline{\mathrm{\Lambda }}\overline{\mathrm{\Lambda }}$ decays based on a sample of $90\times 10^6\mathrm{\Upsilon }(2S)$ and $110\times 10^6\mathrm{\Upsilon }(3S)$ decays collected by the BABAR experiment. No signal is observed, and 90% confidence level limits on the combined $\mathrm{\Upsilon }(2S,3S)\to S\overline{\mathrm{\Lambda }}\overline{\mathrm{\Lambda }}$ branching fraction in the range $(1.2–1.4)\times 10^{-7}$ are derived for $m_S<2.05\mathrm{GeV}$. These bounds set stringent limits on the existence of such exotic particles.

Angular correlations between heavy-flavor decay electrons and charged particles at midrapidity ($|\eta |<0.8$) are measured in $p$-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$. The analysis is carried out for the 0%–20% (high) and 60%–100% (low) multiplicity ranges. The jet contribution in the correlation distribution from high-multiplicity events is removed by subtracting the distribution from low-multiplicity events. An azimuthal modulation remains after removing the jet contribution, similar to previous observations in two-particle angular correlation measurements for light-flavor hadrons. A Fourier decomposition of the modulation results in a positive second-order coefficient ($v_2$) for heavy-flavor decay electrons in the transverse momentum interval $1.5<p_{\mathrm{T}}<4\mathrm{GeV}/c$ in high-multiplicity events, with a significance larger than $5\sigma$. The results are compared with those of charged particles at midrapidity and those of inclusive muons at forward rapidity. The $v_2$ measurement of open heavy-flavor particles at midrapidity in small collision systems could provide crucial information to help interpret the anisotropies observed in such systems.

The physical properties of neutrons emitted from neutron-induced fission are fundamental to our understanding of nuclear fission. However, while state-of-the-art fission models still incorporate isotropic fission neutron spectra, it is believed that the preequilibrium prefission component of these spectra is strongly anisotropic. The lack of experimental guidance on this feature has not motivated incorporation of anisotropic neutron spectra in fission models, though any significant anisotropy would impact descriptions of a fissioning system. In the present work, an excess of counts at high energies in the fission neutron spectrum of ${}^{239}\mathrm{Pu}$ is clearly observed and identified as an excess of the preequilibrium prefission distribution above the postfission neutron spectrum. This excess is separated from the underlying postfission neutron spectrum, and its angular distribution is determined as a function in incident neutron energy and outgoing neutron detection angle. Comparison with neutron scattering models provides the first experimental evidence that the preequilibrium angular distribution is uncorrelated with the fission axis. The results presented here also impact the interpretation of several influential prompt fission neutron spectrum measurements.

Simulations of nonresonant ultrafast x-ray scattering from a molecular wave packet in ${\mathrm{H}}_2$ are used to examine and classify the components that contribute to the total scattering signal. The elastic component, which can be used to determine the structural dynamics of the molecule, is also found to carry a strong signature of an adiabatic electron transfer that occurs in the simulated molecule. The inelastic component, frequently assumed to be constant, is found to change with the geometry of the molecule. Finally, a coherent mixed component due to interferences between different inelastic transitions is identified and shown to provide a direct probe of transient electronic coherences.

Excitation of a bound state in the continuum (BIC) through scattering is problematic since it is by definition uncoupled. Here, we consider a type of dressed BIC and show that it can be excited in a nonlinear system through multiphoton scattering and delayed quantum feedback. The system is a semi-infinite waveguide with linear dispersion coupled to a qubit, in which a single-photon, dressed BIC is known to exist. We show that this BIC can be populated via multiphoton scattering in the non-Markovian regime, where the photon delay time (due to the qubit-mirror distance) is comparable with the qubit’s decay. A similar process excites the BIC existing in an infinite waveguide coupled to two distant qubits, thus yielding stationary entanglement between the qubits. This shows, in particular, that single-photon trapping via multiphoton scattering can occur without band edge effects or cavities, the essential resource being instead the delayed quantum feedback provided by a single mirror or the emitters themselves.

Many turbulent flows undergo drastic and abrupt configuration changes with huge impacts. As a paradigmatic example we study the multistability of jet dynamics in a barotropic beta plane model of atmosphere dynamics. It is considered as the Ising model for Jupiter troposphere dynamics. Using the adaptive multilevel splitting, a rare event algorithm, we are able to get a very large statistics of transition paths, the extremely rare transitions from one state of the system to another. This new approach opens the way for addressing a set of questions that are out of reach through direct numerical simulations. We demonstrate for the first time the concentration of transition paths close to instantons, in a numerical simulation of genuine turbulent flows. We show that the transition is a noise-activated nucleation of vorticity bands. We address for the first time the existence of Arrhenius laws in turbulent flows. The methodology we developed shall prove useful to study many other transitions related to drastic changes for the turbulent dynamics of climate, geophysical, astrophysical, and engineering applications. This opens a new range of studies impossible so far, and bring turbulent phenomena in the realm of nonequilibrium statistical mechanics.

Linear defects, referred to as dislocations, determine the strength, formability, and toughness of crystalline metallic alloys. The associated deformation mechanisms are well understood for traditional metallic materials consisting of one or two prevalent matrix elements such as steels or aluminum alloys. In the recently developed high-entropy alloys (HEAs) containing multiple principal elements, the relationship between dislocations and the mechanical behavior is less understood. Particularly HEAs with a hexagonal close-packed (hcp) structure can suffer from intrinsic brittleness due to their insufficient number of slip systems. Here we report on the surprisingly high formability of a novel high-entropy phase with hcp structure. Through in situ tensile testing and postmortem characterization by transmission electron microscopy we reveal that the hcp phase in a dual-phase HEA (${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{30}{\mathrm{Co}}_{10}{\mathrm{Cr}}_{10}$, at. %) activates three types of dislocations, i.e., $⟨a⟩$, $⟨c⟩$, and $⟨c+a⟩$. Specifically, nonbasal $⟨c+a⟩$ dislocations occupy a high line fraction of $\sim 31\%$ allowing for frequent double cross slip which explains the high deformability of this high-entropy phase. The hcp structure has a $c/a$ ratio of 1.616, i.e., below the ideal value of 1.633. This modest change in the structure parameters promotes nonbasal $⟨c+a⟩$ slip, suggesting that ductile HEAs with hcp structure can be designed by shifting the $c/a$ ratio into regimes where nonbasal slip systems are activated. This simple alloy design principle is particularly suited for HEAs due to their characteristic massive solid solution content which readily allows tuning the $c/a$ ratio of hcp phases into regimes promoting nonbasal slip activation.

Since 2014 the layered semiconductor SnSe in the high-temperature $Cmcm$ phase is known to be the most efficient intrinsic thermoelectric material. Making use of first-principles calculations we show that its vibrational and thermal transport properties are determined by huge nonperturbative anharmonic effects. We show that the transition from the $Cmcm$ phase to the low-symmetry $Pnma$ is a second-order phase transition driven by the collapse of a zone border phonon, whose frequency vanishes at the transition temperature. Our calculations show that the spectral function of the in-plane vibrational modes are strongly anomalous with shoulders and double-peak structures. We calculate the lattice thermal conductivity obtaining good agreement with experiments only when nonperturbative anharmonic scattering is included. Our results suggest that the good thermoelectric efficiency of SnSe is strongly affected by the nonperturbative anharmonicity.

We revisit the enduring problem of the $2\times 2\times 2$ charge density wave (CDW) order in ${\mathrm{TiSe}}_2$, utilizing photon energy-dependent angle-resolved photoemission spectroscopy to probe the full three-dimensional high- and low-temperature electronic structure. Our measurements demonstrate how a mismatch of dimensionality between the 3D conduction bands and the quasi-2D valence bands in this system leads to a hybridization that is strongly $k_z$ dependent. While such a momentum-selective coupling can provide the energy gain required to form the CDW, we show how additional “passenger” states remain, which couple only weakly to the CDW and thus dominate the low-energy physics in the ordered phase of ${\mathrm{TiSe}}_2$.

Demonstrating the topological protection of Andreev states in Josephson junctions is an experimental challenge. In particular the telltale $4\pi$ periodicity expected for the current phase relation has remained elusive, because of fast parity breaking processes. It was predicted that low temperature ac susceptibility measurements could reveal the topological protection of quantum spin Hall edge states by probing their low energy Andreev spectrum at finite frequency. We have performed such a microwave probing of a phase-biased Josephson junction built around a bismuth nanowire, a predicted second order topological insulator, and which was previously shown to host one-dimensional ballistic edge states. We find absorption peaks at the Andreev level crossings, whose temperature and frequency dependencies point to protected topological crossings with an accuracy limited by the electronic temperature of our experiment.

By employing unbiased numerical methods, we show that pulse irradiation can induce unconventional superconductivity even in the Mott insulator of the Hubbard model. The superconductivity found here in the photoexcited state is due to the $\eta$-pairing mechanism, characterized by staggered pair-density-wave oscillations in the off-diagonal long-range correlation, and is absent in the ground-state phase diagram; i.e., it is induced neither by a change of the effective interaction of the Hubbard model nor by simple photocarrier doping. Because of the selection rule, we show that the nonlinear optical response is essential to increase the number of $\eta$ pairs and thus enhance the superconducting correlation in the photoexcited state. Our finding demonstrates that nonequilibrium many-body dynamics is an alternative pathway to access a new exotic quantum state that is absent in the ground-state phase diagram, and also provides an alternative mechanism for enhancing superconductivity.

We report the photoassisted activation of selected $\mathrm{C}─\mathrm{H}$ bonds in individual molecules adsorbed on metal surfaces within the junction of a scanning tunneling microscope. Photons can couple to the $\mathrm{C}─\mathrm{H}$ bond activation of specific hydrocarbons through a resonant photoassisted tunneling process. The molecule to be activated can be selected by positioning the tip with subangstrom resolution. Furthermore, structural tomography of the molecule and its dissociation products are imaged at different heights by the inelastic tunneling probe. The demonstration of single bond dissociation induced by resonant photoassisted tunneling electrons implies the attainment of atomic scale spatial resolution for bond-selected photochemistry.

A prerequisite for isolating diseased cells requires a mechanism for effective mass-based separation. This objective, however, is generally rather challenging because typically no valid correlation exists between the size of the particles and their mass value. We consider an inertial Brownian particle moving in a symmetric periodic potential and subjected to an externally applied unbiased harmonic driving in combination with a constant applied bias. In doing so, we identify a most efficient separation scheme which is based on the anomalous transport feature of negative mobility, meaning that the immersed particles move in the direction opposite to the acting bias. This work is the first of its kind in demonstrating a tunable separation mechanism in which the particle mass targeted for isolation is effectively controlled over a regime of nearly 2 orders of mass magnitude upon changing solely the frequency of the external harmonic driving. This approach may provide mass selectivity required in present and future separation of a diversity of nano- and microsized particles of either biological or synthetic origin.

Dark matter could be composed of compact dark objects (CDOs). We find that the oscillation of CDOs inside neutron stars can be a detectable source of gravitational waves (GWs). The GW strain amplitude depends on the mass of the CDO, and its frequency is typically in the range 3–5 kHz as determined by the central density of the star. In the best cases, LIGO may be sensitive to CDO masses greater than or of order ${10}^{-8}{M}_{\odot }$.

We present first results on the scalar coupling of weakly interacting massive particles (WIMPs) to pions from $1\mathrm{t}\mathrm{yr}$ of exposure with the XENON1T experiment. This interaction is generated when the WIMP couples to a virtual pion exchanged between the nucleons in a nucleus. In contrast to most nonrelativistic operators, these pion-exchange currents can be coherently enhanced by the total number of nucleons and therefore may dominate in scenarios where spin-independent WIMP-nucleon interactions are suppressed. Moreover, for natural values of the couplings, they dominate over the spin-dependent channel due to their coherence in the nucleus. Using the signal model of this new WIMP-pion channel, no significant excess is found, leading to an upper limit cross section of $6.4\times {10}^{-46}{\mathrm{cm}}^2$ (90% confidence level) at $30\mathrm{GeV}/c^2$ WIMP mass.

We report the first observation of the double strange baryon $\mathrm{\Xi }(1620{)}^0$ in its decay to ${\mathrm{\Xi }}^{-}{\pi }^{+}$ via ${\mathrm{\Xi }}_c^{+}\to {\mathrm{\Xi }}^{-}{\pi }^{+}{\pi }^{+}$ decays based on a $980{\mathrm{fb}}^{-1}$ data sample collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The mass and width are measured to be $1610.4\pm 6.0(\text{stat}{)}_{-4.2}^{+6.1}(\text{syst})\mathrm{MeV}/c^2$ and $59.9\pm 4.8(\text{stat}{)}_{-7.1}^{+2.8}(\text{syst})\mathrm{MeV}$, respectively. We obtain $4.0\sigma$ evidence of the $\mathrm{\Xi }(1690{)}^0$ with the same data sample. These results shed light on the structure of hyperon resonances with strangeness $S=-2$.

The first $\gamma$-ray spectroscopy of ${}^{52}\mathrm{Ar}$, with the neutron number $N=34$, was measured using the ${}^{53}\mathrm{K}(p,2p)$ one-proton removal reaction at $\sim 210\mathrm{MeV}/\mathrm{u}$ at the RIBF facility. The $2_1^{+}$ excitation energy is found at 1656(18) keV, the highest among the Ar isotopes with $N>20$. This result is the first experimental signature of the persistence of the $N=34$ subshell closure beyond ${}^{54}\mathrm{Ca}$, i.e., below the magic proton number $Z=20$. Shell-model calculations with phenomenological and chiral-effective-field-theory interactions both reproduce the measured $2_1^{+}$ systematics of neutron-rich Ar isotopes, and support a $N=34$ subshell closure in ${}^{52}\mathrm{Ar}$.