Using a dynamic variant of the Vicsek model, we show that the emergence of disorder from an orderly moving human crowd is a nonequilibrium first-order phase transition. We also show that this transition can be reversed by modifying the dynamics of a few agents, deemed as game changers. Surprisingly, the optimal placement of these game changers is found to be in regions of maximum local crowd speed. The presence of such game changers is effective owing to the discontinuous nature of the underlying phase transition. Thus our generic approach provides strategies to (i) delay crowd crush and (ii) design safe evacuation procedures, two aspects that are of paramount importance in maintaining safety of mass gatherings of people.

The transition between shear-flowing and shear-arrested states of frictional granular matter is studied using constant-stress discrete element simulations. By subjecting a dilute system of frictional grains to a constant external shear stress and pressure, friction-dependent critical shear stress and density are clearly identified with both exhibiting a crossover between low and high friction. The critical shear stress bifurcates two nonequilibrium steady states: (i) steady state shear flow characterized by a constant deformation rate, and (ii) shear arrest characterized by temporally decaying creep to a statically stable state. The onset of arrest below critical shear stress occurs at a time $t_c$ that exhibits a heavy-tailed distribution, whose mean and variance diverge as a power law at the critical shear stress with a friction-dependent exponent that also exhibits a crossover between low and high friction. These observations indicate that granular arrest near critical shear stress is highly unpredictable and is strongly influenced by interparticle friction.

Degradation processes in living systems often take place gradually by front propagation. An important context of such processes is loss of biological productivity in drylands or desertification. Using a dryland-vegetation model, we analyze the stability and dynamics of desertification fronts, identify linear and nonlinear front instabilities, and highlight the significance of these instabilities in inducing self-recovery. The results are based on the derivation and analysis of a universal amplitude equation for pattern-forming living systems for which nonuniform instabilities cannot emerge from the nonviable (zero) state. The results may therefore be applicable to other contexts of animate matter where degradation processes occur by front propagation.

We analyze quantum dynamics of strongly interacting, kinetically constrained many-body systems. Motivated by recent experiments demonstrating surprising long-lived, periodic revivals after quantum quenches in Rydberg atom arrays, we introduce a manifold of locally entangled spin states, representable by low-bond dimension matrix product states, and derive equations of motion for them using the time-dependent variational principle. We find that they feature isolated, unstable periodic orbits, which capture the recurrences and represent nonergodic dynamical trajectories. Our results provide a theoretical framework for understanding quantum dynamics in a class of constrained spin models, which allow us to examine the recently suggested explanation of “quantum many-body scarring” [Nat. Phys. 14, 745 (2018)], and establish a possible connection to the corresponding phenomenon in chaotic single-particle systems.

The cosmic microwave background (CMB) places a variety of model-independent constraints on the strength interactions of the dominant component of dark matter with the standard model. Percent-level subcomponents of the dark matter can evade the most stringent CMB bounds by mimicking the behavior of baryons, allowing for larger couplings and novel experimental signatures. However, in this Letter, we will show that such tightly coupled subcomponents leave a measurable imprint on the CMB that is well approximated by a change to the helium fraction, $Y_{\mathrm{He}}$. Using the existing CMB constraint on $Y_{\mathrm{He}}$, we derive a new upper limit on the fraction of tightly coupled dark matter, $f_{\mathrm{TCDM}}$, of $f_{\mathrm{TCDM}}<0.006$ (95% C.I.). We show that future CMB experiments can reach $f_{\mathrm{TCDM}}<0.001$ (95% C.I.) and confirm that the bounds derived in this way agree with the results of a complete analysis. These bounds provide an example of how CMB constraints on $Y_{\mathrm{He}}$ have applications beyond studying big bang nucleosynthesis, since tightly coupled dark matter plays no direct role in the formation of light nuclei. We briefly comment on the implications for model building, including millicharged dark matter.

We report the first observation of the spontaneous polarization of $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons transverse to the production plane in $e^{+}{e}^{-}$ annihilation, which is attributed to the effect arising from a polarizing fragmentation function. For inclusive $\mathrm{\Lambda }/\overline{\mathrm{\Lambda }}$ production, we also report results with subtracted feed-down contributions from ${\mathrm{\Sigma }}^0$ and charm. This measurement uses a dataset of $800.4{\mathrm{fb}}^{-1}$ collected by the Belle experiment at or near a center-of-mass energy of 10.58 GeV. We observe a significant polarization that rises with the fractional energy carried by the $\mathrm{\Lambda }/\overline{\mathrm{\Lambda }}$ hyperon.

Mapping states with explicit gluonic degrees of freedom in the light sector is a challenge, and has led to controversies in the past. In particular, the experiments have reported two different hybrid candidates with spin-exotic signature, ${\pi }_1(1400)$ and ${\pi }_1(1600)$, which couple separately to $\eta \pi$ and ${\eta }^{\prime }\pi$. This picture is not compatible with recent Lattice QCD estimates for hybrid states, nor with most phenomenological models. We consider the recent partial wave analysis of the ${\eta }^{(\prime )}\pi$ system by the COMPASS Collaboration. We fit the extracted intensities and phases with a coupled-channel amplitude that enforces the unitarity and analyticity of the $S$ matrix. We provide a robust extraction of a single exotic ${\pi }_1$ resonant pole, with mass and width $1564\pm 24\pm 86$ and $492\pm 54\pm 102\mathrm{MeV}$, which couples to both ${\eta }^{(\prime )}\pi$ channels. We find no evidence for a second exotic state. We also provide the resonance parameters of the $a_2(1320)$ and $a_2^{\prime }(1700)$.

The motion of dislocations bridges the atomistic-scale deformation events with the macroscopic strength and ductility of crystalline metals. In particular, screw dislocations, whose Burgers vector is parallel to the line, play crucial roles on plastic flow. Nevertheless, their speed limit and its stress dependence remain controversial. Using large-scale molecular dynamics simulations, we reveal that full screw dislocations and twinning partial screw-type dislocations can glide steadily at the speed of shear wave velocity. Such a scenario is excluded in existing theories due to energy dissipation singularity. We conclude that both types of screw dislocations can move supersonically. We further observe that the motion of a screw dislocation also depends on the shear stress components, which do not contribute to the resolved shear stress (RSS), in contrast to the conventional Schmid’s law, which states that the motion of a dislocation is determined by the RSS.

Single crystal $L$-amino acids can exhibit technologically useful piezoelectric and nonlinear optical properties. Here we predict, using density functional theory, the piezoelectric charge and strain and voltage tensors of the racemic amino acid $DL$ alanine, and use the modeling data to guide the first macroscopic and nanoscopic piezoelectric measurements on $DL$-alanine single crystals and polycrystalline aggregates. We demonstrate voltage generation of up to 0.8 V from $DL$-alanine crystal films under simple manual compression, twice as high as other amino acid crystals. Our results suggest that net molecular chirality is not a prerequisite for piezoelectric behavior in organic crystals. The transducer presented herein demonstrates that $DL$-alanine crystals can be used in applications such as temperature and force measurement in biosensors, data storage in flexible electronic devices, and mechanical actuation in energy harvesters.

We introduce a simple, exactly solvable strong-randomness renormalization group (RG) model for the many-body localization (MBL) transition in one dimension. Our approach relies on a family of RG flows parametrized by the asymmetry between thermal and localized phases. We identify the physical MBL transition in the limit of maximal asymmetry, reflecting the instability of MBL against rare thermal inclusions. We find a critical point that is localized with power-law distributed thermal inclusions. The typical size of critical inclusions remains finite at the transition, while the average size is logarithmically diverging. We propose a two-parameter scaling theory for the many-body localization transition that falls into the Kosterlitz-Thouless universality class, with the MBL phase corresponding to a stable line of fixed points with multifractal behavior.

Neutrinos are key to probing the deep structure of matter and the high-energy Universe. Yet, until recently, their interactions had only been measured at laboratory energies up to about 350 GeV. An opportunity to measure their interactions at higher energies opened up with the detection of high-energy neutrinos in IceCube, partially of astrophysical origin. Scattering off matter inside Earth affects the distribution of their arrival directions—from this, we extract the neutrino-nucleon cross section at energies from 18 TeV to 2 PeV, in four energy bins, in spite of uncertainties in the neutrino flux. Using six years of public IceCube High-Energy Starting Events, we explicitly show for the first time that the energy dependence of the cross section above 18 TeV agrees with the predicted softer-than-linear dependence, and reaffirm the absence of new physics that would make the cross section rise sharply, up to a center-of-mass energy $\sqrt{s}\approx 1\mathrm{TeV}$.

A systematic procedure for constructing classical integrable field theories with arbitrarily many free parameters is outlined. It is based on the recent interpretation of integrable field theories as realizations of affine Gaudin models. In this language, one can associate integrable field theories with affine Gaudin models having arbitrarily many sites. We present the result of applying this general procedure to couple together an arbitrary number of principal chiral model fields on the same Lie group, each with a Wess-Zumino term.

We present an efficient Monte Carlo framework for perturbative calculations of infinite nuclear matter based on chiral two-, three-, and four-nucleon interactions. The method enables the incorporation of all many-body contributions in a straightforward and transparent way, and makes it possible to extract systematic uncertainty estimates by performing order-by-order calculations in the chiral expansion as well as the many-body expansion. The versatility of this new framework is demonstrated by applying it to chiral low-momentum interactions, exhibiting a very good many-body convergence up to fourth order. Following these benchmarks, we explore new chiral interactions up to next-to-next-to-next-to-leading order (${\mathrm{N}}^3\mathrm{LO}$). Remarkably, simultaneous fits to the triton and to saturation properties can be achieved, while all three-nucleon low-energy couplings remain natural. The theoretical uncertainties of nuclear matter are significantly reduced when going from next-to-next-to-leading order to ${\mathrm{N}}^3\mathrm{LO}$.

We report on the first detailed study of motional heating in a cryogenic Penning trap using a single antiproton. Employing the continuous Stern-Gerlach effect we observe cyclotron quantum transition rates of $6(1)\mathrm{quanta}/\mathrm{h}$ and an electric-field noise spectral density below $7.5(3.4)\times {10}^{-20}{\mathrm{V}}^2{\mathrm{m}}^{-2}{\mathrm{Hz}}^{-1}$, which corresponds to a scaled noise spectral density below $8.8(4.0)\times {10}^{-12}{\mathrm{V}}^2{\mathrm{m}}^{-2}$, results which are more than 2 orders of magnitude smaller than those reported by other ion-trap experiments.

We study a cavity-QED setup consisting of a two-level system coupled to a single cavity mode with two-photon relaxation. The system dynamics is modeled via a Lindblad master equation consisting of the Rabi Hamiltonian and a two-photon dissipator. We show that an even-photon relaxation preserves the $Z_2$ symmetry of the Rabi model, and provide a framework to study the corresponding non-Hermitian dynamics in the number-parity basis. We discuss the role of different terms in the two-photon dissipator and show how one can extend existing results for the closed Rabi spectrum to the open case. Furthermore, we characterize the role of the $Z_2$ symmetry in the excitation-relaxation dynamics of the system as a function of light-matter coupling. Importantly, we observe that initial states with even-odd parity manifest qualitatively distinct transient and steady state behaviors, contrary to the Hermitian dynamics that is only sensitive to whether or not the initial state is parity invariant. Moreover, the parity-sensitive dynamical behavior is not a creature of ultrastrong coupling and is present even at weak coupling values.

It is known that the Manakov equation which describes wave propagation in two mode optical fibers, photorefractive materials, etc., can admit solitons which allow energy redistribution between the modes on collision that also leads to logical computing. In this Letter, we point out that the Manakov system can admit a more general type of nondegenerate fundamental solitons corresponding to different wave numbers, which undergo collisions without any energy redistribution. The previously known class of solitons which allows energy redistribution among the modes turns out to be a special case corresponding to solitary waves with identical wave numbers in both the modes and traveling with the same velocity. We trace out the reason behind such a possibility and analyze the physical consequences.

We develop a geometric approach to understand the mechanics of perforated thin elastic sheets, using the method of strain-dependent image elastic charges. This technique recognizes the buckling response of a hole under an external load as a geometrically tuned mechanism of stress relief. We use a diagonally pulled square paper frame as a model system to quantitatively test and validate our approach. Specifically, we compare nonlinear force-extension curves and global displacement fields in theory and experiment. We find a strong softening of the force response accompanied by curvature localization at the inner corners of the buckled frame. Counterintuitively, though in complete agreement with our theory, for a range of intermediate hole sizes, wider frames are found to buckle more easily than narrower ones. Upon extending these ideas to many holes, we demonstrate that interacting elastic image charges can provide a useful kirigami design principle to selectively relax stresses in elastic materials.