Smoking Gun Evidence Research Articles

Dark Matter Annihilation Can Produce a Detectable Antihelium Flux through Λ[over ¯]_{b} Decays.

Recent observations by the Alpha Magnetic Spectrometer (AMS-02) have tentatively detected a handful of cosmic-ray antihelium events. Such events have long been considered as smoking-gun evidence for new physics, because astrophysical antihelium production is expected to be negligible. However, the dark-matter-induced antihelium flux is also expected to fall below current sensitivities, particularly in light of existing antiproton constraints. Here, we demonstrate that a previously neglected standard model process-the production of antihelium through the displaced-vertex decay of Λ[over ¯]_{b}-baryons-can significantly boost the dark matter induced antihelium flux. This process can entirely dominate the production of high-energy antihelium nuclei, increasing the rate of detectable AMS-02 events by 2 orders of magnitude.

Half-Magnetic Topological Insulator with Magnetization-Induced Dirac Gap at a Selected Surface

Topological magnets are a new family of quantum materials providing great potential to realize emergent phenomena, such as quantum anomalous Hall effect and axion-insulator state. Here we present our discovery that stoichiometric ferromagnet MnBi8Te13 with natural heterostructure MnBi2Te4-(Bi2Te3)3 is an unprecedented half-magnetic topological insulator, with the magnetization existing at the MnBi2Te4 surface but not at the opposite surface terminated by triple Bi2Te3 layers. Our angle-resolved photoemission spectroscopy measurements unveil a massive Dirac gap at the MnBi2Te4 surface, and gapless Dirac cone on the other side. Remarkably, the Dirac gap (~28 meV) at MnBi2Te4 surface decreases monotonically with increasing temperature and closes right at the Curie temperature, thereby representing the first smoking-gun spectroscopic evidence of magnetization-induced topological surface gap among all known magnetic topological materials. We further demonstrate theoretically that the half-magnetic topological insulator is desirable to realize the half-quantized surface anomalous Hall effect, which serves as a direct proof of the general concept of axion electrodynamics in condensed matter systems.

Non-thermal neutrinos created by shock acceleration in successful and failed core-collapse supernova

ABSTRACT We present a comprehensive study of neutrino shock acceleration in core-collapse supernova (CCSN). The leading players are heavy leptonic neutrinos, νμ and ντ; the former and latter potentially gain the energy up to ∼100 and ∼200 MeV, respectively, through the shock acceleration. Demonstrating the neutrino shock acceleration by Monte Carlo neutrino transport, we make a statement that it commonly occurs in the early post-bounce phase (≲50 ms after bounce) for all massive stellar collapse experiencing nuclear bounce and would reoccur in the late phase (≳100 ms) for failed CCSNe. This opens up a new possibility to detect high energy neutrinos by terrestrial detectors from Galactic CCSNe; hence, we estimate the event counts for Hyper(Super)-Kamiokande, DUNE, and JUNO. We find that the event count with the energy of ≳80 MeV is a few orders of magnitude higher than that of the thermal neutrinos regardless of the detectors, and muon production may also happen in these detectors by νμ with the energy of ≳100 MeV. The neutrino signals provide a precious information on deciphering the inner dynamics of CCSN and placing a constraint on the physics of neutrino oscillation; indeed, the detection of the high energy neutrinos through charged current reaction channels will be a smoking gun evidence of neutrino flavour conversion.

Boundary-Obstructed Topological High- Tc Superconductivity in Iron Pnictides

Non-trivial topology and unconventional pairing are two central guiding principles in the contemporary search for and analysis of superconducting materials and heterostructure compounds. Previously, a topological superconductor has been predominantly conceived to result from a topologically non-trivial band subject to intrinsic or external superconducting proximity effect. Here, we propose a new class of topological superconductors which are uniquely induced by unconventional pairing. They exhibit a boundary-obstructed higher-order topological character and, depending on their dimensionality, feature unprecedently robust Majorana bound states or hinge modes protected by chiral symmetry. We predict the 112-family of iron pnictides, such as Ca$_{1-x}$La$_x$FeAs$_2$, to be a highly suited material candidate for our proposal, which can be tested by edge spectroscopy. Because of the boundary-obstruction, the topologically non-trivial feature of the 112 pnictides does not reveal itself for a bulk-only torus band analysis without boundaries, and as such had evaded previous investigations. Our proposal not only opens a new arena for highly stable Majorana modes in high-temperature superconductors, but also provides the smoking gun evidence for extended s-wave order in the iron pnictides.

International Trafficking in Persons and the Global Spread of Diseases: Cross-country Evidence from Two Modern Pandemics

This paper exploits cross-country differences in coastline distances (and drug trafficking inflows) to estimate the relative risk of trafficking in persons (TIP) inflows, finding smoking gun evidence that countries with a higher risk of TIP inflows have higher local confirmed cases during the H1N1 and COVID-19 pandemic. Institutional and health factors mainly influence confirmed numbers through the TIP channel, with their effects largely muted in the second stage. In addition, once the instrumented risk of TIP inflow is controlled for, migration flows, including flows originating from the pandemic source country, is no longer positively associated with local confirmed numbers. Controlling for COVID-19 testing numbers also produces the most precise estimates, indicating that the accuracy of reported numbers is indeed increasing in testing efforts.

On the wave optics effect on primordial black hole constraints from optical microlensing search

ABSTRACT Microlensing of stars, e.g. in the Galactic bulge and Andromeda galaxy (M31), is among the most robust, powerful method to constrain primordial black holes (PBHs) that are a viable candidate of dark matter. If PBHs are in the mass range $M_{\rm PBH} {\,\,\lesssim \,\,}10^{-10}\mathrm{ M}_\odot$, its Schwarzschild radius (rSch) becomes comparable with or shorter than optical wavelength (λ) used in a microlensing search, and in this regime the wave optics effect on microlensing needs to be taken into account. For a lensing PBH with mass satisfying rSch ∼ λ, it causes a characteristic oscillatory feature in the microlensing light curve, and it will give a smoking gun evidence of PBH if detected, because any astrophysical object cannot have such a tiny Schwarzschild radius. Even in a statistical study, e.g. constraining the abundance of PBHs from a systematic search of microlensing events for a sample of many source stars, the wave effect needs to be taken into account. We examine the impact of wave effect on the PBH constraints obtained from the r-band (6210 Å) monitoring observation of M31 stars in Niikura et al., and find that a finite source size effect is dominant over the wave effect for PBHs in the mass range MPBH ≃ [10−11, 10−10]M⊙. We also discuss that, if a denser cadence (10 s), g-band monitoring observation for a sample of white dwarfs over a year time-scale is available, it would allow one to explore the wave optics effect on microlensing light curve, if it occurs, or improve the PBH constraints in $M_{\rm PBH} {\,\,\lesssim \,\,}10^{-11}\mathrm{ M}_\odot$ even from a null detection.

Electrically tunable chiral Majorana edge modes in quantum anomalous Hall insulator–topological superconductor systems

Chiral Majorana edge modes are theoretically proposed to perform braiding operations for the potential quantum computation. Here, we suggest a scheme to regulate trajectories of the chiral Majorana fermion based on a quantum anomalous Hall insulator (QAHI)-topological superconductor heterostructure. An applied external gate voltage to the QAHI region introduces a dynamical phase so that the outgoing Majorana fermions can be prominently tuned to different leads. The trajectory is mechanically analyzed and the electrical manipulation is represented by the oscillating transmission coefficients versus the gate voltage. Through the optimization of devices, the conductance is likewise detectable to be periodically oscillating, which means an experimental control of chiral Majorana edge modes. Besides, this oscillating period which is robust against disorder also provides an attainable method of observing the energy dispersion relation of the edge mode of the QAHI. Furthermore, the oscillating behavior of conductance serves as smoking-gun evidence of the existence of the chiral Majorana fermion, which could be experimentally confirmed.

AGN fueling and feedback

AbstractDynamical mechanisms are essential to exchange angular momentum in galaxies, drive the gas to the center, and fuel the central super-massive black holes. While at 100pc scale, the gas is sometimes stalled in nuclear rings, recent observations reaching ∼10pc scale have revealed, within the sphere of influence of the black hole, smoking gun evidence of fueling. Observations of AGN feedback are described, together with the suspected responsible mechanisms. Molecular outflows are frequently detected in active galaxies with ALMA and NOEMA, with loading factors between 1 and 5. When driven by AGN with escape velocity, these outflows are therefore a clear way to moderate or suppress star formation. Molecular disks, or tori, are detected at 10pc-scale, kinematically decoupled from their host disk, with random orientation. They can be used to measure the black hole mass.

Non-exploding and exploding core-collapse supernova models and the multimessenger predictions

AbstractWe present results of full general relativistic (GR), three-dimensional (3D) core-collapse simulation of a massive star with multi-energy neutrino transport. Using a 70Mȯ zero-metallicity star, we show that the black-hole (BH) formation occurs at ∼ 300 ms after bounce. At a few ∼ 10 ms before the BH formation, we find that the stalled bounce shock is revived by neutrino heating from the forming hot proto-neutron star (PNS), which is aided by vigorous convection behind the shock. Our numerical results present the first evidence to validate the BH formation by the so-called fallback scenario. Furthermore we present results from a rapidly rotating core-collapse model of a 27Mȯ star that is trending towards an explosion. We point out that the correlated neutrino and gravitational-wave signatures, if detected, could provide a smoking-gun evidence of rapid rotation of the newly-born PNS.

Circum-nuclear molecular disks: Role in AGN fueling and feedback

AbstractGas fueling AGN (Active Galaxy Nuclei) is now traceable at high-resolution with ALMA (Atacama Large Millimeter Array) and NOEMA (NOrthern Extended Millimeter Array). Dynamical mechanisms are essential to exchange angular momentum and drive the gas to the super-massive black hole. While at 100pc scale, the gas is sometimes stalled in nuclear rings, recent observations reaching 10pc scale (50mas), may bring smoking gun evidence of fueling, within a randomly oriented nuclear gas disk. AGN feedback is also observed, in the form of narrow and collimated molecular outflows, which point towards the radio mode, or entrainment by a radio jet. Precession has been observed in a molecular outflow, indicating the precession of the radio jet. One of the best candidates for precession is the Bardeen-Petterson effect at small scale, which exerts a torque on the accreting material, and produces an extended disk warp. The misalignment between the inner and large-scale disk, enhances the coupling of the AGN feedback, since the jet sweeps a large part of the molecular disk.

Charge-spin response and collective excitations in Weyl semimetals

Weyl semimetals are characterized by unconventional electromagnetic response. We present analytical expressions for all components of the frequency- and wave-vector-dependent charge-spin linear-response tensor of Weyl fermions. The spin-momentum locking of the Weyl Hamiltonian leads to a coupling between charge and longitudinal spin fluctuations, while transverse spin fluctuations remain decoupled from the charge. A real Weyl semimetal with multiple Weyl nodes can show this charge-spin coupling in equilibrium if its crystal symmetry is sufficiently low. All Weyl semimetals are expected to show this coupling if they are driven into a non-equilibrium stationary state with different occupations of Weyl nodes, for example by exploiting the chiral anomaly. Based on the response tensor, we investigate the low-energy collective excitations of interacting Weyl fermions. For a local Hubbard interaction, the charge-spin coupling leads to a dramatic change of the zero-sound dispersion: its velocity becomes independent of the interaction strength and the chemical potential and is given solely by the Fermi velocity. In the presence of long-range Coulomb interactions, the coupling transforms the plasmon modes into spin plasmons. For real Weyl semimetals with multiple Weyl nodes, the collective modes are strongly affected by the presence of parallel static electric and magnetic fields, due to the chiral anomaly. In particular, the zero-sound frequency at fixed momentum and the spin content of the spin plasmons go through cusp singularities as the chemical potential of one of the Weyl cones is tuned through the Weyl node. We discuss possible experiments that could provide smoking-gun evidence for Weyl physics.

Evidence for Optically Thick, Eddington-limited Winds Driven by Supercritical Accretion

Abstract Supercritical accretion onto compact objects powers a massive wind that is optically thick and Eddington-limited. If most of the hard X-rays from the central disk are obscured by the wind, the source will display a blackbody-like spectrum with a luminosity scaled with the mass of the compact object. From the Chandra archive of nearby galaxies, we selected a sample of luminous and very soft sources and excluded contamination from foreground objects and supernova remnants. They are found to be preferentially associated with late-type galaxies. The majority of sources in our sample are either too hot or too luminous to be explained by nuclear burning on the surface of white dwarfs, and are argued to be powered by accretion. The most likely explanation is that they are due to emission from the photosphere of a wind driven by supercritical accretion onto compact objects. Their blackbody luminosity ranges from ∼1037 to nearly 1040 erg s−1, indicative of the presence of both neutron stars and stellar-mass black holes. The blackbody luminosity also shows a possible bimodal distribution, albeit at low significance, peaked around the Eddington limit for neutron stars and stellar-mass black holes, respectively. If this can be confirmed, it will be smoking gun evidence that supercritical accretion powers thick winds. Based on a wind model, the inferred mass accretion rate of these objects is around a few hundred times the Eddington rate, suggesting that they may be intermediate between the canonical ultraluminous X-ray sources and SS 433 in terms of the accretion rate.

Long-lived remnants from binary neutron star mergers

Massive neutron star (NS) with lifetimes of at least several seconds are expected to be the result of a sizable fraction of NS mergers. We study their formation using a large set of numerical relativity simulations. We show that they are initially endowed with angular momentum that significantly exceeds the mass-shedding limit for rigidly-rotating equilibria. We find that gravitational-wave (GW) emission is not able to remove this excess angular momentum within the time over which solid body rotation should be achieved. Instead, we argue that the excess angular momentum could be carried away by massive winds. Long-lived merger remnants are also formed with larger gravitational masses than those of rigidly-rotating NSs having the same number of baryons. The excess mass is likely radiated in the form of neutrinos. The evolution of long-lived remnants on the viscous timescale is thus determined by the interplay of finite-temperature effects, mass ejection, and neutrinos with potentially dramatic consequences for the remnants' properties and stability. We also provide an empirical fit for the spin of the remnant at the end of its viscous evolution as a function of its final mass, and we discuss the implications for the magnetar model of short gamma-ray bursts (SGRBs). Finally, we investigate the possible electromagnetic signatures associated with the viscous ejecta. Massive outflows possibly resulting from the formation of long-lived remnants would power unusually bright, blue kilonova counterparts to GW events and SGRBs whose detection would provide smoking gun evidence for the formation of long-lived remnants.

Closing the Window of Vulnerability: Nuclear Proliferation and Conventional Retaliation

Living with a nuclear-armed enemy is unattractive, but, strangely, states seldom use their military power to prevent the enemy’s entry into the nuclear club. It is puzzling why preventive strikes against nuclear programs have been quite rare. I address this puzzle by considering the role of conventional retaliation, a subfield of deterrence that so far has received scant attention in the literature. I theorize the concept of conventional retaliation and test its explanatory power. First, I explore all historical cases where states struck another state’s nuclear installations and find none occurring when the proliferator threatened conventional retaliation. Second, I explore two cases where a strike was most likely, but the would-be attacker balked and find smoking-gun evidence that the threat of conventional retaliation restrained the would-be attacker. This evidence supports my claim that the threat of conventional retaliation is sufficient to deter a preventive strike against emerging nuclear states.

On the Efficiency of Thermal Conduction in Galaxy Clusters

Abstract Galaxy clusters host a large reservoir of diffuse plasma with radially varying temperature profiles. The efficiency of thermal conduction in the intracluster medium (ICM) is complicated by the existence of turbulence and magnetic fields, and has received a lot of attention in the literature. Previous studies suggest that the magnetothermal instability developed in outer regions of galaxy clusters would drive magnetic field lines to be preferentially radial, resulting in efficient conduction along the radial direction. Using a series of spherically symmetric simulations, here we investigate the impact of thermal conduction on the observed temperature distributions in the outer regions of three massive clusters, and find that thermal conduction substantially modifies the ICM temperature profile. Within 3 Gyr, the gas temperature at a representative radius of 0.3r 500 typically decreases by ∼10%–20% and the average temperature slope between 0.3r 500 and r 500 drops by ∼30%–40%, indicating that the observed ICM would not stay in a long-term equilibrium state in the presence of thermal conduction. However, X-ray observations show that the outer regions of massive clusters have remarkably similar radially declining temperature profiles, suggesting that they should be quite stable. Our study thus suggests that the effective conductivity along the radial direction must be suppressed below the Spitzer value by a factor of 10 or more, unless additional heating sources offset conductive cooling and maintain the observed temperature distributions. Our study provides smoking-gun evidence for the suppression of parallel conduction along magnetic field lines in low-collisionality plasmas by kinetic mirror or whistler instabilities.

Near-infrared Emission Lines in Starburst Galaxies at 0.5 < z < 0.9: Discovery of a Merger Sequence of Extreme Obscurations

Abstract We obtained optical/near-IR rest-frame Magellan FIRE spectra (including Paβ and Paγ) of 25 starburst galaxies at 0.5 < z < 0.9, with average star formation rates (SFRs) seven times above the main sequence (MS). We find that Paschen-to-Balmer line ratios saturate around a constant value corresponding to A V ∼ 2–3 mag, while line-to-IR-luminosity ratios suggest a large range of more extreme obscurations and appear to be uncorrelated with the former. This behavior is not consistent with standard attenuation laws derived for local and distant galaxies, yet is remarkably consistent with observations of starburst cores in which young stars and dust are homogeneously mixed. This model implies A V = 2–30 mag attenuation to the center of starburst cores, with a median of ∼9 mag (a factor of 4000). X-ray hardness ratios for six AGNs in our sample and column densities derived from observed dust masses and radio sizes independently confirm this level of attenuation. In these conditions observed optical/near-IR emission comes from surface regions, while inner starburst cores are invisible. We thus attribute the high [N ii]/Hα ratios to widespread shocks from accretion, turbulence, and dynamic disturbances rather than to AGNs. The large range of optical depths demonstrates that substantial diversity is present within the starburst population, possibly connected to different merger phases or progenitor properties. The majority of our targets are, in fact, morphologically classified as mergers. We argue that the extreme obscuration provides in itself smoking gun evidence of their merger origin, and a powerful tool for identifying mergers at even higher redshifts.

3D AMR hydrosimulations of a compact-source scenario for the Galactic Centre cloud G2

The nature of the gaseous and dusty cloud G2 in the Galactic Centre is still under debate. We present three-dimensional hydrodynamical adaptive mesh refinement (AMR) simulations of G2, modeled as an outflow from a "compact source" moving on the observed orbit. The construction of mock position-velocity (PV) diagrams enables a direct comparison with observations and allow us to conclude that the observational properties of the gaseous component of G2 could be matched by a massive ($\dot{M}_\mathrm{w}=5\times 10^{-7} \;M_{\odot} \mathrm{yr^{-1}}$) and slow ($50 \;\mathrm{km \;s^{-1}}$) outflow, as observed for T Tauri stars. In order for this to be true, only the material at larger ($>100 \;\mathrm{AU}$) distances from the source must be actually emitting, otherwise G2 would appear too compact compared to the observed PV diagrams. On the other hand, the presence of a central dusty source might be able to explain the compactness of G2's dust component. In the present scenario, 5-10 years after pericentre the compact source should decouple from the previously ejected material, due to the hydrodynamic interaction of the latter with the surrounding hot and dense atmosphere. In this case, a new outflow should form, ahead of the previous one, which would be the smoking gun evidence for an outflow scenario.

π Spin Berry Phase in a Quantum-Spin-Hall-Insulator-Based Interferometer: Evidence for the Helical Spin Texture of the Edge States.

The quantum spin Hall insulator is characterized by helical edge states, with the spin polarization of the electron being locked to its direction of motion. Although the edge-state conduction has been observed, unambiguous evidence of the helical spin texture is still lacking. Here, we investigate the coherent edge-state transport in an interference loop pinched by two point contacts. Because of the helical character, the forward interedge scattering enforces a π spin rotation. Two successive processes can only produce a nontrivial 2π or trivial 0 spin rotation, which can be controlled by the Rashba spin-orbit coupling. The nontrivial spin rotation results in a geometric π Berry phase, which can be detected by a π phase shift of the conductance oscillation relative to the trivial case. Our results provide smoking gun evidence for the helical spin texture of the edge states. Moreover, it also provides the opportunity to all electrically explore the trajectory-dependent spin Berry phase in condensed matter.

Numerical simulations of stellar collapse in scalar-tensor theories of gravity

We present numerical-relativity simulations of spherically symmetric core collapse and compact-object formation in scalar-tensor theories of gravity. The additional scalar degree of freedom introduces a propagating monopole gravitational-wave mode. Detection of monopole scalar waves with current and future gravitational-wave experiments may constitute smoking gun evidence for strong-field modifications of general relativity. We collapse both polytropic and more realistic pre-supernova profiles using a high-resolution shock-capturing scheme and an approximate prescription for the nuclear equation of state. The most promising sources of scalar radiation are protoneutron stars collapsing to black holes. In case of a galactic core collapse event forming a black hole, Advanced LIGO may be able to place independent constraints on the parameters of the theory at a level comparable to current solar-system and binary-pulsar measurements. In the region of the parameter space admitting spontaneously scalarised stars, transition to configurations with prominent scalar hair before black-hole formation further enhances the emitted signal. Although a more realistic treatment of the microphysics is necessary to fully investigate the occurrence of spontaneous scalarisation of neutron star remnants, we speculate that formation of such objects could constrain the parameters of the theory beyond the current bounds obtained with solar-system and binary-pulsar experiments.

Footprints of New Strong Dynamics via Anomaly and the 750GeV Diphoton.

The chiral anomaly provides smoking-gun evidence of a new confining gauge theory. Motivated by a reported event excess in a diphoton invariant mass distribution at the LHC, we discuss a scenario that a pseudo-Nambu-Goldstone (PNG) boson of a new QCD-like theory is produced by gluon fusion and decays into a pair of the standard model gauge bosons. Despite the strong dynamics, the production cross section and the decay widths are determined by an anomaly matching condition. The excess can be explained by the PNG boson with mass of around 750GeV. The model also predicts exotic hadrons such as a color-octet scalar and baryons. Some of them are within the reach of the LHC experiment.