Collision Region Research Articles

Particle-in-cell simulations of THz emission from plasma by oblique collision of two-electron beams

We studied the THz radiation generated by a beam-plasma system using two-dimensional (2D) particle-in-cell (PIC) simulations. The Langmuir waves excited by two counterpropagating electron beams, via two-stream instability, collide with each other at an oblique angle, which forms a high beam-density modulation near the collision region, where both beam electrons become trapped. As a result, spatially localized Langmuir wave packets with large longitudinal-electric field amplitudes are formed, which give rise to bursts of electromagnetic radiation. Our 2D PIC simulations of the two thin, low-density, asymmetric, electron beams colliding obliquely show that a strong THz emission is obtained at the second harmonic of the plasma frequency (f = 1.0 THz), with a narrow spectral width (∼0.80%) in vacuum and significantly higher efficiency than the head-on-collision case. The efficiency of power conversion from electron beams to THz radiation measured in vacuum reaches around ∼0.0289, for a continuous injection of beams into the plasma, making it suitable for applications requiring high-power narrow-band THz radiation sources.

Study of Z Bosons Produced in Association with Charm in the Forward Region

Events containing a Z boson and a charm jet are studied for the first time in the forward region of proton-proton collisions. The data sample used corresponds to an integrated luminosity of 6 fb^{-1} collected at a center-of-mass energy of 13 TeV with the LHCb detector. In events with a Z boson and a jet, the fraction of charm jets is determined in intervals of Z-boson rapidity in the range 2.0<y(Z)<4.5. A sizable enhancement is observed in the forwardmost y(Z) interval, which could be indicative of a valencelike intrinsic-charm component in the proton wave function.

Assessment of standing passenger traumatic brain injury caused by ground impact in subway collisions

Human head is the most vulnerable region in subway collisions. To design a safer subway, the head impact biomechanical response should be studied first. This paper aims to investigate the standing passenger head-ground impact dynamic response and traumatic brain injury (TBI) in subway collisions. A standing passenger-subway interior dynamic model was numerically developed by using our previous validated finite element (FE)-multibody (MB) coupled human body model, which was integrated by the Total Human Model for Safety (THUMS) head-neck FE model and the extracted remaining body segments pedestrian MB model of TNO. A parametric study considering the handrail type, standing angle, and friction coefficient between the shoes and ground was performed. Results show that the passenger dynamic response could be divided into two categories according to whether the passenger hit handrails. Passenger TBIs severity could be efficiently alleviated by the passenger body (excluding the head) hitting the handrail first before head-ground impact. The probabilities of DAI in the cerebellum and brain stem were low. A statistical analysis of TBIs demonstrated that the risks of TBIs were sensitive to the handrail type in subway collisions, but did not to the standing angle and friction coefficient. This study provides practical help for improving the interior crashworthiness performance of subways.

Improving the quality of Al-Fe tube joints manufactured via magnetic pulse welding using an inclined-wall field shaper

Due to the mismatched impact parameters between the velocity and the angle, a joint with an unbonded zone in the middle and the microdefects at the interface was fabricated by magnetic pulse welding (MPW). In this study, an oblique angle in the working zone was prefabricated to make an inclined-wall field shaper (FS) for MPW and thereby improve the quality of the joint. The effects of the span and size of the inclined wall on the impact parameters were evaluated by numerical simulations. With increasing span, the location where the maximum impact velocity occurred approached the free end of the joint. With increasing size of angle, the initial impact angle evidently increased. The microstructure indicated that the flat interface was changed into the wavy one, and the thick transition layer with microdefects could be removed using an inclined-wall FS. The double bonding zone was changed into the single one, which formed a continuous range with a length of above 3 mm where the impact parameters matched with each other. The length of the unbonded zone in the initial collision zone decreased from about 1 mm to 0.3 mm via using an inclined-wall FS with the span of 6.4 mm and the size of 5°. Shear test results showed that the zones with high strength were merged in the initial collision region. The maximum shear strength of about 81 MPa could be obtained using an inclined-wall FS with the size of 3° and the span of 6.4 mm, which was increased by 33% higher than that without an inclined wall.

PeV Photon and Neutrino Flares from Galactic Gamma-Ray Binaries

Abstract The high-energy radiation from short period binaries containing a massive star with a compact relativistic companion was detected from radio to TeV γ-rays. We show here that PeV regime protons can be efficiently accelerated in the regions of collision of relativistic outflows of a compact object with stellar winds in these systems. The accelerated proton spectra in the presented Monte Carlo model have an upturn in the PeV regime and can provide very hard spectra of sub-PeV photons and neutrinos by photomeson processes in the stellar radiation field. The recent report of a possible sub-PeV γ-ray flare in coincidence with a high-energy neutrino can be understood in the frame of this model. The γ-ray binaries may contribute substantially to the Galactic component of the detected high-energy neutrino flux.

η Carinae with Fermi-LAT: two full orbits and the third periastron

Context. Colliding-wind binaries are massive stellar systems featuring strong, interacting winds. These binaries may be actual particle accelerators, making them variable γ-ray sources due to changes in the wind collision region along the orbit. However, only two of these massive stellar binary systems have been identified as high-energy sources. The first and archetypical system of this class is η Carinae, a bright γ-ray source with orbital variability peaking around its periastron passage. Aims. The origin of the high-energy emission in η Carinae is still unclear, with both lepto-hadronic and hadronic scenarios being under discussion. Moreover, the γ-ray emission seemed to differ between the two periastrons previously observed with the Fermi-Large Area Telescope. Continuing observations might provide highly valuable information for understanding the emission mechanisms in this system. Methods. We have used almost 12 yr of data from the Fermi-Large Area Telescope. We studied both low- and high-energy components, searching for differences and similarities between both orbits, and we made use of this large dataset to search for emission from nearby colliding-wind binaries. Results. We show how the energy component above 10 GeV of η Carinae peaks months before the 2014 periastron, while the 2020 periastron is the brightest one to date. Additionally, upper limits are provided for the high-energy emission in other particle-accelerating colliding-wind systems. Conclusions. Current γ-ray observations of η Carinae strongly suggest that the wind collision region of this system is perturbed from orbit to orbit, affecting particle transport within the shock.

Space-average electromagnetic fields and electromagnetic anomaly weighted by energy density in heavy-ion collisions

We study the space-averaged electromagnetic (EM) fields weighted by the energy density in the central regions of heavy-ion collisions. These average quantities can serve as a barometer for the magnetic-field induced effects such as the magnetic effect, the chiral separation effect and the chiral magnetic wave. Comparing with the magnetic fields at the geometric center of the collision, the space-averaged fields weighted by the energy density are smaller in the early stage but damp slower in the later stage. The space average of squared fields as well as the EM anomaly $\mathbf{E}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{B}$ weighted by the energy density are also calculated. We give parametrized analytical formulas for these average quantities as functions of time by fitting numerical results for collisions in the collision energy range $7.7--200$ GeV with different impact parameters.

Effect of cross-slot configuration in microfluidics on o/w emulsification at high throughput

The Liquid–liquid dispersion process is experimentally investigated to manufacture oil-in-water emulsion working at high flowrates in the cross-slot type of microfluidics. Two cross layouts, namely symmetric and asymmetric configurations, are compared via characterizing the droplet size and size distribution. Automated granulometry is implemented on the images taken by microscopy observations of the emulsion samples. High-speed shadow photography is carried out to discover the continuous- and dispersed-phase flow interactions in the vicinity of the collision region. The results show that the designed microsystems present great potential in terms of fabricating fine oil droplets distributed in the final emulsion. The arithmetic averaged diameter is less than 10 μm at all tested flow conditions and the minimum mean diameter reaches 3.9 μm at the highest energy consumption case. Because of the higher shear stress and more intensified interaction, the symmetric geometry of the cross-slot is beneficial to create fewer amounts of large droplets and dispersing the oil phase more uniformly at the same hydrodynamic conditions, especially in the low Reynolds flow case in this study. As the flowrate is enhanced, the disparity between them is diminished due to the instability inside the channel reaching a high level. The mean drop diameter for both systems is capable to be scaled with the emulsion velocity-based Weber number. The detrimental effect of the symmetric configuration is that the energy required to burst the dispersed streams is relatively a little higher than with asymmetric one.

Hyperons from Bi + Bi Collisions at MPD-NICA: Preliminary Analysis of Production at Generation, Simulation and Reconstruction Level

An important observable to understand the properties of the matter produced in heavy-ion collisions is its strangeness content. Recent experimental results show that in semi-central collisions, the Λ and $$\bar {\Lambda }$$ global polarization show differences that increase at low energies. This behaviour has been described using a model where these particles may be produced from two distinct density zones in the collision region: the core and the corona where QGP processes and p + p like reactions, respectively, are mainly at work. Using this idea, the polarization can be influenced by the relative abundance of these particles coming from either regions. In this work we show how to test this model in the MPD experiment.

Cosmological particle creation in the little bang

Particle production by expanding in the future light cone scalar quantum field is studied by assuming that the initial state is associated with the quasiequilibrium statistical operator corresponding to fluid dynamics. We calculate particle production from a longitudinally boost-invariant expanding quantum field designed as a simple but reliable model for the central rapidity region of a relativistic collision. Exact diagonalization of the model is performed by introducing a notion of quasiparticles.

Unmanned Aerial Vehicle Mid-Air Collision Detection and Resolution Using Avoidance Maps

Collision avoidance for fixed-wing unmanned aerial vehicles (UAVs), using a novel methodology of avoidance map, is addressed in this work. Avoidance map is defined as a mapping in the control input space of a pair of UAVs, which partitions the control space into avoidance and collision regions, respectively. The choice of maneuvers to execute successful avoidance can be effectively obtained by using the avoidance map. The duration of control application is determined by a simple method that is time optimal and computationally efficient. Different versions of the map portraying the evolution of the concept are also given. Precision control, defined as a gradual reduction in control, is discussed to reduce the control input demand for the entire duration of avoidance. The concept is extended to multiple UAVs, and the additional requirement of the UAVs to return to nominal trajectory after avoidance, using Dubins paths, is also addressed. Finally, simulation results using realistic UAV models are considered, and some needed modifications to the basic avoidance strategy, made necessary by realistic maneuver models, are discussed. Several illustrative simulation results are given to support the applicability of the proposed method.

Determination of dominant loop in dual loop atrial tachycardia with direct graph mapping: an alternative for entrainment mapping?

Abstract Funding Acknowledgements Type of funding sources: None. Background Entrainment mapping (EM) is an important tool to determine the mechanism of complex reentrant atrial tachycardias (ATs), mostly to distinguish dominant from bystander reentrant loops. However, entrainment maneuvers are challenging, time consuming and risk to end the tachycardia. Purpose Recently, we developed a novel method Directed Graph Mapping (DGM), using concepts of network theory, allowing to automatically determine AT reentry loops from the local activation times (LAT) of any clinical mapping system. DGM showed good performance: it correctly finds ablation target (100 % success rate) on simple AT cases and could automatically determine reentry loops confirmed by the expert electrophysiologist with EM in complex AT cases. Out of 32 single loop cases, 62.5 % was identified correctly with automated DGM and out of 6 true double loop cases, 83.3 %. Lower performance for single reentry complex cases compared to EM was mainly because DGM could not distinguish the dominant loop from additional bystander loops found by DGM. Hence, the purpose of this work was to develop additional algorithms which in case of multiple found DGM loops could automatically find the dominant loop and compare it with the results of EM. Methods We performed multiple simulations of various types of double loop reentry on a patient specific model of the left atrium. Based on a clinical case, double loops were simulated around a scar at the anterior wall (localized reentry) and the mitral valve (MV). LAT maps were determined similar as in the clinic. By varying the size of the scar in multiple steps, we obtained a transition from a regime of a dominant loop around the scar (small scar), to a true double loop and further to a regime of a dominant loop around the MV (large scar). We developed a novel DGM algorithm to determine the dominant loop from the region of collision (ROC) found from the vector field of the wavefront graph. The developed method was also tested on 8 clinical cases of double loop ATs with EM measurements. Results Our algorithm found the location of the ROC and determined the correct dominant loop in 100% of the simulated data. We tested this on 8 clinical cases of AT, and accuracy of the method was 75 %. Conclusions Determining the ROC in case of multiple loops in AT could correctly determine the dominant versus bystander loop, leading to the correct ablation target, without the need for further EM.

Relativistic fluid modelling of gamma-ray binaries

Context. In the first paper of this series, we presented a numerical model for the non-thermal emission of gamma-ray binaries in a pulsar-wind-driven scenario. Aims. We apply this model to one of the best-observed gamma-ray binaries, the LS 5039 system. Methods. The model involves a joint simulation of the interaction between the pulsar wind and the stellar wind and the transport of electron pairs from the pulsar wind accelerated at the emerging shock structure. We compute the synchrotron and inverse Compton emission in a post-processing step while consistently accounting for relativistic beaming and γγ-absorption in the stellar radiation field. Results. The wind interaction leads to the formation of an extended, asymmetric wind collision region that develops strong shocks, turbulent mixing, and secondary shocks in the turbulent flow. Both the structure of the collision region and the resulting particle distributions show significant orbital variation. In addition to the acceleration of particles at the bow-like pulsar wind and the Coriolis shock, the model naturally accounts for the re-acceleration of particles at secondary shocks that contribute to the emission at very-high-energy (VHE) gamma-rays. The model successfully reproduces the main spectral features of LS 5039. While the predicted light curves in the high-energy and VHE gamma-ray band are in good agreement with observations, our model still does not reproduce the X-ray to low-energy gamma-ray modulation, which we attribute to the employed magnetic field model. Conclusions. We successfully model the main spectral features of the observed multi-band, non-thermal emission of LS 5039 and thus further substantiate a wind-driven interpretation of gamma ray binaries. Open issues relate to the synchrotron modulation, which might be addressed through a magnetohydrodynamic extension of our model.

A Multiwavelength Study of the Formation of the AFGL 333-Ridge

Abstract We presented a multiwavelength study of the AFGL 333-Ridge. The molecular line data reveals that the AFGL 333-Ridge has two independent velocity components at −50.5 and −48.0 km s−1. In the position–velocity diagram, the bridge feature connects with two parts that are spatially correlated but separated in velocity. This observational evidence supports the scenario that the two velocity components have collided and merged into one molecular cloud. The majority of Class I young stellar objects (YSOs) are distributed within the collision region, suggesting that the cloud–cloud collision has induced the YSOs’ formation in the ridge. Using the radio recombination line (RRL) data obtained by the Five-hundred-meter Aperture Spherical radio Telescope, the RRL velocities of three H ii regions are consistent with that of the AFGL 333-Ridge. By comparing the three H ii regions’ dynamical ages with the collision timescale of the two components, we conclude that the influence of the three H ii regions may not drive the two clouds to merge. The formation of the AFGL 333-Ridge is probably due to the expansion of the giant H ii region W4.

Distribution of nuclear matter and radiation in the fragmentation region

We study the fragmentation (far forward/backward) region of heavy ion collisions by considering an at-rest nucleus which is struck by a relativistic sheet of colored glass. By means of a simple classical model, we calculate the subsequent evolution of baryons and the associated radiation. We confirm that the struck nucleus undergoes a compression and that the dynamics of the early times of the collision are best described by two separate fluids as the produced radiation's velocity distribution is very different to the velocity distribution of the matter in the struck nucleus.

Particle acceleration and non-thermal emission in colliding-wind binary systems

ABSTRACT We present a model for the creation of non-thermal particles via diffusive shock acceleration in a colliding-wind binary. Our model accounts for the oblique nature of the global shocks bounding the wind–wind collision region and the finite velocity of the scattering centres to the gas. It also includes magnetic field amplification by the cosmic ray induced streaming instability and the dynamical back reaction of the amplified field. We assume that the injection of the ions and electrons is independent of the shock obliquity and that the scattering centres move relative to the fluid at the Alfvén velocity (resulting in steeper non-thermal particle distributions). We find that the Mach number, Alfvénic Mach number, and transverse field strength vary strongly along and between the shocks, resulting in significant and non-linear variations in the particle acceleration efficiency and shock nature (turbulent versus non-turbulent). We find much reduced compression ratios at the oblique shocks in most of our models compared to our earlier work, though total gas compression ratios that exceed 20 can still be obtained in certain situations. We also investigate the dependence of the non-thermal emission on the stellar separation and determine when emission from secondary electrons becomes important. We finish by applying our model to WR 146, one of the brightest colliding wind binaries in the radio band. We are able to match the observed radio emission and find that roughly 30 per cent of the wind power at the shocks is channelled into non-thermal particles.

Deadlock-Free Online Plan Repair in Multi-robot Coordination with Disturbances

Multirobot systems are increasingly deployed in environments where they interact with humans. From the perspective of a robot, such interaction could be considered a disturbance that causes a well-planned trajectory to fail. Previous approaches that modify trajectories in the presence of disturbances rearrange the order in which robots pass collision regions and other obstacles, in the laudable attempt to improve the average travel time for all robots. By doing so, however, deadlock may arise. In this paper, we provide a precise definition of deadlock using a graphical representation and prove some of its important properties. We show how to exploit the representation to detect the possibility of deadlock and to characterize conditions under which deadlock may not occur. We provide experiments in simulated environments that illustrate the potential usefulness of our theory of deadlock.

Interaction-Asymptotic Region Decomposition Method for a Triatomic Reactive Scattering with Symmetry Adoption.

For solving the "coordinate problem" in a product state-resolved calculation with the quantum wave packet method, an interaction-asymptotic region decomposition (IARD) method has been proposed for a general triatomic reactive scattering process. In the IARD method, the three asymptotic regions are represented by the corresponding Jacobi coordinates, but the hyperspherical coordinate is applied for representing the interaction region. For a triatomic reaction A + B2 with symmetry, explicit inclusion of all three channels in the calculations is unnecessary. Thus, numerical methods for exploring the symmetry of the A + B2 reaction need to be developed. Due to the symmetry of reactant B2, spherical harmonics with only even or odd number are required for representing the angular degree of freedom in the reactant channel and only one product channel needs to be considered. For representing the interaction region, the symmetry in the hyperspherical coordinate could also be explored to reduce the computational effort. The advantages of the IARD method with symmetry adoption were illustrated by calculating the product state-resolved reaction probabilities of the 16O + 36O2, 18O + 32O2, F + H2, and D+ + H2 reactions in the ultracold collision region. The numerical results calculated using the famous ABC code and the reactant coordinate-based method were provided for comparison.

The effect of hadronic scatterings on the measurement of vector meson spin alignments in heavy-ion collisions * *The work of D. Y. Shen and J. H. Chen was Supported in part by the Strategic Priority Research Program of Chinese Academy of Sciences (XDB34030000), the Guangdong Major Project of Basic and Applied Basic Research (2020B0301030008) and the National Natural Science Foundation

Spin alignments of vector mesons and hyperons in relativistic heavy-ion collisions have been proposed as signals of global polarization. The STAR experiment first observed the polarization. Recently, the ALICE collaboration measured the transverse momentum ( ) and the collision centrality dependence of , and spin alignments during Pb-Pb collisions at = 2.76 TeV. A large signal is observed in the low region of mid-central collisions for , while the signal is much smaller for , and these have not been understood yet. Since vector mesons have different lifetimes and their decay products have different scattering cross sections, they suffer from different hadronic effects. In this paper, we study the effect of hadronic interactions on the spin alignment of , , and mesons in relativistic heavy-ion collisions with a multi-phase transport model. We find that hadronic scatterings lead to a deviation of the observed spin alignment matrix element away from the true value for and mesons (with a bigger effect on ) while the effect is negligible for the meson. The effect depends on the kinematic acceptance: the observed value is lower than the true value when the pseudorapidity ( ) coverage is small, while there is little effect when the coverage is large. Hence, this study provides valuable information to understand the vector meson spin alignment signals observed during the experiments.

Stress–Strain State of the Earth’s Crust in the Central Caucasus

The Caucasus is a region of continental collision between Eurasia and Arabia and one of the Earth’s most seismically active and rapidly deformed continental regions. The wide range of deformation processes occurring in this relatively limited region makes the Caucasus as a whole and its central parts in particular a unique test area, the study of which can improve our understanding of the complexities of the stress–strain state, as well as the associated seismicity. During the instrumental period of observations, many strong earthquakes have been recorded here, e.g., the August 4, 1974 Bezhta earthquake (Mb = 5.4), the 1978 earthquake with a magnitude of 5.7, the February 23, 1981 Gavaz earthquake (Mb = 5), the October 12, 1992 Barisakho earthquake (Mb = 6.1), and the July 5, 2012 Zaqatala (Mb = 5.7). We have constructed the mechanisms of strong earthquakes and compared the results with the tectonic conditions of the studied region. Based on a morphostructural analysis of the modern relief of the central and eastern Greater Caucasus, a diagram of its morphostructural blocks has been compiled. The average earthquake mechanisms were constructed for the distinguished blocks, the prevailing types of slip in earthquake sources in each selected block were identified, and the fault kinematics were reconstructed. The solution was based on generally accepted tectonophysical theoretical views, with the wide application of modern structural–geomorphological methods, modern GIS technologies, and morphological analysis of the modern relief of this region. The modern tectonic stress field was reconstructed based on seismological data on earthquakes recorded during the entire instrumental period (1971–2019).