Transverse Momentum Conservation Research Articles

Symmetric cumulant sc2,4{4} and asymmetric cumulant ac2{3} from transverse momentum conservation and flow

Multiparticle cumulants method can be used to reveal long-range collectivity in small and large colliding systems. The four-particle symmetric cumulant sc2,4{4}, three-particle asymmetric cumulant ac2{3}, and the normalized cumulants nsc2,4{4} and nac2{3} from the transverse momentum conservation and flow are calculated. The interplay between the two effects is also investigated. Our results are in a good agreement with the recent ATLAS measurements of multiparticle azimuthal correlations with the subevent cumulant method, which provides insight into the origin of collective flow in small systems. Published by the American Physical Society 2024

KINEMATIC DESCRIPTION OF ALIGNMENT EFFECT IN COSMIC RAYS

The important role of kinematic constraints for the origin of the alignment of hadron and photon families observed by the Pamir collaboration in emulsion experiments with cosmic rays is discussed. Within the framework of the suggested approach it is shown that the high degree of alignment of the interaction products of the target nuclei and cosmic rays can be a consequence of the selection procedure of the most energetic clusters of particles together with the law of conservation of transverse momentum. The obtained results correctly describe the experimental data for three energetic centers and are also close enough to the measurements in the case of four and five clusters, which indicates encouraging prospects for the proposed method of explaining the alignment phenomenon.

On geometrical interpretation of alignment phenomenon

The observed alignment of spots in the X-ray films in cosmic ray emulsion experiments is analyzed and interpreted in the framework of geometrical approach. It is shown that the high degree of alignment can appear partly due to the selection procedure of most energetic particles itself and the threshold on the energy deposition together with the transverse momentum conservation.

Valley transport in strained silicene heterojunctions with zigzag and armchair interfaces

We study the carrier transport through a strained silicene heterojunction based on the tight-binding Hamiltonian, focusing on the effect of the interfaces, i.e., zigzag or armchair, on the valley polarization. We find that inter-valley transmission is prevented in the zigzag configuration owing to the conservation of transverse momentum, but this restriction is lifted for the armchair configuration. More importantly, the preservation of certain valley symmetries of the Fermi surfaces in the zigzag configuration even under strain distortion results in complete cancellation of the valley polarization after summing over all incident angles. In contrast, such symmetries are absent in the forward-propagating states for the strained armchair configuration, resulting in a sizable net valley polarization even after summation over all incident angles. Our findings provide insight toward the key role of interfaces in strained silicene-based valleytronics.

Multiparticle azimuthal cumulants from transverse momentum conservation and collective flow

We calculate the $n$th order of $2k$-particle azimuthal cumulants $c_n\{2k\}$ based on transverse momentum conservation (TMC) and collective flow $v_n$(n=2,3). We demonstrate that the TMC effect only leads to a nonzero $c_n\{2k\}$ with the sign of $(-1)^{nk}$ and the magnitude inversely proportional to $(N-2k)^{nk}$. The interplay between TMC and collective flow can change the signs of $c_2\{4\}$, $c_3\{2\}$ and $c_3\{4\}$ at some values of multiplicity $N$, which could provide a good probe to study the onset of collectivity and search for the substructure of proton in small colliding systems.

Air Plasma Mitigation of Shock Wave

Theory is presented to show that shock wave structure can be modified via interacting with an air plasma plume generated symmetrically in front of a shock wave generator. Electron jet in the plasma plume deflects the impinging airflow via momentum transfer collisions. Symmetrical deflection makes it easy to satisfy the conservation of transverse momentum between the airflow and plasma. The charge transfer between N2+ and N2 randomizes the impinging airflow to increase the deflection angle. The flow deflection increases the effective cone angle of the shock generator; as the effective cone angle exceeds a critical value, the shock becomes detached and faded.  Experiments were conducted in a Mach 2.5 wind tunnel to mitigate shock wave via air plasma deflection. A non-thermal air plasma was symmetrically generated, in front of a wind tunnel model by on-board 60 Hz periodic electric arc discharge, as a deflector for shock wave mitigation. The results show a transformation of the shock from a well-defined attached shock into a highly curved shock structure, which has increased shock angle and appears in diffused form. Shown in a sequence with increasing discharge intensity, the transformed curve shock in front of the model moves upstream to become detached with increasing standoff distance from the model and is eliminated when the discharge is near the peak. The steady of the incoming flow during the discharge cycle is manifested by the repetition of the baseline shock front. The experimental observations confirm the theory and efficacy of this air plasma mitigator.

A High-performance and Portable All-Mach Regime Flow Solver Code with Well-balanced Gravity. Application to Compressible Convection

Abstract Convection is an important physical process in astrophysics well-studied using numerical simulations under the Boussinesq and/or anelastic approximations. However, these approaches reach their limits when compressible effects are important in the high-Mach flow regime, e.g., in stellar atmospheres or in the presence of accretion shocks. In order to tackle these issues, we propose a new high-performance and portable code called “ARK” with a numerical solver well suited for the stratified compressible Navier–Stokes equations. We take a finite-volume approach with machine precision conservation of mass, transverse momentum, and total energy. Based on previous works in applied mathematics, we propose the use of a low-Mach correction to achieve a good precision in both low and high-Mach regimes. The gravity source term is discretized using a well-balanced scheme in order to reach machine precision hydrostatic balance. This new solver is implemented using the Kokkos library in order to achieve high-performance computing and portability across different architectures (e.g., multi-core, many-core, and GP-GPU). We show that the low-Mach correction allows to reach the low-Mach regime with a much better accuracy than a standard Godunov-type approach. The combined well-balanced property and the low-Mach correction allowed us to trigger Rayleigh–Bénard convective modes close to the critical Rayleigh number. Furthermore, we present 3D turbulent Rayleigh–Bénard convection with low diffusion using the low-Mach correction leading to a higher kinetic energy power spectrum. These results are very promising for future studies of high Mach and highly stratified convective problems in astrophysics.

Evaluation of photocathode emission properties in an electron gun: one-step photoemission from bulk band to vacuum states

A one-step photoemission analysis is developed, using the exact one-dimensional quantum solution for transmission over and through a triangular barrier presented by Forbes and Deane (2011 Proc. R. Soc. A 467 2927), to evaluate the emission properties of a photocathode in an electron gun. The analysis, which employs transverse momentum conservation in electron emission, includes the physical attributes (density of states and energy-momentum dispersion) of both the bulk band emission states and the recipient vacuum states in its evaluation of the mean transverse energy and relative quantum efficiency of the emitted electrons.

A remark on the sign change of the four-particle azimuthal cumulant in small systems

The azimuthal cumulants, $c_2\{2\}$ and $c_2\{4\}$, originating from the global conservation of transverse momentum in the presence of hydro-like elliptic flow are calculated. We observe the sign change of $c_2\{4\}$ for small number of produced particles. This is in a qualitative agreement with the recent ATLAS measurement of multi-particle azimuthal correlations with the subevent cumulant method.

Elliptic flow coefficients from transverse momentum conservation

We calculate the $k$-particle ($k=2,4,6,8$) azimuthal cumulants resulting from the conservation of transverse momentum. We find that $c_2\{k\}>0$ and depending on the transverse momenta, $c_2\{k\}$ can reach substantial values even for a relatively large number of particles. The impact of our results on the understanding of the onset of collectivity in small systems is emphasized.

Ab initio study of mode-resolved phonon transmission at Si/Ge interfaces using atomistic Green's functions

Solid interfaces with exceptionally low or high thermal conductance are of intense scientific and practical interest. However, realizing such interfaces is challenging due to a lack of knowledge of the phonon transmission coefficients between specific modes on each side of the interface and how the coefficients are affected by atomic scale structure. Here, we report an ab initio based study of phonon transmission at Si/Ge interfaces using a recent extension of the atomistic Green's function method that resolves transmission coefficients by mode. These results provide a detailed framework to investigate the precise transmission and reflection processes that lead to thermal resistance and how they depend on phonon frequency as well as incident angle. We find that the transmission and reflection processes can be partly explained with familiar concepts such as conservation of transverse momentum, but we also find that numerous phonons have zero transmission coefficient despite the existence of modes that satisfy transverse momentum conservation. This work provides detailed insights into precisely which phonons transmit or reflect at interfaces, knowledge necessary to design solid interfaces with extreme values of thermal conductance for thermoelectricity and heat management applications.

Relative-phase and time-delay maps all over the emission cone of hyperentangled photon source

High flux of hyperentangled photons entails collecting the two-photon emission over relatively wide extent in frequency and transverse space within which the photon pairs are simultaneously entangled in multiple degrees of freedom. In this paper, we present a numerical approach to determining the spatial-spectral relative-phase and time-delay maps of hyperentangled photons all over the spontaneous parametric down conversion (SPDC) emission cone. We consider the hyperentangled-photons produced by superimposing noncollinear SPDC emissions of two crossed and coherently-pumped nonlinear crystals. We adopt a vectorial representation for all parameters of concern. This enables us to study special settings such as the self-compensation via oblique pump incidence. While rigorous quantum treatment of SPDC emission requires Gaussian state representation, in low-gain regime (like the case of the study), it is well approximated to the first order to superposition of vacuum and two-photon states. The relative phase and time-delay maps are then calculated between the two-photon wavepackets created along symmetrical locations of the crystals. Assuming monochromatic plane-wave pump field, the mutual signal-idler relations like energy conservation and transverse-momentum conservation define well one of the two-photon with reference to its conjugate. The weaker conservation of longitudinal momentum (due to relatively thin crystals) allows two-photon emission directions coplanar with the pump beam while spreading around the perfect phase-matching direction. While prior works often adopt first-order approximation, it is shown that the relative-phase map is a very well approximated to a quadratic function in the polar angle of the two-photon emission while negligibly varying with the azimuthal angle.

Systematic Searches for the Chiral Magnetic Effect and the Chiral Vortical Effect Using Identi ed Particles at RHIC/STAR

STAR measurements of identified-particle correlations in Au+Au collisions at = 200 GeV are used to explore possible hierarchical structures in the particle-dependent correlations due to the chiral magnetic effect (CME) and the chiral vortical effect (CVE). Correlations of p-π (p-Λ), sensitive to the possible CME (CVE), are reported and compared with hadron-hadron (h-h) correlation, yielding a charge separation ordering (p-Λ > p-π ~ h-h). The estimated background strength from transverse momentum conservation (TMC) hardly shows any centrality dependence. For Au+Au collisions at 19.6, 39 and 200 GeV, clear signals above the background are observed for mid-central and peripheral collisions while results from central collisions (< 15%) can not separate from the background. For the results from collisions at 7.7 GeV, all are consistent with the TMC background although the errors are large.

The gauge-invariant canonical energy-momentum tensor

The canonical energy-momentum tensor is often considered as a purely academic object because of its gauge dependence. However, it has recently been realized that canonical quantities can in fact be defined in a gauge-invariant way provided that strict locality is abandoned, the non-local aspect being dictacted in high-energy physics by the factorization theorems. Using the general techniques for the parametrization of non-local parton correlators, we provide for the first time a complete parametrization of the energy-momentum tensor (generalizing the purely local parametrizations of Ji and Bakker-Leader-Trueman used for the kinetic energy-momentum tensor) and identify explicitly the parts accessible from measurable two-parton distribution functions (TMDs and GPDs). As by-products, we confirm the absence of model-independent relations between TMDs and parton orbital angular momentum, recover in a much simpler way the Burkardt sum rule and derive three similar new sum rules expressing the conservation of transverse momentum.

Spatial and spectral properties of entangled photons from spontaneous parametric down-conversion with a focused pump

The spatial and spectral properties of entangled photons from spontaneous parametric down-conversion (SPDC) with a focused pump are investigated with theoretical analysis and numerical simulation. Here, the spatial and spectral properties of down-converted photons are fully calculated without using the transverse momentum conservation assumption. We have obtained the spatial and spectral properties of SPDC photons under focused pumping in various SPDC configurations, including type-I, type-II for both positive and negative uniaxial crystals. Our result not only helps to understand the effect of pump focusing in SPDC better but also can find use in application of SPDC in realizing quantum information protocols.

Electronic transmission throughAB-BAdomain boundary in bilayer graphene

We study the electron transmission through the domain boundary on bilayer graphene separating AB and BA stacking regions. Using the effective continuum model, we calculate the electron transmission probability as a function of the electron energy and the incident angle, for several specific boundary structures. The transmission strongly depends on the crystallographic direction of the boundary and also on the atomic configuration inside. At the low energy, the boundary is either insulating or highly transparent depending on the structure. In insulating cases, the transmission sharply rises when the Fermi energy is increased to a certain level, suggesting that the electric current through the boundary can be controlled by the field effect. The boundary parallel to the zigzag direction generally have different transmission properties between the two different valleys, and this enables to generate the valley polarized current in a certain configuration. We show that those characteristic features can be qualitatively explained by the transverse momentum conservation in the position-dependent band structure in the intermediate region.

Correlations from hydrodynamic flow in pPb collisions

Two-particle correlations in relative rapidity and azimuth are studied for the p-Pb collisions at the LHC energy of 5.02 TeV in the framework of event-by-event 3+1-dimensional viscous hydrodynamics. It is found that for the highest-multiplicity events the observed ridge structures appear in a natural way, suggesting that collective flow may be an important element in the evolution of the system. We also discuss the role of the charge balancing and the transverse-momentum conservation.

Event-by-event background in estimates of the chiral magnetic effect

In terms of the parton hadron string dynamics (PHSD) approach---including the retarded electromagnetic field---we investigate the role of fluctuations of the correlation function in the azimuthal angle $\ensuremath{\psi}$ of charged hadrons that is expected to be a sensitive signal of local strong parity violation. For the early time we consider fluctuations in the position of charged spectators resulting in electromagnetic field fluctuations as well as in the position of participant baryons defining the event plane. For partonic and hadronic phases in intermediate stages of the interaction we study the possible formation of excited matter in electric charge dipole and quadrupole form as generated by fluctuations. The role of the transverse momentum and local charge conservation laws in the observed azimuthal asymmetry is investigated, too. All these above-mentioned effects are incorporated in our analysis based on event-by-event PHSD calculations. Furthermore, the azimuthal angular correlations from Au+Au collisions observed in the recent STAR measurements within the Relativistic Heavy Ion Collider (RHIC) Beam Energy Scan (BES) program are studied. It is shown that the STAR correlation data at the collision energies of $\sqrt{{s}_{NN}}=7.7$ and 11.5 GeV can be reasonably reproduced within the PHSD. At higher energies the model fails to describe the $\ensuremath{\psi}$ correlation data resulting in an overestimation of the partonic scalar field involved. We conclude that an additional transverse anisotropy fluctuating source is needed which with a comparable strength acts on both in- and out-of-plane components.

Collins fragmentation function within the NJL-jet model

The NJL-jet model is extended to accommodate hadronization of a transversely polarized quark in order to explore the Collins effect within a multihadron emission framework. This is accomplished by calculating the polarized quark spin flip probabilities after a pseudoscalar hadron emission and the elementary Collins functions. The model is used to calculate the number densities of the hadrons produced in the polarized quark's decay chain. The full Collins fragmentation function is extracted from the sine modulation of the polarized number densities with respect to the polar angle between the initial quark's spin and hadron's transverse momentum. Two cases are studied here. First, a toy model for elementary Collins function is used to study the features of the transversely polarized quark-jet model. Second, a full model calculation of transverse momentum dependent pion and kaon Collins functions is presented. The remarkable feature of our model is that the 1/2 moments of the favored Collins fragmentation functions are positive and peak at large values of z but decrease and oscillate at small values of z. The 1/2 moments of the unfavored Collins functions have comparable magnitude and opposite sign to the favored functions, vanish at large z and peak at small values of z. This feature is observed for both the toy and full models and can be attributed to the quark-jet picture of hadronization. Moreover, the transverse momentum dependencies of the model Collins functions differ significantly from the Gaussian form widely used in the empirical parametrizations. Finally, a naive interpretation of the Schafer-Teryaev sum rule is proven not to hold in our model, where the transverse momentum conservation is explicitly enforced. This is attributed to the sizable average transverse momentum of the remnant quark that needs to be accounted for to satisfy the transverse momentum sum-rule.

Directed Flow at Midrapidity insNN=2.76 TeVPb+PbCollisions

We analyze published data from the ALICE Collaboration in order to obtain the first extraction of the recently proposed rapidity-even directed flow observable v(1). An accounting of the correlation due to the conservation of transverse momentum restores the factorization seen by ALICE in all other Fourier harmonics and thus indicates that the remaining correlation gives a reliable measurement of directed flow. We then carry out the first viscous hydrodynamic calculation of directed flow, and show that it is less sensitive to viscosity than higher harmonics. This allows for a direct extraction of the dipole asymmetry of the initial state, providing a strict constraint on the nonequilibrium dynamics of the early-time system. A prediction is then made for v(1) in Au-Au collisions at RHIC.