Momentum Spectra Research Articles

Compton ionization momentum spectrum from neutral hydrogenlike atoms

We investigate the ionization momentum spectrum of a bound system composed of two oppositely charged particles, which interact via electromagnetic forces, under nonrelativistic Compton scattering. Our analysis focuses on positronium, a unique system that can be experimentally realized. We find that as the incident photon energy increases, the ionization momentum spectrum of positrons and electrons converges to their respective bound-state momentum distributions. This property distinguishes positronium from the hydrogen atom and can be attributed to the mass ratio ${R}_{m}$ of the two particles in the system. We provide a detailed analysis of the effect of ${R}_{m}$ and demonstrate the existence of hydrogenlike atoms with ${R}_{m}\ensuremath{\ne}1$ that exhibit similar behavior in Compton scattering. Such atoms correspond to exotic systems found in nature.

Unveiling the effects of multiple soft partonic interactions in pp collisions at s=13.6 TeV using a new event classifier

Event classifiers based either on the charged-particle multiplicity or the event shape have been extensively used in proton-proton (pp) collisions by the ALICE collaboration at the LHC. The use of these tools became very instrumental since the observation of fluid-like behavior in high-multiplicity pp collisions. In particular, the study as a function of the charged-particle multiplicity registered in the forward V0 ALICE detector allowed for the discovery of strangeness enhancement in high-multiplicity pp collisions. However, one drawback of the multiplicity-based event classifiers is that requiring a high charged-particle multiplicity biases the sample towards hard processes like multi-jet final states. These biases make it difficult to perform jet-quenching searches in high-multiplicity pp collisions. In this context, the present paper explores the use of the new event classifier, flattenicity; which uses the multiplicity calculated in the forward pseudorapidity region. To illustrate how this tool works, pp collisions at $\sqrt{s}=13.6$ TeV simulated with PYTHIA~8 are explored. The sensitivity of flattencity to multi-partonic interactions as well as to the ``hardness'' of the collision are discussed. PYTHIA 8 predictions for the transverse momentum spectra of light- and heavy-flavored hadrons as a function of flattenicity are presented.

Distortion Compensation for Orbital Angular Momentum Beams: From Probing to Deep Learning

Beams carrying orbital angular momentum (OAM), has attracted wide interests recently, due to their favorable potential in communications, optical tweezers, remote detection, etc. However, distortion will come once OAM beams propagate through inhomogeneous media like atmospheric turbulence. Such distortion will broaden the orbital angular momentum spectrum, thus introduce interchannel crosstalk. These phenomena will adversely affect practical applications, for instance, the rising bit-error-rate in OAM-based data transmission. Therefore, distortion correction for OAM beams, an important means to improve the robustness of OAM beams when propagating in atmospheric turbulence, must be developed. In this paper, we overview recent advances on distortion compensation of OAM beams, including the influence of atmospheric turbulence on OAM beams, and multiple wavefront correction schemes. In addition, toward the engineering realization of OAM based free space data-transmission, we propose to apply TSVMD-AR model to predict atmospheric turbulence, and expect to further improve the distortion correction efficiency.

Spectral phase effects in above threshold ionization

We present theoretical studies of above threshold ionization (ATI) using sculpted laser pulses. The time-dependent Schrödinger equation is solved to calculate the ATI energy and momentum spectra, and a qualitative understanding of the electron motion after ionization is explored using the simple man’s model and a classical model that solves Newton’s equation of motion. Results are presented for Gaussian and Airy laser pulses with identical power spectra, but differing spectral phases. The simulations show that the third order spectral phase of the Airy pulse, which can alter the temporal envelope of the electric field, causes changes to the timing of ionization and the dynamics of the rescattering process. Specifically, the use of Airy pulses in the ATI process results in a shift of the Keldysh plateau cutoff to lower energy due to a decreased pondermotive energy of the electron in the laser field, and the side lobes of the Airy laser pulse change the number and timing of rescattering events. This translates into changes to the high-order ATI plateau and intra- and intercycle interference features. Our results also show that laser pulses with identical carrier envelope phases and nearly identical envelopes yield different photoelectron momentum distributions, which are a direct result of the pulse’s spectral phase.

Scattering characteristics of electrically large arbitrarily shaped targets illuminated by an off-axis vortex electromagnetic beam

For the application of vortex electromagnetic (EM) beams in practical detection scenes, the scattering characteristics of electrically large arbitrarily shaped targets illuminated by an off-axis Laguerre–Gaussian (LG) vortex beam are investigated and compared to the on-axis incidence case. The vector potential method is used to extract the electric and magnetic field components of the LG beam in different polarization states. The physical optics algorithm is adopted to calculate the scattering fields of four typical targets with the shape of a sphere, NASA almond, blunt cone, and blade model. The results revealed that as the beam center offset and the topological charge of the incident vortex beam increase, the scattering field distorts, and the obvious orbital angular momentum (OAM) spectrum mixing occurs. In addition, OAM spectrum aliasing occurs for asymmetric targets, even at on-axis incidence. These results elucidate the mechanism of vortex EM scattering and provide a reference for applying vortex beams for target detection and recognition.

Worldline instantons for the momentum spectrum of Schwinger pair production in spacetime dependent fields

We show how to use the worldline-instanton formalism to calculate the momentum spectrum of the electron-positron pairs produced by an electric field that depends on both space and time. Using the Lehmann-Symanzik-Zimmermann (LSZ) reduction formula with a worldline representation for the propagator in a spacetime field, we make use of the saddle-point method to obtain a semiclassical approximation of the pair-production spectrum. To check the final result, we integrate the spectrum and compare with the results obtained using a previous instanton method for the imaginary part of the effective action.

Comprehensive study of muon-catalyzed nuclear reaction processes in the dtμ molecule

Muon catalyzed fusion ($\ensuremath{\mu}\mathrm{CF}$) has recently regained considerable research interest owing to several new developments and applications. In this regard, we have performed a comprehensive study of the most important fusion reaction, namely, ${(dt\ensuremath{\mu})}_{J=v=0}\ensuremath{\rightarrow}\ensuremath{\alpha}+n+\ensuremath{\mu}+17.6\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ or ${(\ensuremath{\alpha}\ensuremath{\mu})}_{nl}+n+17.6\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. The coupled-channel Schr\"odinger equation for the reaction is thus solved, satisfying the boundary condition for the muonic molecule ${(dt\ensuremath{\mu})}_{J=v=0}$ as the initial state and the outgoing wave in the $\ensuremath{\alpha}n\ensuremath{\mu}$ channel. We employ the $dt\ensuremath{\mu}$- and $\ensuremath{\alpha}n\ensuremath{\mu}$-channel coupled three-body model. All the nuclear interactions, the $d\text{\ensuremath{-}}t$ and $\ensuremath{\alpha}\text{\ensuremath{-}}n$ potentials, and the $dt\text{\ensuremath{-}}\ensuremath{\alpha}n$ channel-coupling nonlocal tensor potential are chosen to reproduce the observed low-energy ($1--300$ keV) astrophysical $S$ factor of the reaction $d+t\ensuremath{\rightarrow}\ensuremath{\alpha}+n+17.6\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$, as well as the total cross section of the $\ensuremath{\alpha}+n$ reaction at the corresponding energies. The resultant $dt\ensuremath{\mu}$ fusion rate is $1.15\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{4pt}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Substituting the obtained total wave function into the $T$ matrix based on the Lippmann-Schwinger equation, we have calculated absolute values of the fusion rates ${\ensuremath{\lambda}}_{\mathrm{f}}^{\mathrm{bound}}$ and ${\ensuremath{\lambda}}_{\mathrm{f}}^{\mathrm{cont}.}$ going to the bound and continuum states of the outgoing $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\mu}$ pair, respectively. We then derived the initial $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\mu}$ sticking probability ${\ensuremath{\omega}}_{S}^{0}={\ensuremath{\lambda}}_{\mathrm{f}}^{\mathrm{bound}}/({\ensuremath{\lambda}}_{\mathrm{f}}^{\mathrm{bound}}+{\ensuremath{\lambda}}_{\mathrm{f}}^{\mathrm{cont}.})=0.857%$, which is $\ensuremath{\approx}7%$ smaller than the literature values $(\ensuremath{\approx}0.91%--0.93%)$ and can explain the recent observations (2001) at high D-T densities. We have much improved the sticking-probability calculation by employing the $D$-wave $\ensuremath{\alpha}\text{\ensuremath{-}}n$ outgoing channel with the nonlocal tensor-force $dt\text{\ensuremath{-}}\ensuremath{\alpha}n$ coupling and by deriving ${\ensuremath{\omega}}_{S}^{0}$ based on the absolute values of the ${\ensuremath{\lambda}}_{\mathrm{f}}^{\mathrm{bound}}$ and ${\ensuremath{\lambda}}_{\mathrm{f}}^{\mathrm{cont}.}$. We also calculate the absolute values for the momentum and energy spectra of the muon emitted during the fusion process. The most important result is that the peak energy is 1.1 keV although the mean energy is 9.5 keV owing to the long higher-energy tail. This is an essential result for the ongoing experimental project to realize the generation of an ultraslow negative muon beam by utilizing the $\ensuremath{\mu}\mathrm{CF}$ for various applications, e.g., a scanning negative muon microscope and an injection source for the muon collider.

K*0 production in Au+Au collisions at sNN=7.7 , 11.5, 14.5, 19.6, 27, and 39 GeV from the RHIC beam energy scan

We report the measurement of $K^{*0}$ meson at midrapidity ($|y|<$ 1.0) in Au+Au collisions at $\sqrt{s_{\rm NN}}$~=~7.7, 11.5, 14.5, 19.6, 27 and 39 GeV collected by the STAR experiment during the RHIC beam energy scan (BES) program. The transverse momentum spectra, yield, and average transverse momentum of $K^{*0}$ are presented as functions of collision centrality and beam energy. The $K^{*0}/K$ yield ratios are presented for different collision centrality intervals and beam energies. The $K^{*0}/K$ ratio in heavy-ion collisions are observed to be smaller than that in small system collisions (e+e and p+p). The $K^{*0}/K$ ratio follows a similar centrality dependence to that observed in previous RHIC and LHC measurements. The data favor the scenario of the dominance of hadronic re-scattering over regeneration for $K^{*0}$ production in the hadronic phase of the medium.

Simulating the Enrichment of Fossil Radio Electrons by Multiple Radio Galaxies

We simulate the evolution of relativistic electrons injected into the intracluster medium by five radio galaxies. We study the spatial transport and the emission properties of the injected radio plasma over a ∼5 Gyr period, and the sequence of cooling and re-acceleration events experienced by electrons, using a Lagrangian approach joined with a numerical method to model the evolution of momentum spectra of relativistic electrons. When compared with electrons injected by shock waves, electrons injected by radio galaxies (here limited to a single injection event) in our tests are unable to fuel large ∼Mpc-sized radio relics with fossil electrons, as required by current theoretical models, while electrons previously seeded by other shocks can do this. On the other hand, the combination of seeding from radio galaxies and of re-acceleration events from plasma perturbation can produce detectable, small-scale, and filamentary emissions in the proximity (≤100–200 kpc) of radio galaxies.

Directed flow and global polarization in Au+Au collisions across energies covered by the beam energy scan at RHIC

We study the directed flow of identified particles in Au+Au collisions at $\sqrt{s_\text{NN}}=7.7$ to 62.4 GeV. The Glauber model is extended to include both a tilted deformation of the QGP fireball with respect to the longitudinal direction and a non-zero longitudinal flow velocity gradient in the initial state. By combining this improved initial condition with a (3+1)-dimensional viscous hydrodynamic model calculation, we obtain a satisfactory description of the transverse momentum spectra and the rapidity dependent directed flow coefficient of different hadron species. Our calculation indicates the sensitivity of the hadron directed flow, especially its splitting between protons and antiprotons, to both the initial geometry and the initial longitudinal flow velocity. Therefore, the combination of directed flow of different hadrons can provide a tight constraint on the initial condition of nuclear matter created in heavy-ion collisions. The initial condition extracted from the directed flow is further tested with the global polarization of $\Lambda$ and $\bar{\Lambda}$ within the same theoretical framework, where we obtain a reasonable description of these hyperon polarization observed at different collision energies at RHIC.

Fragmentation dynamics of electron-impact double ionization of helium

We study the double ionization dynamics of a helium atom impacted by electrons with full-dimensional classical trajectory Monte Carlo simulation. The excess energy is chosen to cover a wide range of values from 5 eV to 1 keV for comparative study. At the lowest excess energy, i.e., close to the double-ionization threshold, it is found that the projectile momentum is totally transferred to the recoil-ion while the residual energy is randomly partitioned among the three outgoing electrons, which are then most probably emitted with an equilateral triangle configuration. Our results agree well with experiments as compared with early quantum-mechanical calculation as well as classical simulation based on a two-dimensional Bohr’s model. Furthermore, by mapping the final momentum vectors event by event into a Dalitz plot, we unambiguously demonstrate that the ergodicity has been reached and thus confirm a long-term scenario conceived by Wannier. The time scale for such few-body thermalization, from the initial nonequilibrium state to the final microcanonical distribution, is only about 100 attoseconds. Finally, we predict that, with the increase of the excess energy, the dominant emission configuration undergoes a transition from equilateral triangle to T-shape and finally to a co-linear mode. The associated signatures of such configuration transition in the electron-ion joint momentum spectrum and triple-electron angular distribution are also demonstrated.

The Theoretical Description of the Transverse Momentum Spectra: A Unified Model

Analysis of transverse momentum distributions is a useful tool to understand the dynamics of relativistic particles produced in high-energy collisions. Finding a proper distribution function to approximate the spectra is a vastly developing area of research in particle physics. In this work, we have provided a detailed theoretical description of the unified statistical framework in high-energy physics. We have tested the applicability of this framework on experimental data by analyzing the transverse momentum spectra of pion produced in heavy-ion collision at RHIC and LHC. We have also attempted to explain the transverse momentum spectra of charged hadrons formed in pp collision at different energies using the unified statistical framework. This formalism has been proved to nicely explain the spectra of particles produced in soft processes as well as hard scattering processes in a consistent manner.

Spin Hall Effect of Double-Index Cylindrical Vector Beams in a Tight Focus

We investigate the spin angular momentum (SAM) of double-index cylindrical vector beams in tight focus. Such a set of beams is a generalization of the conventional cylindrical vector beams since the polarization order is different for the different transverse field components. Based on the Richards-Wolf theory, we obtain an expression for the SAM distribution and show that if the polarization orders are of different parity, then the spin Hall effect occurs in the tight focus, which is there are alternating areas with positive and negative spin angular momentum, despite linear polarization of the initial field. We also analyze the orbital angular momentum spectrum of all the components of the focused light field and determine the overwhelming angular harmonics. Neglecting the weak harmonics, we predict the SAM distribution and demonstrate the ability to generate the focal distribution where the areas with the positive and negative spin angular momentum reside on a ring and are alternating in pairs, or separated in different semicircles. Application areas of the obtained results are designing micromachines with optically driven elements.

Pion, kaon, and (anti)proton production in U+U collisions at sNN=193 GeV measured with the STAR detector

We present the first measurements of transverse momentum spectra of $\pi^{\pm}$, $K^{\pm}$, $p(\bar{p})$ at midrapidity ($|y| < 0.1$) in U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV with the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The centrality dependence of particle yields, average transverse momenta, particle ratios and kinetic freeze-out parameters are discussed. The results are compared with the published results from Au+Au collisions at $\sqrt{s_{NN}} =$ 200 GeV in STAR. The results are also compared to those from A Multi Phase Transport (AMPT) model.

Production of Strange and Charm Hadrons in Pb+Pb Collisions at sNN = 5.02 TeV

Using a quark combination model with the equal-velocity combination approximation, we study the production of hadrons with strangeness and charm flavor quantum numbers in Pb+Pb collisions at sNN= 5.02 TeV. We present analytical expressions and numerical results for these hadrons’ transverse momentum spectra and yield ratios. Our numerical results agree well with the experimental data available. The features of strange and charm hadron production in the quark–gluon plasma at the early stage of heavy ion collisions are also discussed.

Measurement of structure dependent radiative K+ → e+νγ decay at J-PARC

The J-PARC E36 experiment is aiming at searching for lepton universality violation by accurately measuring the ratio of branching ratio of the K + → e + ν(γ) (K e2(γ)) to K + → μ + ν(γ) (K μ2(γ)) decays. The experiment was performed at J-PARC employing a stopped K + method in conjunction with a 12-sector iron-core superconducting toroidal spectrometer. Charged particle momenta were determined by reconstructing charged particle trajectories in the spectrometer. Particle identification for e + and μ + was carried out using an aerogel Cherenkov counter and a lead-glass Cherenkov counter, as well as by measuring the time-of-flight between TOF counters. The peak structure due to K e2(γ) decay was successfully observed in the e + momentum spectrum after the e + requirement in the PID analysis. The structure dependentradiative K + → e + νγ () events were selected by imposing photon hits in the CsI(Tl) calorimeter or the GSC counter. The experimental spectra were accurately reproduced by the Monte Carlo simulation, indicating a correct understanding of the experimental conditions. The Br() value relative to Br(K e2(γ)) was determined from the and K e2(γ) yields corrected for the detector acceptances. The Br()/Br(K e2(γ)) ratio was derived to be 1.19 ± 0.07 stat ± 0.04 syst in the CsI(Tl) analysis and 1.25 ± 0.14 stat ± 0.04 syst in the GSC analysis.

KOTO II at J-PARC : toward measurement of the branching ratio of

The KOTO II experiment at J-PARC is a next-generation experiment that is under design aiming at the measurement of the branching ratio of the decay. The KOTO II experiment is planned in the extended Hadron Experimental Facility at J-PARC using the second production target. With the second production target, the KL extraction angle of 5 degrees is chosen instead of 16 degrees in order to obtain a larger KL flux and harder KL momentum spectrum while keeping the same ratio of neutron and KL fluxes. To realize the extraction angle, the KOTO II detector is behind the primary beam dump. The KOTO II detector has a longer decay volume and a larger calorimeter to achieve a larger decay probability and higher acceptance of the KL decay. With the designed beam line and the detector, 40 signal events are expected for the branching ratio of 3×10−11 with 60 background events in the running period of 3 × 107 s, which corresponds to approximately 5-σ discovery of the decay.

Pair production in inhomogeneous electric fields with phase modulation

Electron-positron pair production in spatial inhomogeneous electric fields with sinusoidal phase modulation is studied within the Dirac-Heisenberg-Wigner formalism. The focus is on discussing the effects of the modulation parameters on the momentum spectrum and the reduced particle number at various spatial scales. For the momentum spectrum, the interference effect becomes more and more remarkable with the increase of modulated amplitude or frequency, while the symmetry is severely destroyed with modulated amplitude. For the reduced particle number, it is greatly enhanced by about a few times and evenly one order of magnitude when modulation parameters are applied. Moreover, the effect of spatial scales on the reduced particle number is carefully examined, and it is found that it increases rapidly at small spatial scales, while it tends to be a constant at large spatial scales. We also obtain the optimal pair production that can be achieved through different modulations. These results can provide a possibility for realizing the optimal pair production by combining the advantages of field spatial inhomogeneity with different choices of phase modulation.

Nondipole Effects in Atomic Ionization Induced by an Intense Counter‐Rotating Bicircular Laser Field

AbstractNondipole effects occurring in the process of atomic ionization by an intense, mid‐infrared, counter‐rotating bicircular laser field are investigated using the strong‐field approximation with leading‐order nondipole corrections. The time integrals appearing in the expression for the differential ionization rate are computed in two ways: numerically, and by applying the saddle‐point approximation. The nondipole corrections introduce an asymmetry in the photoelectron momentum distribution along the field propagation direction. The asymmetry is quantified by the partial average value of the propagation‐direction momentum component of the photoelectrons and by the normalized difference of the differential ionization rates computed including and excluding the nondipole corrections. Using the saddle‐point approximation, it is investigated how the nondipole corrections change the solutions for direct photoelectrons and how this affects the momentum spectra. The impact of nondipole corrections increases with increasing photoelectron energy. Analysis of the complete photoelectron spectra including both direct and rescattered photoelectrons shows that, in the low‐energy region, a shift against the propagation direction occurs. The partial average of the propagation–direction momentum component in the rescattering region exhibits a plateau structure and also a local minimum structure that was recently observed in an experiment with a linearly polarized laser field (Lin et al., Phys Rev. Lett. 128, 023201 (2022)).

Centrality and Transverse Spherocity Dependent Study of Charged-Particle Production in Xe–Xe Collisions at √SNN=5.44 TeV Using PYTHIA8 Angantyr and AMPT Models

Transverse spherocity is an event structure variable which provide an effective way to disentangle the data into hard and soft components of the processes corresponding to events with small and large numbers of multiparton interactions (MPI), respectively. Recent experimental results in small systems from the LHC suggest the importance of the transverse spherocity variable in the classification of the events. In this contribution, we have studied the dynamics of identified particle production in Xe–Xe collisions at √SNN = 5.44 TeV using A Multi-Phase Transport Model (AMPT) and the recently developed Angantyr model, which is incorporated within PYTHIA8. A study of the transverse momentum spectra of the identified particles is presented for soft (isotropic) and hard (jet-like) events in different centrality intervals.