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1. Deep Minimum and a Vortex for Positronium Formation in Low-Energy Positron-Helium Collisions

   https://doi.org/10.3390/atoms9030056  

   Alrowaily, Albandari W.; Ward, Sandra J.; Van Reeth, Peter (September 2021, Atoms)

   We find a zero in the positronium formation scattering amplitude and a deep minimum in the logarithm of the corresponding differential cross section for positron–helium collisions for an energy just above the positronium formation threshold. Corresponding to the zero, there is a vortex in the extended velocity field that is associated with this amplitude when one treats both the magnitude of the momentum of the incident positron and the angle of the scattered positronium as independent variables. Using the complex Kohn variational method, we determine accurately two-channel K-matrices for positron–helium collisions in the Ore gap. We fit these K-matrices using both polynomials and the Watanabe and Greene’s multichannel effective range theory taking into account explicitly the polarization potential in the Ps-He+ channel. Using the fitted K-matrices we determine the extended velocity field and show that it rotates anticlockwise around the zero in the positronium formation scattering amplitude. We find that there is a valley in the logarithm of the positronium formation differential cross section that includes the deep minimum and also a minimum in the forward direction. 

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2. Dipolar focusing in laser-assisted positronium formation

   https://doi.org/10.1088/1361-6455/ad3f5a  

   Ambalampitiya, H. B.; Matfunjwa, M. K.; Fabrikant, I. I. (April 2024, Journal of Physics B: Atomic, Molecular and Optical Physics)

   Abstract We consider positronium formation in collisions of positrons with excited hydrogen atoms H(n) in an infrared laser field theoretically. This process is assisted by the dipolar focusing effect: a positron moving in a superposition of a laser field and the dipolar field can approach the atomic target even if its trajectory starts with a very large impact parameter, leading to a significant enhancement of the Ps formation cross section. The classical trajectory Monte Carlo method, which is justified for $n\text{⩾}3$, allows efficient calculation of this enhancement. A similar effect can occur in collisions of positrons with other atoms in excited states, which can lead to improvements in the efficiency of positronium formation. 

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3. Parametric study of the polarization dependence of nonlinear Breit–Wheeler pair creation process using two laser pulses

   https://doi.org/10.1063/5.0165788  

   Qian, Qian; Seipt, Daniel; Vranic, Marija; Grismayer, Thomas E; Blackburn, Thomas G; Ridgers, Christopher P; Thomas, Alexander_G R (October 2023, Physics of Plasmas)

   With the rapid development of high-power petawatt class lasers worldwide, exploring physics in the strong field QED regime will become one of the frontiers for laser–plasma interactions research. Particle-in-cell codes, including quantum emission processes, are powerful tools for predicting and analyzing future experiments where the physics of relativistic plasma is strongly affected by strong field QED processes. The spin/polarization dependence of these quantum processes has been of recent interest. In this article, we perform a parametric study of the interaction of two laser pulses with an ultrarelativistic electron beam. The first pulse is optimized to generate high-energy photons by nonlinear Compton scattering and efficiently decelerate electron beam through the quantum radiation reaction. The second pulse is optimized to generate electron–positron pairs by the nonlinear Breit–Wheeler decay of photons with the maximum polarization dependence. This may be experimentally realized as a verification of the strong field QED framework, including the spin/polarization rates. 

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4. Simulation of the formation of antihydrogen via magnetobound positronium

   https://doi.org/10.1063/5.0060707  

   Aguirre, F_F; Ordonez, C_A (September 2021, AIP Advances)

   Antihydrogen formation involving magnetobound positronium is simulated by computing classical trajectories. Simulated collisions between electrons and positrons generate magnetobound positronium, which consists of electron–positron pairs that are not energetically bound but that have spatially correlated trajectories within a magnetic field. Simulations show that antihydrogen can form if such electron–positron pairs pass near antiprotons. In addition, the possibility of forming antihydrogen atomic ions or antihydrogen molecular ions via magnetobound positronium or magnetobound antihydrogen is discussed. 

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5. Deep Minima in the Triply Differential Cross Section for Ionization of Atomic Hydrogen by Electron and Positron Impact

   https://doi.org/10.3390/atoms8020026  

   DeMars, C.; Ward, S.; Colgan, J.; Amami, S.; Madison, D. (June 2020, Atoms)

   We investigate ionization of atomic hydrogen by electron- and positron-impact. We apply the Coulomb–Born (CB1) approximation, various modified CB1 approximations, the three body distorted wave (3DW) approximation, and the time-dependent close-coupling (TDCC) method to electron-impact ionization of hydrogen. For electron-impact ionization of hydrogen for an incident energy of approximately 76.45 eV, we obtain a deep minimum in the CB1 triply differential cross section (TDCS). However, the TDCC for 74.45 eV and the 3DW for 74.46 eV gave a dip in the TDCS. For positron-hydrogen ionization (breakup) we apply the CB1 approximation and a modified CB1 approximation. We obtain a deep minimum in the TDCS and a zero in the CB1 transition matrix element for an incident energy of 100 eV with a gun angle of 56.13 ° . Corresponding to a zero in the CB1 transition matrix element, there is a vortex in the velocity field associated with this element. For both electron- and positron-impact ionization of hydrogen the velocity field rotates in the same direction, which is anticlockwise. All calculations are performed for a doubly symmetric geometry; the electron-impact ionization is in-plane and the positron-impact ionization is out-of-plane. 

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