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1. 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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2. Dipolar focusing in laser-assisted positronium formation

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

   Ambalampitiya, Harindranath ; Matfunjwa, Mbuso ; Fabrikant, Ilya ( April 2024 , Journal of Physics B: Atomic, Molecular and Optical Physics)

   We consider theoretically positronium formation in collisions of positrons with excited hydrogen atoms H($n$) in an infrared laser field. 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. Classical trajectory Monte Carlo method, which is justified for $n\geq 3$, 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 efficiency of positronium formation.

    

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3. Inclusive $$\text {J}/\psi $$ production at midrapidity in pp collisions at $$\sqrt{s} = 13$$ TeV

   https://doi.org/10.1140/epjc/s10052-021-09873-4  

   Acharya, S. ; Adamová, D. ; Adler, A. ; Aglieri Rinella, G. ; Agnello, M. ; Agrawal, N. ; Ahammed, Z. ; Ahmad, S. ; Ahn, S. U. ; Ahuja, I. ; et al ( December 2021 , The European Physical Journal C)

   Abstract We report on the inclusive $$\text {J}/\psi $$ J / ψ production cross section measured at the CERN Large Hadron Collider in proton–proton collisions at a center-of-mass energy $$\sqrt{s}~=~13$$ s = 13  TeV. The $$\text {J}/\psi $$ J / ψ mesons are reconstructed in the $$\text {e}^{+}\text {e}^{-}$$ e + e - decay channel and the measurements are performed at midrapidity ( $$|y|<0.9$$ | y | < 0.9 ) in the transverse-momentum interval $$0 more » « less
4. Centrality dependence of J/ψ and ψ(2S) production and nuclear modification in p-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ = 8.16 TeV

   https://doi.org/10.1007/JHEP02(2021)002  

   Acharya, S. ; Adamová, D. ; Adler, A. ; Adolfsson, J. ; Aggarwal, M. M. ; Agha, S. ; Aglieri Rinella, G. ; Agnello, M. ; Agrawal, N. ; Ahammed, Z. ; et al ( February 2021 , Journal of High Energy Physics)

   null (Ed.)

   A bstract The inclusive production of the J/ ψ and ψ (2S) charmonium states is studied as a function of centrality in p-Pb collisions at a centre-of-mass energy per nucleon pair $$ \sqrt{s_{\mathrm{NN}}} $$ s NN = 8 . 16 TeV at the LHC. The measurement is performed in the dimuon decay channel with the ALICE apparatus in the centre-of-mass rapidity intervals − 4 . 46 < y cms < − 2 . 96 (Pb-going direction) and 2 . 03 < y cms < 3 . 53 (p-going direction), down to zero transverse momentum ( p T ). The J/ ψ and ψ (2S) production cross sections are evaluated as a function of the collision centrality, estimated through the energy deposited in the zero degree calorimeter located in the Pb-going direction. The p T -differential J/ ψ production cross section is measured at backward and forward rapidity for several centrality classes, together with the corresponding average 〈 p T 〉 and $$ \left\langle {p}_{\mathrm{T}}^2\right\rangle $$ p T 2 values. The nuclear effects affecting the production of both charmonium states are studied using the nuclear modification factor. In the p-going direction, a suppression of the production of both charmonium states is observed, which seems to increase from peripheral to central collisions. In the Pb-going direction, however, the centrality dependence is different for the two states: the nuclear modification factor of the J/ ψ increases from below unity in peripheral collisions to above unity in central collisions, while for the ψ (2S) it stays below or consistent with unity for all centralities with no significant centrality dependence. The results are compared with measurements in p-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ s NN = 5 . 02 TeV and no significant dependence on the energy of the collision is observed. Finally, the results are compared with theoretical models implementing various nuclear matter effects. 

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5. Measurement of the central exclusive production of charged particle pairs in proton-proton collisions at $$ \sqrt{s} $$ = 200 GeV with the STAR detector at RHIC

   https://doi.org/10.1007/JHEP07(2020)178  

   Adam, J. ; Adamczyk, L. ; Adams, J. R. ; Adkins, J. K. ; Agakishiev, G. ; Aggarwal, M. M. ; Ahammed, Z. ; Alekseev, I. ; Anderson, D. M. ; Aparin, A. ; et al ( July 2020 , Journal of High Energy Physics)

   null (Ed.)

   A bstract We report on the measurement of the Central Exclusive Production of charged particle pairs h + h − ( h = π, K, p ) with the STAR detector at RHIC in proton-proton collisions at $$ \sqrt{s} $$ s = 200 GeV. The charged particle pairs produced in the reaction pp → p ′ + h + h − + p ′ are reconstructed from the tracks in the central detector and identified using the specific energy loss and the time of flight method, while the forward-scattered protons are measured in the Roman Pot system. Exclusivity of the event is guaranteed by requiring the transverse momentum balance of all four final-state particles. Differential cross sections are measured as functions of observables related to the central hadronic final state and to the forward-scattered protons. They are measured in a fiducial region corresponding to the acceptance of the STAR detector and determined by the central particles’ transverse momenta and pseudorapidities as well as by the forward-scattered protons’ momenta. This fiducial region roughly corresponds to the square of the four-momentum transfers at the proton vertices in the range 0 . 04 GeV 2 < −t 1 , −t 2 < 0 . 2 GeV 2 , invariant masses of the charged particle pairs up to a few GeV and pseudorapidities of the centrally-produced hadrons in the range |η| < 0 . 7. The measured cross sections are compared to phenomenological predictions based on the Double Pomeron Exchange (DPE) model. Structures observed in the mass spectra of π + π − and K + K − pairs are consistent with the DPE model, while angular distributions of pions suggest a dominant spin-0 contribution to π + π − production. For π + π − production, the fiducial cross section is extrapolated to the Lorentz-invariant region, which allows decomposition of the invariant mass spectrum into continuum and resonant contributions. The extrapolated cross section is well described by the continuum production and at least three resonances, the f 0 (980), f 2 (1270) and f 0 (1500), with a possible small contribution from the f 0 (1370). Fits to the extrapolated differential cross section as a function of t 1 and t 2 enable extraction of the exponential slope parameters in several bins of the invariant mass of π + π − pairs. These parameters are sensitive to the size of the interaction region. 

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