Positron Collisions Research Articles

State selective total and angular differential cross sections for positronium formation in [formula omitted]+[formula omitted] collisions

State-selective total and angular differential cross sections for positronium formation are presented in the collision of positron with helium atoms in their ground state by using the four-body formalism of the boundary-corrected continuum-intermediate-states approximation. The main purpose of the present study is to investigate the relative importance of intermediate ionization continuum states that has been incorporated with a suitable choice of distorting potential. In addition, the nine-dimensional integrals of the transition amplitude are analytically reduced to a one-dimensional real integral, which can be easily calculated numerically. The influence of the target static electron correlations on the capture probability is investigated using the different bound-state wave functions of He. The theoretical model for the calculation is fairly accurate and predicts a number of interesting features of the state-selective total and differential cross sections in the range of positron impact energy varies from 20 to 250eV. The total cross sections are highly peaked at an impact energy of 50 eV. The results are compared with those of other theories and the available experimental data. The present theoretical results and other measurements show satisfactory agreement in the wide energy range. Moreover, surface plots for the state-selective differential cross sections are also presented to illustrate the interesting feature of the positronium formation in this collision process.

Electron and Positron Impact Ionization of Molecules

We review our group’s most significant results concerning the collision of positrons and electrons with small molecules. Total and triple differential cross sections for the ionization of these targets were calculated in the distorted wave Born approximation using Gaussian molecular orbitals. Different models were tested. The obtained theoretical results reproduced, in most cases, the features observed in the experimental data.

Complex scaling calculation of phase shifts for positron collisions with positive ions

Complex scaling calculation of phase shifts for positron collisions with positive ions

Virtual photon exchange vs electron transfer in interparticle Coulombic electron capture.

We have investigated Interparticle Coulombic Electron Capture (ICEC) using an abinitio approach for two systems, H+ + H2O and H + H2O+. In this work, we have determined the contribution of virtual photon exchange and electron transfer to the total ICEC cross section as a function of the distance between the charged and neutral particles. Furthermore, we have shown that the relative orientation of the electron acceptor and neighbor systems affects the magnitude of the ICEC cross sections by at least two orders at relatively small distances. This geometry dependence, present even for distances as large as 10 a0, is due to the electron transfer contribution. The relative magnitude of each contribution to ICEC seems to depend on the system studied. By replacing the projectile electron with a positron, we have confirmed that electron transfer also takes place in positron collisions and that the charge of the projectile has a noticeable effect on the process, particularly at low scattering energies.

Dipolar focusing in laser-assisted positronium formation

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⩾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.

Energy and angular distributions in 250 eV electron and positron collisions with argon atom

We present energy and angular differential cross sections for single-ionization in collisions between electrons and positrons with argon atoms at 250 eV. We treat the collision classically using the three body approximation where the target atoms are described within the single active electron approximation using a Garvey model potential and only the outermost electron is involved in the collision dynamics. Our present classical trajectory Monte Carlo model is shown to describe the ionization cross sections reasonably well and agree with existing experimental data. We show that the energy distributions, both for electron and positron impact, have the same shape and structure. In contrast, the angular distributions for electron and positron impact behave completely different which it maybe be attributed to the projectile-target core interaction. We present also the ionization probabilities as a function of impact parameter. We found that for the case of positron impact the distribution is symmetric, while for the case of electron impact the distribution is asymmetric.

UKRmol-scripts: A Perl-based system for the automated operation of the photoionization and electron/positron scattering suite UKRmol+

UKRmol-scripts is a set of Perl scripts to automatically run the UKRmol+ codes, a complex software suite based on the R-matrix method to model fixed-nuclei photoionization and electron- and positron-scattering for polyatomic molecules. Starting with several basic parameters, the scripts operatively produce all necessary input files and run all codes for electronic structure and scattering calculations as well as gather the more frequently required outputs. The scripts provide a simple way to run such calculations for many molecular geometries concurrently and collect the resulting data for easier post-processing and visualization. We describe the structure of the scripts and the input parameters as well as provide examples for photoionization and electron and positron collisions with molecules. The codes are freely available from Zenodo. Program summaryProgram Title: UKRmol-scriptsCPC Library link to program files:https://doi.org/10.17632/cpvc2473pt.1Developer's repository link:https://gitlab.com/Uk-amor/UKRMol/UKRmol-scriptsLicensing provisions: GPLv3Programming language: PerlNature of problem: Performing ab initio photoionization and low energy electron- and positron-scattering on polyatomic molecules requires selecting and setting a significant number of input parameters in order to model the physics in a numerically accurate way. These scripts streamline setting up and analyzing calculations based on the R-matrix method [1] using the UKRmol+ suite [2].Solution method: The scripts provide automatic generation of input files and execution of programs from the UKRmol+ suite [2] using a number of parameters that describe both physical models and machine-dependent settings, and also outputs of the previous programs in the suite which are automatically read and analyzed. The resulting output files are then post-processed to collect target and scattering data for further analysis and simple plotting.Additional comments including restrictions and unusual features: The scripts should be used with releases 3.2 of UKRmol-in and UKRmol-out [3] although they are compatible with earlier versions.

Theoretical investigations of positron collisions with phosphorus-containing compounds

A theoretical investigation of positron scattering from phosphorus-containing compounds (viz., PH3, P2H4, PCl3, PF3, PBr3, POF3, POCl3, and H2PO4) is reported in this article. The quantum mechanical potential scattering approach is utilized to calculate integral elastic, excitation, momentum transfer, direct ionization, positronium formation, total ionization, inelastic, differential, and total cross sections on a fine energy grid from 1 to 5000 eV. The ionization contribution in the inelastic scattering is estimated using the complex scattering potential-ionization contribution technique. Prior research on positron collisions with these targets is scarce; as a result, the purpose of this study is to make up, at least in part, for this deficiency in cross-section data. In addition to being pertinent to positron transport analyses, such as Monte Carlo methods, the current results should be useful to benchmark the accuracy and validity of positron molecule collision computations and, more significantly, to compare these calculations with related electron scattering outcomes. Furthermore, the calculated cross sections of PH3 are compared with NH3 and other phosphorus-containing compounds. The analysis makes it abundantly evident that the atoms on the periphery of a molecule have a substantially larger impact on the scattering process than the central atom. To analyze the scattering dynamics of positrons and their anti-particle electrons, a comparative study of cross sections of H2PO4 and H2SO4 is also presented. For most of these targets, positron calculations are carried out for the first time.

Low-Energy Electron and Positron Scattering by para-Difluorobenzene.

The Schwinger multichannel method is employed in calculations of the elastic integral and differential cross sections for collisions of low-energy electrons and positrons with para-difluorobenzene (1,4-C6H4F2). The present calculations involving electron scattering have been performed using both static-exchange and static-exchange plus polarization levels of approximation. Our results indicate the presence of three resonances of π*-character in the low-energy region and a fourth resonance, of σ*-character, at higher energy levels. With respect to the positron scattering, the calculations were conducted using the static plus polarization approximation at three different polarization levels. The present calculated electron and positron cross sections show a reasonable agreement with the total cross sections measured by Makochekanwa et al. (J. Phys. B, 2004, 37, 1841). It is also important to highlight that our computed integral cross section for electron scattering indicates the presence of a Ramsauer-Townsend minimum, while the integral cross section for positron scattering indicates the presence of a virtual state.

Measurements of Normalized Differential Cross Sections of Inclusive π^{0} and K_{S}^{0} Production in e^{+}e^{-} Annihilation at Energies from 2.2324 to 3.6710GeV.

Based on electron positron collision data collected with the BESIII detector operating at the BEPCII storage rings, the differential cross sections of inclusive π^{0} and K_{S}^{0} production as a function of hadron momentum, normalized by the total cross section of the e^{+}e^{-}→hadrons process, are measured at six center-of-mass energies from 2.2324 to 3.6710GeV. Our results, which cover a relative hadron energy range from 0.1 to 0.9, significantly deviate from several theoretical calculations based on existing fragmentation functions.

Top and bottom quark forward-backward asymmetries at next-to-next-to-leading order QCD in (un)polarized electron positron collisions

We consider, at order {alpha}_s^2 in the QCD coupling, top-quark pair production in the continuum at various center-of-mass energies and b-quark pair production at the Z resonance by (un)polarized electron and positron beams. For top quarks we compute the forward-backward asymmetry with respect to the top-quark direction of flight, the associated polar angle distribution, and we analyze the effect of beam polarization on the QCD corrections to the leading-order asymmetry. We calculate also the polarized forward-backward asymmetry. For b-quark production at the Z peak we explore different definitions of AFB. In particular, we analyze b jets defined by the Durham and the flavor-kT clustering algorithms. We compute the inclusive b-jet and two-jet asymmetry with respect to the b-jet direction. For the latter asymmetry the QCD corrections to order {alpha}_s^2 are small. That predestines it to act as a precision observable.

New methods to achieve meson, muon and gamma light sources through asymmetric electron positron collisions

We propose methods to produce gamma light source, muon and energetic meson beams such as charged and neutral Kaons, which are boosted to be collimated and with relatively long lifetime. The first type of method is based on asymmetric electron positron collisions with a center-of-mass energy of, e.g. 1020[Formula: see text]MeV, and Kaons can be produced at a rate of [Formula: see text]. The electron and positron beams are either asymmetric in energy, e.g. 10[Formula: see text]GeV electron beam with 26[Formula: see text]MeV positron beam, or asymmetric in space, e.g. 10[Formula: see text]GeV electron and positron beams collisions with an angle around 0.05 radius. Such proposals should be able to be achieved with a reasonable budget. The other type of method relies on TeV positron on target experiment, where Kaon beams can be achieved at around [Formula: see text] per bunch crossing. Such Kaon beams are clean with small contamination, and can have great physics potential on, e.g. hyperon searches through Kaon nuclei collision, Kaon rare decay measurement, and Kaon proton or Kaon lepton collisions. The same technique can also be extended to other final states such as pions and tau leptons.

Antiproton collisions with excited positronium

We present results of calculations of several processes resulting from positronium (Ps) collisions with antiprotons: antihydrogen formation, Ps breakup, and ${n}_{\mathrm{Ps}}$-changing collisions. Calculations utilize the quantum convergent close-coupling (CCC) method and the classical trajectory Monte Carlo (CTMC) method. We identify a region of Ps principal quantum numbers ${n}_{\mathrm{Ps}}$ and Ps energies where the classical description is valid and where the CCC calculations become computationally too expensive. This allows us to present the most complete and reliable set of cross sections in a broad range of ${n}_{\mathrm{Ps}}$ and initial orbital momentum quantum numbers ${l}_{\mathrm{Ps}}$ which are necessary for experiments with antihydrogen at CERN.

Concept of Very-Asymmetric lepton Collider for dark matter search

Accelerator-based searches for dark matter are aiming for high sensitivity and need an experimental setup with high luminosity. This field of research is often called intensity frontier physics. One of the best motivated portals of interaction between dark matter and ordinary matter is a dark photon which could be observed as a resonance in the invariant mass of the decay products. Electron–positron collisions are known to be the cleanest interaction for such a study. In this paper we propose a scheme for a collider which allows for a luminosity of a few orders of magnitude higher than could be obtained in a conventional symmetric collider and with a few times higher accessible mass than is possible using a positron beam and a fixed target approach. The key concept is based on asymmetric energies: a high-energy circulating positron beam and a low-energy high-intensity electron beam, and optimization of the beam interaction region. We present here a configuration of a collision region of 10 MeV electron and 4 GeV positron beams.

Discrete symmetry tests using hyperon-antihyperon pairs

The BESIII experiment has accumulated millions of spin entangled hyperon–antihyperon pairs from the decays of the J/ψ and ψ(2S ) mesons produced in electron–positron collisions. Using multi-dimensional methods to analyze the angular distributions in the sequential decays the hyperon and antihyperon decay amplitudes are studied and tests of the combined charge-conjugation–parity (CP) symmetry are performed. Implications of the recent results using J/ψ decays into Λ$ \bar \Lambda $ and Ξ−$ \bar {\Xi}^{+} $ are presented. For the cascade decay chain, the exclusive measurement allows for three independent CP tests and the first direct determination of the weak phases.

Particle identification with the cluster counting technique for the IDEA drift chamber

IDEA (Innovative Detector for an Electron–positron Accelerator) is a general-purpose detector concept, designed to study electron–positron collisions in a wide energy range in a very large circular leptonic collider. Its drift chamber is designed to provide an efficient tracking, a high precision momentum measurement and an excellent particle identification by exploiting the application of the cluster counting technique. To investigate the potential of the cluster counting techniques on physics events, a simulation of the ionization cluster generation is needed, therefore we developed an algorithm which can use the energy deposit information provided by the Geant4 toolkit to reproduce, in a fast and convenient way, the cluster number and cluster size distributions. The results obtained confirm that the cluster counting technique allows to reach a resolution two times better than the traditional dE/dx method. A beam test has been performed during November 2021 at CERN on the H8 beam line to validate the simulations results, to define the limiting effects for a fully efficient cluster counting and to count the number of electron clusters released by an ionizing track at a fixed βγ as a function of the track angle. The simulation and the beam test results will be described briefly in this issue.

Positron Interactions with Some Human Body Organs Using Monte Carlo Probability Method

In this study, mean free path and positron elastic-inelastic scattering are modeled for the elements hydrogen (H), carbon (C), nitrogen (N), oxygen (O), phosphorus (P), sulfur (S), chlorine (Cl), potassium (K) and iodine (I). Despite the enormous amounts of data required, the Monte Carlo (MC) method was applied, allowing for a very accurate simulation of positron interaction collisions in live cells. Here, the MC simulation of the interaction of positrons was reported with breast, liver, and thyroid at normal incidence angles, with energies ranging from 45 eV to 0.2 MeV. The model provides a straightforward analytic formula for the random sampling of positron scattering. ICRU44 was used to compile the elemental composition data. In this work, elastic cross sections (ECS) and inelastic cross-sections (ICS) for positron interaction in human tissues were studied. The elastic scattering is obtained from the Rutherford differential cross-section. Gryzinski's excitation function is used within the first-born approximation to determine the core and valence of ICS. The results are presented graphically. The ECS increases rapidly as the scattering energy approaches zero and becomes dependent on the atomic number of elements in organs. The ICS has reached a maximum value of around 100 eV. Increasing positron energy leads to an increase in the elastic and inelastic mean free paths. The simulations agree with many other studies dealing with the same parameters and conditions.

Laser-assisted charge transfer in positronium collisions with protons and antiprotons

We study the process of laser-assisted charge transfer in collisions of positronium atoms with protons (antiprotons) with formation of Rydberg hydrogen (antihydrogen) atoms by the use of classical trajectories Monte Carlo simulations of Ps-$p$ dynamics in a linearly polarized infrared field. We do not observe a drastic enhancement of the cross sections similar to that predicted before in laser-assisted electron bremsstrahlung and electron recombination since in the present charge-transfer process the Coulomb focusing effect is absent. Still we see a substantial enhancement up to a factor of 3 in the energy range between ${10}^{\ensuremath{-}4}$ and 0.1 eV for the field of intensity between 10 $\mathrm{MW}/{\mathrm{cm}}^{2}$ and 10 $\mathrm{GW}/{\mathrm{cm}}^{2}$. The effect depends weakly on the orientation of the incident Ps velocity relative to the field polarization vector. Rydberg states of the produced hydrogen atoms whose orbits are close to circular can survive against ionization by laser field and decay spontaneously to lower states. This is favorable for spectroscopic studies of antihydrogen atoms.

ECCPA: Calculation of classical and quantum cross sections for elastic collisions of charged particles with atoms

ECCPA: Calculation of classical and quantum cross sections for elastic collisions of charged particles with atoms

R-matrix calculations for elastic electron and positron scattering from pyrazine: effect of the polarization description

We present R-matrix calculations of electron and positron low energy scattering from the highly polarizable pyrazine molecule. We compare integral and differential elastic cross sections with experimental results and assess the quality of the models used for describing collisions of either projectile. Static-exchange-plus-polarization models give a good description of electron scattering (including that of shape resonances), whereas both the integral and small-angle differential cross sections are underestimated for positron collisions for the same models. We discuss whether the absence of a permanent dipole moment improves the comparison with experiment for this molecule, as well as future calculations that may improve the description of polarization effects and thus positron scattering.Graphical abstract