Positron Collisions Research Articles

Low-energy positron scattering by pyrimidine.

This work reports elastic integral and differential cross sections for positron collisions with pyrimidine, for energies up to 20 eV. The cross sections were computed with the Schwinger multichannel method in the static plus polarization approximation. We also employed the Born closure procedure to account for the long range potential due to the permanent dipole moment of the molecule. Our results are compared with the experimental total cross section of Zecca et al. [J. Phys. B 43, 215204 (2010)], the experimental grand-total, quasi-elastic integral and differential cross section of Palihawadana et al. [Phys. Rev. A 88, 12717 (2013)]. We also compare our results with theoretical integral and differential cross sections obtained by Sanz et al. [Phys. Rev. A 88, 62704 (2013)] with the R-matrix and the independent atom model with screening-corrected additivity rule methods, and with the results computed by Franz and Gianturco [Phys. Rev. A 88, 042711 (2013)] using model correlation-polarization potentials. The agreement between the theory and the experiment is encouraging.

Novel proposal for a low emittance muon beam using positron beam on target

Muon beams are customarily obtained via K/πdecays produced in proton interaction on target. In this paper we investigate the possibility to produce low emittance muon beams from electron–positron collisions at centre-of-mass energy just above the μ+μ− production threshold with maximal beam energy asymmetry, corresponding to a positron beam of about 45GeV interacting on electrons on target. We present the main features of this scheme with an outline of the possible applications.

Positronium collisions with molecular hydrogen

Positronium collisions with molecular hydrogen

Positron collisions with C3H6 isomers

We report calculated elastic integral (ICS) and differential (DCS) cross sections for positron collisions with C3H6 isomers, propene and cyclopropane. The ICSs computations were performed with the Schwinger multichannel method, at the static plus polarization level of approximation from 0.001 up to 8 eV. These ICSs are compared with the only experimental total cross sections (TCSs) available in literature for low energy positron scattering by these isomers [Makochekanwa C et al 2008 Phys. Rev. A 77 042717]. Even though there are significant differences between the two molecules (for instance propene has a dipole moment while cyclopropane has not), the ICSs for both systems are remarkably similar. This is mainly due to the fact that the dipole moment of propene is very small (0.39 D), and the increase of cyclopropane’s ICS at low impact energy is possibly due to the presence of a virtual state. This same resemblance occurs for the DCSs at lower energies, with the differences between the isomers becoming clearer as the energy increases.

An investigation of antiprotons collisions with positronium atom in Debye plasma environments

The effects of Debye plasmas on antihydrogen formation and ionization processes are investigated in antiprotons ( p¯) and positronium (Ps) collisions. The classical trajectory Monte Carlo method with Debye Huckel potentials has been used for cross section calculations. In this process, antihydrogen formation and ionization cross sections have been calculated in unscreened as well as in Debye plasmas conditions in energies ranging from 1 to 500 keV. Partial cross sections for antihydrogen formation are also calculated which show the largest cross sections correspond to production of antihydrogen in the n = 2 (2s, 2p) states. Comparative study has been carried out to determine the differences of cross sections in screening and unscreening cases. The results show that the cross sections for both antihydrogen formation and positronium ionization depend on Debye screening lengths as well as on collision energies. The effects of plasmas conditions on antihydrogen formation and positronium ionization are explain...

Cross sections for positron and electron collisions with an analog of the purine nucleobases: Indole

Quantitative information about positron and electron collisions with nucleobases is required in charged-particle track simulations to accurately model and assess any radiation damage at the subcellular level in biological systems. However, scattering experiments have so far been restricted to the pyrimidine nucleobases. In this paper we report on total-cross-section measurements for positron impact on indole, a parent molecule of the purine nucleobases, at impact energies between 1 and 25 eV. We also present theoretical cross sections for elastic and total scattering, positronium formation, electronic excitations, and direct ionization between 1 and 500 eV, as calculated with the independent-atom model with the screening-corrected additivity rule. Rotational excitation cross sections are additionally calculated within a Born framework over that same energy range. A significant discrepancy is found between the measured and computed total cross sections, which cannot be entirely accounted for by the lack of forward-angle-scattering discrimination in the experiment. The present results are also compared to the available theoretical cross sections for the purine nucleobases. In addition, total cross sections for electron-indole collisions computed within our theoretical formalism are provided.

New limit for the noncommutativity parameter of the noncommutative standard model

We discuss the limits on the scale of the noncommutative (NC) parameter Λ NC via studying the top-quark pair production through electron–positron collision in the framework of the minimal noncommutative standard model (mNCSM), using the Seiberg–Witten (SW) maps to the first order of the NC parameter Θμν. In this work we assume an ansatz for the NC parameter Θμν and we find new limit on the NC scale Λ NC , which is in the range 0.1–0.2 TeV. We confirm the results obtained in muon pair production.

Production of the spin partner of the [formula omitted] in [formula omitted] collisions

We study the production of the spin partner of the X(3872), which is a D⁎D¯⁎ bound state with quantum numbers JPC=2++ and named X2(4012) here, with the associated emission of a photon in electron–positron collisions. The results show that the ideal energy region to observe the X2(4012) in e+e− annihilations is from 4.4 GeV to 4.5 GeV, due to the presence of the S-wave D¯⁎D1(2420) and D¯⁎D2(2460) thresholds, respectively. We also point out that it will be difficult to observe the γX(4012) at the e+e− center-of-mass energy around 4.26 GeV.

Recent positron-atom cross section measurements and calculations

We review recent cross section results for low-energy positron scattering from atomic targets. A comparison of the latest measurements and calculations for positron collisions with the noble gases and a brief update of the newest studies on other atoms is presented. In particular, we provide an overview of the cross sections for elastic scattering, positronium formation, direct and total ionisation, as well as total scattering, at energies typically between about 0.1 and a few hundred eV. We discuss the differences in the current experimental data sets and compare those results to the available theoretical models. Recommended data sets for the total cross section are also reported for each noble gas. A summary of the recent developments in the scattering from other atoms, such as atomic hydrogen, the alkali and alkaline-earth metals, and two-electron systems is finally provided.

Modelling the behavior of the positron plasma temperature in antihydrogen experimentation

Antihydrogen is now routinely produced at CERN by overlapping clouds of positrons and antiprotons. The mechanisms responsible for antihydrogen formation (radiative capture and the three-body reaction) are both dependent on the temperature of the positrons (T e ), though with a different weight. Here we present a simple model of the behavior of the positron temperature based on the main processes involved during antihydrogen synthesis, namely: antiproton–positron collisions, positron heating due to plasma expansion and cooling via the emission of synchrotron radiation. The time evolution of T e has been simulated by changing the relevant parameters of the mechanisms involved in order to highlight the importance of the different (competing) effects.

Positronium formation from positron impact on hydrogen and helium targets

Charge-exchange cross sections are presented for collisions of positrons with hydrogen and neutral and singly ionized helium targets using a variant of the classical trajectory Monte Carlo approach. As a check on the method a comparison is made with the corresponding proton results. An extended error analysis is presented. Reasonable agreement with available experimental data is found, and the charge-exchange cross section for positrons on ${\text{He}}^{+}$ is predicted.

Four-body treatment of the K-shell positronium formation from multi-electron atoms

Abstract The four-body Coulomb-Born distorted-wave approximation with correct boundary conditions (CBDW-4B) is applied to the K-shell positronium formation from multi-electron atoms at intermediate and high impact energies. In the present approach, both K-shell electrons are treated as active electrons. For collisions of positrons with helium, carbon, and neon atoms, both the post and prior forms of the transition amplitude are calculated and the corresponding differential and integral cross sections are compared with the results of the three-body version of the formalism (CBDW-3B). In order to investigate the effects of the static electronic correlations on the process, initial bound states of the active electrons in helium atoms are described by Hylleraas and Silverman wave functions. Also for positronium formation from helium atoms the obtained cross sections are compared with the available experimental data and also with the results of the other theories.

Ionization cross-sections for positron collisions with N2

The total and direct ionization cross-sections for positron impact on molecular nitrogen have been measured in the energy range from 5 eV to 850 eV. The results are compared with other experimental and theoretical determinations.

Elastic Cross-Section of Positron Scattering from Helium Ions

The collisions of positrons with singly charged helium ion He + (1s) at ground state have been investigated using Harris-Nesbet variational method. This method is demonstrated for total elastic cross sections which corresponding to S-, P-, and D-wave scatterings below positronium formation threshold. The incident energies is considered up to 40 eV (positron momentum k  1.7 a.u.). The results show oscillating behavior of the cross sections in S-wave, where the existence of a resonance is expected, the first peak starts in S-wave and moving toward higher values of k as the angular momentum L increases. We encourage the experimental physicists to carry out further positron-ions experiments.

Exploring Nonstandard Neutrino–Electron Interactions due to a New Light Spin-1 Boson

It is possible that nonstandard interactions of neutrinos arise from a new spin-1 particle with mass of tens of GeV or lower. Various existing data on neutrino-electron scattering and single-photon production in electron–positron collisions can impose considerable and complementary constraints on its couplings to the neutrinos and electron. If the new boson is sufficiently light, the constraints may significantly depend on its width as well as mass.

Higgs production in two-photon process and transition form factor

The Higgs production in the two-photon fusion process is investigated where one of the photons is off-shell while the other one is on-shell. This process is realized in either electron–positron collision or electron–photon collision where the scattered electron or positron is detected (single tagging) and described by the transition form factor. We calculate the contributions to the transition form factor of the Higgs boson coming from top-quark loops and W-boson loops. We then study the Q2 dependence of each contribution to the total transition form factor and also of the differential cross section for the Higgs production.

Positron scattering and annihilation in hydrogenlike ions

Diagrammatic many-body theory is used to calculate the scattering phase shifts, normalized annihilation rates ${Z}_{\mathrm{eff}}$, and annihilation $\ensuremath{\gamma}$ spectra for positron collisions with the hydrogenlike ions He${}^{+}$, Li${}^{2+}$, B${}^{4+}$, and F${}^{8+}$. Short-range electron-positron correlations and longer-range positron-ion correlations are accounted for by evaluating nonlocal corrections to the annihilation vertex and the exact positron self-energy. The numerical calculation of the many-body theory diagrams is performed using $B$-spline basis sets. To elucidate the role of the positron-ion repulsion, the annihilation rate is also estimated analytically in the Coulomb-Born approximation. It is found that the energy dependence and magnitude of ${Z}_{\mathrm{eff}}$ are governed by the Gamow factor that characterizes the suppression of the positron wave function near the ion. For all of the H-like ions, the correlation enhancement of the annihilation rate is found to be predominantly due to corrections to the annihilation vertex, while the corrections to the positron wave function play only a minor role. Results of the calculations for $s$-, $p$-, and $d$-wave incident positrons of energies up to the positronium-formation threshold are presented. Where comparison is possible, our values are in excellent agreement with the results obtained using other, e.g., variational, methods. The annihilation-vertex enhancement factors obtained in the present calculations are found to scale approximately as $1+(1.6+0.46\ensuremath{\ell})/{Z}_{i}$, where ${Z}_{i}$ is the net charge of the ion and $\ensuremath{\ell}$ is the positron orbital angular momentum. Our results for positron annihilation in H-like ions provide insights into the problem of positron annihilation with core electrons in atoms and condensed matter systems, which have similar binding energies.

Positron and electron collisions with nitrous oxide: Measured and calculated cross sections

We present cross-section results from an experimental and theoretical study on positron scattering from nitrous oxide. Total cross sections (TCSs) have been measured at incident energies between 0.1 and 70 eV with a linear-transmission-based positron spectrometer. Elastic differential and integral cross sections, as well as inelastic integral cross sections and TCSs, have been computed with two different theoretical approaches: the independent atom model with screening-corrected additivity rule (IAM-SCAR), at energies in the 1- to 1000-eV range, and the Schwinger multichannel method at energies between 0.1 and 10 eV. Note that the latter method specifically reports cross sections for the elastic channel. We find good qualitative agreement between the theories and the TCS experiment at all common energies. That level of accord is found to also become quantitative above the ionization energy of nitrous oxide. Electron-impact TCSs calculated with the IAM-SCAR approach are also presented and compared to the existing results in order to uncover any similarities or differences in the scattering processes between these two leptons and nitrous oxide.

Low-energy positron scattering from iodomethane

We report experimental total cross sections (TCSs) and calculated elastic integral cross sections (ICSs) for positron collisions with iodomethane (methyl iodide, CH3I). The experimental TCSs were obtained with a linear transmission technique, for energies from 0.1 up to 50 eV. The present TCS data agree well with those previously reported (Kimura et al 2001 J. Chem. Phys. 115 7442) at higher energies (above 7 eV), but significant discrepancies are found at the lower energies. The present ICS computations were performed with the Schwinger multichannel method (SMC) and the Born dipole approximation in the incident energy range from 0.1 eV up to 10 eV. Iodomethane poses a great challenge to theoretical descriptions of the collisions dynamics. In addition to the neglect of inelastic channels, the main difficulty found in the SMC approach is related to numerical limitations that prevent a thorough description of correlation–polarization effects. Although our ICS calculations do not compare well with the present TCS data, the results are encouraging, as iodomethane would challenge all the presently available computational approaches.

Production of doubly charmed tetraquarks with exotic color configurations in electron–positron collisions

Structure and production of doubly charmed tetraquarks Tcc (ccu¯d¯) are studied from the viewpoint of color configurations. Based on the diquark correlation, the tetraquark Tcc with I(JP)=0(1+) is considered to be stable against strong decay. We discuss that the mixing probability of color antitriplet and sextet cc components in Tcc is suppressed by 1/mc2, so the two configurations are separately realized in the heavy quark limit. Utilizing the nonrelativistic QCD framework, we evaluate the production cross sections of Tcc in electron–positron collisions. The momentum dependence of the cross section of color antitriplet is found to be different from that of sextet, which can be used to discriminate the color structure of the Tcc states in experimental measurements.