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Journal of Physics B Atomic Molecular and Optical Physics

Recombination of antiprotons with positrons at low temperatures

Bernard Zygelman

Published Dec 20, 2002• 11 citations
Publisher

Abstract

The formation of antihydrogen atoms via three-body recombination (TBR) of antiprotons with positrons is considered. A collisional radiative model for the recombination process is employed to study the time dependence of state-resolved antihydrogen populations. We show that at low temperatures T −9/2 scaling of the TBR coefficient does not characterize the total recombination rate, and an alternative expression for the three-body rate coefficient is proposed.

DOI
10.1088/0953-4075/36/1/105

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The formation of antihydrogen atoms via three-body recombination (TBR) of antiprotons with positrons is considered. A collisional radiative model for the recombination process is employed to study the time dependence of state-resolved antihydrogen populations. We show that at low temperatures T −9/2 scaling of the TBR coefficient does not characterize the total recombination rate, and an alternative expression for the three-body rate coefficient is proposed.
A quantum-mechanical approach is proposed for the formation of antihydrogen (H‾)(\overline{H})(H) in the ground and excited states (2s,2p)(2s,2p)(2s,2p) via the mechanism of three-body recombination (TBR) inside a trapped plasma of antiprotons (p‾)(\overline{p})(p​) and positrons (e+)({e}^{+})(e+) or in the collision between the two beams of them. Variations of the differential as well as the total formation cross sections are studied as a function of the incident energies of both the active and the spectator e+{e}^{+}e+'s. Significantly large cross sections are found at very low incident energies in the TBR process as compared to other processes leading to antihydrogen. The present (H‾)(\overline{H})(H) formation cross section decreases with increasing positron energy (temperature) but no simple power law could be predicted for it covering the entire energy range $(\ensuremath{\sim}5--50\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, corroborating the experimental findings qualitatively, the latter being at a much lower energy regime $(\ensuremath{\sim}{10}^{\ensuremath{-}4}\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. The formation cross sections are found to be much higher for unequal energies of the two e+{e}^{+}e+'s than for equal energies, as expected physically.
Antihydrogen is at the focus of antimatter research.One of the main aims is to perform a direct test of the fundamental CPT symmetry by comparing the spectroscopic properties of the ground-state antihydrogen atom with those of the hydrogen atom.In current experiments synthetized antihydrogen is formed mainly via the three-body recombination process after mixing positron and antiproton clouds together in a cryogenic trap.The major three-body formation process populates mainly highly excited Rydberg states, therefore before the antihydrogen atoms reach the ground-state additional processes may take place, such as collisional (de)excitation, ionization and spontaneous or stimulated radiative transitions.We have used a classical-trajectory Monte Carlo (CTMC) code along with regular atomic codes to generate a scattering database of rate coefficients in various experimentally achievable magnetic field strengths and in low temperature positron plasma conditions.We used these rates to calculate the evolution of level population of antihydrogen during formation, scattering and flight within our experimental conditions [1].The model and the results are presented.
In the present paper two-body radiative recombination rate for the production of antihydrogen Π) in a merged beam of slow positrons (e+) and antiprotons (p -) is studied in the light of a two-step process, which consists of capture in an excited state of II with subsequent decay to the ground state and emission of a photon.Computation is done using the field theory and the Coulomb gauge.Importance of the two-step radiative recombination process relative to the well-known spontaneous photorecombination process, on the two-body radiative recombination rate for antihydrogen formation, is discussed.The present result predicts higher contribution from the two-step radiative recombination process as compared to the spontaneous photorecombination process to the rate of cold antihydrogen formation witl the relative colfision energy below 0.01 Rydberg, near which experiments are being conducted.However, above 0.1 Rydberg the spontaneous photorecombination process dominates over the two-step radiative recombination process.The present result is valid, as well, for the formation of hydrogen atom due to collision between slow edectron and proton.
Abstract Antihydrogen is produced by mixing an antiproton and a positron plasma in a cryogenic electromagnetic trap. The dominant antihydrogen formation mechanism is three-body recombination, while the subsequent level population evolution is governed by various processes, mainly collisional (de)excitation, ionisation and radiative decay. In this work the impact of various positron plasma lengths on the level population evolution is investigated. The main interest is the ground-state antihydrogen atom yield. It is found that the ground state level population shows different power-law behaviors at short or longer positron plasma lengths.
Laser- assisted antihydrogen formation cross sections (differential and total) via the three - body recombination (TBR) process in a plasma of antiprotons and positrons are studied for the ground and excited states . Significantly large cross sections are obtained for low energies. The ground state antihydrogen cross section dominates throughout the energy range, essential for high precision spectroscopics studies in order to test the CPT theorem. The formation cross section are found to be much higher for unequal energies of the two positrons than for equal energies.
We have simulated the formation of antihydrogen through three-body recombination using classical trajectories of antiprotons and positrons. The simulations include several effects which are important in current antihydrogen experiments: the full motion of the antiproton repeatedly passing into and out of the positron plasma, the energy loss of antiprotons due to the interaction with the positron plasma, and the field-ionization of antihydrogen en route from the plasma to the detector. We find that whereas the overall simulated rate of formation of antihydrogen has a density dependence close to n 2 e , the rate of antihydrogen detection follows a power law less than 2. The difference is due to the effect of density dependent field ionization.