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Physical Review A

Comment on “Laser-enhanced antihydrogen formation”

Lars Bojer Madsen, P. Lambropoulos

Published May 3, 2000• 6 citations
Publisher

Abstract

The formation of antihydrogen in the laser-assisted charge-transfer reaction between the antiproton and positronium is investigated within the first Born approximation, including dressing of the asymptotic scattering states by the laser field. Through a quantitative study it is shown that, if the field intensity is sufficiently high so as to begin influencing the charge-exchange cross section, the positronium will break up and therefore lower the production of antihydrogen. This conclusion is in contrast to results recently reported by Li et al. [Phys. Rev. A 59, 1697 (1999)], where an enhancement effect is predicted using a similar model for the antihydrogen formation, but neglecting the role of the photobreakup channel.

DOI
10.1103/physreva.61.067401

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Nature Communications•Nov 10, 2017•30 citations
The formation of antihydrogen in the laser-assisted charge-transfer reaction between the antiproton and positronium is investigated within the first Born approximation, including dressing of the asymptotic scattering states by the laser field. Through a quantitative study it is shown that, if the field intensity is sufficiently high so as to begin influencing the charge-exchange cross section, the positronium will break up and therefore lower the production of antihydrogen. This conclusion is in contrast to results recently reported by Li et al. [Phys. Rev. A 59, 1697 (1999)], where an enhancement effect is predicted using a similar model for the antihydrogen formation, but neglecting the role of the photobreakup channel.
The formation of antihydrogen atoms from three-body collisions between positronium atoms and antiprotons plays a crucial role in the gravitational behavior of antihydrogen at rest (GBAR) experiment devoted to the measure of the action of gravity on antimatter. One of the main challenges facing this experiment is to find the means for increasing the production of antihydrogens which are produced by the reaction $\overline{p}+\text{Ps}\ensuremath{\rightarrow}\overline{\text{H}}+{e}^{\ensuremath{-}}$ with an extremely low rate. Along these lines, we investigate here the possibility to influence the collision process by using a laser field. A perturbative approach combining the Coulomb-Born approximation for modeling the charged-particle collision and a first-order perturbation theory for describing laser-atom interactions is employed to estimate laser-mediated charge-exchange cross sections. By carefully considering the laser specifications compatible with the experimental constraints, we present an extensive study of the influence of the laser parameters (laser wavelength and intensity) on the cross sections in the energy range of interest for GBAR. We show that under special irradiation conditions the rate of antihydrogen production may be significantly increased by the presence of the laser field.
In a recent paper, Whitehead, McCann, and Shimamura [Phys. Rev. A 64, 023401 (2001)] presented classical-trajectory Monte Carlo (CTMC) calculations of the effects of laser fields on the cross section for antihydrogen formation in collisions of antiprotons with positronium. They also use the same method to calculate the photoionization rate of isolated positronium in a strong field. The purpose of this Comment is to point out (i) the criteria for validity of the quasiclassical approximation are not satisfied for all the conditions used; (ii) some of their numerical results do not agree with the values obtained with a corroborated code; (iii) the ambiguity of a cross section when the target can ionize even before the projectile arrives; and (iv) the essential effect of quantum-mechanical tunneling on the strong-field ionization rate. A recently published method allowing tunneling in a CTMC calculation [J. S. Cohen, Phys. Rev. A 64, 043412 (2001)] is used for (iv). It is concluded that the laser effect on formation of antihydrogen may be much greater than that found by Whitehead et al., but the pulse must be better characterized for quantitative predictions.
Laser-assisted antihydrogen (H‾)(\overline{\mathrm{H}})(H) formation cross sections (differential and total) for collisions of antiprotons with positronium (Ps) are studied in the framework of the eikonal approximation for two geometries, when the field polarization is parallel $({\ensuremath{\Vert}}^{L})$ or perpendicular $({\ensuremath{\perp}}^{r})$ to the incident Ps momentum. The variations of the H‾\overline{\mathrm{H}}H formation cross sections with respect to the field strength and the laser photon energy are studied for the multiphoton (absorption and emission) processes. The contribution of the atomic (both Ps and H‾\overline{\mathrm{H}}H) dressing terms to the enhancement of the H‾\overline{\mathrm{H}}H formation cross section is studied for both the geometries (${\ensuremath{\Vert}}^{L}$ and ${\ensuremath{\perp}}^{r}$). The most important prediction from the present work is the enhancement of the field-free (FF) H‾\overline{\mathrm{H}}H formation cross sections particularly at lower incident energies when the system (Ps+p‾)(\mathrm{Ps}+\overline{p})(Ps+p​) is irradiated by a single mode, linearly polarized laser, the enhancement being more pronounced for a wider range of incident energy in the ${\ensuremath{\perp}}^{r}$ geometry than in the ${\ensuremath{\Vert}}^{L}$ one.
Abstract The interaction of antiprotons with low-energy positronium atoms is a fundamental three-body problem whose significance is its utility for formation of antihydrogen. Particular importance resides in understanding processes involving excited positronium states. Until recently such studies were performed using classical techniques. However, they become inapplicable in the low-energy domain. Here we report the results of comprehensive quantum calculations, which include initial excited positronium states with principal quantum numbers up to ni = 5. Contrary to expectation from earlier work, there are only muted increases in the cross-sections for antihydrogen formation for ni > 3. We interpret this in terms of quantum suppression of the reaction at higher angular momenta. Furthermore, the cross-sections for elastic scattering are around two orders of magnitude higher, which we attribute to the degeneracy of the positronium states. We outline some experimental consequences of our results.