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Physical Review A

Comment on “Laser-assisted formation of antihydrogen”

James S. Cohen

Published Jan 31, 2003• 6 citations

Abstract

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.

DOI: 10.1103/physreva.67.017401

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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.

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.

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.

Laser-assisted antihydrogen

(\overline{\mathrm{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

\overline{\mathrm{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

\overline{\mathrm{H}}

) dressing terms to the enhancement of the

\overline{\mathrm{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)

\overline{\mathrm{H}}

formation cross sections particularly at lower incident energies when the system

(\mathrm{Ps}+\overline{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.