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Physical Review A

Laser-enhanced antihydrogen formation

Shumin Li, Zhang-Jin Chen, Zi-Fang Zhou, Qingqi Wang

Published Feb 1, 1999• 10 citations
Publisher

Abstract

In the first Born approximation, antihydrogen formation in the laser-assisted collision between an antiproton and a positronium is studied. It is found that the antihydrogen formation cross section is notably enhanced at lower impact energy due to the laser modification. For a geometry of laser polarization parallel to the incident direction, the result is modified to the maximum.

DOI
10.1103/physreva.59.1697

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Antihydrogen formation in antiproton collisions with positronium: the adiabatic approach

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The cross section for antihydrogen formation in collision of an antiproton with ground-state positronium has been calculated using the adiabatic hidden-crossing method. At collision velocities below the cross section maximum (3 keV) it is shown that the adiabatic approach provides a very simple description of the positron transfer process, in that capture occurs by a two-state mechanism involving coupling between the 1sσ and 2pσ potential curves only. Good numerical agreement with previous estimates is obtained. 1.

Journal of Physics B Atomic Molecular and Optical Physics•Mar 23, 2001•10 citations
In the first Born approximation, antihydrogen formation in the laser-assisted collision between an antiproton and a positronium is studied. It is found that the antihydrogen formation cross section is notably enhanced at lower impact energy due to the laser modification. For a geometry of laser polarization parallel to the incident direction, the result is modified to the maximum.
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.
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.
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.
Three body recombination (TBR) process for the formation of antihydrogen () inside a trapped plasma of antiprotons () and positrons (e+), supposed to be the most efficient mechanism for the production of cold and trapped antihydrogen is studied quantum mechanically. Present study also involves a background laser field anticipating further enhancement of the formation cross sections. Significantly large cross sections are obtained in the presence of the field at lower incident energies as compared to the field free ones. Cross sections are enhanced for higher excited states of ().
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 cross section for antihydrogen formation in collision of an antiproton with ground-state positronium has been calculated using the adiabatic hidden-crossing method. At collision velocities below the cross section maximum (3 keV) it is shown that the adiabatic approach provides a very simple description of the positron transfer process, in that capture occurs by a two-state mechanism involving coupling between the 1sσ and 2pσ potential curves only. Good numerical agreement with previous estimates is obtained. 1.