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Journal of Physics B Atomic Molecular and Optical Physics

Proposal for making a beam of antihydrogen by two charge exchange events

F. Robicheaux

Published Dec 8, 2009• 2 citations
Publisher

Abstract

We have performed calculations of two successive charge transfers in a geometry that could generate a beam of antihydrogen atoms () or reliably produce cold without having to reach extremely cold plasma temperatures. The basic idea is similar to that proposed by Hessels et al (1998 Phys. Rev. A 57 1668) except that the order of the charge transfers is reversed. A beam of highly-excited (Rydberg) Cs atoms passes through an antiproton () plasma where a charge transfer can take place; the result is an exotic Rydberg atom (Cs+ ion and a bound ) which has approximately the original velocity of the Cs atom since the Cs+ mass is much greater than that of the . This exotic Rydberg atom travels into a positron plasma where a second charge exchange gives antihydrogen (). The velocity distribution of the resulting is directly related to the original velocity of the Rydberg Cs atom. The binding energy of the is roughly that of the original Cs Rydberg atom; thus, the starting state of the can be controlled by choosing the initial state of the Cs atom. Because the Cs+ binding energy can be controlled in the charge transfer, this first step, by itself, could be of interest to the exotic atom community.

DOI
10.1088/0953-4075/43/1/015202

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We have performed calculations of two successive charge transfers in a geometry that could generate a beam of antihydrogen atoms () or reliably produce cold without having to reach extremely cold plasma temperatures. The basic idea is similar to that proposed by Hessels et al (1998 Phys. Rev. A 57 1668) except that the order of the charge transfers is reversed. A beam of highly-excited (Rydberg) Cs atoms passes through an antiproton () plasma where a charge transfer can take place; the result is an exotic Rydberg atom (Cs+ ion and a bound ) which has approximately the original velocity of the Cs atom since the Cs+ mass is much greater than that of the . This exotic Rydberg atom travels into a positron plasma where a second charge exchange gives antihydrogen (). The velocity distribution of the resulting is directly related to the original velocity of the Rydberg Cs atom. The binding energy of the is roughly that of the original Cs Rydberg atom; thus, the starting state of the can be controlled by choosing the initial state of the Cs atom. Because the Cs+ binding energy can be controlled in the charge transfer, this first step, by itself, could be of interest to the exotic atom community.
This work reports on experiments in which antihydrogen atoms have been produced in cryogenic Penning traps from antiproton and positron plasmas by two different methods and on experiments that have been carried out subsequently in order to investigate the antihydrogen atoms. By the first method antihydrogen atoms have been formed during the process of positron cooling of antiprotons in so called nested Penning traps and detected via a field ionization method. A linear dependence of the number of detected antihydrogen atoms on the number of positrons has been found. A measurement of the state distribution has revealed that the antihydrogen atoms are formed in highly excited states. This suggests along with the high production rate that the antihydrogen atoms are formed by three-body recombination processes and subsequent collisional deexcitations. However current theory cannot yet account for the measured state distribution. Typical radii of the detected antihydrogen atoms lie in the range between 0.4 µm and 0.15 µm. The deepest bound antihydrogen atoms have radii below 0.1 µm. Antihydrogen atoms with that size have chaotic positron orbits so that for the first time antihydrogen atoms have been detected that cannot be described by the GCA-model. The kinetic energy of the weakest bound antihydrogen atoms has been measured to about 200 meV, which corresponds to an antihydrogen velocity of approximately 6200 m/s. A simple model suggests that these atoms are formed from only one deexcitation collision and methods that might lead to a decrease of the antihydrogen velocity are presented. By the second method antihydrogen atoms have been synthesized in charge-exchange processes. Lasers are used to produce a Rydberg cesium beam within the cryogenic Penning trap that collides with trapped positrons so that Rydberg positronium atoms are formed via charge-exchange reactions. Due to their charge neutrality the Rydberg positronium atoms are free to leave the positron trapping region. The Rydberg positronium atoms that collide with nearby stored antiprotons form antihydrogen atoms in charge-exchange reactions. So far, 14 +/- 4 antihydrogen atoms have been detected background-free via a field-ionization method. The antihydrogen atoms produced via the two-step charge-exchange mechanism are expected to have a temperature of 4.2 K, the temperature of the antiprotons from which they are formed. A method is proposed by which the antihydrogen temperature can be determined with an accuracy of better than 1 K from a measurement of the time delay between antihydrogen annihilation events and the laser pulse that initiates the antihydrogen production via the production of Rydberg cesium atoms. First experiments have been carried out during the last days of the 2004 beam time, but the number of detected antihydrogen annihilations has been too low for a determination of the antihydrogen temperature. Trapped antiprotons have been directly exposed to laser light delivered by a Titanium:Sapphire laser in order to investigate if the laser light causes any loss on the trapped antiprotons. Experiments have shown that no extra loss occurs for laser powers of less than 590 mW. This is an important result against the background of the future plan to confine antihydrogen atoms in a combined Penning-Ioffe trap and then to carry out laser spectroscopy on these atoms, since it reveals that laser light does not cause an increase of the pressure in the trapping region to the extend that annihilations with the background gas become noticeable. The ATRAP Collaboration plans to precisely investigate antihydrogen atoms. The ultimate goal is to test the CPT-theorem by a high precision measurement of the 1S-2S transition of antihydrogen and a comparison with the precisely known value of the corresponding transition in hydrogen. This thesis presents the achievement of the first step towards this challenging goal: the production of cold antihydrogen itself.
Our ATRAP collaboration has now demonstrated a second technique for antihydrogen (H̄) production. Lasers are used for the first time to control the production of H̄ atoms in our cryogenic apparatus at CERN. As suggested in ref. [2] and first reported in ref. [1], lasers excite a thermal beam of cesium (Cs) atoms to a Rydberg state. In a first charge exchange collision one of these laser‐excited Cs atoms (Cs*) and a cold e+ produces positronium (Ps). Our measurements at Harvard([3]) and at CERN([1]) confirm CTMC simulations([2]) that the laser‐selected binding energy in the Cs atom is preserved by the collision and results in Ps with the selected binding energy. A second charge exchange is between one of these Ps atoms and a trapped p̄ . H̄ is produced by this second collision and is expected to again have the same binding energy. One advantage of this technique as discussed in ref. [2] is that the H̄ produced is expected to be extremely cold, at the temperature of the trapped p̄ , allowing for possible confinement of the H̄ atoms in a magnetic trap.
An efficient method for production of cold antihydrogen $(\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{})$ is proposed. Alkali-metal atoms laser excited to a Rydberg state are charge exchanged with cold trapped positrons, producing Rydberg states of positronium. In a second Rydberg-state charge exchange, the positronium atoms give up their Rydberg positrons to cold trapped antiprotons, producing Rydberg states of $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}$. These $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}$ atoms soon decay down to the ground state, and, because they are cold, could be trapped in a magnetic trap. The efficiency of the process results from the extremely large cross sections for Rydberg charge exchange. Classical trajectory Monte Carlo calculations indicate an $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}$ instantaneous production rate of 106/{}^{6}/6/s.
We have performed calculations of two successive charge transfers from Rydberg states in a strong magnetic field. In the first charge transfer, a positron interacts with a highly excited atom to form positronium. In the second stage, the positronium interacts with an antiproton to give antihydrogen. For many parameters, our results are in qualitative agreement with previous calculations with no magnetic field. However, we do find that there are important changes which may affect the usefulness of the method for efficient formation of antihydrogen that can be trapped.
We have performed calculations of two successive charge transfers from Rydberg states in a strong magnetic field. In the first charge transfer, a positron interacts with a highly excited atom to form positronium. In the second stage, the positronium interacts with an antiproton to give antihydrogen. For many parameters, our results are in qualitative agreement with previous calculations with no magnetic field. However, we do find that there are important changes which may affect the usefulness of the method for efficient formation of antihydrogen that can be trapped.
Antihydrogen, the simplest pure-antimatter atomic system, holds the promise of direct tests of matter-antimatter equivalence and CPT invariance, two of the outstanding unanswered questions in modern physics. Antihydrogen is now routinely produced in charged-particle traps through the combination of plasmas of antiprotons and positrons, but the atoms escape and are destroyed in a minuscule fraction of a second. The focus of this work is the production of a sample of cold antihydrogen atoms in a magnetic atom trap. This poses an extreme challenge, because the state-of-the-art atom traps are only approximately 0.5 K deep for ground-state antihydrogen atoms, much shallower than the energies of particles stored in the plasmas. This thesis will outline the main parts of the ALPHA experiment, with an overview of the important physical processes at work. Antihydrogen production techniques will be described, and an analysis of the spatial annihilation distribution to give indications of the temperature and binding energy distribution of the atoms will be presented. Finally, we describe the techniques needed to demonstrate confinement of antihydrogen atoms, apply them to a data taking run and present the results, making a definitive identification of trapped antihydrogen atoms.