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Journal of Physics B Atomic Molecular and Optical Physics

Simulation of the formation of antihydrogen in a nested Penning trap: effect of positron density

S. Jonsell, Dirk Peter van der Werf, M. Charlton, F. Robicheaux

Published Oct 26, 2009• 35 citations
Publisher

Abstract

Detailed simulations of antihydrogen formation have been performed under the conditions of the ATHENA experiment, using several densities of the positron plasma in the range n e = 5 × 10 13 m −3 to 10 15 m −3 . The simulations include only collisional effects, typically resulting in the formation of weakly bound antihydrogen via the three-body process, . (Radiative processes, which are much slower than collisional effects, are neglected.) The properties of these weakly bound anti-atoms are affected not only by further collisions in the plasma but also by the inherent electric fields. The role of field ionization in influencing the distribution of binding energies of the antihydrogen is clarified and the mechanism for this process in the strong B -field nested Penning trap used in the experiment is elucidated. The fate of antihydrogen is explained and the properties of the population detected after having reached the wall of the Penning trap electrodes, as well as those field ionized, are recorded. We find that the yield of detected antihydrogen varies with positron density roughly as n 1.7 e , rather than the n 2 e expected from the underlying formation process. As n e is increased, antihydrogen formation is sufficiently rapid that epithermal effects begin to play an important role. In general, the simulated timescales for antihydrogen formation are much shorter than those found from the experiment.

DOI
10.1088/0953-4075/42/21/215002

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AIP conference proceedings•Jan 1, 2007•1 citation
Detailed simulations of antihydrogen formation have been performed under the conditions of the ATHENA experiment, using several densities of the positron plasma in the range n e = 5 × 10 13 m −3 to 10 15 m −3 . The simulations include only collisional effects, typically resulting in the formation of weakly bound antihydrogen via the three-body process, . (Radiative processes, which are much slower than collisional effects, are neglected.) The properties of these weakly bound anti-atoms are affected not only by further collisions in the plasma but also by the inherent electric fields. The role of field ionization in influencing the distribution of binding energies of the antihydrogen is clarified and the mechanism for this process in the strong B -field nested Penning trap used in the experiment is elucidated. The fate of antihydrogen is explained and the properties of the population detected after having reached the wall of the Penning trap electrodes, as well as those field ionized, are recorded. We find that the yield of detected antihydrogen varies with positron density roughly as n 1.7 e , rather than the n 2 e expected from the underlying formation process. As n e is increased, antihydrogen formation is sufficiently rapid that epithermal effects begin to play an important role. In general, the simulated timescales for antihydrogen formation are much shorter than those found from the experiment.
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.
The formation of antihydrogen atoms from antiprotons injected into a positron plasma is simulated, focussing on the fraction that fulfil the conditions necessary for confinement of anti-atoms in a magnetic minimum trap. Trapping fractions of around 10 −4 are found under conditions similar to those used in recent experiments, and in reasonable accord with their results. We have studied the behaviour of the trapped fraction at various positron plasma densities and temperatures and found that collisional effects play a beneficial role via a redistribution of the antihydrogen magnetic moment, allowing enhancements of the yield of low-field seeking states that are amenable to trapping.
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.
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.
Some years have passed since the report of the first production of cold antihydrogen by the Athena Collaboration and the Atrap Collaboration at CERN, but no clear answer has been given about the roles of the two mechanisms responsible for antihydrogen formation. A new preliminary analysis of the data acquired by the Athena Collaboration in different experimental conditions seems to suggest that three-body recombination mechanism is dominant in the first tens of seconds of the overlapping of the injected antiproton cloud with the positron plasma in the nested Penning trap, while radiative capture starts to become dominant afterwards.
Numerous antihydrogen atoms are created at CERN, by ATRAP and ATHENA experiments, by bringing together positrons and antiprotons in a magnetic Penning trap. Most of these atoms are created in exotic, highly excited states, such that the magnetic forces on positrons are greater than the Coulomb attraction of antiprotons. This paper presents an overview of the recent progress made toward theoretical understanding of the complicated dynamics which leads to the formation and detection of antihydrogen atoms. There is no formal difference between the plasmas described here and normal, electron‐proton, matter plasmas, except the reversed sign of electrical charges. The next generation of experiments need to bring the antihydrogen atoms to the ground state and to cool them to sub‐milliKelvin temperature. Only then, high resolution spectroscopy can expose differences between matter and antimatter due to CPT violations. Suggestions are made for possible pathways toward this goal.