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AIP conference proceedings

Formation of Antihydrogen Rydberg atoms in strong magnetic field traps

Thomas Pohl, H. R. Sadeghpour, Yasuyuki Kanai, Y. Yamazaki

Published Jan 1, 2008• 2 citations
Publisher

Abstract

It is shown that several features of antihydrogen production in nested Penning traps can be described with accurate and efficient Monte Carlo simulations. It is found that cold deeply‐bound Rydberg states of antihydrogen (H̄) are produced in three‐body capture in the ATRAP experiments and an additional formation mechanism ‐Rydberg charge transfer‐, particular to the nested Penning trap geometry, is responsible for the observed fast (hot) H̄ atoms. Detailed description of the numerical propagation technique for following extreme close encounters is given. An analytic derivation of the power law behavior of the field ionization spectrum is provided.

DOI
10.1063/1.2977838

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New Interpretations of Measured Antihydrogen Velocities and Field Ionization Spectra

Thomas Pohl, H. R. Sadeghpour, G. Gabrielse

We present extensive Monte Carlo simulations, showing that cold antihydrogen (H) atoms are produced when antiprotons (p) are gently heated in the side wells of a nested Penning trap. The observed H with high energies, that had seemed to indicate otherwise, are instead explained by a surprisingly effective charge-exchange mechanism. We shed light on the previously measured field-ionization spectrum, and reproduce both the characteristic low-field power law as well as the enhanced H production at higher fields. The latter feature is shown to arise from H toms too deeply bound to be described as guiding center atoms, atoms with internally chaotic motion.

Physical Review Letters•Oct 5, 2006•41 citations

Antiprotons and Cold Antihydrogen

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The author's ATRAP team has managed to simultaneously accumulate cold antiprotons and cold positrons. Cold antihydrogen is produced when antiprotons are repeatedly driven into collisions with cold positrons within a nested Penning trap. Efficient antihydrogen production takes place during many cycles of positron cooling of antiprotons. A first measurement of a distribution of antihydrogen states is made using a preionizing electric field between separated production and detection regions. Surviving antihydrogen is stripped in an ionization well that captures and stores the free antiproton for background-free detection. A background-free observation of cold antihydrogen atoms was made using field ionization followed by antiproton storage, a detection method that provided the first experimental information about antihydrogen atomic states. More antihydrogen atoms can be field ionized in an hour than all the anti-matter atoms that have been previously reported, and the production rate per incident high energy antiproton is higher than ever observed. The high rate and the high Rydberg states suggest that the antihydrogen is formed via three-body recombination. This report presents bibliographic citations for 10 publications and 36 lectures that were produced during 2004 under this contract. The author also was the recipient of the George Ledlie Prize at Harvard University in 2004.

•Jan 1, 2001•0 citations

Observations of Cold Antihydrogen

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ATRAP’s e + cooling of p in a nested Penning trap has led to reports of cold Hproduced during such cooling by the ATHENA and ATRAP collaborations. ATHENA uses coincident annihilation detection and ATRAP uses field ionization followed by p storage. Advantages of the later method include the complete absence of a substantial background of false events intrinsic to the former, and the first way to measure which H states are produced. ATRAP enhances the H production rate by driving many cycles of e + cooling in the nested trap, with more H counted in an hour than the sum of all the other antimatter atoms ever reported. The number of Hcounted per incident high energy p is also higher than ever observed. The first measured distribution of H states is made using a pre-ionizing electric field between separated production and detection regions. The high rate and the high Rydberg states suggest that the H is formed via three-body recombination, as expected. 1

APS Division of Atomic, Molecular and Optical Physics Meeting Abstracts•May 1, 2003•0 citations

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

S. Jonsell, Dirk Peter van der Werf, M. Charlton +1 more

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.

Journal of Physics B Atomic Molecular and Optical Physics•Oct 26, 2009•35 citations

The Production and Investigation of Cold Antihydrogen Atoms

H. Pittner

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.

•Jan 1, 2005•0 citations

Simulations of antihydrogen formation

F. Robicheaux

The results of simulations of antihydrogen formation in a Penning trap are reported. The antihydrogen atoms are formed by three-body capture. We find that the arrested nature of the three-body capture in the trap greatly reduces the expected binding energy of the antihydrogen. Typically, the formed antihydrogen has larger velocity along the magnetic field than across the field and a binding energy below ${k}_{B}T$.

Physical Review A•Aug 20, 2004•66 citations
It is shown that several features of antihydrogen production in nested Penning traps can be described with accurate and efficient Monte Carlo simulations. It is found that cold deeply‐bound Rydberg states of antihydrogen (H̄) are produced in three‐body capture in the ATRAP experiments and an additional formation mechanism ‐Rydberg charge transfer‐, particular to the nested Penning trap geometry, is responsible for the observed fast (hot) H̄ atoms. Detailed description of the numerical propagation technique for following extreme close encounters is given. An analytic derivation of the power law behavior of the field ionization spectrum is provided.
We present extensive Monte Carlo simulations, showing that cold antihydrogen (H) atoms are produced when antiprotons (p) are gently heated in the side wells of a nested Penning trap. The observed H with high energies, that had seemed to indicate otherwise, are instead explained by a surprisingly effective charge-exchange mechanism. We shed light on the previously measured field-ionization spectrum, and reproduce both the characteristic low-field power law as well as the enhanced H production at higher fields. The latter feature is shown to arise from H toms too deeply bound to be described as guiding center atoms, atoms with internally chaotic motion.
The author's ATRAP team has managed to simultaneously accumulate cold antiprotons and cold positrons. Cold antihydrogen is produced when antiprotons are repeatedly driven into collisions with cold positrons within a nested Penning trap. Efficient antihydrogen production takes place during many cycles of positron cooling of antiprotons. A first measurement of a distribution of antihydrogen states is made using a preionizing electric field between separated production and detection regions. Surviving antihydrogen is stripped in an ionization well that captures and stores the free antiproton for background-free detection. A background-free observation of cold antihydrogen atoms was made using field ionization followed by antiproton storage, a detection method that provided the first experimental information about antihydrogen atomic states. More antihydrogen atoms can be field ionized in an hour than all the anti-matter atoms that have been previously reported, and the production rate per incident high energy antiproton is higher than ever observed. The high rate and the high Rydberg states suggest that the antihydrogen is formed via three-body recombination. This report presents bibliographic citations for 10 publications and 36 lectures that were produced during 2004 under this contract. The author also was the recipient of the George Ledlie Prize at Harvard University in 2004.
ATRAP’s e + cooling of p in a nested Penning trap has led to reports of cold Hproduced during such cooling by the ATHENA and ATRAP collaborations. ATHENA uses coincident annihilation detection and ATRAP uses field ionization followed by p storage. Advantages of the later method include the complete absence of a substantial background of false events intrinsic to the former, and the first way to measure which H states are produced. ATRAP enhances the H production rate by driving many cycles of e + cooling in the nested trap, with more H counted in an hour than the sum of all the other antimatter atoms ever reported. The number of Hcounted per incident high energy p is also higher than ever observed. The first measured distribution of H states is made using a pre-ionizing electric field between separated production and detection regions. The high rate and the high Rydberg states suggest that the H is formed via three-body recombination, as expected. 1
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.
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.
The results of simulations of antihydrogen formation in a Penning trap are reported. The antihydrogen atoms are formed by three-body capture. We find that the arrested nature of the three-body capture in the trap greatly reduces the expected binding energy of the antihydrogen. Typically, the formed antihydrogen has larger velocity along the magnetic field than across the field and a binding energy below kBT{k}_{B}TkB​T.