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Physical Review A

Laser-driven production of the antihydrogen molecular ion

Mark C. Zammit, M. Charlton, S. Jonsell, James P. Colgan, Jeremy S. Savage, Dmitry V. Fursa, Alisher S. Kadyrov, Igor Bray, Robert C. Forrey, Christopher J. Fontes, Jeffery A. Leiding, David P. Kilcrease, P. Hakel, Eddy Timmermans

Published Oct 24, 2019• 20 citations
Publisher

Abstract

The feasibility of producing the molecular antihydrogen anion ${{\overline{\mathrm{H}}}_{2}}^{\ensuremath{-}}$ in the laboratory is investigated. Utilizing reaction rates calculated here involving the interaction of laser excited-state antihydrogen atoms held in magnetic minimum traps, key processes are identified that could lead to anion production, as well as competing effects leading to anti-atom loss. These are discussed in the context of present-day and near-future experimental capabilities.

DOI
10.1103/physreva.100.042709

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Formation of antihydrogen molecular ions by associative ionization

Josiah Taylor, Brandon Vargo, Daniel Hoffman +2 more

Recent calculations have suggested that it may be feasible to produce antihydrogen molecular ions in the near future [Zammit et al., Phys. Rev. A 100, 042709 (2019)]. The proposed scheme would laser excite cold antihydrogen $\overline{\text{H}}(1s)$ atoms held in a magnetic-field minimum trap and the resulting metastable $\overline{\text{H}}(2s)$ atoms would interact to form the antihydrogen molecular ion, ${\overline{\text{H}}}_{2}{}^{\ensuremath{-}}$, by associative ionization. Estimates for the efficiencies of the proposed formation and depletion processes relied on low-energy extrapolations of rate coefficients that were computed at thermal energies. In the present work, we provide quantum calculations of the rate coefficients for associative ionization and for the competing Penning ionization process, which would deplete the population of trapped $\overline{\text{H}}$. In contrast to the low-energy extrapolations, it is found that Penning ionization is the dominant ionization process at all temperatures and the relative efficiency of associative ionization is within a factor of 3 at the low temperatures required by the experimental scheme. Rate constants for single and double excitation transfer are computed as possible loss mechanisms and all rates are found to be small compared to spin-flip decay rates from Stark-induced mixing with $\overline{\text{H}}(2p$) states.

Physical Review A•May 15, 2024•2 citations

The Production and Investigation of Cold Antihydrogen Atoms

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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

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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 10${}^{6}/$s.

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Antihydrogen formation from laser-assisted antiproton-positronium collisions

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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.

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Laser cooling of antihydrogen atoms

C. J. Baker, William Alan Bertsche, A. Capra +55 more

Abstract The photon—the quantum excitation of the electromagnetic field—is massless but carries momentum. A photon can therefore exert a force on an object upon collision1. Slowing the translational motion of atoms and ions by application of such a force2,3, known as laser cooling, was first demonstrated 40 years ago4,5. It revolutionized atomic physics over the following decades6–8, and it is now a workhorse in many fields, including studies on quantum degenerate gases, quantum information, atomic clocks and tests of fundamental physics. However, this technique has not yet been applied to antimatter. Here we demonstrate laser cooling of antihydrogen9, the antimatter atom consisting of an antiproton and a positron. By exciting the 1S–2P transition in antihydrogen with pulsed, narrow-linewidth, Lyman-α laser radiation10,11, we Doppler-cool a sample of magnetically trapped antihydrogen. Although we apply laser cooling in only one dimension, the trap couples the longitudinal and transverse motions of the anti-atoms, leading to cooling in all three dimensions. We observe a reduction in the median transverse energy by more than an order of magnitude—with a substantial fraction of the anti-atoms attaining submicroelectronvolt transverse kinetic energies. We also report the observation of the laser-driven 1S–2S transition in samples of laser-cooled antihydrogen atoms. The observed spectral line is approximately four times narrower than that obtained without laser cooling. The demonstration of laser cooling and its immediate application has far-reaching implications for antimatter studies. A more localized, denser and colder sample of antihydrogen will drastically improve spectroscopic11–13 and gravitational14 studies of antihydrogen in ongoing experiments. Furthermore, the demonstrated ability to manipulate the motion of antimatter atoms by laser light will potentially provide ground-breaking opportunities for future experiments, such as anti-atomic fountains, anti-atom interferometry and the creation of antimatter molecules.

Nature•Mar 31, 2021•84 citations
The feasibility of producing the molecular antihydrogen anion H‾2\ensuremath−{{\overline{\mathrm{H}}}_{2}}^{\ensuremath{-}}H2​\ensuremath− in the laboratory is investigated. Utilizing reaction rates calculated here involving the interaction of laser excited-state antihydrogen atoms held in magnetic minimum traps, key processes are identified that could lead to anion production, as well as competing effects leading to anti-atom loss. These are discussed in the context of present-day and near-future experimental capabilities.
Recent calculations have suggested that it may be feasible to produce antihydrogen molecular ions in the near future [Zammit et al., Phys. Rev. A 100, 042709 (2019)]. The proposed scheme would laser excite cold antihydrogen H‾(1s)\overline{\text{H}}(1s)H(1s) atoms held in a magnetic-field minimum trap and the resulting metastable H‾(2s)\overline{\text{H}}(2s)H(2s) atoms would interact to form the antihydrogen molecular ion, ${\overline{\text{H}}}_{2}{}^{\ensuremath{-}}$, by associative ionization. Estimates for the efficiencies of the proposed formation and depletion processes relied on low-energy extrapolations of rate coefficients that were computed at thermal energies. In the present work, we provide quantum calculations of the rate coefficients for associative ionization and for the competing Penning ionization process, which would deplete the population of trapped H‾\overline{\text{H}}H. In contrast to the low-energy extrapolations, it is found that Penning ionization is the dominant ionization process at all temperatures and the relative efficiency of associative ionization is within a factor of 3 at the low temperatures required by the experimental scheme. Rate constants for single and double excitation transfer are computed as possible loss mechanisms and all rates are found to be small compared to spin-flip decay rates from Stark-induced mixing with H‾(2p\overline{\text{H}}(2pH(2p) states.
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.
Abstract The production of beams of antihydrogen atoms using the dipole force provided by a travelling optical lattice to accelerate a sample of the anti-atoms held in a magnetic gradient atom trap is investigated. By considering current and near-future antihydrogen trapping capabilities we find that useful fluxes of the anti-atoms can be achieved with directional properties that can be manipulated using laser parameters such as pulse duration and frequency chirp rate. Applications of the beams are briefly discussed.
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.
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.
Abstract The photon—the quantum excitation of the electromagnetic field—is massless but carries momentum. A photon can therefore exert a force on an object upon collision1. Slowing the translational motion of atoms and ions by application of such a force2,3, known as laser cooling, was first demonstrated 40 years ago4,5. It revolutionized atomic physics over the following decades6–8, and it is now a workhorse in many fields, including studies on quantum degenerate gases, quantum information, atomic clocks and tests of fundamental physics. However, this technique has not yet been applied to antimatter. Here we demonstrate laser cooling of antihydrogen9, the antimatter atom consisting of an antiproton and a positron. By exciting the 1S–2P transition in antihydrogen with pulsed, narrow-linewidth, Lyman-α laser radiation10,11, we Doppler-cool a sample of magnetically trapped antihydrogen. Although we apply laser cooling in only one dimension, the trap couples the longitudinal and transverse motions of the anti-atoms, leading to cooling in all three dimensions. We observe a reduction in the median transverse energy by more than an order of magnitude—with a substantial fraction of the anti-atoms attaining submicroelectronvolt transverse kinetic energies. We also report the observation of the laser-driven 1S–2S transition in samples of laser-cooled antihydrogen atoms. The observed spectral line is approximately four times narrower than that obtained without laser cooling. The demonstration of laser cooling and its immediate application has far-reaching implications for antimatter studies. A more localized, denser and colder sample of antihydrogen will drastically improve spectroscopic11–13 and gravitational14 studies of antihydrogen in ongoing experiments. Furthermore, the demonstrated ability to manipulate the motion of antimatter atoms by laser light will potentially provide ground-breaking opportunities for future experiments, such as anti-atomic fountains, anti-atom interferometry and the creation of antimatter molecules.