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Physical Review Letters

Background-Free Observation of Cold Antihydrogen with Field-Ionization Analysis of Its States

G. Gabrielse, N. S. Bowden, Paul Oxley, Andrew J. Speck, C. H. Storry, Jun Tan, M. Wessels, D. Grzonka, W. Oelert, G. Schepers, T. Sefzick, J. Walz, H. Pittner, Theodor W. Hänsch, Eric A. Hessels

Published Oct 31, 2002• 562 citations
Publisher

Abstract

A background-free observation of cold antihydrogen atoms is made using field ionization followed by antiproton storage, a detection method that provides the first experimental information about antihydrogen atomic states. More antihydrogen atoms can be field ionized in an hour than all the antimatter 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.

DOI
10.1103/physrevlett.89.213401

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Antihydrogen, the opposite number of hydrogen, is the simplest antimatter, and stable in vacuum, which guarantees a long observation time for high precision spectroscopy. This review discusses a short history of antimatter followed by the motivation of the cold antihydrogen research, and then a couple of latest results, which open a new era of antihydrogen research enabling high precision spectroscopy for the first time. A comparison of the spectroscopic properties of antihydrogen with those of hydrogen constitutes a stringent test of the CPT symmetry, the most fundamental law of physics. One of the important achievements of cold antihydrogen study in the last couple of years is the trapping of antihydrogen atoms in an octupole magnetic bottle. Considering the bottle depth, the trapped antihydrogen atoms were really cold, less than 0.5K in their temperature. High precision laser spectroscopy of 1S-2S transition is foreseen once antihydrogen atoms can be laser-cooled. The other milestone development was a successful synthesis of antihydrogen atoms in a so-called cusp trap, where an anti-Helmholtz coil configuration is adopted. The cusp trap enables extraction of antihydrogen atoms in a field-free region as an intensified spin-polarized beam,which realizes high resolution microwave spectroscopy of ground state hyperfine transitions.

Molecular Science•Jan 1, 2012•0 citations
A background-free observation of cold antihydrogen atoms is made using field ionization followed by antiproton storage, a detection method that provides the first experimental information about antihydrogen atomic states. More antihydrogen atoms can be field ionized in an hour than all the antimatter 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.
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.
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 freed antiproton for background-free detection.
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.
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
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.
Antihydrogen, the opposite number of hydrogen, is the simplest antimatter, and stable in vacuum, which guarantees a long observation time for high precision spectroscopy. This review discusses a short history of antimatter followed by the motivation of the cold antihydrogen research, and then a couple of latest results, which open a new era of antihydrogen research enabling high precision spectroscopy for the first time. A comparison of the spectroscopic properties of antihydrogen with those of hydrogen constitutes a stringent test of the CPT symmetry, the most fundamental law of physics. One of the important achievements of cold antihydrogen study in the last couple of years is the trapping of antihydrogen atoms in an octupole magnetic bottle. Considering the bottle depth, the trapped antihydrogen atoms were really cold, less than 0.5K in their temperature. High precision laser spectroscopy of 1S-2S transition is foreseen once antihydrogen atoms can be laser-cooled. The other milestone development was a successful synthesis of antihydrogen atoms in a so-called cusp trap, where an anti-Helmholtz coil configuration is adopted. The cusp trap enables extraction of antihydrogen atoms in a field-free region as an intensified spin-polarized beam,which realizes high resolution microwave spectroscopy of ground state hyperfine transitions.