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Physica status solidi. C, Conferences and critical reviews/Physica status solidi. C, Current topics in solid state physics

ATRAP antihydrogen experiments

C. H. Storry, M. M. Aggarwal, A. Akbari, F. Al‐Rahawi, C. Amole, A. Batrachenko, Adam O. Bebko, Adam C. Carew, M. Chalfin, D. Comeau, G. Gabrielse, F. Garofalo, M. C. George, Frank Goldenbaum, D. Grzonka, Nicholas D. Guise, Y. Gura, Theodor W. Hänsch, Eric A. Hessels, Daniel Kolbe, S. Kotlhammer, Irma Kuljanishvili, P. Larochelle, D. Lesage, B. Levitt, Kasey Lewis, B. Lishak, Frank Markert, F. Nillius, W. Oelert, S. Patel, P. Popescu, Martin Scheid, T. Sefzick, Andrew J. Speck, D. Swierad, J. Walz, M. Weel, Jonathan Wrubel, Zhen-qun Zang

Published Jul 11, 2007• 3 citations
Publisher

Abstract

Abstract Antihydrogen (Hbar) was first produced at CERN in 1996. Over the past decade our ATRAP collaboration has made massive progress toward our goal of producing large numbers of cold Hbar atoms that will be captured in a magnetic gradient trap for precise comparison between the atomic spectra of matter and antimatter. The AD at CERN provides bunches of 3 × 107 low energy Pbars every 100 seconds. We capture and cool to 4 K, 0.1% of these in a cryogenic Penning trap. By stacking many bunches we are able to do experiments with 3 × 105 Pbars. ∼100 e+/sec from a 22Na radioactive source are captured and cooled in the trap, with 5 × 106 available experiments.We have developed 2 ways to make Hbar from these cold ingredients, namely 3‐body collisions, and 2‐stage Rydberg charge exchange. In the first case, Pbars are injected into a nested trap containing e+. Hbar is formed when 2 e+ and 1 Pbar collide. In 2‐stage Rydberg charge exchange, laser‐excited caesium (Cs) enters the trap through a small hole. Rydberg positronium is formed when a e+ captures an e‐from a Cs. These atoms exit the trap, some passing through a nearby cloud of cold Pbars. A 2nd charge‐exchange results when a Pbar captures the e+, forming Hbar. We have also developed techniques to measure the excited‐state distribution of the Hbar and measure their velocity. I will present results from these experiments and discuss the next generation of apparatus to be commissioned this year. This new apparatus includes a e+ accumulator built at York University providing many more e+. The new Pbar annihilation detector provides spatial information of annihilations. Windows allow lasers to enter the trap for spectroscopic measurements and for laser cooling of the Hbar. Possibly the most exciting inclusion in this new apparatus is the inclusion of a neutral particle trap which may, for the first time, capture the Hbar and lead to the first atomic spectrum from antimatter. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

DOI
10.1002/pssc.200675759

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Cryogenic Particle Accumulation In ATRAP And The First Antihydrogen Production Within A Magnetic Gradient Trap For Neutral Antimatter

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ATRAP has made many important improvements since CERN's Antiproton Decelerator (AD) was restarted in 2006. These include substantial increases in the number of positrons (e+) and antiprotons (Pbars) used to make antihydrogen (Hbar) atoms, a new technique for loading electrons (e−) that are used to cool Pbars and e+, implementation of a completely new, larger and more robust apparatus in our second experimental zone and the inclusion of a quadrupole Ioffe trap intended to trap the coldest Hbar atoms produced. Using this new apparatus we have produced large numbers of Hbar atoms within a Penning trap that is located within this quadrupole Ioffe trap using a new technique which shows promise for producing even colder atoms. These observed Hbar atoms resolve a debate about whether positrons and antiprotons can be brought together to form atoms within the divergent magnetic fields of a quadrupole Ioffe trap.

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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.

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The production and study of cold antihydrogen will require the manipulation of dense and cold, single component plasmas of antiprotons and positrons. The undertaking will build upon the experience of the nonneutral plasma physics community. Annihilations of the antimatter particles in the plasmas can be imaged, offering unique diagnostic opportunities not available to this community when electrons and protons are used. The techniques developed by our TRAP collaboration to capture and cool antiprotons will certainly be used by our expanded ATRAP collaboration, and by the competing ATHENA Collaboration, both working at the nearly completed AD facility of CERN. We recently demonstrated a new techniques for accumulating cold positrons directly into a cryogenic vacuum system. The closest we have come to low energy antihydrogen so far is to confine cold positrons and cold antiprotons within the same trap structure and vacuum container. Finally, we mention that stored electrons have been cooled to 70 mK, the first time that elementary particles have been cooled below 4 K. In such an apparatus it should be possible to study highly magnetized plasmas of electrons or positrons at this new low temperature.

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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

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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G. Gabrielse

Only our ATRAP Collaboration is yet able to accumulate and store 4.2 K antiprotons and positrons. The antiprotons come initially from the new Antiproton Decelerator facility at CERN. Good control of such cold antimatter plasmas is key to aspirations to produce and study antihydrogen atoms that are cold enough to confine by their magnetic moments. In the closest approach to cold antihydrogen realized to date, the cold positrons have been used to cool antiprotons, the first time that positron cooling has ever been observed. The Penning-loffe trap, one possibility for simultaneously confining antihydrogen and the cold ingredients from which it is formed, is introduced and discussed.

AIP conference proceedings•Jan 1, 2002•0 citations
Abstract Antihydrogen (Hbar) was first produced at CERN in 1996. Over the past decade our ATRAP collaboration has made massive progress toward our goal of producing large numbers of cold Hbar atoms that will be captured in a magnetic gradient trap for precise comparison between the atomic spectra of matter and antimatter. The AD at CERN provides bunches of 3 × 107 low energy Pbars every 100 seconds. We capture and cool to 4 K, 0.1% of these in a cryogenic Penning trap. By stacking many bunches we are able to do experiments with 3 × 105 Pbars. ∼100 e+/sec from a 22Na radioactive source are captured and cooled in the trap, with 5 × 106 available experiments.We have developed 2 ways to make Hbar from these cold ingredients, namely 3‐body collisions, and 2‐stage Rydberg charge exchange. In the first case, Pbars are injected into a nested trap containing e+. Hbar is formed when 2 e+ and 1 Pbar collide. In 2‐stage Rydberg charge exchange, laser‐excited caesium (Cs) enters the trap through a small hole. Rydberg positronium is formed when a e+ captures an e‐from a Cs. These atoms exit the trap, some passing through a nearby cloud of cold Pbars. A 2nd charge‐exchange results when a Pbar captures the e+, forming Hbar. We have also developed techniques to measure the excited‐state distribution of the Hbar and measure their velocity. I will present results from these experiments and discuss the next generation of apparatus to be commissioned this year. This new apparatus includes a e+ accumulator built at York University providing many more e+. The new Pbar annihilation detector provides spatial information of annihilations. Windows allow lasers to enter the trap for spectroscopic measurements and for laser cooling of the Hbar. Possibly the most exciting inclusion in this new apparatus is the inclusion of a neutral particle trap which may, for the first time, capture the Hbar and lead to the first atomic spectrum from antimatter. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
ATRAP has made many important improvements since CERN's Antiproton Decelerator (AD) was restarted in 2006. These include substantial increases in the number of positrons (e+) and antiprotons (Pbars) used to make antihydrogen (Hbar) atoms, a new technique for loading electrons (e−) that are used to cool Pbars and e+, implementation of a completely new, larger and more robust apparatus in our second experimental zone and the inclusion of a quadrupole Ioffe trap intended to trap the coldest Hbar atoms produced. Using this new apparatus we have produced large numbers of Hbar atoms within a Penning trap that is located within this quadrupole Ioffe trap using a new technique which shows promise for producing even colder atoms. These observed Hbar atoms resolve a debate about whether positrons and antiprotons can be brought together to form atoms within the divergent magnetic fields of a quadrupole Ioffe trap.
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.
The production and study of cold antihydrogen will require the manipulation of dense and cold, single component plasmas of antiprotons and positrons. The undertaking will build upon the experience of the nonneutral plasma physics community. Annihilations of the antimatter particles in the plasmas can be imaged, offering unique diagnostic opportunities not available to this community when electrons and protons are used. The techniques developed by our TRAP collaboration to capture and cool antiprotons will certainly be used by our expanded ATRAP collaboration, and by the competing ATHENA Collaboration, both working at the nearly completed AD facility of CERN. We recently demonstrated a new techniques for accumulating cold positrons directly into a cryogenic vacuum system. The closest we have come to low energy antihydrogen so far is to confine cold positrons and cold antiprotons within the same trap structure and vacuum container. Finally, we mention that stored electrons have been cooled to 70 mK, the first time that elementary particles have been cooled below 4 K. In such an apparatus it should be possible to study highly magnetized plasmas of electrons or positrons at this new low temperature.
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
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.
Only our ATRAP Collaboration is yet able to accumulate and store 4.2 K antiprotons and positrons. The antiprotons come initially from the new Antiproton Decelerator facility at CERN. Good control of such cold antimatter plasmas is key to aspirations to produce and study antihydrogen atoms that are cold enough to confine by their magnetic moments. In the closest approach to cold antihydrogen realized to date, the cold positrons have been used to cool antiprotons, the first time that positron cooling has ever been observed. The Penning-loffe trap, one possibility for simultaneously confining antihydrogen and the cold ingredients from which it is formed, is introduced and discussed.