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Journal of Physics Conference Series

Antimatter Matters: Progress in Cold Antihydrogen Research

Y. Yamazaki

Published Nov 5, 2012• 6 citations
Publisher

Abstract

The purpose of cold antihydrogen research is briefly reviewed together with the latest developments of manipulating antihydrogen atoms. Two major progresses last year were the trapping of antihydrogen atoms in a magnetic bottle and synthesis of antihydrogen atoms in a cusp trap, where a spin-polarized antihydrogen beam can be extracted as an intensified beam. The magnetic bottle consists of an octupole coil and a pair of mirror coils, which improved the magnetic field uniformity near the axis, and so the stability of trapping antiprotons and positrons. Eventually, antihydrogen atoms were trapped for more than 1000s, which is ready to be testified with high precision laser spectroscopy. The cusp trap consists of a superconducting anti-Helmholtz coil and a stack of multiple ring electrodes. This success opens a new path to make a stringent test of the CPT symmetry via high precision microwave spectroscopy of ground-state hyperfine transitions of antihydrogen atoms.

DOI
10.1088/1742-6596/388/1/012002

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The purpose of cold antihydrogen research is briefly reviewed together with the latest developments of manipulating antihydrogen atoms. Two major progresses last year were the trapping of antihydrogen atoms in a magnetic bottle and synthesis of antihydrogen atoms in a cusp trap, where a spin-polarized antihydrogen beam can be extracted as an intensified beam. The magnetic bottle consists of an octupole coil and a pair of mirror coils, which improved the magnetic field uniformity near the axis, and so the stability of trapping antiprotons and positrons. Eventually, antihydrogen atoms were trapped for more than 1000s, which is ready to be testified with high precision laser spectroscopy. The cusp trap consists of a superconducting anti-Helmholtz coil and a stack of multiple ring electrodes. This success opens a new path to make a stringent test of the CPT symmetry via high precision microwave spectroscopy of ground-state hyperfine transitions of antihydrogen atoms.
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.
Performing measurements of the properties of antihydrogen, the bound state of an antiproton and a positron, and comparing the results with those for ordinary hydrogen, has long been seen as a route to test some of the fundamental principles of physics. There has been much experimental progress in this direction in recent years, and antihydrogen is now routinely created and trapped and a range of exciting measurements probing the foundations of modern physics are planned or underway. In this contribution we review the techniques developed to facilitate the capture and manipulation of positrons and antiprotons, along with procedures to bring them together to create antihydrogen. Once formed, the antihydrogen has been detected by its destruction via annihilation or field ionization, and aspects of the methodologies involved are summarized. Magnetic minimum neutral atom traps have been employed to allow some of the antihydrogen created to be held for considerable periods. We describe such devices, and their implementation, along with the cusp magnetic trap used to produce the first evidence for a low-energy beam of antihydrogen. The experiments performed to date on antihydrogen are discussed, including the first observation of a resonant quantum transition and the analyses that have yielded a limit on the electrical neutrality of the anti-atom and placed crude bounds on its gravitational behaviour. Our review concludes with an outlook, including the new ELENA extension to the antiproton decelerator facility at CERN, together with summaries of how we envisage the major threads of antihydrogen physics will progress in the coming years.
The ASACUSA CUSP experiment plans a high precision spectroscopy of the ground-state hyperfine splitting of antihydrogen to test CPT symmetry.To perform this, a Rabi-like antiatomic beam method has been developed with a novel double-cusp trap.Antihydrogen atoms were synthesized in the double-cusp trap by directly injecting antiprotons from an accumulation trap into a positron plasma.By adjusting the energy difference between the incoming antiproton beam and the positron plasma, a high-rate production of antihydrogen atoms was observed.
The recent demonstration of trapped atomic antihydrogen for 15 to 1000 seconds is a milestone towards precise spectroscopy for tests of CPT invariance. The confinement of a total of 105±21 atoms in a quadrupole magnetic trap was made possible by several improved methods. Improved accumulation techniques give us the largest numbers of constituent particles yet: up to 10 million antiprotons and several billion positrons. A novel cooling protocol leads to 3.5 K antiprotons, the coldest ever observed. Characterizing and controlling the geometry and density of these confined antimatter plasmas allow for consistency in antihydrogen production. Continued use of these methods along with the larger trap depth of a unique second-generation magnet are expected to yield greater numbers of trapped antihydrogen. The new magnet generates both quadrupole and octupole trap geometries, which should make it possible to reduce charged particle loss and will prove useful for laser cooling and spectroscopy. The ultra-low inductances of the magnet have been shown to vastly reduce turn-off times, which will optimize single-atom detection. Finally, improved detector characterization already makes us sensitive to smaller numbers of trapped antihydrogen atoms than before.
The creation of cold antihydrogen atoms by the controlled combination of positrons and antiprotons has opened up a new window on fundamental physics. More recently, techniques have been developed that allow some antihydrogen atoms to be created at low enough kinetic energies that they can be held inside magnetic minimum neutral atom traps. With confinement times of many minutes possible, it has become feasible to perform experiments to probe the properties of the antiatom for the first time. We review the experimental progress in this area, outline some of the motivation for studying basic aspects of antimatter physics and provide an outlook of where we might expect this field to go in the coming years.