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Physical Review A

Using electric fields to prevent mirror-trapped antiprotons in antihydrogen studies

Philip Richerme, G. Gabrielse, S. Malbrunot-Ettenauer, R. Kalra, E. R. Tardiff, D. W. Fitzakerley, M. C. George, Eric A. Hessels, C. H. Storry, M. Weel, A. Müllers, J. Walz

Published Feb 25, 2013• 10 citations
Publisher

Abstract

The signature of trapped antihydrogen ($\overline{\mathrm{H}}$) atoms is the annihilation signal detected when the magnetic trap that confines the atoms is suddenly switched off. This signal would be difficult to distinguish from the annihilation signal of any trapped $\overline{p}$ that is released when the magnetic trap is switched off. This work deduces the large cyclotron energy ($>$137 eV) required for magnetic trapping of $\overline{p}$, considers the possibility that such $\overline{p}$ are produced, and explores the effectiveness of an electric field applied to clear charged particles from the trapping volume before $\overline{\mathrm{H}}$ detection. No mechanisms are found that can give a $\overline{p}$ such a large cyclotron energy and allow it to mimic an $\overline{\mathrm{H}}$ annihilation. The method used to release $\overline{\mathrm{H}}$ atoms from their magnetic trap without removing the magnetic field gradient that could possibly confine $\overline{p}$ with a high cyclotron energy is also discussed.

DOI
10.1103/physreva.87.023422

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The signature of trapped antihydrogen (H‾\overline{\mathrm{H}}H) atoms is the annihilation signal detected when the magnetic trap that confines the atoms is suddenly switched off. This signal would be difficult to distinguish from the annihilation signal of any trapped p‾\overline{p}p​ that is released when the magnetic trap is switched off. This work deduces the large cyclotron energy (>>>137 eV) required for magnetic trapping of p‾\overline{p}p​, considers the possibility that such p‾\overline{p}p​ are produced, and explores the effectiveness of an electric field applied to clear charged particles from the trapping volume before H‾\overline{\mathrm{H}}H detection. No mechanisms are found that can give a p‾\overline{p}p​ such a large cyclotron energy and allow it to mimic an H‾\overline{\mathrm{H}}H annihilation. The method used to release H‾\overline{\mathrm{H}}H atoms from their magnetic trap without removing the magnetic field gradient that could possibly confine p‾\overline{p}p​ with a high cyclotron energy is also discussed.
Recently, antihydrogen atoms were trapped at CERN in a magnetic minimum (minimum-B) trap formed by superconducting octupole and mirror magnet coils. The trapped antiatoms were detected by rapidly turning off these magnets, thereby eliminating the magnetic minimum and releasing any antiatoms contained in the trap. Once released, these antiatoms quickly hit the trap wall, whereupon the positrons and antiprotons in the antiatoms annihilate. The antiproton annihilations produce easily detected signals; we used these signals to prove that we trapped antihydrogen. However, our technique could be confounded by mirror-trapped antiprotons, which would produce seemingly identical annihilation signals upon hitting the trap wall. In this paper, we discuss possible sources of mirror-trapped antiprotons and show that antihydrogen and antiprotons can be readily distinguished, often with the aid of applied electric fields, by analyzing the annihilation locations and times. We further discuss the general properties of antiproton and antihydrogen trajectories in this magnetic geometry, and reconstruct the antihydrogen energy distribution from the measured annihilation time history.
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.
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Antihydrogen, the simplest pure-antimatter atomic system, holds the promise of direct tests of matter-antimatter equivalence and CPT invariance, two of the outstanding unanswered questions in modern physics. Antihydrogen is now routinely produced in charged-particle traps through the combination of plasmas of antiprotons and positrons, but the atoms escape and are destroyed in a minuscule fraction of a second. The focus of this work is the production of a sample of cold antihydrogen atoms in a magnetic atom trap. This poses an extreme challenge, because the state-of-the-art atom traps are only approximately 0.5 K deep for ground-state antihydrogen atoms, much shallower than the energies of particles stored in the plasmas. This thesis will outline the main parts of the ALPHA experiment, with an overview of the important physical processes at work. Antihydrogen production techniques will be described, and an analysis of the spatial annihilation distribution to give indications of the temperature and binding energy distribution of the atoms will be presented. Finally, we describe the techniques needed to demonstrate confinement of antihydrogen atoms, apply them to a data taking run and present the results, making a definitive identification of trapped antihydrogen atoms.
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.
Antihydrogen, a positron bound to an antiproton, is the simplest anti-atom. Its structure and properties are expected to mirror those of the hydrogen atom. Prospects for precision comparisons of the two, as tests of fundamental symmetries, are driving a vibrant programme of research. In this regard, a limiting factor in most experiments is the availability of large numbers of cold ground state antihydrogen atoms. Here, we describe how an improved synthesis process results in a maximum rate of 10.5 ± 0.6 atoms trapped and detected per cycle, corresponding to more than an order of magnitude improvement over previous work. Additionally, we demonstrate how detailed control of electron, positron and antiproton plasmas enables repeated formation and trapping of antihydrogen atoms, with the simultaneous retention of atoms produced in previous cycles. We report a record of 54 detected annihilation events from a single release of the trapped anti-atoms accumulated from five consecutive cycles.Antihydrogen studies are important in testing the fundamental principles of physics but producing antihydrogen in large amounts is challenging. Here the authors demonstrate an efficient and high-precision method for trapping and stacking antihydrogen by using controlled plasma.