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arXiv:1201.3944 · New Journal of Physics

Discriminating between antihydrogen and mirror-trapped antiprotons in a minimum-B trap

C. Amole, Gorm Bruun Andresen, M. D. Ashkezari, M. Baquero-Ruiz, William Alan Bertsche, Eoin Butler, Claudio Lenz Cesar, S Chapman, M. Charlton, Adam Deller, S. Eriksson, J. Fajans, Tim Friesen, M. Fujiwara, D. R. Gill, Andrea Gutierrez, Jeffrey Scott Hangst, W. N. Hardy, M. E. Hayden, Andrew James Humphries, Richard Hydomako, L. L. Kurchaninov, S. Jonsell, N. Madsen, S. Menary, Paul J. Nolan, K. Olchanski, Art Olin, Alexander P. Povilus, Petteri Pusa, F. Robicheaux, E. Sarid, Daniel Miranda Silveira, Chukman So, James William Storey, R. I. Thompson, Dirk Peter van der Werf, Jonathan Syrkin Wurtele

Published Jan 31, 2012• 28 citations

Abstract

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.

DOI: 10.1088/1367-2630/14/1/015010

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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.

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.

We have demonstrated storage of plasmas of the charged constituents of the antihydrogen atom, antiprotons and positrons, in a Penning trap surrounded by a minimum-B magnetic trap designed for holding neutral antiatoms. The neutral trap comprises a superconducting octupole and two superconducting, solenoidal mirror coils. We have measured the storage lifetimes of antiproton and positron plasmas in the combined Penning-neutral trap, and compared these to lifetimes without the neutral trap fields. The magnetic well depth was 0.6 T, deep enough to trap ground state antihydrogen atoms of up to about 0.4 K in temperature. We have demonstrated that both particle species can be stored for times long enough to permit antihydrogen production and trapping studies.

Antihydrogen production in a neutral atom trap formed by an octupole-based magnetic field minimum is demonstrated using field-ionization of weakly bound anti-atoms. Using our unique annihilation imaging detector, we correlate antihydrogen detection by imaging and by field-ionization for the first time. We further establish how field-ionization causes radial redistribution of the antiprotons during antihydrogen formation and use this effect for the first simultaneous measurements of strongly and weakly bound antihydrogen atoms. Distinguishing between these provides critical information needed in the process of optimizing for trappable antihydrogen. These observations are of crucial importance to the ultimate goal of performing CPT tests involving antihydrogen, which likely depends upon trapping the anti-atom.

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.

The ALPHA apparatus is designed to produce and trap antihydrogen atoms. The device comprises a multifunction Penning trap and a superconducting, neutral atom trap having a minimum‐B configuration. The atom trap features an octupole magnet for transverse confinement and solenoidal mirror coils for longitudinal confinement. The magnetic trap employs a fast shutdown system to maximize the probability of detecting the annihilation of released antihydrogen. In this article we describe the first attempts to observe antihydrogen trapping.