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Physical Review A

Producing ultracold and trappable antihydrogen atoms

S. X. Hu

Published Jan 29, 2007• 4 citations
Publisher

Abstract

Both experiments and simulations have shown that the temperature of antihydrogen atoms $(\overline{\mathrm{H}})$ formed in a strongly magnetized positron-antiproton plasma is mainly determined by the heavy particles' temperature. A routine to keep cooling antiprotons $(\overline{p})$ by using ultracold $(<1\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ electrons is proposed under attainable experimental conditions. When cold positrons are loaded into such a low-temperature electron-antiproton mixture, ultracold antihydrogen atoms can be produced at a temperature below $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Our large-scale molecular-dynamics simulations, which involve $20\phantom{\rule{0.2em}{0ex}}000$ particles (${e}^{\ensuremath{-}}$, ${e}^{+}$, and $\overline{p}$) evolving up to $1\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{s}$, have confirmed the generation of trappable $\overline{\mathrm{H}}$ atoms.

DOI
10.1103/physreva.75.010501

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Both experiments and simulations have shown that the temperature of antihydrogen atoms (H‾)(\overline{\mathrm{H}})(H) formed in a strongly magnetized positron-antiproton plasma is mainly determined by the heavy particles' temperature. A routine to keep cooling antiprotons (p‾)(\overline{p})(p​) by using ultracold (<1K)(<1\phantom{\rule{0.3em}{0ex}}\mathrm{K})(<1K) electrons is proposed under attainable experimental conditions. When cold positrons are loaded into such a low-temperature electron-antiproton mixture, ultracold antihydrogen atoms can be produced at a temperature below \ensuremath∼1K\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{K}\ensuremath∼1K. Our large-scale molecular-dynamics simulations, which involve 2000020\phantom{\rule{0.2em}{0ex}}00020000 particles (e\ensuremath−{e}^{\ensuremath{-}}e\ensuremath−, e+{e}^{+}e+, and p‾\overline{p}p​) evolving up to 1\ensuremathμs1\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{s}1\ensuremathμs, have confirmed the generation of trappable H‾\overline{\mathrm{H}}H atoms.
The formation of antihydrogen atoms from antiprotons injected into a positron plasma is simulated, focussing on the fraction that fulfil the conditions necessary for confinement of anti-atoms in a magnetic minimum trap. Trapping fractions of around 10 −4 are found under conditions similar to those used in recent experiments, and in reasonable accord with their results. We have studied the behaviour of the trapped fraction at various positron plasma densities and temperatures and found that collisional effects play a beneficial role via a redistribution of the antihydrogen magnetic moment, allowing enhancements of the yield of low-field seeking states that are amenable to trapping.
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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.
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 merging of antiprotons with a positron plasma is the predominant and highest efficient method for cold antihydrogen formation used to date. We present experimental evidence that this method has serious disadvantages for producing antihydrogen cold enough to be trapped. Antihydrogen is neutral but may be trapped in a magnetic field minimum. However, the depth of such traps are of order 1 K, shallow compared to the kinetic energies in current antihydrogen experiments. Studying the spatial distribution of the antihydrogen emerging from the ATHENA positron plasma we have, by comparison with a simple model, extracted information about the temperature of the antihydrogen formed. We find that antihydrogen is formed before thermal equilibrium is attained between the antiprotons and the positrons, and thus that further positron cooling may not be sufficient for producing antihydrogen cold enough to be trapped. We discuss the implications for trapping of antihydrogen in a magnetic trap, important for ongoing work by the ALPHA collaboration.
A comparison of the 1S-2S transitions of hydrogen and antihydrogen will yield a stringent test of CPT conservation. Necessarily, the antihydrogen atoms need to be trapped to perform high precision spectroscopy measurements. Therefore, an approximately 0.75 T deep neutral atom trap, equivalent to about 0.5 K for ground state (anti)hydrogen atoms, has been superimposed on a Penning-Malmberg trap in which the anti-atoms are formed. The antihydrogen atoms are produced following a number of steps. A bunch of antiprotons from the CERN Antiproton Decelerator is caught in a Penning-Malmberg trap and subsequently sympathetically cooled and then compressed using rotating wall electric fields. A positron plasma, formed in a separate accumulator, is transported to the main system and also compressed. Antihydrogen atoms are then formed by mixing the antiprotons and positrons. The velocity of the anti-atoms, and their binding energies, will strongly depend on the initial conditions of the constituent particles, for example their temperatures and densities, and on the details of the mixing process. In this paper the complete lifecycle of antihydrogen atoms will be presented, starting with the production of the constituent antiparticles and the description of the manipulations necessary to prepare them appropriately for antihydrogen formation. The latter will also be described, as will the possible fates of the anti-atoms.