Antihydrogen atoms

The proposed scenario for antihydrogen production involves stopping a bunch of approximately 109 antiprotons in dense helium this is followed shortly afterwards by the injection of a bunch of approximately 10s positrons into the same medium. A detailed discussion was given by Ito et al. (1993) this included a treatment of the differences between the positrons and the positronium atoms which result from the positron injection, in terms of the known behaviour of these species in helium gas see Chapters 6 and 7. Under optimum conditions, the number of antihydrogen atoms formed could be as great as 103-104 per antiproton bunch. 

Fedichev, P.O. (1997). Formation of antihydrogen atoms in an ultra-cold positron-antiproton plasma. Phys. Lett. A 226 289-292. 

Ito, Y., Widmann, E. and Yamazaki, T. (1993). Possible formation of antihydrogen atoms from metastable antiprotonic helium atoms and positrons/positroniums. Hyperfine Interactions 76 163-173. 

Munger, C.T., Brodsky, S.J. and Schmidt, I. (1994). Production of relativistic antihydrogen atoms by pair production with positron capture. Phys. Rev. D 49 3228-3235. 

Wolf, A. (1993). Laser-stimulated formation and stabilization of antihydrogen atoms. Hyperfine Interactions 76 189-201. 

While experiments with an antihydrogen atom (after experimental success in its trapping, cooling and keeping for a while in the trap) look like routine measurements (but an extremely accurate one), another kind of search for new physics is a search for exotic events which are forbidden within the Standard model but can nevertheless occur within its extensions. A few of them deal with simple atoms. 

Since the ultimate goal consists of high precision spectroscopy on (trapped) antihydrogen, the main focus of future experiments must be on the production and storage of antihydrogen atoms at very low energies. Thus, the recombination technique used should have the prospect to ... 

Once an antiproton and positron have combined, electrical confinement forces cease and the antihydrogen atom will escape, hit an electrode, and annihilate. During the early stages of the experiment no attempt will be made to trap the produced antihydrogen and the annihilation signature will be one of the most important diagnostics tools. 

The second phase will be designed and constructed based on the results of Phase 1. While the focus is on 2-photon laser spectroscopy of magnetically trapped antihydrogen atoms, other measurements (e.g. a measurement of the hyperfine structure using an atomic antihydrogen beam) are being explored for this program. 

A number of open questions arise in this context, like the stability of the charged plasmas in such a magnetic field, or the achievable well depth in relation to the energy distribution of the produced antihydrogen atoms using different recombination scenarios. An active R D program will be necessary to clarify... 

Initially the focus will be upon producing cold antihydrogen atoms. The rate for spontaneous radiative recombination of antiprotons and positrons is rather low because the emission of photons is a slow process on the time scale of collisions. Laser-stimulated recombination can increase the antihydrogen formation rate by orders of magnitude [14]. Other avenues towards antihydrogen production at low energies are pulsed-field recombination [15] or collisions of antiprotons with positronium [16]. 

The resonant absorption cross section for radiation at Lyman-a can be as high as 3A /27r [39]. Consider a volume of 1 mm diameter being illuminated with InW resonant radiation at Lyman-a. The excitation rate for an atom is then 5s-1. Suppose that we would like to cool antihydrogen atoms in a magnetic... 

Shelving spectroscopy thus involves many decisions whether the antihydrogen atom has been excited to the metastable 2 2S /2 state or not. These decisions have to be made somewhat quicker than the natural lifetime of the metastable state and are based on the observation or the non-observation of fluorescent light at Lyman-a. The detection efficiency for fluorescent light from an antihydrogen sample in a magnetic trap with superconducting coils is probably rather... 

Fig. 8. Trajectories of antihydrogen atoms through two sextupole magnets as calculated by a Monte-Carlo method. Please not the the X and Z axis do not have he same scale...

Typical trajectories calculated by this Monte Carlo method are shown in Fig. 8. The overall result is that about 7 x 10-5 of all antihydrogen atoms initially formed in the trap region can be transported to the H detector after S2. At expected formation rates of about 200/s [32] this would result in a count rate of 1 event per 2 minutes on resonance. This seems rather small, but is feasible since the antihydrogen atoms can be easily detected with unity efficiency from the annihilation of their constituents. 

The antihydrogen atom puts both the CPT symmetry principle and the equivalence principle to the most exacting test now conceived. Before considering how this can be done, it is appropriate to consider briefly how the antiproton and the positron can be brought together to form antihydrogen. Dan Kleppner said in 1992 at a workshop in Munich, in the past six years the creation of antihydrogen has advanced from the totally visionary to the merely very difficult. Since 1992, the merely very difficult remains very difficult. 

Figure 21.1 The experimental apparatus at Fermilab that produced the first antihydrogen atom.

Antimatter was first produced at the European research centre CERN, near Geneva, in 1995 by interaction of antiprotons with a beam of Xe atoms. The antihydrogen atoms p"e+ disappeared quickly after a short lifetime of about 30 ns by annihilation and liberation of large amounts of energy. 

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