Antihydrogen formation

The recent advances in producing, trapping and cooling antiprotons and positrons opened the possibility of antihydrogen formation in laboratory. This may allow the studies of antimatter and tests of fundamental physical principles such as charge - parity - time ( CPT ) invariance or the weak equivalence principle (WEP) for antiparticles. Such experiments are planned at the newly built CERN AD (Antiproton Decelerator) within ASACUSA, ATRAP and ATHENA projects, which have just started their operations. 

All these contributions add up to a total antihydrogen formation cross section of approximately 2 x 10-15 cm2 in the antiproton energy range 2-10 keV where the charge-exchange production mechanism is likely to be most effective. This value is consistent with results obtained by Ermolaev, Bransden and Mandal (1987), who used the classical trajectory Monte Carlo method, and also with the results of a recent experiment (Merrison et al., 1997) which measured the hydrogen atom formation cross section via reaction (8.22). 

Fig. 8.10. Cross sections for antihydrogen formation in collisions of stationary antiprotons with positronium atoms (from Igarashi, Toshima and Shirai, 1994). (a) is for IS positronium and (b) is for the 2P state (note the changes in scale). Key (same for both figures) dotted curve with crosses, formation into the nfl = 1 state short-broken line plus squares, formation into the ns = 2 state long-broken line plus triangles, formation into the nn = 3 state very-long-broken line plus inverted triangles, formation into the ns = 4 states. The solid curve with circles is the total cross section summed over all ns states and the double chain curve is this quantity as calculated by Mitroy and Stelbovics (1994).

We now turn to reaction (8.14), the trapped plasmas combination reaction, in which the excess energy is removed by an extra positron as shown in Figure 8.9(c). The use of this ternary reaction for antihydrogen formation was first suggested by Gabrielse et al. (1988), who noted that its matter equivalent had been studied, mainly in relation to high temperature plasmas, for some time. These authors were therefore able to write the antihydrogen production rate, which corresponds to the recombination rate in conventional plasma physics, as... 

Fig. 8.11. Illustration of the principle of nested Penning traps for holding antiprotons and positrons in close proximity, in order to promote antihydrogen formation.

Charlton, M. (1996). Possibilities for antihydrogen formation by antiproton-positronium collisions. Can. J. Phys. 73 483-489. 

Igarashi, A., Toshima, N. and Shirai, T. (1994). Hyperspherical coupled-channel calculation for antihydrogen formation in antiproton-positronium collisions. J. Phys. B At. Mol. Opt. Phys. 27 L497-L501. 

Neumann, R., Poth, H., Winnacker, A. and Wolf, A. (1983). Laser-induced electron-ion capture and antihydrogen formation. Z. Phys. A 313 253-262. 

Poth, H. (1987). Antihydrogen formation through positron-antiproton radiative combination. Appl. Phys. A 43 287-293. 

Yamazaki, T. (1992). A possible way to promote antihydrogen formation via metastable antiprotonic helium atoms. Z. Phys. A 341 223-225. 

A different approach to enhance the rate of antihydrogen formation is based on three-body recombination (TBR) [21] ... 

Following the discovery of metastable bound states of antiprotons in helium gas [29] the use of this product as an intermediate step towards antihydrogen formation has been discussed ... 

These values agree within a factor two or better with more elaborate calculations [33] as well as with experiments at the Test Storage Ring in Heidelberg [34], A more general discussion of these calculations, as applicable to antihydrogen formation, has been given by Muller and Wolf [35],... 

Since antihydrogen formation rates increase at least linearly with the positron density, it is advantageous to use of a copious source of cold positrons. We have chosen a design based on a scheme developed by C. Surko et al. at the University of California at San Diego [48]. This type of positron accumulator supplies (depending on the strength of the radioactive source) 107-108 positrons with a repetition time of few minutes, well adapted to the AD cycle. 

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]. 

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