Antiproton collisions

Figure 8. Longitudinal momentum distribution for single ionization of helium by 945-keV antiproton (data points) in comparison with proton collision (full curve), (a) Electron momentum data [26] (b) recoil-ion data [26], The theoretical calculations represent antiproton collisions dotted curve, CDW results [26] broken curve, CTMC result [26], Here pze and pzr are equivalent to the notation of pey and pRy of Figs. 1 and 2, respectively.

A. Igarashi, N. Toshima, Application of hyperspherical close-coupling method to antiproton collisions with muonic hydrogen, Eur. Phys. J. D 46 (2008) 425. 

When a particle and its antiparticle, such as an electron and a positron, or a proton and an antiproton, are used in head-on collision experiments, acceleration of the particles can be accomplished in one ring. This is because electrons and positrons, for example, behave m the same way in terms of their response to magnetic and electric fields. Thus, both particles can be injected into the same ring, one to follow an orbit in a clockwise direction the other in a counterclockwise direction. Upon injection of a cluster of each type of particle, collisions occur at two points diametrically opposed. This arrangement provides maximum utilization of the equipment. 

Studies of positron collisions with atoms and molecules are of interest not only for their own sake but also because comparisons with the results obtained using other projectiles, such as electrons, protons and antiprotons, provide information about the effects on the scattering process of different masses and charges. 

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).

Charlton, M. (1990). Antihydrogen production in collisions of antiprotons with excited states of positronium. Phys. Lett. A 143 143-146. 

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

Ermolaev, A.M., Bransden, B.H. and Mandal, C.R. (1987). Theoretical cross sections for formation of antihydrogen in p-Ps collisions in the antiproton energy range 2-100 keV lab. Phys. Lett. A 125 44-46. 

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. 

Bakalov et al. treated the trajectories of the helium atom in collision with pHe+ in a semiclassical way, and calculated the pressure shifts and broadening. They obtained numerical values for a numer of transitions, as presented in Table 2. For the precisely known transitions (39,35) —> (38,34) and (37,34) —> (36,33) their theoretical values with realistic collision trajectories (not the linear approximation) turned out to be in excellent agreement with the experimental values. The theoretical treatment of Bakalov et al. was the first quantum chemistry type calculation on the interaction of antiprotonic helium with other atoms and molecules. 

The purpose of the detector surrounding the antihydrogen trap is to discriminate between signals due to the annihilation of antihydrogen and those due to the trapped clouds of antiprotons and positrons. For this, one needs to provide temporal and spatial coincident detection of the annihilation of an antiproton and a positron. It should also allow reconstruction of the annihilation vertex with sufficient resolution to discriminate between annihilations on the wall of the charged particle traps and those resulting from possible collisions with residual gas atoms. In addition it must have a sufficiently high rate capability to allow the study of the time evolution of the recombination process. 

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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