Abstract
In collisions of antiprotons with hydrogen atoms or molecules, antiprotonic hydrogen (protonium) is produced mostly in very high principal (n) and angular momentum (ℓ) quantum number states, which are practically stable against particle–antiparticle annihilation. However, since annihilation is very fast for the ns state, ℓ mixing induced by surrounding electric fields or by collisions has a serious influence on the stability of the produced atoms, as was first pointed out by Day et al (1959 Phys. Rev. Lett. 3 61). This paper calculates the cross sections for annihilation which stem from ℓ-changing transition by ion collisions. Because the collision time can be comparable to the lifetime against ns annihilation in the present case, collisional transition and annihilation must be treated in a coherent manner. We carry out an impact-parameter close-coupling calculation, simultaneously including the annihilation channel by means of an optical potential model. At low collision energies, the ℓ mixing due to the polarization effect makes the annihilation cross section enormously large.
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