Collisional quenching of antiprotonic helium atoms in gaseous helium

The cross sections and rates for quenching of thermalized antiprotonic helium atoms in binary collisions with ordinary helium atoms are calculated in perturbation theory and in the coupled-channel formalism at the temperature and the number density of the helium gas, T=6 K and N=10(21) cm(-3), respectively. It is shown that quenching is unlikely to be relevant for the metastable states of the antiprotonic helium atom with 32less than or equal tonless than or equal to40. Comparison with experiment shows a good quantitative agreement for the (n,l)=(39,35) state of antiprotonic helium and a large disagreement for the (37,34) state. As a byproduct of this work, the existence of a series of bound states of the He-(p) over bar He-4(+) dimer is demonstrated numerically for ngreater than or equal to38.