Structure and formation of deeply-bound pionic atoms

Abstract

We have calculated spectra of deeply-bound pionic atoms using the standard pion-nucleus optical potential. The widths of the deeply-bound states are found to be narrow and are considered as quasi-stable states even for the 1s state in 208 Pb. These unexpected results are caused by the repulsive pion-nucleus optical potential which pushes the pionic wave functions outwards so that the pion absorption by the nucleus is weakened. These deeply-bound π states have hybrid character, somewhere between pionic nuclei and pionic atoms constituting a pion halo around the nucleus. This result remains relatively unchanged by the use of different optical potentials, as far as they reproduce shallow pionic atoms. We propose to produce these deeply-bound pionic atoms, hitherto untouched region, by using the “pion transfer” (n, p) reaction, where π is transferred to the target nucleus. We have calculated the cross sections for the formation of various deeply bound states via the (n, p) reaction within the framework of a plane-wave approximation. The calculated cross section increases rapidly with the nuclear charge and indicates that the detection of unexplored deeply bound pionic atoms is feasible. We comment also on the use of complex projectiles for the ‘pion-transfer’ reactions such as (d, 2 He).