Hadron-nucleus bound states Original Research Article

A new type of nuclear spectroscopy to study hadron-nucleus bound states is described. The first successful experiment was to search for deeply bound π− states in heavy nuclei using the 208Pb(d,3He) reaction at GSI, in which a narrow peak arising from the 2p π− orbital coupled with the neutron-hole states was observed at 135 MeV excitation energy. An improved experiment has just been carried out to separately identify the 1s and 2p π− states. These experiments provide important information on the local potential strength, from which the effective mass of π− is deduced to be 20 MeV. This method will be extended to search for η and ω bound states as well as for K− bound states. The advantage of the bound-state spectroscopy versus invariant mass spectroscopy is emphasized.