Deeply bound π^- states in ²⁰⁷Pb formed in the ²⁰⁸Pb(d, ³He) reaction. I. Experimental method and results

The deeply bound pionic states (1s,2p)_π-⊗ ²⁰⁷Pb were observed for the first time, by investigating the ²⁰⁸Pb(d,³He) reaction at a beam energy of T_df=604.3 MeV and at ejectile angles around 0°. The measured momentum distribution of the ³He particles was used to determine the Q value spectrum in the region - 145 MeV<Q<-120 MeV with a systematic uncertainty of ±120 keV and a full width at half maximum resolution of 480±60keV. This spectrum shows a prominent peak at Q≃-135.5 MeV, corresponding to the pionic 2p state, coupled mainly to the 3P_1/2 and 3p_3/2 neutron hole states in ²⁰⁷Pb. A shoulder is seen on the tail of this dominant peak, and is attributed to the population of the pionic 1s state. The cross section for the continuum component at these high excitation energies is d²σ/(dΩdE) = 4.7 μb/srMeV. The net cross section for production of π^- states in the bound pion region - 140 MeV<Q<-133 MeV above the continuum amounts to dσ/dΩ = 76 μb/sr.