Direct Observation of Proton-Neutron Short-Range Correlation Dominance in Heavy Nuclei

High-momentum transfer measurements have shown that nucleons in nuclear ground state can form temporary pairs with large relative momentum and small center-of-mass momentum, referred as short range correlated (SRC) pairs. Properties of SRC pairs are primarily inferred from measurements of exclusive electron-induced triple-coincidence reactions. A high missing-momentum nucleon is knocked out of the nucleus via a high-momentum transfer reaction and detected in coincidence with the scattered probe and a recoil nucleon balancing the large missing momentum. Previous measurements of such reactions in ⁴He and ¹²C, showed that neutron-proton (np) SRC pairs are nearly 20 times as prevalent as proton-proton (pp) pairs. This was explained as being due to the dominance of the tensor part of the nucleon-nucleon force at short distances. For heavy nuclei, the predominance of np-SRC was never extracted directly from measurements of the exclusive A(e,e'pp) and A(e,e'np). Based on our data from CLAS detector at Jefferson Laboratory we present, the triple coincidence measurement on C, Al, Fe, and Pb. The data verified, for the first time on neutron rich nuclei, that the number of pp-pairs is small than the np-pairs by about a factor of 20, independent of the neutron excess in the nucleus.