The mass splitting between the quarkonium spin-singlet state h (JPC=1+-) and the spin average of the quarkonium spin-triplet states χ (JPC=0++,1++,2++) is seen to be astonishingly small, not only in the charmonium and bottomonium cases where the relevant masses have been measured, but in positronium as well. We find, both in nonrelativistic quark models and in nonrelativistic quantum chromodynamics (NRQCD), that this hyperfine splitting is so small that it can be used as a test of the pure QQ- content of the states. We discuss the 2P states of charmonium in the vicinity of 3.9 GeV, where the putative exotics X(3872) and X(3915) have been seen and a new χc0(2P) candidate has been observed at Belle.
ASJC Scopus subject areas
- Physics and Astronomy (miscellaneous)