Charmed Hadron Spectroscopy from Focus

With very accurate measurements of the parameters of the ground state charmed mesons, interest has shifted to the array of excited charm meson states, especially with the interesting discoveries of the last year in the Ds sector. In the limit of infinite quark masses, D mesons may be treated as a “hydrogen atom” type object where the heavy quark does not contribute to the orbital degrees of freedom. In this treatment the quantum numbers of the heavy and light quarks decouple. The heavy quark is described by its spin, sQ, and the light quark degrees of freedom are described by jq = sq + L where sq is the spin of the light quark and L is its angular momentum. For L = 1 we have jq = 1/2, 3/2. Combined with sQ = 1/2, we obtain four states, one with J = 0, one with J = 2, and two with J = 1 (one each with jq = 1/2 and 3/2). In the Heavy Quark Symmetry limit, conservation of parity and jq requires that the strong decays D (∗) J (jq = 3/2) → D (∗)π proceed via D-wave decays while D (∗) J (jq = 1/2) → D (∗)π proceed via S-wave decays. States decaying via S-wave decays are expected to be broad while those decaying via D-wave are expected to be narrow.