CLEO discovers second D_sJ particle

The CLEO collaboration has discovered a new particle, tentatively named the D_sJ(2463), which decays to D^*_sπ⁰ with a decay width less than 7 MeV. The search that led to this discovery was motivated by BaBar’s discovery of an unexpected new narrow state called D^*_sJ(2317), which decays to D_sπ⁰. CLEO has also confirmed the existence of the D^*_sJ(2317).

In the simplest interpretation of these results both particles are excited bound states of a charm quark, c, and a strange antiquark, sbar. It was thought that such states would be massive enough to decay to a D or D^* meson and a K meson. The biggest surprise of the D^*_sJ is that it is too light to decay via any of these modes. The decay of D^*_sJ to D_sπ⁰ is suppressed because it violates isospin symmetry, leading to the small decay width observed. Such suppressed isospin violating decays are not unknown in the c-sbar system – in 1995 CLEO found that about 6% of the decays of the D^*_s are to D_sπ⁰, instead of the dominant D_sγ mode. On the other hand, the D_sJ is above the thresholds for decay to DK and D_sπ⁰, but apparently does not decay through either mode. If the spin parity of the D_sJ is 1⁺, these modes would be forbidden and D^*_sπ⁰ would be allowed but also suppressed by isospin, again leading to a small decay width.

The analysis that determined the existence of the second state was complicated by an unusual kinematic property of the two states – the two new particles and the D^* are narrow and the mass difference between the D_sJ and D^*_s is essentially identical to the mass difference between the D^*_sJ and the D_s (see figure). Since the dominant decay mode of D^*_s is D_sγ, a real D^*_s decay can appear as a D_s if the photon from the D^*_s decay is lost, and a real D_s and random photon can appear as a D^*_s. This means that the two states can “feed into each other” as photons are missed or randomly acquired. The observed D_sπ⁰ signal thus has a background from real D^*_sπ⁰ events and vice-versa, so separating the two sources requires careful and subtle analysis. CLEO found that multiple analysis techniques applied to the D_sπ⁰ and D^*_sπ⁰ signals led to the conclusion that both states exist and resulted in consistent measurements of their masses and widths.

The preferred spin parity of the D^*_sJ is 0⁺ because it decays into D_sπ⁰ not D^*_sπ⁰. Predating the discovery of the D^*_sJ and D_sJ, there were at least two theoretical models that coupled heavy quark effective theory with chiral symmetry and predicted light c-sbar states. In these models the mass difference between a 1⁺D_sJ and the 1^–D^*_s would be equal to the difference between a 0⁺D^*_sJ and the 0^–D_s, in accord with CLEO’s observation.