The CLEO collaboration has discovered a new particle, tentatively named the DsJ(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 Dsπ0. 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 Dsπ0 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 Dsπ0, instead of the dominant Dsγ mode. On the other hand, the DsJ is above the thresholds for decay to DK and Dsπ0, but apparently does not decay through either mode. If the spin parity of the DsJ 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 DsJ and D*s is essentially identical to the mass difference between the D*sJ and the Ds (see figure). Since the dominant decay mode of D*s is Dsγ, a real D*s decay can appear as a Ds if the photon from the D*s decay is lost, and a real Ds 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 Dsπ0 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 Dsπ0 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 Dsπ0 not D*sπ0. Predating the discovery of the D*sJ and DsJ, 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+DsJ and the 1–D*s would be equal to the difference between a 0+D*sJ and the 0–Ds, in accord with CLEO’s observation.
D Besson et al. http://arxiv.org/abs/hep-ex/0305100.