The distribution of the D0 K+
combined mass for all candidates in the
data sample including Anti-particle combinations (D0bar K–).
There are
two clear peaks. The lower, at a mass of 2570 MeV/c2, is the known
DsJ(2573) meson, discovered in 1994. This peak’s width is more than the
detector resolution showing the the “natural width (Gamma)” of this state
due to its short lifetime. The value measured for the natural width of 14
MeV/c2 is consistent with previous measurements. The detector resolution
is better by a factor of 2 in this D0 K+ decay mode than in the Ds+
eta0 mode making Selex more sensitive to the lifetimes of these state in
this decay mode.
The second peak is at a mass of 2632 MeV/c2 – the same mass (within
uncertainties) seen in the Ds+ eta0 decay mode. The width of this peak
is consistent with the resolution of the detector. This state very likely
lives several times longer than the lower mass state. Both peaks have
high statistical significance (more than 5 standard deviations).