The last* CLEO-c double tag event

(* last = last confidently-identified fully-reconstructed D_s D_s^* event)

CLEO-c stopped taking data earlier this week.  We looked in the last data run (7:38 am to 8 am) of just over 61 thousand events for collisions that produced D_s mesons, and we actually found one.  Even better, we found an event where you could see both the D_s⁺ and D_s^–, and where the photon from the D_s^*+ → γ D_s⁺ transition was visible.  Above, you can see the event display, with all the tracks labeled; the event is consistent with the following sequence of events:

• e⁺ e^– → γ → D_s^*+ D_s^–
  • D_s^*+ → γ D_s⁺
    • D_s⁺ → K^– K⁺ π⁺ π⁺ π^–
  • D_s^– → K_S K^–
    • K_S → π⁺ π^–

It was actually unlikely that we’d find such a nice event.  For the amount of data in the last run, we would expect roughly 75 D_s^* D_s events.  Our full reconstruction efficiency (getting both D_s candidates) is somewhat less than 1%, so we had a good chance of winding up with zero events like this.  It’s nice to be lucky though.