Direct measurement of

The absolute inclusive branching fraction of D s+→ φ X+ has been measured from data collected by the BES detector at a center-of-mass energy of 4.03 GeV, corresponding to an integrated luminosity of 22.3 pb− 1. At this energy, direct pair production e+ e−→ D s+ D s− has been observed. We have selected D s candidate events by reconstructing five hadronic decay modes D s+→ φ π+, K 0* K+, K 0 K+, f 0 π+ and K 0 K− π+ π+ and have searched for inclusive φ’s in the recoiling D s−. We observed three recoiling φ’s in the 166.4±31.8 D s candidate events, which leads to the absolute branching fraction B (D s+→ φ X+)=(17.8− 7.2+ 15.1− 6.3+ 0.6)% and B (D s+→ φ π+)=(3.6− 1.6+ 3.1− 1.3+ 0.4)%.