6 v 1 1 0 N ov 1 99 8 Measurement of the D s Lifetime

We report the results of a precise measurement of the Ds meson lifetime based on 1662± 56 fully reconstructed Ds → φπ decays, from the charm hadroproduction experiment E791 at Fermilab. Using an unbinned maximum likelihood fit, we measure the Ds lifetime to be 0.518 ± 0.014 ± 0.007 ps. The ratio of the measured Ds lifetime to the world average D0 lifetime [1] is 1.25 ± 0.04. This result differs from unity by six standard deviations, indicating significantly different lifetimes for the Ds and the D 0. PACS: 13.25.Ft, 14.40.Lb Precise measurements of the lifetimes of the weakly decaying charm mesons are useful for understanding the contributions of various weak decay mechanisms. Despite the fact that they are all tied to the charm quark decay, the decays of the ground-state charm mesons can have different contributions from the four first-order processes (two spectator, W-annihilation, and W-exchange), and the lifetimes are, in fact, quite different [1]: τ(D) : τ(D) : τ(Ds) = 2.5 : 1 : 1.1 The least well measured among these lifetimes is that of theDs, which is known only to about the 4% level [2]. It is clear that there is a very large difference between the lifetime of the D and that of the D or Ds. However, the current measurements are only suggestive of a difference between the lifetimes of the D and the Ds: τ(Ds) τ(D0) = 1.13± 0.04 ( 3σ difference from unity ). A more precise measurement of the lifetime of the Ds could more clearly