– 1 – Τ Branching Fractions

To accommodate the 9 new experimental papers listed in the τ References for this edition, only a few changes were made to the τ Listings. There were new measurements of the τ charged-prong topological branching fractions by the DELPHI [1] and L3 [2] collaborations. Early measurements of these branching fractions tended to define charged tracks from K 0 S → π − π + decays as charged prongs, while later measurements usually considered them to be secondary particles. To accommodate both choices, we have defined decay modes for each case. For example, the first two τ-decay modes are: Γ 1 τ − → particle − ≥ 0 neutrals ≥ 0K 0 ν τ (" 1-prong ") Γ 2 τ − → particle − ≥ 0 neutrals ≥ 0K 0 L ν τ. In previous editions, we have attached the label " 1-prong " to Γ 2 , but given that all recent measurements have called Γ 1 the 1-prong topological branching fraction, in this edition we have moved the label to Γ 1. Similarly, we have moved the label " 3-prong " from Γ 52 to Γ 53. However, we do not consider charged pions from K 0 S → π − π + decays to be secondary particles unless they are explicitly listed as being excluded in a decay mode definition. For example, they are included in the definition of Γ 81 (τ − → K − π + π − ν τ), but are excluded in the definition of Γ 82 (τ − → K − π + π − ν τ (ex. K 0)). See the 1996 edition of this Review [3] for a complete description of our notation for naming τ-decay modes.] We have also made a few changes to the constrained fit to tau branching fractions. A description of the constrained fit is given below. The constrained fit to τ branching fractions: The Lep-ton Summary Table and the List of τ-Decay Modes contain branching fractions for 109 conventional τ-decay modes and upper limits on the branching fractions for 27 other conventional τ-decay modes. Of the 109 modes with branching fractions, 79 are derived from a constrained fit to τ branching fraction data. The goal of the constrained fit is to make optimal use CITATION: K. Hagiwara et al.