At the Frascati phi-factory Dafne e + and e − beams collide in the center of mass energy of the the φ-meson which decays into anti-collinear K \( \bar K \) pairs. In the laboratory this remains approximately true because of the small crossing angle of the e + e − beams. Therefore the detection of a K(\( \bar K \) ) tags the presence of a \( \bar K \) (K) of given momentum and direction. The decay products of the K ± pair define two spatially well separated regions called the tag and the signal hemispheres. Identified K ∓ decays tag a K ± beam and provide an absolute count, using the total number of tags as normalization. This procedure is a unique feature of a φ-factory and provides the means for measuring absolute branching ratios. Charged kaons are tagged using the two body decays K ± → μ± v μ and K ± → π ± π 0. Since the two body decays correspond to about 85% of the charged kaon decays  and since BR(φ → K + K −) ≃ 49% , there are about 1.5 × 106 K + K − events/pb−1. The two body decays are identified as peaks in the momentum spectrum of the secondary tracks in the kaon rest frame and in the pion mass hypothesis p*(m π) In order to minimize the impact of the trigger efficiency, the tagging kaon by itself must provide the event trigger (self-trigger).
KeywordsBody Decay Decay Vertex Kaon Decay Charged Kaon Secondary Track
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