Background on the Search for Rare Kaon Decays
K+ => pi+,nu,nubar event in the E787
apparatus
In the upper left a K+ enters near the center of the concentric circles
representing a cylindrical tracking chamber. It loses energy and stops in
a segmented target of scintillating plastic counters, then decays to a
pi+, which leaves the target and is tracked through the chambers (small
circles indicate the path) into an array of large scintillation counters
surrounding the chamber. The counters in the path of the pi+ are shown. It
stops in the 15th counter on the path. On the upper right is a blowup of
the stopping target region. The filled red squares indicate target
elements hit by the K+, the blue outlines are those hit by the pi+ leaving
the target. The curved element indicates a trigger scintillation counter
adjacent to the target which the pi+ also hit on its way out. The curve on
the lower right is the pulse of energy left by the K+ in the target
element in which it stoped. The curve on the lower left shows the pattern
of energy deposition in the last counter on the pi+ trajectory. The pulse
of energy left by the stopping pion is shown by the muon to which it
decayed after about 15 billionths of a second. The lack of other tracks or
significant energy depositions indicates that the pi+ was unaccompanied by
any detectable particles (neutrinos are not detectable in this apparatus).
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