J-PARC Experiment E36 at the Nuclear and Particle Physics Facillity

Precise Measurement of Γ (K+→ e+ν)/Γ(K+→ μ+ν)
using Stopped Positive Kaons

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Time of Flight (TOF)

Particle discrimination between the e+ and μ+ mainly for the low energy region is carried out by a time-of-flight measurement between TOF1 and TOF2. TOF1 is located surrounding the K+ target system and TOF2 is located at the exit of the spectrometer (behind C4). The time difference between the e+ and μ+ from the Ke2 and Kμ2 decays is approximately 0.5 ns for a typical trajectory. The uncertainty of the light propagation time from the particle hit position to the photo multiplier is suppressed by reading signals from both ends of the TOF counters. For optimal separation of Ke2 and Kμ2, a timing resolution of σt = 100 ps for the TOF measurement is needed. We will construct the new TOF system with a timing resolution of σ = 100 ps and install the aerogel Cherenkov counter for e+ discrimination. Note, the TOF2 counters are used for the Kμ2 trigger and TOF1 counters are used as the detector to define the fiducial region of the K+ stopping target in the beam direction.

Figure (Left) The expected mass squared spectrum (M2 TOF) from the TOF measurement calculated under the assumption of the timing resolution of σ = 100 ps. (Right) Schematic drawing of TOF setup.