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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Aerogel Cherenkov counter

A threshold type aerogel Cherenkov counter is used for the e+ identification, which is installed surrounding the TOF1 counter, as shown in the followings. This type of a Cherenkov counter is widely used for particle identification by many experiments such as BELLE. The β value of μ+ from K μ2 is 0.913, therefore a Cherenkov radiation material with refractive index (n) from 1.01 to 1.095 is necessary to discriminate e+ and μ+. Normally, the refractive index of aerogel ranges from 1.01 to 1.05, and an increase of Cherenkov photons with increasing refractive index is expected. The typical number of Cherenkov photons per e+ passing through this counter is estimated to be about 500, and a wavelength region of photons measured by this counter ranges from 350 nm to 700 nm.

The performance of this Cherenkov counter is the most critical factor for the proposed experiment; it is essential to achieve reliable e+ triggers with nearly 100% efficiency and negligible misidentification for any possible crossing track of e+. A prototype is proposed to be made as soon as possible and tested at an e+ beam facility.