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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K1.1BR beamline

A separated K+ beam with low momentum (0.8 GeV/c) from the K1.1BR will be used. The layout of the beamline is shown in the followings. Details of this beam are described in the report submitted to the FIFC in 2007. The beam optics were designed by J. Doornbos of the TREK collaboration by keeping the upstream layout design of K1.1, as its short branch with a total length of 20.3 m. Although there will be only a single electro-static separator (ESS), the existence of a vertical focus (IFY) before the ESS will play an important role to increase the K/π ratio. The horizontal focus after the sector type magnet B3 is also essential to suppress background pions. The K+ beam intensity can be evaluated from the beamline acceptance of 4.5 msr%[Δp/p], and the comparison with LESB3 at BNL-AGS, to be 2 × 105/s at 30 kW of accelerator beam power. We will be able to obtain a round beam spot at the final focus (FF) with a full width at half maximum of 1.0 cm horizontally and vertically. The momentum bite is ±3%. The spot is free from dispersion (R16), but with finite R26, 2 on which the design of the new target is based.

Commisioning of the beamline

The beamline is already completed and commissioned. The E36 collaboration is responsible for the beam tuning. This initial version of the beamline is modified from the original optics design in a few points mainly due to tight budgetary constraints. For the ESS the old K5 separator (with 2.0 m length) is recycled in place of the final 2.5 m one. The last sector bending magnet B3 is a 45 degree bender instead of 50 degrees. The necessity of 2.5 m thick iron shielding makes the final focus point far from the last element Q8.