Magnets

To achieve sufficient separation of pairs of oppositely charged, high-energy Standard Model particles originating from decays of new physics particles, FASER is equipped with strong magnets designed and manufactured by the CERN magnet group to fulfill technical requirements dictated by the architecture of the TI12 tunnel.

FASER uses 0.55 T permanent dipole magnets (see Fig. 1) based on the Halbach array design with SmCo magnetic material. The largest, 1.5 m-long magnet surrounds a decay volume in the upstream part of the detector. Following it are two 1 m-long magnets in the FASER tracker. The LHC beam collision axis line of sight (LOS) passes through the magnet centers.

The resulting magnetic field in FASER is strong enough to guarantee good physics performance, while remaining contained within the magnet volumes as shown in Fig. 2. Only a small field strays outside the magnet openings as shown in Fig. 3. To minimize the potential impact of this stray field on photomultiplier tubes (PMTs) installed on the scintillator layers, additional mu-metal shielding is used for the PMTs. The use of permanent magnets minimizes the services needed for maintaining high quality performance.

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Fig. 1: Sketch of the FASER magnet design.
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Fig. 2: Field lines in a FASER magnet.
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Fig. 3: The stray magnetic field (in mT) outside of the FASER magnet estimated from the magnet design model 10 cm from the magnet end.