The incoming beam contains charged pions (rest-frame lifetime = 2.6 x 10-8 seconds) decaying to muons (rest-frame lifetime = 2.2 x 10-6 seconds) and electrons. These different particles are identified by the various detectors (Cerenkov, Trackers, KL, EMR) enabling excellent identity attribution as verified by the time of flight graph.
In the rest frame of the decaying pion, the muon production is isotropic - both "forward" along the beam direction and "backwards" towards the target. In the lab frame, "forward" or "backward" muons can be selected by appropriate currents in the bending dipoles D1 and D2. When the field in D2 is half the field in D1, the "backward" muons are selected and the beam in the MICE apparatus is almost purely muons with around 1% contamination by pions. When the fields in D1 and D2 are equal, the "forward" muons are selected and the beam contains more pions and electrons; in this "calibration" mode, the detectors can be calibrated.
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