The muon is an elementary particle similar to the electron, but with a mass 207 times larger. Due to their greater mass, muons lose much less energy flying through the matter and penetrate far more deeply into matter than electrons. KL (k-long) is a weak eigenstate of the neutral kaon K0, which decays into three pions, therefore has a large lifetime and escapes inner detectors undetected. What is common between muon and KL is a large distance they can travel inside Belle II detector.
To identify muons and KL the largest Belle II subdetector is used, KLM (K-Long-Muon). It occupies a space in gaps of the magnet yoke. Due to the distance from the interaction point and amount of metal in the yoke no other particle but muon and KL can effectively reach and penetrate KLM.
KLM consists of the barrel part with 15 layers and two endcaps, forward and backward, with 14 and 12 layers, respectively. Two inner barrel layers and entire endcaps are instrumented with scintillator strips; 13 outer barrel layers are instrumented with Resistive Plate Counters (RPC).
A typical muon will pass through all of the KLM layers in the barrel or an endcap, leaving a clean trail of hits to mark its passage. A typical K-long will collide with an iron nucleus, resulting in debris that leaves a concentrated cluster of KLM hits.