STAR refers to the Solenoidal Tracking instrument At RHIC (the Relativistic Heavy Ion Collider). For momenta above 500 MeV/c charged kaons are not separated from pions within STAR's Main TPC (Time Projection Chamber) by track density alone and they are poorly separated below 500 MeV/c, even when using information from other sources like the vertex tracker. Within the TPC large numbers of kaons and pions decay into muons (and undetected neutrinos). Earlier work has shown parent pions and kaons whose decays are detected within a TPC may be distinguished uniquely from each other in a two-dimensional plot of muon-emission angle versus momentum difference (between each parent meson and its decay muon). Since pions and kaons have zero spin, each muon decay-product emerges isotropically in its parent meson's rest frame. Identification of particle type provides the parent meson's rest mass and, thus, its total energy. This means the measurement of each decay event is kinematically complete. Thus, Lorentz Transformations may be used to transform each component of the decaying muon's laboratory four-momentum into the "rest frame" of its parent meson, where the muon decay is isotropic. An aggregated plot of muon directions from many "parent rest frames" will be isotropic in each (selected) sub-volume of the TPC unless there is a problem within the TPC or in its tracking algorithms. Continuous monitoring of a TPC is possible using this subset of detected charged particles.
Braithwaite, Wilfred J. and Braithwaite, Edwin S.
"Continuous Monitoring of STAR's Main Time Projection Chamber,"
Journal of the Arkansas Academy of Science: Vol. 52
, Article 6.
Available at: https://scholarworks.uark.edu/jaas/vol52/iss1/6