Back-to-back topology & track multiplicity

Illustration of a back-to-back event topology of two &tau; leptons (to define acoplanarity or azimuthal separation &Delta;&phi; in the transverse plane), and multiplicity of pileup (PU) and hard scattering (HS) tracks in a small window along the beam axis (z). Made for this CMS paper on the measurement of the &gamma;&gamma; &rarr; &tau;&tau; process in proton-proton collisions and the measurement…