  Jet radiation patterns are indispensable for the purpose of discriminating
partons' with different quantum numbers. However, they are also vulnerable to
various contaminations from the underlying event, pileup, and radiation of
adjacent jets. In order to maximize the discrimination power, it is essential
to optimize the jet radius used when analyzing the radiation patterns. We
introduce the concept of jet radiation radius which quantifies how the jet
radiation is distributed around the jet axes. We study the color and momentum
dependence of the jet radiation radius, and discuss two applications:
quark-gluon discrimination and $W$ jet tagging. In both cases, smaller (sub)jet
radii are preferred for jets with higher PTs, albeit due to different
mechanisms: the running of the QCD coupling constant and the boost to a color
singlet system. A shrinking cone W jet tagging algorithm is proposed to achieve
better discrimination than previous methods.
