The subjet multiplicity in quark and gluon jets

We calculate the mean number of subjets in quark and gluon jets in the final state of e+e− annihilation. Since “quark” and “gluon” jets are scheme-dependent objects, we stress the importance of using the same definition as in experimental analyses. We define the jets using the k⊥ algorithm at a coarse scale y1, and the subjets using a finer scale y0, in the s algorithm. Gluon jets are anti-tagged by the presence of heavy quarks in both other jets. Our result is exact to leading order in αs, and resums leading and next-to-leading logarithmic terms in the ratio y1y0 to all orders in αs.