$\alpha_s$ from hadron multiplicities via SUSY-like relation between anomalous dimensions

Abstract

We recover in QCD an amazingly simple relationship between the anomalous dimensions, resummed through next-to-next-to-leading-logarithmic order, in the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution equations for the first Mellin moments $D_{q,g}(\mu^2)$ of the quark and gluon fragmentation functions, which correspond to the average hadron multiplicities in jets initiated by quarks and gluons, respectively. This relationship, which is independent of the number of quark flavors, dramatically improves previous treatments by allowing for an exact solution of the evolution equations. So far, such relationships have only been known from supersymmetric QCD, where $C_F/C_A=1$. This also allows us to extend our knowledge of the ratio $D_g^-(\mu^2)/D_q^-(\mu^2)$ of the minus components by one order in $\sqrt{\alpha_s}$. Exploiting available next-to-next-to-next-to-leading-order information on the ratio $D_g^+(\mu^2)/D_q^+(\mu^2)$ of the dominant plus components, we fit the world data of $D_{q,g}(\mu^2)$ for charged hadrons measured in $e^+e^-$ annihilation to obtain $\alpha_s^{(5)}(M_Z)=0.1205\genfrac{}{}{0pt}{}{+0.016}{-0.0020}$.