Measurement of the gluon fragmentation function and a comparison of the scaling violation in gluon and quark jets

Abstract.

The fragmentation functions of quarks and gluons are measured in various three-jet topologies in Z decays from the full data set collected with the Delphi detector at the Z resonance between 1992 and 1995. The results at different values of transverse momentum-like scales are compared. A parameterization of the quark and gluon fragmentation functions at a fixed reference scale is given. The quark and gluon fragmentation functions show the predicted pattern of scaling violations. The scaling violation for quark jets as a function of a transverse momentum-like scale is in a good agreement with that observed in lower energy ${\rm e}^+{\rm e}^-$ annihilation experiments. For gluon jets it appears to be significantly stronger. The scale dependences of the gluon and quark fragmentation functions agree with the prediction of the DGLAP evolution equations from which the colour factor ratio $C_A/C_F$ is measured to be:

\begin{eqnarray*} \frac{C_A}{C_F} = 2.26 \pm 0.09_{stat.} \pm 0.06_{sys.} \pm 0.12_{clus.,scale}\, . \end{eqnarray*}