A measurement of the QCD colour factors and a limit on the light gluino

Using data collected from 1992 to 1995 with the ALEPH detector at LEP, a measurement of the colour factor ratios CACF and TFCF and the strong coupling constant s=CFs(MZ)(2) has been performed by fitting theoretical predictions simultaneously to the measured differential two-jet rate and angular distributions in four-jet events. The result is found to be in excellent agreement with QCD, \begin{eqnarray*} \frac{C_A}{C_F} &=& 2.20 \pm 0.09\, (stat) \pm 0.13\, (syst), \\ \frac{T_F}{C_F} &=& 0.29 \pm 0.05\, (stat) \pm 0.06\, (syst). \end{eqnarray*} Fixing CACF and TFCF to the QCD values permits a determination of s(MZ) and nf, the number of active flavours. With this measurement the existence of a gluino with mass below 6.3 GeV/c2 is excluded at 95 % confidence level.