First measurement of the quark-to-photon fragmentation function

Earlier measurements at LEP of isolated hard photons in hadronicZ decays, attributed to radiation from primary quark pairs, have been extended in the ALEPH experimentto include hard photon productioninside hadronjets. Events are selected where all particles combine democratically to form hadronjets, one of which contains a photon with a fractional energyz≥0.7. After statistical subtraction of non-prompt photons, the quark-to-photon fragmentation function,D(z), is extracted directly from the measured 2-jet rate. By taking into account the perturbative contributions toD(z) obtained from anO(ααs) QCD calculation, the unknown non-perturbativecomponent ofD(z) is then determined at highz. Provided due account is taken of hadronizationeffects nearz=1, a good description of the other event topologies is then found.