Studies of strong and electroweak interactions using final state photon emission in hadronicZ ⁰ decays

Abstract

The cross section and jet rates ofZ ⁰ decays into photons and quarks are compared with matrix element Monte Carlos ofO(αα_s). Good agreement is found between data and theoretical predictions. From the ratio of events with three jets plus a photon to those with two jets plus a photon, α^s in first order is determined to be 0.176±0.010. Combining the cross section of final state photon events with the LEP average hadronic partial decay width of theZ ⁰, the widths of theZ ⁰ into up and down type quarks can be calculated. The results

$$\begin{gathered} \Gamma _{u - type} = 242 \pm 46MeV; \hfill \\ \Gamma _{d - type} = 419 \pm 30MeV \hfill \\ \end{gathered} $$

are in good agreement with the standard model expectation. A comparison of the measured cross section as a function ofy _cut with predictions of QCD shower models shows that, at the current level of accuracy ARIADNE and, to a lesser extent, HERWIG and JETSET can reproduce the measurement.