Measurement of $Z^0 \to e^{+} e^{-} + N$ Jet Cross-Sections in $\bar{p} p$ Collisions at $\sqrt{s}$ = 1.8-TeV

Abstract

The Collider Detector at Fermilab (CDF) has collected 106 $pb^{- 1}$ of $p\bar{p}$ data from the 1992-1995 Tevatron run, at a center-of-mass energy of $\sqrt{s}$ = 1.8 TeV. From this data sample we extract 6708 $Z \to e^+e^-$ decay candidates, the largest sample available from hadronic collisions. The size of the dataset is sufficient to allow the first measurement of the $Z \to e^+e^-$ cross section as a function of associated jet multiplicity. In calculating $B(Z \to e^+e^-)\sigma(Z^0 + \ge$ N jets), we make use of a previously measured inclusive $Z \to e^+ e^-$ cross section to obtain relative cross sections for the higher jet multiplicities. We measure $B(Z \to e^+e^-)\sigma$ for jet multiplicities N = 1-4, and study the production properties of the jets produced in association with the $Z^0$ boson. The data sample is examined for evidence of b-quark secondary decay vertices, and no excess of b-tags over Standard Model expectations is found. We compare our results with QCD predictions obtained with the heavy boson plus jet tree-level matrix element calculation VECBOS. The VECBOS program generates $Z^0$ + N parton events for N = 1-3. The generated partons are fragmented with a HERWIG routine which adds an underlying event and gluon radiation and introduces color coherence effects in the fragmentation. The events are then processed through a detector simulation. The cross sections of these fully reconstructed events are compared to the measured cross sections. The ratio of the measured to calculated cross section is computed for jet multiplicities 1-3. We find that the value of this ratio depends on the renormalization scale ( $Q^2$ ) used in generation, and on the magnitude of the allowed gluon radiation at fragmentation. The best agreement with the data is achieved with $Q^2$ = $(pt)^2$ of the partons, and essentially unlimited gluon radiation. The kinematic distributions of the boson and jets in these QCD events are compared to the data. We find good agreement between the predicted and measured distributions for all jet production variables.