W + Jets Production

Hadronic production of W and Z bosons provides a clean probe of perturbative QCD calculations. Three analyses were presented from the DØ and CDF collaborations utilising the large sample of $p\overline{p}$ collisions accumulated from the 1994-1995 Tevatron Collider run [54].

The first analysis from the CDF collaboration was a measurement of the $W/Z + \geq n$ Jets cross sections for n=1-4. Figure 12(a) shows the inclusive associated jet multiplicity distribution for W and Z bosons. The uncorrected data are compared to the CDF detector simulation incorporating the VECBOS [55] LO QCD calculation plus HERWIG [6] parton shower and hadronisation. The CTEQ3M [30] parton densities were used. The band in the theoretical predictions represents the effect of varying the renormalisation and factorisation scales from Q²=M²+p_T² of the boson to the $\langle p_T\rangle^2$ of the partons. Using the hard scale, M²+p_T², the LO QCD predictions are about a factor 1.7 lower than the data, for all jet multiplicities. On the other hand the predictions for the softer scale, $\langle p_T\rangle^2$ , are in better agreement with the data.

The DØ collaboration reported results on the ratio of the production cross sections for W + 1 Jet to W + 0 Jets, ${\cal R}^{10}$ , as a function of the minimum jet transverse energy, shown in Fig. 12(b). The data between 20 and 60 GeV are consistently higher than the DYRAD NLO predictions by about a factor of two. This is a rather curious result since it is in a domain where one generally expects QCD to work well.

**Figure 12:** CDF and DØ results for W/Z + Jets production.
$\begin{figure} \centering \mbox{ \subfigure[Cross sections for $W/Z + \geq n$\s... ...^{min}$] { \psfig {figure=r_etmin-newx.eps,width=.45\textwidth} } }\end{figure}$

DØ also investigated color coherence effects in W + Jets events. For this study events with a W boson and opposing jet were selected and the distribution of soft particles around the colorless W boson and the jet (colored parton) was measured. The color coherence signal is observed by comparing the multiplicity distributions of calorimeter towers with E_T > 250 MeV around the W and around the jet. It is concluded that both angular ordering and string fragmentation are needed in PYTHIA [5] to describe the data.