JET RESULTS FROM THE TEVATRON

Results were presented on subjet multiplicity in quark and gluon jets at DØ [41]. Jets are identified using the k_T jet algorithm, which has the advantage of being IR-safe and allows a more direct comparison between theory and measurements. The gluon jet fraction was determined at $\sqrt{s}$ = 1800 GeV and $\sqrt{s}$ = 620 GeV. The results indicate that there are more subjets at $\sqrt{s}$ = 1800 GeV as well as more subjets in gluon jets.

In the joint session with the structure function working group, both CDF and DØ presented results on the inclusive jet cross section [42] [43]. When the CDF run IA results were published, they showed an excess of events at high E_T with respect to QCD expectations using particular parton density functions. This excess generated a lot of interest, and explanations ranged from quark substructure to modified parton density functions. The preliminary CDF measurement from the IB run is based on an integrated luminosity of 87.7 pb^-1 and is in agreement with the Run IA measurement. DØ presented recently published results which are consistent with QCD predictions. Improved energy calibrations at DØ allowed them to reduce the systematic errors to 10% at low E_T and to about 30% at high E_T. A comparison of CDF data with DØ data is shown in Fig. 19, where the results are seen to be consistent.

**Figure 19:** The CDF and DØ measurements of the inclusive jet cross section.
$\begin{figure} \psfig{figure=incJet.eps,width=75mm,height=65mm}\vspace{-10mm} \end{figure}$

A consistency check of $\alpha_{s}^{}$ from jet data was presented by CDF [43]. The technique extracts $\alpha_{s}^{}$ from a third-order equation where the coefficients are calculated assuming a particular PDF and value of $\alpha_{s}^{}$ . By varying $\alpha_{s}^{}$ one can check for a consistent solution where the extracted $\alpha_{s}^{}$ equals the input value. The method depends on the choice of PDF since different PDFs result in a different $\alpha_{s}^{}$ . The results show the running of $\alpha_{s}^{}$ in one experiment and yield a result consistent with measured $\alpha_{s}^{}$ from other experiments.

Both CDF and DØ presented the ratio of the scaled cross section for a centre-of-mass energy of 630 and 1800 GeV as a function of x_T [42] [43]. The ratio allows a reduction of the uncertainty due to theory and experiment. Above values of x_T = 0.1 the CDF and DØ measurements agree, while at lower x_T values the measurements diverge. The DØ data tend to higher ratio values while the CDF results tend to lower values as is shown in Fig. 20.

**Figure 20:** Ratio of the scaled cross section for centre-of-mass energies of 1800 and 630 GeV as a function of x_T as measured by CDF and DØ.
$\begin{figure} \psfig{figure=xtscaling.ps,width=78mm,height=65mm}\vspace{-10mm} \end{figure}$

DØ also presented the rapidity dependence of the inclusive jet cross section for three different $\eta$ bins up to $\eta$ < 1.5 [42]. Results were in good agreement with NLO QCD calculations.

Both CDF and DØ presented measurements on the dijet mass distribution [44] [45]. Results from the two experiments are in good agreement in both shape and normalization. DØ has used the measurement to place limits on quark compositeness as is shown in Fig 21. Dijet angular distributions provide a sensitive test of new physics and have the advantage that the distributions are less sensitive to the energy measurement uncertainty. Results were presented from CDF and used to place limits on quark compositeness.

**Figure 21:** Limits placed on compositeness models using the DØ dijet mass data
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DØ presents the differential dijet cross section separately for opposite-side jets and same-side jets [44]. Both jets are required to sit in the same $\eta$ bin. Results were compared to the JETRAD calculation using different PDFs. Dijet differential cross sections from CDF were shown where the central jet was used to measure the E_T of the event [45]. A second jet is allowed to fall in one of four $\eta$ bins. A quantitative comparison of different PDFs is under way. The differential dijet measurement covers a plane in the x-Q² space, making it more sensitive to the shape of the cross section determined by different PDFs. The data will provide a useful input to QCD fits in order to determine refined PDFs.

**Figure 22:** Triple differential dijet cross section.
$\begin{figure} \psfig{figure=dijet_india.ps,width=78mm,height=70mm}\vspace{-10mm} \end{figure}$