A measurement of the dijet mass spectrum can be used to verify the predictions of quantum chromodynamics (QCD) for parton-parton scattering and to constrain the parton distribution functions (pdf) of the proton. Additionally, new physics, such as quark compositeness, would be revealed by an excess of events in the dijet mass spectrum at high masses with respect to the predictions of QCD. The DØ Collaboration has made a new measurement of the inclusive dijet mass spectrum and improved limits on the contact interaction scale. The inclusive dijet mass spectrum (plot 1) is compared to a prediction made with the NLO parton event generator JETRAD. Given the experimental and theoretical uncertainties the predictions are in agreement with the data (c²=5.0 for 15 degrees of freedom).

The ratio of mass spectra for |h_jet| < 0.5 and 0.5 < |h_jet| < 1.0, (plot 4), exploits the high correlations in the uncertainty. The resulting cancellation of uncertainties leads to a systematic error that is less than 8% for all masses. The JETRAD prediction is in good agreement with the data. The ratio of the mass spectra places limits on quark compositeness. A mass scale L, characterizes the strength of the quark substructure binding and the physical size of the composite state. The 95% lower limits on the mass scale are L⁺ >2.7 TeV and L^- > 2.4 TeV.

Plot 1: The inclusive dijet mass spectrum for central pseudorapidities (|h_jet| < 1.0) with mass greater than 200 GeV/c² which is compared with the NLO JETRAD prediction (eps file)
Plot 2: The correlations between the systematic errors in bins of the dijet mass spectrum (eps file).
Plot 3: The difference between the data and the prediction JETRAD divided by the prediction for |h_jet| < 1.0. The points represent the comparison to the calculation using the CTEQ3M pdf with m=0.5E_T^max. The shaded region represents the ±1s systematic errors. The effect of changing the renormalization scale and choosing a different pdf is also shown (each curve shows the difference between the alternative prediction and the standard prediction) (eps file).
Plot 4: The ratio of mass spectra for |h_jet| < 0.5 and 0.5 < |h_jet| < 1.0, and the prediction. The vertical error bars show the statistical and systematic errors added in quadrature, the cross-bar shows the size of the statistical error (eps file).

Down-loadable Cross Section Files
The inclusive dijet mass cross section for |h_jet| < 1.0. (DØ data file, Correlations, Jetrad Predictions)
The ratio of mass spectra for |h_jet| < 0.5 over 0.5 < |h_jet| < 1.0. (DØ data file,Correlations, Jetrad Predictions)

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