Invariant Mass Distribution of Jet Pairs Produced in Association with a $W$ Boson in $p\overline{p}$ Collisions at $\sqrt{s}=1.96\mathrm{TeV}$

We report a study of the invariant mass distribution of jet pairs produced in association with a $W$ boson using data collected with the CDF detector which correspond to an integrated luminosity of $4.3{\mathrm{fb}}^{-1}$. The observed distribution has an excess in the $120–160\mathrm{GeV}/c^2$ mass range which is not described by current theoretical predictions within the statistical and systematic uncertainties. In this Letter, we report studies of the properties of this excess.

Figure 1

The dijet invariant mass distribution. The sum of electron and muon events is plotted. In the left plots we show the fits for known processes only (a) and with the addition of a hypothetical Gaussian component (c). In the right plots we show, by subtraction, only the resonant contribution to $M_{jj}$ including $WW$ and $WZ$ production (b) and the hypothesized narrow Gaussian contribution (d). In (b) and (d), data points differ because the normalization of the background changes between the two fits. The band in the subtracted plots represents the sum of all background shape systematic uncertainties described in the text. The distributions are shown with a $8\mathrm{GeV}/c^2$ binning, while the actual fit is performed by using a $4\mathrm{GeV}/c^2$ bin size.

Figure 2

The dijet invariant mass distribution for the sum of electron and muon events is shown after subtraction of fitted background components with the exception of the resonant contribution to $M_{jj}$ including $WW$ and $WZ$ production and the hypothesized narrow Gaussian contribution (a). With respect to Fig. 1, the subtracted background components are chosen as the systematic combination that best fit the data (see the text). The fit ${\chi }^2/\mathrm{ndf}$ is $62.0/81$. (b) $\Delta R_{jj}$ distribution for events with $M_{jj}<$ 115 and $M_{jj}>175\mathrm{GeV}/c^2$ of the data compared to the background estimation that corresponds to the same systematic combination of (a). The uncertainty band corresponds to background statistical uncertainty.