The (filtered and processed) data

Figure 1 reports the upper plots

Figure: CDF data[9] before and after `arithmetic' background subtraction

of figure 1 of the cited CDF paper. The left side one shows the histogram of the `data',<sup>3</sup>the jet-jet mass distribution in 8 GeV bins, for a total of about 10800 events<sup>4</sup> (points with vertical bars). The colored regions show the predictions split into several contributions, the most important of which is due to the production of two W bosons, or of a W boson together with a Z boson (red). We see that at around 140 GeV there are more events than `expected' (an expression which we shall return to later).

The right side plot shows the data after the contributions called here `background' (all but the red one of the left plot) were subtracted `arithmetically'.⁵In the five bins between 120 and 160 GeV there are about 230 events (but in the side bins there are even `negative events' whose meaning is only mathematical). That was `the excess'.