A new look at W+jets

This plot shows the dijet mass distribution in W/Z + 2 jets, overlaid, in the combined electron and muon data sets. The plot incorporates improved jet energy scale corrections for simulated quark and gluon jets and tunings of the multijet-background modeling. The bottom panel shows data with all fit background contributions subtracted, except those from diboson production.

Three years ago, CDF saw an excess of events containing a W boson and two jets — a peak in the 140 GeV/c2 mass range with a statistical significance of 3.2 sigma. A peak indicates that a particle may exist at that particular mass.

However, the DZero and CMS experiments did not see the peak in their data. It was quite clear that more checks would be needed before claiming that a new particle was lurking around 140 GeV/c2.

Based on the Standard Model, our expectation is to see no peak near 140 GeV/c2. We expect four different event types to contribute to the background, and we understand three of them very well.

The fourth contribution, called the multijet background, consists of multiple jets produced in strong force interactions. For such a jet to be considered as part of the 140-GeV/c2 signal, it must be wrongly identified as an electron or a muon in the detector.

This background is not easy to understand. Scientists use data-driven models to identify it, but the models have a hard time making a good prediction for the dijet mass distribution.

After more than one year, additional studies of potential systematic effects have led to additional tunings of the data-driven modeling for the multijet-background contributions. CDF applied a subtle correction to the energy of jets mimicking an electron and contributing a fake background. At the same time, CDF incorporated specific jet energy scale corrections for simulated quark and gluon jets to model the simulation-driven backgrounds.

The analysis of the full CDF data set is similar to our earlier analysis, but we have introduced above-mentioned correction factors for quark and gluon jets and a new estimation of the multijet background. The new result is shown in the figure, and it is clear that there is no dijet peak at a mass near 140 GeV/c2.

The Standard Model wins again.

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edited by Andy Beretvas

These CDF physicists contributed to this data analysis. Top row from left: Giorgio Bellettini (U Pisa, INFN), Matteo Cremonesi (Oxford), Giuseppe Latino (U Siena, INFN). Second row: Vadim Rusu (Fermilab), Marco Trovato (Scuola Normale Superiore, Pisa, INFN and Fermilab), Caterina Vernieri (Scuola Normale Superiore, Pisa, INFN).