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 fb^{−1}. The observed distribution has an excess in the 120-160 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.
The published paper, Phys.Rev.Lett.106:171801 (2011), is available here and from arxiv here. The analysis was presented by Viviana Cavaliere at a Joint Experimental-Theoretical Seminar at FNAL on April-6. A copy of that talk is here.
Additional studies of these W+dijet event are presented in the thesis of Viviana Cavaliere and below we include plots of additional distributions not included in the PRL submission due to space restrictions.
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). On 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 plot (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 GeV/c^{2} binning while the actual fit is performed using a 4 GeV/c^{2} bin size.
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 resonant contribution to M_{JJ} including WW and WZ production and the hypothesized narrow Gaussian contribution (a). With respect to Figure 1, the subtracted background components are chosen as the systematic combination that best fit data. The fit χ^{2}/ndf is 62.0/81. (b) ΔRjj distribution for events with M_{JJ} < 115 and M_{JJ} > 175 GeV 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.
These are used to determine the normalization of the multijet QCD events and a preliminary estimate of the W+jets fraction. The MET distribution of the data is fitted to a sum of background templates. The samples with known cross section (Z+jets, diboson, ttbar and single top) are fixed to their NLO cross sections, while the normalization of the QCD and W+jets samples are allowed to float in the fit. The parameters of the fit are the total normalization and the QCD fraction.
Kinematic distribution of events with 3 or more jets which enhances the top contribution to the sample.
W+jet/QCD discriminants :
Δφ in the transverse plane between the purely calorimetric
MET and MET evaluated using tracks.
Δφ defined as the
angle in the transverse plane between MET and the closest jet with raw
E_{T} > 5 GeV.
Z+jets. Z+2 jet events are selected by requiring one tight lepton (same selection of W+jets) and one lepton with P_{T} > 10 GeV/c and the invariant mass of the two leptons to be in the range 81-110 GeV/c^{2}.
ΔR distributions.
The ΔR distribution is studied to investigate possible effects
that could result in a mismodeling of the dijet invariant mass
distribution. Two control regions are studied: the first defined by
events with M_{JJ} < 115 and M_{JJ} > 175
GeV/c^{2} and the second defined by events with
pT_{JJ} < 40 GeV/c. We use these regions to derive a
correction as a function of ΔR so that ΔR agrees with the
Monte-Carlo and apply this reweight to the events in the excess
region. We find that the statistical significance of the result is
reduced by one sigma for reweightings from the M_{JJ}
sidebands and increased by one sigma using reweights based on the
pT_{JJ} < 40 GeV/c sample.
The significance of the excess for the subsample of events selected with pT_{JJ}> 60 GeV is 2.2 σ. The M_{JJ} distribution is shown below.
The plots below show the diboson, single top, and t-tbar backgrounds. Left: normalised to unit area, and Right: normalised to expected cross sections
The M_{JJ} and background subtracted M_{JJ} distributions are shown below when the njets=2 cut is relaxed i.e. these distributions are with njets ≥ 2.