We estimate the bb background using a Pythia Monte Carlo sample of pp-> bb events and normalize to data using the B±->J/&psi K± mass peak. A large source of systematic error in the Pythia prediction is uncertainty of the relative cross sections for different QCD processes that produce bb. The Pythia Monte Carlo produces bb by the following highest order QCD processes: flavor creation (FC) where two quarks or gluons annihilate to produce bb, flavor excitation (FE) where a b quark from a bb pair in the sea scatters off of a parton, and gluon splitting (GS) where a gluon scatters and then splits to a bb pair. We study the Pythia sample's agreement with data by fitting the &Delta&phi distribution for unvertexed J/&psi&mu events in Pythia to data while allowing the relative fractions of FC, FE and GS in Pythia to float. |
Before fitting the Pythia &Delta&phi distributions to data we fit individual &Delta&phi templates for FC, FE and GS which can be seen in Figure 10 . The unvertexed J/&psi&mu also contains contributions from fake muon, fake J/&psi, fake muon + fake J/&psi and Bc->J/&psi &mu &nu events. We fit templates for these additional sources and estimate the size of their contributions (except for Bc). The templates can be seen in Figure 11. |
Figure 10: J/&psi&mu &Delta&phi distributions and template fits for Pythia QCD processes. Left: Flavor creation. Center: Flavor Excitation. Right Gluon Splitting. |
Figure 11: &Delta&phi distributions for other sources of J/&psi&mu and template fits. Top Left: Fake muon. Top Right: Fake J/&psi. Bottom Left: Fake muon + Fake J/&psi. Bottom Right: Bc->J/&psi &mu . |
Fitting the &Delta&phi templates to data, we see the fit results shown in Figure 12 and extract scale factors for FC, FE and GS which can be seen in Table 3. All three scale factors are consistent with 1, so we use the default Pythia sample to estimate the bb background. We estimate the sytematic uncertainty using the uncertainties from our &Delta&phi study. We predict a bb background of 12.7 ± 1.7(stat.) ± 5.7(syst.) in the 4.0-6.0 GeV/c2 mass window. |
Figure 12: Results fitting J/&psi&mu &Delta&phi distributions from Pythia to data. |
Table 3: Pythia QCD process fractions before &Delta&phi fitting and fractions and scale factors after fitting. |