Vertices with low p_T tracks

Dijet

Two jet Pythia events (produced with PythiaZ700dijets.cfi). Generated, simulated and reconstructed with CMSSW_1_2_0_pre1 with special code for low p_T reconstruction. The distribution of number of found vertices (NVertices) with several NTrkMin settings, keeping ZSeparation=0.05cm and PtMin=0. While with NTrkMin=5 there are a lot of events with multiple vertices, with NTrkMin=10 their frequency is below 1%.

Event gallery

EventGalleryDijet

Minimum bias

Minimum bias Pythia p+p events at 14 TeV. Generated, simulated and reconstructed with CMSSW_1_2_0_pre1 with special code for low p_T reconstruction.

Event gallery

EventGallery

Vertex resolution

Distribution of vertex residuals with the standard setting (PtMin=1GeV/c) and the low p_T setting (PtMin=0).

The resolution has strong multiplicity dependence, shown here as function reconstructed track multiplicity NRecTracks.

Multiple vertices

Distribution of the number of reconstructed (charged pixel) tracks for several NVertices values is shown, that is for different number of found vertices. The following four plots are done with different vertex reconstruction parameters in RecoPixelVertexing/PixelVertexFinding/data/PixelVertexes.cff (ZSeparation, NTrkMin, PtMin).

Standard parameters

Only high multiplicity events give vertex.

Lowering PtMin to 0

Now all tracks take part in vertex finding. Even low multiplicity events give a vertex, if, say, NRecTracks > 8.

Even looser

Lowering NTrkMin to 2. This choice gives vertex to very low multiplicity events, but at the same time many events will have multiple vertices.

Even looser

Increasing ZSeparation to 0.5 cm. A possible cure of the previous problem.

-- FerencSikler - 12 Nov 2006