L3 Primary Vertex Tool

Miscellaneous Plots

This document: http://www-d0.fnal.gov/~lima/vertex/l3fvertex_plots.html
Guilherme Lima
Last update: September 28, 1999

Documentation on L3 Primary Vertex Tool

Miscellaneous Plots

Performance
This plot shows a scatter plot of generated (x-axis) vs. reconstructed (y-axis) primary vertices, for about 5000 ttbar events. Well reconstructed vertices lie along the diagonal. Off-diagonal are misreconstructed vertices. The top plot gives a better idea about the relative number of times the right primary vertex gets reconstructed. The gaussian-like distribution is the expected distribution of ppbar collisions along the z-axis.
Efficiency plots for several physics processes:
Higher granularity, 1mm-wide bins show clearly that the uncertainty in PV position could be calculated by fitting a gaussian to these high-granularity histograms. This is not practical though, as the algorithm gets slower with 1mm-wide bins. Also a gaussian fit to such histograms takes some CPU time...
This plot compares the L3 primary vertex determination using a simple averaging and a weighted averaging, where weights are taken from the uncertainties in the z-projection of each hit pair: weight = 1/sigma**2. No significant improvement is achieved from weigthed average, so simple arithmetic average is used for PV calculation at L3.
This plot compares the distributions for number of hits in several physics processes:
- Top plot:
  - solid line: ttbar events
  - dashed line: B into Psi Ks, Psi to e+e- events
  - dotted line: Prompt Psi to e+e- events
- Bottom plot: Same as top one, but with different scales for comparison.
  - solid line: Prompt Psi to e+e- events
  - dashed line: Gamma + Jet events
  - dotted line: ttbar events
Performance for multiple interaction events
The plots below show results for ttbar events superimposed to 2 minbias interactions:
- General comparison between 5000 single-interaction ttbar events (left plots) and 100 multiple-interaction ttbar (+2 minbias) events (right plots)
  - Top plots: number of hits per event
  - Middle plots: number of reconstructed vertices
  - Bottom plots: probability associated to the best vertex candidate (quality parameter)
- Performance. The bottom plot shows a scatter plot for generated (x-axis) and reconstructed (y-axis) primary vertices. Well reconstructed vertices lie along the diagonal. Off-diagonal vertices are well reconstructed minbias vertices which got better probability than generated ttbar vertex, generally due to the latter falling outside SMT acceptance.
- Higher granularity. This plot shows results of the L3 Primary Vertex tool using 1mm-wide bins, for 10 ttbar+2minbias events. Minbias and ttbar vertices are clearly seen in many of the events.

Back to my activities on Vertex Reconstruction at DØ
DØ Vertex Reconstruction Page
DØ Run II Algorithms Page

Comments: Guilherme Lima