7.3 Vertex Reconstruction

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Goals of this page:

This tutorial describes:

• the pixel standalone reconstruction of tracks and primary vertices (for HLT),
• the fitting of tracks to a common vertex,
• the offline primary vertex reconstruction that uses tracks reconstructed from both pixel and silicon strip hits.

Contents

Introduction

Vertex reconstruction usually involves 2 steps, vertex finding and vertex fitting. Vertex finding involves grouping tracks into vertex candidates. The vertex-finding algorithms can be very different depending on the physics case (primary or secondary vertex finding, reconstruction of exclusive decays, etc.). Vertex fitting involves determining the best estimate of the vertex parameters (position, covariance matrix, and track parameters constrained by the vertex position and their covariances) for a given set of tracks, as well as indicators of the fit quality (total chi2, number of degrees of freedom, or track weights).

Pixel standalone track and vertex reconstruction is fast enough to be used at HLT. Fitting a single vertex from reconstructed tracks is a common step in the reconstruction of high-level physics objects such as tau's and B's. Offline primary vertex reconstruction yields ultimate vertex position resolution as well as the best possible estimation of the primary vertex position error matrix.

Set up your Environment

Set your runtime environment (shown for release %WBRELEASENEW%):
cd CMSSW_%WBRELEASENEW%/src
cmsenv

• Full instructions
• Summary for every login session
• Access modules from the CVS repository.

Data samples

For this tutorial, any data sample reconstructed with the standard sequence can be used, as long as it countains tracks and primary vertices. These are available in both the RECo and AOD samples. Typically the user wishes to analyze an already generated and reconstructed sample, such as PhysicsValidation or in CMS.ReleaseValidation samples. To find out these samples, please refer to the WorkBook Locating Data Samples section.

In some cases the user may want to analyze reconstructed Tracks in a private production sample: some examples of configuration files to generate data samples can be found in SWGuideTrackRecoSequences (particle gun muons and H->ZZ->4mu).

Individual Tutorial Sections

Offline primary vertex finding

Vertex Fitting

Several other topics are described in the Vertex Reconstruction section of the SWGuide:

Beam-spot reconstruction

Pixel track finding and fitting (for HLT)

Pixel primary vertex finding and tagging (for HLT)

Vertex Analysis Tools

Information Sources

CMS Note 2006/026, "Track reconstruction, primary vertex finding and seed generation with the Pixel Detector", S.Succiarelli et al.
CMS Note 2006/032, "Vertex Fitting in the CMS Tracker", T.Speer et al.
CMS Note 2006/029, "Impact of CMS Silicon Tracker Misalignment on Track and Vertex Reconstruction", P. Vanlaer et al.

Review status

Reviewer/Editor and Date (copy from screen)	Comments
AnneHeavey - 28 Jul 2006	Jenny moved Pascal's and Thomas Speer's tutorial initial page to WB; I moved indiv tutorial pages in, fixed TOC, made many edits, added lots of red comments for authors to address! Needs review by experts.
ThomasSpeer - 19 February 2008	Update samples
ThomasSpeer - 27 Feb 2009	review and update

Responsible: Thomas Speer