# Vertex Reconstruction Offline Guide

## Contacts

For vertex fitting, secondary vertex finding & offline primary vertex finding (B-tag POG):

For beam-spot determination, V0 reconstruction and online & offline primary vertex finding (Tracking POG):

## 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).

For a basic introduction to vertex reconstruction, see the Vertexing Section of the CMSSW Workbook.

## RECO/AOD collection and the standard sequence.

Only he primary vertex reconstruction step is part of the default sequence, and it will write a VertexCllection into the event. Information on the vertex data format can be found in:

The full list of products can be found in the following pages:

## Algorithms and Software Architecture

A beginners guide to vertex finding and fitting is given in the CMSSW Workbook Vertex reconstruction. More detailed information is given below:

Note that for all high-level algorithms (b-tagging, vertex reconstruction), TransientTracks have to be used instead of the persistent reco::Tracks.

### Online (HLT) vertex reconstruction

In the HLT, very fast tracking and vertexing is used, based on the Pixel Tracker alone. This is described below:

## Vertex Analysis and Validation

A software package exists for producing vertex performance plots. This is useful both to evalulate performance and to verify new software releases. It is described in SWGuideBtagValidation.

## Tutorial

• An introduction to vertex reconstruction: link (Data and analysis flow tutorials, October 2008)
• A complete introduction to vertex reconstruction, including some details on the internal framework: link (b Tag and Vertexing Workshop (FNAL), July 2007)

### Old documents

• Some information on the basic elements on the framework (mostly ORCA, but most is still valid in CMSSW) are available on the old vertexReco page (here). It contains the the presentations from past workshops, tutorials and various other information from ORCA/P-TDR times. Some of it may be a bit outdated though

## Review status

Reviewer/Editor and Date (copy from screen) Comments
KatiLassilaPerini - 09 Feb 2007 created page from VertexReconstruction page
ThomasSpeer - 27 Feb 2009 Review and update
VictorBazterra - 2009-08-27 Updating the page with documentation to access MC information.

Responsible: ThomasSpeer

Topic revision: r67 - 2011-02-10 - MaximeGouzevitch

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