RS Tracking Algorithm

Complete: 4

Goal of the page

This page gives an overview of the RoadSearch tracking algorithm. Readers should understand generally how the algorithm works and how to find these tracks in an event.

Contacts

KevinBurkett

Introduction

The RoadSearch (RS) is an alternative algorithm for reconstructing tracks. It was originally designed to reconstruct tracks in collisions, especially in a pixel-less scenario. It has since been modified to reconstruct tracks from cosmic rays. The algorithm is characterized by a navigation scheme based on treating silicon modules as rings in the r-z plane, where r and z are the distances off and along the CMS beam axis respectively. A road is a pre-defined set of rings, determined by the expected linear trajectory of a particle in the r-z plane. Tracks are seeded based on pairs of RecHits in inner and outer seed layers.

The algorithm starts by looping over modules in the inner and outer seed layers, and selecting pairs of RecHits in these modules. A B-field dependent constraint on the Delta(phi) between the two seed RecHits ensures that only sensible combinations are retained. In the next stage, RecHits are collected in the other silicon layers inside a narrow window around the expected trajectory based on the seed RecHits. Extraneous RecHits are removed by building a candidate trajectory from the collection of RecHits. The final track fit is identical to the one used by the Combinatorial Track Finder. All good reconstructed tracks are retained.

Usage of the algorithm

The RS can be used for reconstruction of tracks in cosmics or collisions. It is a backup algorithm for the CTF reconstruction in collisions.

The sequence for RS tracking is called "rstracks" and is defined in RecoTracker/Configuration/python/RecoTracker_cff.py. The RS sequence can be included in the standard reconstruction by using reconstruction_withRS instead of reconstruction, as defined in Configuration/StandardSequences/python/Reconstruction_cff.py.

Tracks found by the RS will be in the collection rsWithMaterialTracks. This is a collection of standard recoTracks.

Software architecture

The RS follows the same sequence of other tracking algorithms of:

  • seed finding
  • pattern recognition
  • final track fit

Seed finding is controlled by the RoadSearchSeedFinder module. The standard seeding uses hits from the inner two (matched) layers of the TIB and TID for inner seed hits, and hits in the outer two layers of TOB and TEC for outer seed hits. Additional modules along the outer edge (in Z) of the TOB and TEC are also used for the outer seed layers.

Pattern recognition is performed by the RoadSearchCloudMaker and RoadSearchTrackCandidateMaker. The cloud maker uses a triplet of hits in the four layers to build an initial trajectory. The trajectory will be in r-phi or r-z based on the precision coordinate of the outer hits. Hits in the other layers are collected in a window around this trajectory. This collection of hits is the cloud and the cloud maker outputs a collection of RoadSearchClouds.

The second part of pattern recognition is performed by the RoadSearchTrackCandidateMaker. In each cloud, a trajectory is first built in the low-occupancy layers. Ideally, the trajectory would be built in layers with only one hit per layer. The standard trajectory building tools, as used in the CTF, are used. This trajectory is built inside-out and then smoothed. The smoothed trajectory is then extrapolated back to the beamline and the trajectory is then built inside-out again, this time adding the compatible hits in the higher occupancy layers. The output is a collection of TrackCandidates.

In the final step, the track candidates are fit with the same fit as for the CTF, creating the final collection of tracks.

Review status

Reviewer/Editor and Date (copy from screen) Comments
KatiLassilaPerini - 29 Jan 2008 created template page
KevinBurkett - 16 Oct 2009 first draft of RS information

Responsible: KevinBurkett
Last reviewed by: Most recent reviewe

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Topic revision: r2 - 2009-10-16 - Burkett
 
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