RS for Cosmic Algorithm

Complete: 4

Goal of the page

This page should help users understand the way cosmic tracks are reconstructed by the CMS.RoadSearch algorithm. Further details about the RS algorithm in general can be found at the main CMS.RoadSearch page.




The RS is an alternative algorithm for reconstructing tracks. It was originally designed to reconstruct tracks in collisions, especially in a pixel-less scenario. It was 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.

Usage of the algorithm

The RS for Cosmics can be used for reconstructing cosmic tracks in both B=0 and B=3.8T data. It can be used along with the CTF and CosmicTrackFinder to compare the performances of the different algorithms.

The RS cosmic reconstruction is defined by the rstracksP5 sequence in RecoTracker/Configuration/python/, which is part of the standard cosmic reconstruction sequence, used on CRAFT data.

The output of the RS cosmic reconstruction is the recoTrack collection rsWithMaterialTracksP5.

Software architecture

The algorithm is a modified version of the one used to reconstruct tracks in collisions. Below we describe the modifications:
  • Instead of using 3 hits from 4 seeding layers (2 inner, 2 outer) pairs of hits are used, one from the inner seed layers and one from the outer seed layers
  • For cosmics, the outer seed layers are the two outermost layers of the TOB and TEC. The inner layers are either the two inner layers of the TOB or the two inner layers of the TIB. The inner layers of the TOB are used to improve efficiency for cosmics that do not pass close to the center of the detector.
  • The cloud is built on the assumption of a straight line trajectory. This works well for B=0 cosmics, but causes a low PT inefficiency in B=3.8T data.
  • All roads are considered as r-phi because the r-z straight line approximation is invalid for tracks with large impact parameters.
  • For B=0 cosmics, since there is no way to determine the PT, we arbitrarily set the seed PT to 5 GeV


The RS has been used extensively for reconstruction of cosmics in CRUZET and CRAFT in 2008 and CRAFT in 2009. The algorithm is well tested in CMSSW 22X and 31X.

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 reviewer

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Topic revision: r4 - 2009-10-19 - GiuseppeCerati
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