Integrated Tracker Task Force Review

This document is meant to help the Integrated Tracker Task Force (ITTF) review committee in their task.

Introduction/History

As the STAR experiment evolved, its detector system was upgraded with several new sub-systems to enhance tracking precision and/or to enhance our ability to search for secondary particles. It soon became clear that a new tracking approach was required in order to fully take advantage of the information provided by all sub-systems. In fact, while the current approach of tracking in the TPC-only was found to be adequate at the previous review (June 2000), and indeed, has proven to be tremendously successful in the processing and analysis of the past year's data, the same review commitee recommended the formation of an R&D group to start work on an integrated tracking scheme. A call for investigating new algorithms for road-finding and track extrapolation (as the two main example topics) was made and the Integrated-Tracking Task Force (ITTF) was formed to achieve this goal. The initial tasks for the group were to produce a tracker for STAR that

1. provides highly efficient and minimum-contamination information on particles emitted into the STAR acceptance.

2. incorporates all tracking detectors taking into account their detailed geometry, calibration, material location and thickness, as well as magnetic field effects.

3. provide also tools which allow to extrapolate from one position on the track to any other position along the flight path of the particle with high accuracy (track extrapolation).

The effort is lead by Claude Pruneau from Wayne State University. Since several experiments have worked on this subject in the recent past and solutions have been developed and implemented by ALICE, Atlas and BaBar (to name but a few), it was pointed out that the group may consider the re-use of such existing code or algorithms were applicable and justified. However, freedom in the choice of approach was given and the ITTF members only had to follow some basic guidelines such as

• The group should stay in close contact with ongoing efforts in the analysis and reconstruction of STAR data and address apparent issues in their algorithms.

• The implementation of an integrated tracker requires a sufficiently detailed description of the detector to evaluate energy loss and multiple scattering. It is generally considered that GEANT is too complex, slow, and too detailed to be usable for this purpose. The development of an independent slim geometry interface was identified as one topic of close collaboration between the ALICE collaboration and STAR.

• The group should pursue this joint effort and take a leading role in its development.

• The task force leader should stay in close contact to the STAR reconstruction leader. They both report to the STAR computing leader.

• All code is to be written in C++, where necessary compatible with the existing STAR infrastructure.

Requirements for the STAR Integrated Tracker (STI)

The ITTF members were give a set of guidelines as well as requirements on several front. They will be described in this section.

Functional Requirements

1. Track finder/reconstruction

• Include flexible/polymorphic interface to enable access to data from various detectors such as, in STAR, the TPC, SVT, SSD, and possibly other detectors such as the FTPC, and even the TOF, and EMC.

• Enable a certain degree of flexibility on the detector geometry in order to accommodate upgrades.

• Use a Kalman track follower to extend tracks

• Allow for many-to-many point to track relationships.

• Develop an efficient ghost track rejector

• Use error matrices (covariance)

2. Track Fitting

• Use a robust track model

• Possibly allow for usage of different track model if needed by using an abstract track model interface

• Use a Kalman Filter technique for track model fitting

3. Geometry

• Use a flexible and polymorphic geometry interface to enable track propagation and multiple scattering and energy loss calculations across various detector and volumes.

4. Data Store

• Use a polymorphic data store with selectable (coarse/fine ) granularity to allow for efficient retrieval of data hits and data tracks.

Performance Requirements

• STI should enable good track reconstruction to optimize the reconstruction efficiency while minimizing ghost or false tracks.

• STI should handle errors properly so that fit chi-square are meaningful

• STI should be faster than the existing STAR tracker

• STI should make efficient use of memory to limit the size of objects - however emphasis is on "speed" and reconstruction quality.

Code and Language Requirements

• Code to be written entirely in C++ Code to be developed with an object oriented design

• Code to be multi-platform portable

• Code to be compatible with ROOT

• Code to adhere to STAR code development standards

• Code to be documented as much as possible. Documentation to include class, method level specifications as well as usage examples.

• Code to be archived using the STAR archival system

Committee Charges

At the end of the set of presentations by the ITTF members and STI testers, the review committee should be informed on and be able to comment on the following items :

1. The general code design. In particular, but not limited to,

• the practicality of adding new geometry for new detector sub-systems (SVT, SSD, TOF, EMC, CTB, FTPC, ...)

• the implementation of track extensions methods for projection into other volumes

• the approach for extending the tracker with new tracking models.

2. The state of the geometry implementation and treatments of energy loss and multiple scattering.

3. The STI tracker’s efficiencies and the global tracking performance (as per the original Performance Requirements specified above)

• Evaluate reconstruction efficiencies for both primary and secondary tracks using results from Monte Carlo, embedding, and real data for low and high track densities.

• Compare (where possible) to the current STAR tracking software. Tracking inefficiencies should be clearly understood and suggestions to compensate for and/or cure the observed discrepancies made.

4. Integration plans and global tracking feasibility for final implementation

• Incorporating the STI tracker into the STAR framework

• The approach for global detector alignment and the handling of coordinate frames and transformations.

• The time line for integration.

5. The STI readiness and suitability to replace the current STAR tracker.

In addition from reporting on the above issues, the review committee is invited to address the following items as needed:

1. Any unforeseen tracking issues inherent to the new approach and make recommendations for possible improvements and accounting of items not addressed in the review presentations.

2. Identify any tracking issues not addressed in either the current or the STI tracker but which need to be taken into account in future upgrades and developments.

3. Comment and suggest improvements or other tracking methods/models.

The review committee is asked to return their written report within two weeks following the review to the STAR Software and Computing leader.

The review committee

The review committee will be composed of the following members :

Developers and Testers

The ITTF was composed of

• Claude Pruneau, project leader and coordinator, WSU

• Matthias Messer, Integration coordinator, BNL

• Andrew Rose, WSU

• Ben Norman, KSU

• Manuel Calderon de la Barca Sanchez, BNL

• Mike Miller, Yale

Testers who have made this effort possible are

• Dan Magestro, OSU

• Jennifer Klay, LBL

• Lee Barnby, KSU

• Manuel Calderon, BNL

• Mark Heinz, Yale

• Mercedes Lopez-Noriega, OSU

• Richard Witt, Yale

• Sergei Panitkin, BNL

• Zhangbu Xu, BNL

This document is available at

http://www.star.bnl.gov/STAR/comp/meet/ITTFReviewSept2002/ITTF-review-charges.html

http://www.star.bnl.gov/STAR/comp/meet/ITTFReviewSept2002/ITTF-review-charges.ps