From e47b442474a8de5d3e2037d50cdf2a5c2b2f1da7 Mon Sep 17 00:00:00 2001
From: Whitney Armstrong <warmstrong@anl.gov>
Date: Mon, 23 Aug 2021 22:32:21 -0500
Subject: [PATCH] 	modified:   README.md

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 # Guide to Tracking Development in Juggler for ATHENA Collaboration
 
+- [Overview](#overview)
+  * [What this guide is not](#what-this-guide-is-not)
+  * [What this guide is](#what-this-guide-is)
+- [Data Model](#data-model)
+- [Repositories and Workflow](#repositories-and-workflow)
+  * [Repositories](#repositories)
+  * [Pipelines and Artifacts](#pipelines-and-artifacts)
+    + [Pipeline](#pipeline)
+    + [Artifacts](#artifacts)
+
 ## Overview
 
 This guide assumes the reader has some basic knowledge of tracking and computing.
@@ -95,5 +105,44 @@ Artifacts can be browsed via the web interface, for example, the latest in recon
 
 
 
+## Software Toolkit
+
+### ACTS
+
+"[Acts](https://acts.readthedocs.io/en/latest/) is an experiment-independent toolkit for (charged) particle track reconstruction in (high energy) physics experiments implemented in modern C++."
+
+#### The ACTS way of tracking
+
+First, the geometry has to be constructed.  Assuming this is already done, we describe the data processing.
+
+##### Source Links and Measurements
+
+```
+  Tracker Hit  +  Geometry --> Measurement
+  Hit CellID   +  Surface  --> Source Link
+```
+
+A `SourceLinker` is an algorithm that looks up the surface and maps it to the hit's `cellID`.
+Naturally the same algorithm outputs measurements which are also mapped to the hit and contain the position 
+and sensor size information (via covariance matrix).
+
+
+##### Proto tracks
+
+Both track finding and fitting will use the information contained in the source links and measurements.
+Track finding produces `proto tracks` or groupings of hits.  Each proto track is simply a `std::vector<int>` storing the index of the
+hits associated with a track seed.
+
+##### Initial Track parameters and Seeding
+
+A Kalman filter needs a starting point and those are the `Initial Track Parameters`. These can be determined many different ways. 
+Conceptually the process of determining these parameters begins with track seeding.
+
+
+
+### GenFit2
+
+[GenFit](https://github.com/GenFit/GenFit) is a generic track fitting tool built on ROOT and the tgeo library.
+
 
 
-- 
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