Start of part5 analysis docs

src/docs/part5/reconstruction_analysis.md

+---
+title: 'Reconstruction Analysis'
+---
+
+This part of the tutorial outlines how reconstruction output from full simulations can be used for analysis by physics working groups.
+
+At the end of this tutorial you should able to:
+- determine the location of relevant full simulation reconstruction files,
+- understand the file structure of the full simulation reconstruction output,
+- use one of several approaches to perform low-level analysis on the reconstruction output.
+
+## Location of full simulation reconstruction output
+
+Full simulation outputs are stored on one of several locations:
+- S3: https://dtn01.sdcc.bnl.gov:9000/minio/eictest/ATHENA/ (web interface)
+- xrootd: root://sci-xrootd.jlab.org//osgpool/eic/ATHENA/ (no preview currently available)
+
+For details on accessing these locations, please refer to https://doc.athena-eic.org/en/latest/howto/.
+
+Data at these location is organized in a predicatable structure:
+- `EVGEN/` contains all initial generated events (in hepmc3 or other format),
+- `FULL/` contains the raw full simulation output without any reconstruction,
+- `RECO/` contains the reconstruction output.
+
+Under the different top-level directories, you can find the different physics processes:
+- `SINGLE/` contains single particle initial states,
+- `DIS/` contains DIS events.
+
+## File structure of the full simulation reconstruction output
+
+Each reconstruction output file has essentially the same structure, defined by the EIC Data Model. This structure can be retrieved with `rootls`, e.g.
+```console
+rootls root://sci-xrootd.jlab.org///osgpool/eic/ATHENA/RECO/SINGLE/pi+/1GeV/45to135deg/pi+_1GeV_45to135deg.0001.root
+```
+which produces the following output, two trees:
+```console
+events  metadata
+```
+
+The `events` tree is of course what we are interested in. We can explore its top-level structure as follows. We first start a ROOT session:
+```console
+root -l root://sci-xrootd.jlab.org///osgpool/eic/ATHENA/RECO/SINGLE/pi+/1GeV/45to135deg/pi+_1GeV_45to135deg.0001.root
+```
+and then list the top-level branches in the `events` tree:
+```console
+events->Print("toponly")
+```