Use Cases

Last updated on 2026-07-10 | Edit this page

Overview

Questions

  • How do users interact with EIC/ePIC data?

Objectives

  • Explore different use cases for ePIC simulation data and how users work with EIC/ePIC data
  • Discover how simulation files can be utilised in further analysis
  • Know how to download files if needed (and when it might be needed)

In this episode, we will explore a few common use cases and how users may want to interact with simulation campaign output in each case. Examples of carrying out some common tasks associated with each use case will be included.

Physics Analyser - Novice

This use case explores a user new to analysing ePIC data to try and look at a specific physics process. They will likely want to find and identify a specific physics process to pass through their analysis code. Their requirements are likely to include:

  • Reconstructed output files (RECO)
  • A general type of physics process (or group of processes)
  • The latest available files to test
  • A specific collider (energy and ion species) configuration

They may also want to only test a small subset of data to test and develop their analysis. This use case is one example where downloading a small number of files locally may be beneficial.

To find files that meet their requirements they could utilise the following tags:

  • software_release
  • requester_pwg
  • electron_beam_energy_gev
  • ion_beam_energy_gev
  • ion_species
  • data_level

We can use these tags to filter through the DIDs and find datasets of interest:

BASH 

rucio did list --filter 'software_release==XXX, requester_pwg==YYY, electron_beam_energy_gev==ZZ, ion_beam_energy_gev==iii, ion_species==jjj' 'epic:*'

Where we can substitute in our chosen values for each in place of XXX, YYY, ZZ, iii and jjj.

Callout

Beam Energies

Whilst we can enter any number for the electron_beam_energy_gev and ion_beam_energy_gev values, there are only certain combinations actually in use. electron_beam_energy_gev is typically 5, 9, 10 or 18 GeV ion_beam_energy_gev is typically 41, 100, 130, 250 or 275 GeV for protons. For other ion species, 110 and 166 may also be used.

Once we have identified a specific dataset of interest, we can look at the files within it using:

BASH 

rucio did content list scope:name

and we can get locations of the files within the dataset via:

BASH 

rucio replica list file --protocols root --pfns --rses isopenaccess scope:name_of_file
rucio replica list file --protocols root --pfns --rses isopenaccess scope:name_of_Dataset

as we saw in the last episode. We can get just the location of a specific file OR the location of all files within the dataset, depending upon which we specify. We could download this file locally using

BASH 

xrdcp FILEPATH ./

where FILEPATH is the path to one specific file from the output of one of the rucio commands above.

Challenge

Exercise

Using the suggested tags, find the latest available datasets for:

  • Neutral current (NC) DIS events for 10 GeV electrons colliding with 130 GeV protons
  • Download one file from this dataset of your choice

Filter on the relevant tags (using the latest software_release available), for example:

BASH 

rucio did list --short --filter 'software_release=26.*, requester_pwg==inclusive, electron_beam_energy_gev==10, ion_beam_energy_gev==130, ion_species==p, data_level==reconstruction' 'epic:*'

Pick a NC DIS dataset from the returned list, list its files with rucio did content list --short scope:name, get a file location with rucio replica list file --protocols root --pfns --rses isopenaccess scope:name_of_file, then copy a single file locally with xrdcp FILEPATH ./.

Physics Analyser - Experienced

In this use case, we consider an experienced physics analyser that has a well developed analysis script that they want to run on a large number of files, possibly even a full dataset, for a specific physics process they’re interested in. Their requirements are likely to include:

  • Reconstructed output files (RECO)
  • A specific physics process (or group of processes)
  • The latest available files to test
  • A specific collider (energy and ion species) configuration
  • A datasets with machine backgrounds embedded in the simulated output files

To find files that meet their requirements they could utilise the following tags:

  • software_release
  • requester_pwg
  • electron_beam_energy_gev
  • ion_beam_energy_gev
  • generator
  • data_level

They may also want to use the q2_min_gev2 and q2_max_gev2 tags, along with the ion_species tags to narrow down to an even more specific subset of files. They may also want to analyse files with or without background enabled.

As they want to process a large number of files, it is unlikely (and not recommended) that they will want to download a large number of files to process them locally. Instead, they may want to stream their files directly in their analysis script. They could do this via:

CPP 

auto f = TFile::Open("FILEPATH");
auto tree = f->Get<TTree>("events");

or if they’re using python:

PYTHON 

import uproot
import XRootD
file_path = "FILEPATH"
root_file = uproot.open(file_path)

As they may wish to process a full dataset, they might want to feed their script a full list of files to stream and run. They could print the full list of files in a dataset:

BASH 

rucio replica list file --protocols root --pfns --rses isopenaccess scope:name_of_Dataset > FileList

This could then be processed in the script:

CPP 

void FileListProcess(){
  string line;
  ifstream fstream ("FileList");
  int FileCount = 0;
  //TChain *AnalysisChain = new TChain("events"); // We could define a chain to process our files too and add them as we scan over our list
  while(getline(fstream, line)){
    if (FileCount > 5) continue; // Stop loop after 5 files, comment out to read full file
    // Check file exists
    TString tmpFile{line};
    auto RootFile = TFile::Open(tmpFile);
    if(!RootFile){ // Check file exists
      cout << "File not found:"<<tmpFile << endl;
      continue;
    }
    cout << "Found file - " << line << endl;
    //AnalysisChain->Add(tmpFile) // Add to our chain if we want
    FileCount++;
  }
}

or if they’re using python:

PYTHON 

import ROOT
import uproot
import XRootD
import awkward as ak

Files=[]

with open('FileList', 'r') as file:
    lines_list = file.readlines()
    for line in lines_list[:5]: # Read only lines 0:5 - remove [:5] to read all or change 5 to N where N is the number of lines you want
        file_path = line.rstrip() # rstrip to remove trailing white space/new lines
        try:
            with uproot.open(file_path) as file:
                Files.append(file_path) # Add file path to array
                print("Found file - ", file_path, "and appended to list for processing.")
        except Exception as e:
            print(f"Could not open file: {e}")

# Use the uproot iterate method to process our list of files - See https://uproot.readthedocs.io/en/stable/uproot.behaviors.TBranch.iterate.html
#for chunk in uproot.iterate({f: "events" for f in Files}, expressions=["MCParticles.PDG"]): # Open files in array f and process events tree with branches specified
    # Process each chunk - Do something
    # print(ak.type(chunk))

Note that we have restricted these examples to only print out the first five files in the list we created. We can comment out or change the lines as noted to process the full list (or adjust the cutoff value in the condition to process a different number).

Challenge

Exercise

Using the suggested tags, find the latest available dataset for:

  • Deeply Virtual Compton Scattering (DVCS) events from the EpIC event generator for 10 GeV electrons colliding with 130 GeV protons without background included
  1. Stream one file from this dataset in a script, check the number of events in this file
  2. Print all of the files in this dataset to a text file
  3. Stream five of the files in this dataset in a script, check the total number of events contained in all five files.

Hint - See the example scripts in the last episode for how to get the number of events in a root file in a few different ways.

Filter on the EpIC generator and the required beam energies, requiring no background mixing, for example:

BASH 

rucio did list --short --filter 'software_release=26.*, generator==epic, electron_beam_energy_gev==10, ion_beam_energy_gev==130, is_background_mixed==False' 'epic:*'
  1. Take one file location from rucio replica list file ... scope:name and stream it with a Test.C or Test.py script as shown in the previous episode; print tree->GetEntries() (or num_entries in uproot).
  2. Print the full file list with rucio replica list file --protocols root --pfns --rses isopenaccess scope:name_of_Dataset > FileList.
  3. Use the FileListProcess C++/python loop from this episode, leaving the 5-file cutoff in place, and sum the entries from each opened file.

Detector Designer/Optimiser, Algorithm/Reconstruction Development

In this use case, someone updating the design of a detector in DD4hep, or adjusting a reconstruction algorithm for a detector, may not want full reconstructed data. Instead, they may want more raw, hit level information. They may also want a specific detector configuration for comparison. In terms of physics process, they may not be looking at an actual reaction at all, but a particle gun simulation. To summarise, they may want:

  • Simulated (FULL) as well as reconstructed output files (RECO)
  • Particle gun studies with specific single particles
    • Specific momentum or angular ranges
  • Different versions of a dataset to track changes between software releases
  • Specific geometry files in use for the simulation

Some tags they might use to find their data include:

  • software_release
  • data_level
  • requester_dsc
  • geometry_config
  • gun_particle
  • gun_momentum_min_gev
  • gun_momentum_max_gev
  • gun_theta_min_deg
  • gun_theta_max_deg
  • gun_phi_min_deg
  • gun_phi_max_deg
  • gun_distribution
Challenge

Exercise

Using combinations of the suggested tags, find the latest available dataset(s) for:

  • K- single particle gun simulations
  • Determine the available momentum and angular ranges available for this/these dataset(s)
  • Do non-reconstructed files exist for this/these dataset(s)?

Filter on the kaon- particle gun using the latest software_release, for example:

BASH 

rucio did list --short --filter 'software_release=26.*, generator==single_particle, gun_particle==kaon-' 'epic:*'

Inspect the returned DID names (and their gun_momentum_*/gun_theta_* tags) to read off the available momentum and angular ranges. To check whether non-reconstructed files exist, repeat the query with data_level==simulation (FULL) instead of data_level==reconstruction, or look for matching DIDs whose name begins with /FULL/ rather than /RECO/.

Conclusion and Comments

That wraps up our introduction to using Rucio and some example use cases and scenarios.

New tags may be added in the future. We’re welcome to take on board any suggestions or changes as we roll out Rucio and it becomes more widely used. Get in touch via stephen.kay@york.ac.uk

or on the ePIC software-tutorials Mattermost channel with suggestions, comments and feedback.

Remember to consider whether you need full datasets before downloading them and keep an eye on whether your files have multiple open access copies when making file lists.

Also, if you find any nice tricks or develop short scripts (maybe one which makes a file list for the latest version of a dataset based upon inputs?) then feel free to share them too!

Key Points
  • Files from datasets can be directly streamed in analysis scripts
  • Files (or whole datasets) can be downloaded locally, but this is usually not needed