Understanding large track parameter values

FCCAnalysis
1

Dear experts! Me and my analysis group are trying to reconstruct secondary vertices with this algorithm: FCCAnalyses/analyzers/dataframe/src/VertexFinderLCFIPlus.cc at d0abc8d76e37630ea157f9d5c48e7867a86be2e2 · HEP-FCC/FCCAnalyses · GitHub . While debugging some vertices that are reconstructed at extremely large radii (>1.5m), we observed some strange tracks that we would like help to understand. For two example tracks, the track parameters obtained from VertexingUtils::get_trackParam have the following values:

Track1: (d0, phi0, omega, z0, tanLambda) = (-590.709, 2.04753, -0.000197501, -268.031, -0.380197)
Track2: (d0, phi0, omega, z0, tanLambda) = (169.184, 1.23833, 5.05816e-05, 29.8117, -0.489485)

We think that the d0 and z0 values (in mm) are really big for the process which we are running (e+e- → Z(ee) + H(bb) @ sqrt(s) = 240 GeV), and would like to ask if you have any idea why the values are so big?

2

Dear Axel,

Thanks for contacting us! Are you running on a Delphes sample or a full sim sample? Can you do a matching to MCParticles to see if you have any particle in the MC record that could lead to such tracks?

Cheers,
Brieuc

3

Dear Brieuc,

The sample we’re running on is the winter2023 delphes sample for IDEA. Regarding the matching, could you elaborate a bit (possibly with example code)?

I assume that one would use something like get_EventPrimaryVertex or possibly get_list_of_particles_from_decay, but how do I get the ROOT::VecOps::RVec<edm4hep::MCParticleData> object on track level (i.e. in e.g. VertexFinderLCFIPlus.cc?

Thanks,
Axel

4

Dear Axel,

I do not see any helper methods in FCCanalyses to print the vertex origin of MCParticles (FCCAnalyses users may comment further). But, for this kind of debugging where you want to dissect a pathological event, I would be tempted to simply run something like:

import ROOT
from podio import root_io
input_file_path = "path_to_your_file_with_pathological_event"
podio_reader = root_io.Reader(input_file_path)
for event in podio_reader.get("events"):
  for mc_particle in event.get("Particle"):
    if mc_particle.getCharge() != 0:
      mc_particle_vertex = mc_particle.getVertex()
      print(f"x: {mc_particle_vertex.x}, y: {mc_particle_vertex.y}, z: {mc_particle_vertex.z}")

(let me know if you want the C++ equivalent), and see if you have a charged particle with a displacement compatible with what you have from VertexingUtils. To develop further the above example, the following Doxygen will be helpful: EDM4hep: edm4hep::MCParticle Class Reference .

If you do not see a MC particle compatible with the displacement, further debugging of VertexingUtils might be needed.

Hoping this helps,
Brieuc