Simulation from Scoring Plane Hits

Available in ldmx-sw v4.8.2 and later.

Two-stage simulation is helpful when two parts of the simulation are physically separate enough that the physical effects are approximately one direction. For example, the tracking, target, and trigger scintillator ("tracking") region compared to the ecal and hcal ("calorimeter") region. These two regions are separated into two distinct areas along the beam line and (excluding the lower rate and lower energy "backsplash"), particles pass through the upstream region and then pass through the downstream calorimeter region. Another example would be if we wanted to simulate some beamline components upstream of our detector apparatus.

Since these two stages are physically separate, we can simulate them separately and get physically-reasonable events (even if the events we simulate are not identical to if we simulated the two regions together at once). Below, I walk through an example of doing a two-stage simulation where we first simulate the tracking region alone and then simulate the calorimeter region. We separate out the tracking region because, since it has less mass in the detector, it doesn't cause the beam particle to shower and so the simluation has less particles to deal with and therefore completes much faster. In this way, we can simulate many more events in the tracking region and then filter out events from that sample that are particularly troublesome that we want to study in more detail with the calorimeter simulation as well.

Stage One: "simtrk"

SimCore/exampleConfigs/stage-one-no-cal.py

In this stage, we do a normal simulation but we omit the detector volumes and sensitive detectors (SDs) that are not within the region we care about. The tracking region already has a GDML detector model constructed that is equivalent to the full v15 detector but without the two calorimeters, so we can just use that model.

For example,

from LDMX.SimCore import simulator, generators, sensitive_detectors
sim = simulator.Simulator('sim')
sim.set_detector('ldmx-det-v15-8gev-no-cals')
sim.generators = [
    generators.single_8gev_e_upstream_tagger()
]
sim.description = 'single electron 8gev beam, no calorimeters, no biasing or filtering'
# manually add the sensitive detectors that we want
sim.sensitive_detectors = [
    sensitive_detectors.TrackerSD.tagger(),
    sensitive_detectors.TrackerSD.recoil(),
    sensitive_detectors.TrigScintSD.target(),
    sensitive_detectors.TrigScintSD.pad1(),
    sensitive_detectors.TrigScintSD.pad2(),
    sensitive_detectors.TrigScintSD.pad3(),
    sensitive_detectors.ScoringPlaneSD.tracker(),
    sensitive_detectors.ScoringPlaneSD.target(),
    sensitive_detectors.ScoringPlaneSD.ecal()
]

We want to remove all of the SDs that pertain to subsystems we are not including (e.g. EcalSD and HcalSD are omitted), but we do want to include the scoring planes for the area from which we want to start stage two (in this case, ScoringPlaneSD.ecal() contains one "plane" at the front of the calorimeter region, ScoringPlaneSD.hcal() has a similar one which is larger in the transverse dimensions if you need to capture wider scatters).

Stage Two: "simcal"

SimCore/exampleConfigs/stage-two-cal.py

The FromScoringPlane generator requires an input file to be given to the process so that (a) the prior pass's information is copied into the output file for analysis and (b) so this generator can access the prior pass's scoring plane hits to start the second stage of simulation.

You should also make sure the pass name is different for the stage two run so that both sets of simulation collections (e.g. SimParticles) are able to be stored in the output event file.

p = ldmxcfg.Process("simcal")
p.input_files = ["output-file-from-stage-one.root"]

And then the simulation can use the special generator that uses hits from the input file to create primary particles.

# sim is a normal simulator.Simulator
from LDMX.SimCore.generators import FromScoringPlane
sim.generators = [ FromScoringPlane.ecal() ]

You can do a similar thing as stage one where you tune the sim.sensitive_detectors to only the subsystems that are of-interest in this stage. Technically, that is not necessary but it would be helpful to avoid downstream confusion during analysis.

Analysis Comments

The file output by "Stage Two" of the simulation is complicated since it was constructed in an abnormal way.

• Be Careful with Pass Names: Remember that both stages could have contributed a collection with the same name, so make sure the two stages have different pass names and you choose the correct pass name when analyzing the resulting file. For example, I know that the RecoilSimHits collection from the simcal pass will only be back-splash hits while the RecoilSimHits collection from the simtrk pass will be near-normal.
• Neglecting Backspash: While we expect backsplash from the calorimeters confusing the recoil tracker to be a small/neglible effect, this two-stage simulation completely omits this possibility. In some sense, the tracking in the recoil detector is therefore a bit "too easy" compared to real events.
• Re-Sampling: Since the fate of particles is inherently probabilistic, you may want to simulate the same scoring-plane events many times to try to understand the distribution of possible signals in the calorimeters. This can be achieved by using the same input file and simply using a different run number in stage two so that the random number generator is seeded differently.