Low hit multiplicity in first layer from ACTS test framework generic Fatras example (was ACTS-315)

Original author Steven Andrew Farrell @sfarrell

Hello,

I'm using the generic Fatras example in the ACTS test framework to write hits files, but in the output I'm seeing strangely low hit multiplicity in the first pixel layer.

Setup: lxplus, LCG_88, gcc62, custom ROOT build with Pythia8 enabled. ACTS version: 3c303b6d https://gitlab.cern.ch/acts/a-common-tracking-sw/tree/3c303b6d37ea7fd3cef5da9bbb3a206c5420de63

I had to customize a little bit the test fw code to fix compilation issues and to add the event, volume, layer, module IDs to the output text file. I also dropped the cell information: sfarrell/acts-test-fw@master...heptrkx

Now, if I analyze some 10-event output, I identify that the first pixel layers are volume ID 8 with layer IDs 2, 4, 6, 8. I count the number of hits on each of these layers: {noformat} Volume Layer Hits 8 2 16751 8 4 90004 8 6 71995 8 8 64904 {noformat} I.e., the first layer has less than 1/5 as many hits as the 2nd layer. I'm attaching a snippet of code to show how I calculate the above. I'll also attach the slides I sent to the ACTS developers that showcases this issue plus the other quirks I've seen.

I'm probably forgetting some important details so let me know if some more information will help.

Thanks, Steve