How to make a simulation including MCH

Only MCH

Like all O2 simulations, it's a two stage process : first create the hits and then digitize them.

Hit creation

 o2-sim -n 10 -g fwmugen -e TGeant3 -m MCH

will generate 10 events using the fwmugen generator (1 forward muon per event), using Geant3 as transport, and with only MCH geometry.

 CAUTION: by specifying -m MCH you only get MCH geometry, no absorber, no dipole, no beam shield, etc... So that's only a relevant option for quick debug, not for any kind of physics analysis.

This steps creates (among others) an o2sim_HitsMCH.root file with the MCH hits, as well as o2sim_geometry.root and o2sim_grp.root which are used later on by the digitizer.

Digitization

In the same directory where the hits were created, use:

 o2-sim-digitizer-workflow

This will creates mchdigits.root, updates o2sim_grp.root and generates a collisioncontext.root. This last file can be used to redo the digitization for the very same collisions, but with e.g. different digitization parameters.

Various options can be specified using the --configKeyValues. For example for the MCH digitizer:

 o2-sim-digitizer-workflow --configKeyValues "MCHDigitizer.noiseOnlyProba=1.e-6;MCHDigitizer.timeSigma=6."

For the list of possible parameters see the DigitizerParam and ResponseParam structs.

More realistic simulations

For anything but debug, you should include more modules at the hit creation stage:

 o2-sim -m HALL MAG DIPO COMP PIPE ABSO SHIL MCH

Of course, add other detectors (MID,MFT,ITS,...) if need be.

Note that there is a way to simply include the geometry of a detector, just to have its material budget in the picture, without also having it participating to the hit creation, see PR: https://github.com/AliceO2Group/AliceO2/pull/8149 for the options.