Wrong decay volume starting position for DP production

[amagnan]

The dark photon decay vertex should be placed within the decay volume, it seems it is using incorrect value for both the end of the balloon and for the start, using target z0 which is the start of the target ??

FairShip/macro/run_simScript.py

Lines 349 to 350 in ba0b073

	P8gen.SetLmin((ship_geo.Chamber1.z - ship_geo.chambers.Tub1length) - ship_geo.target.z0 )
	P8gen.SetLmax(ship_geo.TrackStation1.z - ship_geo.target.z0 )

@olantwin How can we best set them in the current FairShip definitions, such that it would not rely too much on what is put in front: maybe the end position using the SST first layer - decay volume length ?
Or maybe I misunderstand those params, but that is how they are used in the DPPythia8Generator class:

FairShip/shipgen/DPPythia8Generator.cxx

Line 260 in 4814bbf

Double_t LS = gRandom->Uniform(fLmin, fLmax); // mm, G4 and Pythia8 units

So should be start and end of vessel, simply...

[olantwin]

ship_geo.decayVolume.z0 should be the start of the decay volume.

[amagnan]

ok great, but do you know how it ended up being set at those values ? And I guess it should also be the same def for HNL ?

[olantwin]

When we were changing the coordinate system, we basically had to redefine all of these variables. Probably the definition of the chamber-variables slightly changes.

Why those were chosen, who knows. These variables are terribly named, there are no or few comments and no documentation to those lines. Maybe git blame can help.

[amagnan]

OK, thanks ! We will fix it but in another PR and once @constanzaval will have tested everything properly :)

[olantwin]

Related: #1179