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Hi, Norman and I spoke a bit about which G4 version should be used for HPS. Since the discussion awhile back about the single scattering tail distribution being wrong in G4 version > 9.3, we have changed our working method about how events are input to SLIC. Now, Takashi is simulating this distrubution using EGS and then these generated events are put into SLIC using StdHep files. I don't even think that our test beam simulations have a target geometry component in them, as it is physically upstream of the simulated "world" volume. As I understand it (from Norman), multiple scattering will occur in the tracker and other geometry components, rather than single scattering. Though I would assume some single scattering will be invoked by the physics engine, this is not going to influence the primary e+e- events generated from target interactions, which are input externally. So the effect should be small/negligible. (Can we confirm this somehow?) Consequently, my advice is that we should move HPS SimDist builds to the cvs head, which uses, by default, Geant4 9.5.1 rather than 9.3. Doing this will have the benefit that you do not need to try and compile SimDist using an older G4 version, which is likely to fail anyways. Because I stopped trying to make it backward compatible due to their API changes between releases (the second version number). This way we will also get any bug fixes, bells and whistles, etc. in the newer versions rather than being stuck on an old one. It is true that if we use Geant4 to simulate target interactions, we will get the wrong tail in the scattering distribution. If we want to do this, then, in the long term, we may want to create our own HPS physics list that modifies the parameters for the distribution. Takashi sent me an email about how to do this. It is not at all difficult (e.g. a few lines of code changed in a G4 EM model file), though I am not yet sure whether these parameter changes could be done through an API or if we would need to copy/paste the Geant4 file and then modify it. This bears further investigation. Anyone have anything to add to this, perhaps something I've missed about how we currently do things? --Jeremy ######################################################################## Use REPLY-ALL to reply to list To unsubscribe from the HPS-SOFTWARE list, click the following link: https://listserv.slac.stanford.edu/cgi-bin/wa?SUBED1=HPS-SOFTWARE&A=1