An event generator using histograms as input

[WenjieWu-Sci]

I'm trying to write an event generator using histograms as input. The goal is to use the background histograms PR 25 to generate the primary vertex. There are two backgrounds available: muons and neutrons.

• Direction: we don't have direction information. We'll probably just set them to the +z direction.
• Position: we can use TH2::GetRandom2 to draw (x, y) from the fluence histogram, and set z at the entrance of the cavern. If we're setting the direction to be +z, then we can limit the (x, y) area to no more than the cross-section of the detector or cavern.
• Energy: we can use TH1::GetRandom to draw the energy from the flux histograms based on its x position.

@mvicenzi Does this make sense to you? I was looking at the flux histograms you generated in the background folder. It looks like the ones with (200 cm < x < 300 cm) are missing for both mu_minus and mu_plus. The same with the neutron flux. Could you help to take a look if that's also true for you?

[mvicenzi]

• We should have some information on the direction: the direction cosines of the particles... maybe we could turn them into 1D histograms as well. Looking at the distributions (from here: /dune/data/users/mvicenzi/FPF_backgrounds/muons/flux_v2/) they seem peaked at 0, so particles are mostly towards +z, but we should apply the normalization to see if it remains true. Generally speaking, it might be interesting to replicate the correct directions to see how many muons interact in the rock around FLArE (and if they produce secondary backgrounds).
• I fully agree with your plan on how to set the position and energy. I think it's the most reasonable thing to do with what we have. We could also do a 2D binning for the energy, i.e. creating a 1D flux histograms for each (x,y) position, but I don't know if we would gain something or not.
• I checked those missing histograms... I think it's an issue with kGray which is not properly displayed. This explains why it happens on the same x-bin in all files. It happened to me in the past, but I never figured out why... I remember once moving a window from one screen to another would make it disappear, so it might be screen-dependent. What happens if you set a different line color?