Detector scattering of muons

[GilesStrong]

Currently muons are only scattered by material in the passive volume. @MaximeLagrange has investigated the degree to which the detectors themselves can scatter muons (see attached slides). In general, only a small fraction of muons undergo a significant scattering, and that these are normally muons with a high angle of incidence; and as such are likely to miss the bulk of the detectors.

It was also suggested that scattering between detector layers could:

• spoil inference, since now the reco hit positions are a function of the resolution and the scattering, so we can no longer assume a straight-line fit.
• Be used to help infer momentum, using e.g. the chi2 of the fit.

Adding the muon scattering in the detectors is doable, however accounting for the scattering in the trajectory fitting would be complex. Computing the chi2 etc. of the fit is also doable, and may be useful as an input to the X0 GNN.

Given the relatively low effect, I don't think that we need to prioritise this at the moment, and can point to Maxime's plots in a publication to justify this, if the scattering is not included by then.
TomOpt_detectorScattering_8_12_21.pdf