Hi, thanks a lot for reporting this interesting problem!

This problem seems to be specific to putting a `RooProdPdf`

into a `RooFFTConvPdf`

. We are investigating this problem right now and hope to find a real fix. If you need a quick and dirty solution in the meantime, you can just add zero to the product such that it’s class is `RooRealSumPdf`

, which works in the convolution. Here is what you need to do for that:

```
# calculate product of RooBreitWigner and RooLognormal
# give it a temporary name because we will turn it into a RooRealSumPdf in the next step
modifedbw_tmp = ROOT.RooProdPdf("modifedbw_tmp" + name + str(width), "modifedbw_tmp", ROOT.RooArgList(bw, ln))
# Using a RooProdPdf directly in RooFFTConvPdf gives wrong results.
# But passing a RooRealSumPdf to RooFFTConvPdf works, so we turn the RooProdPdf into a RooRealSumPdf
# by adding a dummy pdf with coefficient zero (the second zero coefficient will be inferred from
# the coefficient of the first pdf, which is one)
dummy_for_sum_pdf = ROOT.RooUniform("dummy_for_sum_pdf", "dummy_for_sum_pdf", x)
unity_for_sum_pdf = ROOT.RooRealVar("unity_for_sum_pdf", "unity_for_sum_pdf", 1)
modifedbw = ROOT.RooRealSumPdf("modifedbw" + name + str(width), "modifedbw", ROOT.RooArgList(modifedbw_tmp, dummy_for_sum_pdf),
ROOT.RooArgList(unity_for_sum_pdf))
```

The complete script with the change is here:

test_add_zero.py (3.3 KB)