Ok, that looks correct.
So in the end the question is why the negative tails. Well, these are needed:
- The s plot is meant to “subtract” the background from signal by assigning negative weights to the background.
- This can be used on an independent distribution (i.e. not the mass) to subtract the background.
- This, however, means that in the mass distributions, the background regions need to be negative. Otherwise, the subtraction doesn’t work.
- In the signal region of the mass distribution, you should therefore see more than the bare number of signal events, because only the sum of the signal and the negative background should yield the initial number of signal events.
From all I can see, the method works correctly.