Dear all

I am using a RooRealSumPdf with custom “fractions” definition and i enforce via RooFormulaVars that the sum of component fractions is always 1.

Doing so i can achieve a morphing of the shape scaling with negative and positive factors one of the sub-pdf of a RooRealSumPdf ensuring it still normalize to 1.

I.e

```
PDF_1 = frac1 * PDF1_1 + (1-frac1) * PDF1_2 + ( 1- frac1 -frac2) * PDF1_3
```

, and my morphing is achieved applying

```
frac1 --> ScaleFactor * frac1
```

and when i do this i propagate

```
frac2 --> ( 1- ScaleFactor * frac1 ) * frac2Original
frac3 --> ( 1- newFrac1 - newFrac2 )
```

Now this PDF morphed (`PDF_1 --> PDF_1_Scaled`

(and another one where the same is done (`PDF_1_Scaled`

), like this is used inside a `RooAddPdf`

with fixed fractions assignments and passing n-1 coefficiencies :

```
FULL MODEL = RooAddPdf( fracFull1[fix] , fracFull2[fix] , PDF_1_Scaled , PDF_2_Scaled, OtherPDF)
```

My question is if the scale factor goes negative for some reason in the scaled PDF morphed shapes,

Is this an issue when `FULL_MODEL.Generate`

is executed?

( The question is a continuation of what i proposed as solution in RooSumPdf and RecursiveFraction with a scale factor on one of sub-components - #5 by jblomer ) with the difference that i encountered some bias in the `Scale`

factor parameter when running toys and using RooAddPdf, since sometimes that parameter converged to 0 , the boundary. When that was left to go negative, but still using RooAddPdf i instead observed other weird behaviours.

So in practice, the use case i am doing of a negative yield is to achieve a morphing of the shape rather than actually fit its amount. Is this something safe one can achieve with RooRealSumPdf ?

Cheers

Renato

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