Dear experts,
I do unbinned fit of the dimuon mass in the interval 1250 GeV, not including a signal region
2530 GeV using polynomial function. The goal is to have background model from the sideband data.
Sometime I got a messages like:
p.d.f value is less than zero (0.389207), forcing value to zero @ x=dimuonmass2sideband=48.3537, coefList=(a0 = 3.82232,a1 = 0.139657,a2 = 0.00124115)
p.d.f value is less than zero (0.939635), forcing value to zero @ x=dimuonmass2sideband=48.9306, coefList=(a0 = 3.82232,a1 = 0.139657,a2 = 0.00124115)
but at the end fit converges with the messages
MIGRAD MINIMIZATION HAS CONVERGED.
MIGRAD WILL VERIFY CONVERGENCE AND ERROR MATRIX.
COVARIANCE MATRIX CALCULATED SUCCESSFULLY
FCN=246.245 FROM MIGRAD STATUS=CONVERGED 163 CALLS 164 TOTAL
EDM=8.7068e05 STRATEGY= 1 ERROR MATRIX ACCURATE
…
** 9 **HESSE 2000
COVARIANCE MATRIX CALCULATED SUCCESSFULLY
FCN=246.245 FROM HESSE STATUS=OK 23 CALLS 187 TOTAL
EDM=9.43288e05 STRATEGY= 1 ERROR MATRIX ACCURATE
and my printout at the end:
Number of data events = 233
log(L) at minimum = 246.245
chi2 = 0.679778
 RooRealVar:: a0 = 3.85915 +/ 0.455439
 RooRealVar:: a1 = 0.139186 +/ 0.0142921
 RooRealVar:: a2 = 0.0012522 +/ 0.000196147
 RooRealVar:: norm = 306.049 +/ 78.7228
norm = 306.049 +/ 78.7228
I got background fitted function which describes the sideband data well:
SR2mod13_2017UL_FTEST.pdf (21.0 KB)
My questions are:

should I worry then about a such messages reporting a negative pdfs during minimization ? It does not lead to the failure of the fit and minimization gives me finally a reasonable function positively defined in my mass range.

what RooFit is doing once at some step it is getting to a negative pdfs ?
Thanks, Sasha.