doc/v620/rf604__constraints_8py.html

from __future__ import print_function

import ROOT


# Create model and dataset

# ----------------------------------------------


# Construct a Gaussian p.d.f

x = ROOT.RooRealVar("x", "x", -10, 10)


m = ROOT.RooRealVar("m", "m", 0, -10, 10)

s = ROOT.RooRealVar("s", "s", 2, 0.1, 10)

gauss = ROOT.RooGaussian("gauss", "gauss(x,m,s)", x, m, s)


# Construct a flat p.d.f (polynomial of 0th order)

poly = ROOT.RooPolynomial("poly", "poly(x)", x)


# model = f*gauss + (1-f)*poly

f = ROOT.RooRealVar("f", "f", 0.5, 0., 1.)

model = ROOT.RooAddPdf(

    "model",

    "model",

    ROOT.RooArgList(

        gauss,

        poly),

    ROOT.RooArgList(f))


# Generate small dataset for use in fitting below

d = model.generate(ROOT.RooArgSet(x), 50)


# Create constraint pdf

# -----------------------------------------


# Construct Gaussian constraint p.d.f on parameter f at 0.8 with

# resolution of 0.1

fconstraint = ROOT.RooGaussian(

    "fconstraint",

    "fconstraint",

    f,

    ROOT.RooFit.RooConst(0.8),

    ROOT.RooFit.RooConst(0.1))


# Method 1 - add internal constraint to model

# -------------------------------------------------------------------------------------


# Multiply constraint term with regular p.d.f using ROOT.RooProdPdf

# Specify in fitTo() that internal constraints on parameter f should be

# used


# Multiply constraint with p.d.f

modelc = ROOT.RooProdPdf(

    "modelc", "model with constraint", ROOT.RooArgList(model, fconstraint))


# Fit model (without use of constraint term)

r1 = model.fitTo(d, ROOT.RooFit.Save())


# Fit modelc with constraint term on parameter f

r2 = modelc.fitTo(

    d,

    ROOT.RooFit.Constrain(

        ROOT.RooArgSet(f)),

    ROOT.RooFit.Save())


# Method 2 - specify external constraint when fitting

# ------------------------------------------------------------------------------------------


# Construct another Gaussian constraint p.d.f on parameter f at 0.8 with

# resolution of 0.1

fconstext = ROOT.RooGaussian("fconstext", "fconstext", f, ROOT.RooFit.RooConst(

    0.2), ROOT.RooFit.RooConst(0.1))


# Fit with external constraint

r3 = model.fitTo(d, ROOT.RooFit.ExternalConstraints(

    ROOT.RooArgSet(fconstext)), ROOT.RooFit.Save())


# Print the fit results

print("fit result without constraint (data generated at f=0.5)")

r1.Print("v")

print("fit result with internal constraint (data generated at f=0.5, is f=0.8+/-0.2)")

r2.Print("v")

print("fit result with (another) external constraint (data generated at f=0.5, is f=0.2+/-0.1)")

r3.Print("v")