Notebook

df104_HiggsToTwoPhotons¶

The Higgs to two photons analysis from the ATLAS Open Data 2020 release, with RDataFrame.

This tutorial is the Higgs to two photons analysis from the ATLAS Open Data release in 2020 (http://opendata.atlas.cern/release/2020/documentation/). The data was taken with the ATLAS detector during 2016 at a center-of-mass energy of 13 TeV. Although the Higgs to two photons decay is very rare, the contribution of the Higgs can be seen as a narrow peak around 125 GeV because of the excellent reconstruction and identification efficiency of photons at the ATLAS experiment.

The analysis is translated to a RDataFrame workflow processing 1.7 GB of simulated events and data.

Author: Stefan Wunsch (KIT, CERN)
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Tuesday, May 27, 2025 at 04:10 PM.

In [1]:

import ROOT
import os

Enable multi-threading

In [2]:

ROOT.ROOT.EnableImplicitMT()

Create a ROOT dataframe for each dataset

In [3]:

path = "root://eospublic.cern.ch//eos/opendata/atlas/OutreachDatasets/2020-01-22"
df = {}
df["data"] = ROOT.RDataFrame("mini", (os.path.join(path, "GamGam/Data/data_{}.GamGam.root".format(x)) for x in ("A", "B", "C", "D")))
df["ggH"] = ROOT.RDataFrame("mini", os.path.join(path, "GamGam/MC/mc_343981.ggH125_gamgam.GamGam.root"))
df["VBF"] = ROOT.RDataFrame("mini", os.path.join(path, "GamGam/MC/mc_345041.VBFH125_gamgam.GamGam.root"))
processes = list(df.keys())

Apply scale factors and MC weight for simulated events and a weight of 1 for the data

In [4]:

for p in ["ggH", "VBF"]:
    df[p] = df[p].Define("weight",
            "scaleFactor_PHOTON * scaleFactor_PhotonTRIGGER * scaleFactor_PILEUP * mcWeight");
df["data"] = df["data"].Define("weight", "1.0")

Select the events for the analysis

In [5]:

for p in processes:
    # Apply preselection cut on photon trigger
    df[p] = df[p].Filter("trigP")

    # Find two good muons with tight ID, pt > 25 GeV and not in the transition region between barrel and encap
    df[p] = df[p].Define("goodphotons", "photon_isTightID && (photon_pt > 25000) && (abs(photon_eta) < 2.37) && ((abs(photon_eta) < 1.37) || (abs(photon_eta) > 1.52))")\
                 .Filter("Sum(goodphotons) == 2")

    # Take only isolated photons
    df[p] = df[p].Filter("Sum(photon_ptcone30[goodphotons] / photon_pt[goodphotons] < 0.065) == 2")\
                 .Filter("Sum(photon_etcone20[goodphotons] / photon_pt[goodphotons] < 0.065) == 2")

Compile a function to compute the invariant mass of the diphoton system

In [6]:

ROOT.gInterpreter.Declare(
"""
using namespace ROOT;
float ComputeInvariantMass(RVecF pt, RVecF eta, RVecF phi, RVecF e) {
    ROOT::Math::PtEtaPhiEVector p1(pt[0], eta[0], phi[0], e[0]);
    ROOT::Math::PtEtaPhiEVector p2(pt[1], eta[1], phi[1], e[1]);
    return (p1 + p2).mass() / 1000.0;
}
""")

Out[6]:

True

Define a new column with the invariant mass and perform final event selection

In [7]:

hists = {}
for p in processes:
    # Make four vectors and compute invariant mass
    df[p] = df[p].Define("m_yy", "ComputeInvariantMass(photon_pt[goodphotons], photon_eta[goodphotons], photon_phi[goodphotons], photon_E[goodphotons])")

    # Make additional kinematic cuts and select mass window
    df[p] = df[p].Filter("photon_pt[goodphotons][0] / 1000.0 / m_yy > 0.35")\
                 .Filter("photon_pt[goodphotons][1] / 1000.0 / m_yy > 0.25")\
                 .Filter("m_yy > 105 && m_yy < 160")

    # Book histogram of the invariant mass with this selection
    hists[p] = df[p].Histo1D(
            ROOT.RDF.TH1DModel(p, "Diphoton invariant mass; m_{#gamma#gamma} [GeV];Events", 30, 105, 160),
            "m_yy", "weight")

Run the event loop

RunGraphs allows to run the event loops of the separate RDataFrame graphs concurrently. This results in an improved usage of the available resources if each separate RDataFrame can not utilize all available resources, e.g., because not enough data is available.

In [8]:

ROOT.RDF.RunGraphs([hists[s] for s in ["ggH", "VBF", "data"]])

ggh = hists["ggH"].GetValue()
vbf = hists["VBF"].GetValue()
data = hists["data"].GetValue()

Create the plot

Set styles

In [9]:

ROOT.gROOT.SetStyle("ATLAS")

Create canvas with pads for main plot and data/MC ratio

In [10]:

c = ROOT.TCanvas("c", "", 700, 750)

upper_pad = ROOT.TPad("upper_pad", "", 0, 0.35, 1, 1)
lower_pad = ROOT.TPad("lower_pad", "", 0, 0, 1, 0.35)
for p in [upper_pad, lower_pad]:
    p.SetLeftMargin(0.14)
    p.SetRightMargin(0.05)
    p.SetTickx(False)
    p.SetTicky(False)
upper_pad.SetBottomMargin(0)
lower_pad.SetTopMargin(0)
lower_pad.SetBottomMargin(0.3)

upper_pad.Draw()
lower_pad.Draw()

Fit signal + background model to data

In [11]:

fit = ROOT.TF1("fit", "([0]+[1]*x+[2]*x^2+[3]*x^3)+[4]*exp(-0.5*((x-[5])/[6])^2)", 105, 160)
fit.FixParameter(5, 125.0)
fit.FixParameter(4, 119.1)
fit.FixParameter(6, 2.39)
fit.SetLineColor(2)
fit.SetLineStyle(ROOT.kSolid)
fit.SetLineWidth(2)
data.Fit("fit", "0", "", 105, 160)

Out[11]:

<cppyy.gbl.TFitResultPtr object at 0x559fa950b280>

****************************************
Minimizer is Minuit2 / Migrad
Chi2                      =      19.9699
NDf                       =           26
Edm                       =  1.42782e-10
NCalls                    =          161
p0                        =        94325   +/-   72.0526     
p1                        =     -1777.22   +/-   0.778155    
p2                        =      11.5606   +/-   0.00536059  
p3                        =   -0.0256281   +/-   2.66824e-05 
p4                        =        119.1                      	 (fixed)
p5                        =          125                      	 (fixed)
p6                        =         2.39                      	 (fixed)

Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S
Error in <TFitResultPtr>: TFitResult is empty - use the fit option S

Draw data

In [12]:

upper_pad.cd()
data.SetMarkerStyle(20)
data.SetMarkerSize(1.2)
data.SetLineWidth(2)
data.SetLineColor("black")
data.SetMinimum(1e-3)
data.SetMaximum(8e3)
data.GetYaxis().SetLabelSize(0.045)
data.GetYaxis().SetTitleSize(0.05)
data.SetStats(0)
data.SetTitle("")
data.Draw("E")

Draw fit

In [13]:

fit.Draw("SAME")

Draw background

In [14]:

bkg = ROOT.TF1("bkg", "([0]+[1]*x+[2]*x^2+[3]*x^3)", 105, 160)
for i in range(4):
    bkg.SetParameter(i, fit.GetParameter(i))
bkg.SetLineColor(4)
bkg.SetLineStyle(ROOT.kDashed)
bkg.SetLineWidth(2)
bkg.Draw("SAME")

Scale simulated events with luminosity * cross-section / sum of weights and merge to single Higgs signal

In [15]:

lumi = 10064.0
ggh.Scale(lumi * 0.102 / 55922617.6297)
vbf.Scale(lumi * 0.008518764 / 3441426.13711)
higgs = ggh.Clone()
higgs.Add(vbf)
higgs.Draw("HIST SAME")

Draw ratio

In [16]:

lower_pad.cd()

ratiobkg = ROOT.TH1I("zero", "", 100, 105, 160)
ratiobkg.SetLineColor(4)
ratiobkg.SetLineStyle(ROOT.kDashed)
ratiobkg.SetLineWidth(2)
ratiobkg.SetMinimum(-125)
ratiobkg.SetMaximum(250)
ratiobkg.GetXaxis().SetLabelSize(0.08)
ratiobkg.GetXaxis().SetTitleSize(0.12)
ratiobkg.GetXaxis().SetTitleOffset(1.0)
ratiobkg.GetYaxis().SetLabelSize(0.08)
ratiobkg.GetYaxis().SetTitleSize(0.09)
ratiobkg.GetYaxis().SetTitle("Data - Bkg.")
ratiobkg.GetYaxis().CenterTitle()
ratiobkg.GetYaxis().SetTitleOffset(0.7)
ratiobkg.GetYaxis().SetNdivisions(503, False)
ratiobkg.GetYaxis().ChangeLabel(-1, -1, 0)
ratiobkg.GetXaxis().SetTitle("m_{#gamma#gamma} [GeV]")
ratiobkg.Draw("AXIS")

ratiosig = ROOT.TH1F("ratiosig", "ratiosig", 5500, 105, 160)
ratiosig.Eval(fit)
ratiosig.SetLineColor(2)
ratiosig.SetLineStyle(ROOT.kSolid)
ratiosig.SetLineWidth(2)
ratiosig.Add(bkg, -1)
ratiosig.Draw("SAME")

ratiodata = data.Clone()
ratiodata.Add(bkg, -1)
for i in range(1, data.GetNbinsX()):
    ratiodata.SetBinError(i, data.GetBinError(i))
ratiodata.Draw("E SAME")

Add legend

In [17]:

upper_pad.cd()
legend = ROOT.TLegend(0.55, 0.55, 0.89, 0.85)
legend.SetTextFont(42)
legend.SetFillStyle(0)
legend.SetBorderSize(0)
legend.SetTextSize(0.05)
legend.SetTextAlign(32)
legend.AddEntry(data, "Data" ,"lep")
legend.AddEntry(bkg, "Background", "l")
legend.AddEntry(fit, "Signal + Bkg.", "l")
legend.AddEntry(higgs, "Signal", "l")
legend.Draw()

Add ATLAS label

In [18]:

text = ROOT.TLatex()
text.SetNDC()
text.SetTextFont(72)
text.SetTextSize(0.05)
text.DrawLatex(0.18, 0.84, "ATLAS")
text.SetTextFont(42)
text.DrawLatex(0.18 + 0.13, 0.84, "Open Data")
text.SetTextSize(0.04)
text.DrawLatex(0.18, 0.78, "#sqrt{s} = 13 TeV, 10 fb^{-1}")

Out[18]:

<cppyy.gbl.TLatex object at 0x559f971f5520>

Save the plot

In [19]:

c.SaveAs("df104_HiggsToTwoPhotons.png")
print("Saved figure to df104_HiggsToTwoPhotons.png")

Saved figure to df104_HiggsToTwoPhotons.png

Info in <TCanvas::Print>: png file df104_HiggsToTwoPhotons.png has been created

Draw all canvases

In [20]:

from ROOT import gROOT 
gROOT.GetListOfCanvases().Draw()