An introductional notebook to HEP analysis in C++

In this notebook you can find an easy set of commands that show some basic computing techniques commonly used in High Energy Physics (HEP) analyzes.

It also shows how to create an histogram, fill it and draw it. Moreover it is an introduction to ROOT too. The final output is a plot with the number of leptons.

Check the description of the varibles inside the dataset at the end of this notebook

All done with less that 20 lines of code!

An introduction to the ATLAS public datasets

This is a notebook using the ROOT Prompt kernel that using c++ language, is intended to show the internal content and the way to call and interact with the datasets released by the ATLAS experiment with focus in Education and Training activities:

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We use data recorded (simulated) by the ATLAS detector (experiment)

where physics objects can be represented as below

Cell #1

At first we have to include several helpers that will support our analysis:



In [1]:

    
#include <iostream>
#include <string>
#include <stdio.h>

Cell #2

In order to activate the interactive visualisation of the histogram that is later created we can use the JSROOT magic:



In [2]:

    
%jsroot on

Cell #3

Next we have to open the data that we want to analyze. It is stored in a _*.root_ file that consists of a tree having branches and leaves.

As you can see , we are reading the data directly from the source! but you can read the file locally too.



In [3]:

    
TFile *file = TFile::Open("http://opendata.atlas.cern/release/samples/MC/mc_147770.Zee.root");

Cell #4

The next step is to define a tree named (we called _tree_) to get the data out of the _*.root_ file, that is into a tree called _mini_:



In [4]:

    
TTree *tree = (TTree*) file->Get("mini");

Cell #5

Now we extract the varibles we want from the dataset. As mentioned before, we will plot the number of leptons:

(Note to know more about the content of the ATLAS Open Data datasets, please go to our documentation)



In [5]:

    
UInt_t lepton_n = -1;
tree->SetBranchAddress("lep_n", &lepton_n);

Cell #6

After the data is opened we create a canvas on which we can draw a histogram. If we do not have a canvas we cannot see our histogram at the end. Its name is _Canvas_ and its header is _a first way to plot a variable_. The two following arguments define the width and the height of the canvas:



In [6]:

    
TCanvas *canvas = new TCanvas("Canvas","a first way to plot a variable",800,600);

Cell #7

Now we define a histogram that will later be placed on this canvas. Its name is _variable_ and the header of the histogram is _Example plot: Number of leptons_. The three following arguments indicate that this histogram contains 4 so called bins which have a range from 0 to 4.



In [7]:

    
TH1F *hist = new TH1F("variable","Example plot: Number of leptons",4,0,4);

Cell #8

The following lines are a loop that goes over the data that is stored in the tree and fills the histogram _h_ that we already defined. In this first notebook we don't do any cuts to keep it simple. Accordingly the loop fills the histogram for each event stored in the tree. After the program has looped over all the data it prints the word

Done!.



In [8]:

    
int nentries, nbytes, i;
nentries = (Int_t)tree->GetEntries();

for (i = 0; i < nentries; i++)
{
    nbytes = tree->GetEntry(i);
    hist->Fill(lepton_n);
}

std::cout << "Done!" << std::endl;









    



Done!

Cell #9

After filling the histogram we want to see the results of the analysis. First we draw the histogram on the canvas and then the canvas on which the histogram lies:



In [9]:

    
hist->Draw();
canvas->Draw();

Cell #10

Description of the Variables inside the _mini_ tree in the ATLAS Open Data samples

# variable	branchname	type	description
01	runNumber	int	runNumber
02	eventNumber	int	eventNumber
03	channelNumber	int	channelNumber
04	lbNumber	int	lbNumber
05	rndRunNumber	int	randomized run number mimicking run number distribution in data
06	mu	float	average interactions per bunch crossing
07	mcWeight	float	weight of an MC event
08	pvxp_n	int	number of primary vertices
09	isGoodEvent	int	summary of diverse quality flags like hfor
10	scaleFactor	float	overall scale factor for the preselected event
11	trigE	bool	boolean whether a standard trigger has fired in the egamma stream
12	trigM	bool	boolean whether a standard trigger has fired in the muon stream
13	passGRL	bool	signifies whether event passes the GRL may be put in isGoodEvent
14	hasGoodVertex	bool	signifies whether the event has at least one good vertex
15	lep_n	int	number of preselected leptons
16	lep_truthMatched	vector	boolean indicating whether the lepton is matched to a truth lepton
17	lep_trigMatched	vector	boolean signifying whether the lepton is the one triggering the event
18	lep_pt	vector	transverse momentum of the lepton
19	lep_eta	vector	pseudo-rapidity of the lepton
20	lep_phi	vector	azimuthal angle of the lepton
21	lep_E	vector	energy of the lepton
22	lep_z0	vector	z-coordinate of the track associated to the lepton wrt. the primary vertex
23	lep_charge	vector	charge of the lepton
24	lep_isTight	vector	boolean indicating whether the lepton is of tight quality
25	lep_flag	vector	bitmask implementing object cuts of the top group
26	lep_type	vector	number signifying the lepton type (e, mu, tau) of the lepton
27	lep_ptcone30	vector	ptcone30 isolation for the lepton
28	lep_etcone20	vector	etcone20 isolation for the lepton
28	lep_trackd0pvunbiased	vector	d0 of the track associated to the lepton at the point of closest approach (p.o.a.)
29	lep_tracksigd0pvunbiased	vector	d0 signifcance of the track associated to the lepton at the p.o.a.
30	met_et	float	Transverse energy of the missing momentum vector
31	met_phi	float	Azimuthal angle of the missing momentum vector
32	jet_n	int	number of selected jets
33	jet_pt	vector	transverse momentum of the jet
34	jet_eta	vector	pseudorapidity of the jet
35	jet_phi	vector	azimuthal angle of the jet
36	jet_E	vector	energy of the jet
37	jet_m	vector	invariant mass of the jet
38	jet_jvf	vector	JetVertexFraction of the jet
39	jet_flag	vector	bitmask implementing object cuts of the top group
40	jet_trueflav	vector	true flavor of the jet
41	jet_truthMatched	vector	information whether the jet matches a jet on truth level
42	jet_SV0	vector	SV0 weight of the jet
43	jet_MV1	vector	MV1 weight of the jet
44	scaleFactor_BTAG	float	scalefactor for btagging
45	scaleFactor_ELE	float	scalefactor for electron efficiency
46	scaleFactor_JVFSF	float	scalefactor for jet vertex fraction
47	scaleFactor_MUON	float	scalefactor for muon efficiency
48	scaleFactor_PILEUP	float	scalefactor for pileup reweighting
49	scaleFactor_TRIGGER	float	scalefactor for trigger
50	scaleFactor_ZVERTEX	float	scalefactor for z-vertex reweighting