Example of analysis class for the H1 data.
This file uses 4 large data sets from the H1 collaboration at DESY Hamburg. One can access these data sets (277 MBytes) from the standard Root web site at: ftp:/// root.cern.ch/root/h1analysis
The Physics plots below generated by this example cannot be produced when using smaller data sets.
There are several ways to analyze data stored in a Root Tree
- Using TTree::Draw: This is very convenient and efficient for small tasks. A TTree::Draw call produces one histogram at the time. The histogram is automatically generated. The selection expression may be specified in the command line.
- Using the TTreeViewer: This is a graphical interface to TTree::Draw with the same functionality.
- Using the code generated by TTree::MakeClass: In this case, the user creates an instance of the analysis class. They have the control over the event loop and he can generate an unlimited number of histograms.
- Using the code generated by TTree::MakeSelector. Like for the code generated by TTree::MakeClass, the user can do complex analysis. However, they cannot control the event loop. The event loop is controlled by TTree::Process called by the user. This solution is illustrated by the current code. The advantage of this method is that it can be run in a parallel environment using PROOF (the Parallel Root Facility).
A chain of 4 files (originally converted from PAW ntuples) is used to illustrate the various ways to loop on Root data sets. Each data set contains a Root Tree named "h42" The class definition in h1analysis.h has been generated automatically by the Root utility TTree::MakeSelector using one of the files with the following statement:
h42->MakeSelector("h1analysis");
This produces two files: h1analysis.h and h1analysis.C (skeleton of this file) The h1analysis class is derived from the Root class TSelector.
The following members functions are called by the TTree::Process functions.
- Begin(): Called every time a loop on the tree starts. A convenient place to create your histograms.
- Notify(): This function is called at the first entry of a new Tree in a chain.
- Process(): Called to analyze each entry.
- Terminate(): Called at the end of a loop on a TTree. A convenient place to draw/fit your histograms.
To use this file, try the following sessions
Case A: Create a TChain with the 4 H1 data files
The chain can be created by executed the short macro h1chain.C below:
{
chain.
Add(
"$H1/dstarmb.root");
chain.
Add(
"$H1/dstarp1a.root");
chain.
Add(
"$H1/dstarp1b.root");
chain.
Add(
"$H1/dstarp2.root");
}
### Case B: Loop on all events
Case C: Same as B, but in addition fill the entry list with selected entries.
The entry list is saved to a file "elist.root" by the Terminate function. To see the list of selected events, you can do elist->Print("all")
. The selection function has selected 7525 events out of the 283813 events in the chain of files. (2.65 per cent)
Case D: Process only entries in the entry list
The entry list is read from the file in elist.root generated by step C
Case E: The above steps have been executed via the interpreter.
You can repeat the steps B, C and D using the script compiler by replacing "h1analysis.C" by "h1analysis.C+" or "h1analysis.C++" in a new session (see F).
### Case F: Create the chain as in A, then execute
The same analysis can be run on PROOF. For a quick try start a PROOF-Lite session
create (if not already done) the chain by executing the 'h1chain.C' macro mentioned above, and then tell ROOT to use PROOF to process the chain:
You can then repeat step B above. Step C can also be executed in PROOF. However, step D cannot be executed in PROOF as in the local session (i.e. just passing option 'useList'): to use the entry list you have to
### Case G: Load first in the session the list form the file
set it on the chain:
call Process as in step B. Of course this works also for local processing.
{
if (x <= 0.13957) return 0;
+ par[2] / 2.5066/par[4]*
TMath::Exp(-xp3/2/par[4]/par[4]));
return res;
}
{
if (x <= 0.13957) return 0;
+ par[1] / 2.5066/sigma*
TMath::Exp(-xp3/2/sigma/sigma));
return res;
}
{
Info(
"Begin",
"starting h1analysis with process option: %s", option.
Data());
delete gDirectory->GetList()->FindObject(
"elist");
elist =
new TEntryList(
"elist",
"H1 selection from Cut");
elist = 0;
}
Info(
"Begin",
"creating an entry-list");
}
} else {
}
}
}
{
"starting h1analysis with process option: %s (tree: %p)", option.
Data(),
tree);
hdmd =
new TH1F(
"hdmd",
"dm_d",40,0.13,0.17);
h2 =
new TH2F(
"h2",
"ptD0 vs dm_d",30,0.135,0.165,30,-3,6);
if (elist)
else
}
}
}
{
if (!useList) {
if (nhitrp[ik]*nhitrp[ipi] <= 1)
return kFALSE;
if (rend[ik] -rstart[ik] <= 22)
return kFALSE;
if (rend[ipi]-rstart[ipi] <= 22)
return kFALSE;
}
}
{
}
{
if (hdmd == 0 || h2 == 0) {
Error(
"Terminate",
"hdmd = %p , h2 = %p", hdmd, h2);
return;
}
hdmd->GetXaxis()->SetTitle("m_{K#pi#pi} - m_{K#pi}[GeV/c^{2}]");
hdmd->GetXaxis()->SetTitleOffset(1.4);
if (
gROOT->GetListOfFunctions()->FindObject(
"f5"))
delete gROOT->GetFunction(
"f5");
hdmd->Fit("f5","lr");
if (
gROOT->GetListOfFunctions()->FindObject(
"f2"))
delete gROOT->GetFunction(
"f2");
Info(
"Fit Slices",
"Restricting fit to two bins only in this example...");
h2->FitSlicesX(f2,10,20,10,"g5 l");
if (!elist)
if (elist) {
Printf(
"Entry list 'elist' created:");
TFile efile(
"elist.root",
"recreate");
} else {
Error(
"Terminate",
"entry list requested but not found in output");
}
}
}
- Author
- Rene Brun
Definition in file h1analysis.C.