ROOT Exercise Session C

This is the guide for the ROOT Course exercise session C. The exercises cover the following areas:

Session C is an example of an analysis using real physics data.

ROOT is a C++ framework; it is not only written in C++, it also uses C++ as the scripting language. To use ROOT effectively you will need to know the basics of C++. The User's Guide chapter 6 "A Little C++", explains the basic concepts.

In addition, there is a "C++ Guide for ROOT Users" at:

http://www-root.fnal.gov/root/CPlusPlus/index.html

To install ROOT on your own machine see Appendix A in the ROOT User's Guide, or the ROOT website:

http://root.cern.ch/root/Availability.html

Make sure you correctly set the ROOTSYS, PATH and LD_LIBRARY_PATH environment variables.

The files needed for the exercises can be found either in $ROOTSYS/tutorials or in ftp://root.cern.ch/ftp/exercises.tar.gz. 

 

 

In this session you will:

·         Study an example analysis from DESY

·         Learn about the TTree::MakeSelector method to automatically create a class that loops over each entry in a tree.

·         Save and retrieve a canvas to and from a ROOT file

·         Compute the integral of a histogram

·         Compute the integral of a function within a range

The data are taken from the DESY H1 micro-DSTs. Copy the file dstarmb.root from: ftp://root.cern.ch/root/h1analysis/ to your $HOME/tutorials directory

The example data file is in your directory $H1/dstarmb.root.

Example Script from TTree::MakeSelector

First, you need to understand how the TTree::MakeSelector method creates a class that loops over the entries in the TTree. Please read the Chapter 12 "Example Analysis" in the User's Guide "Example Analysis" pg.218.

Change directory to $HOME/tutorials, the directory you copied in the last session. Now open the script $HOME/tutorials/h1analysis.C and read the comments.

The script shows how to use a class (the skeleton has been automatically generated by the TTree::MakeSelector method) to loop on the files, perform some cuts and fill two histograms with some physics quantities.

     Start a root session and open the file $H1/dstarmb.root.

Using the TBrowser, display some of the variables in the TTree named h42.

Execute the script h1analysis.C on the h42 tree.

 

Saving and Retrieving a Canvas

Save the canvas with the dm_d histogram as a ROOT file ("c1.root") with the File:Save as canvas.root menu option. Start a new session. Read the saved ROOT file.

 

Computing Integrals

Get a pointer to the Dstar histogram named “hdmd”. 

Get a pointer to the fitted function “f5”.

Compute hint = the histogram bin integral. 

Compute fint = the f5 integral where the function is positive (0.139 – 0.17).

Use the TH1 and TF1 class description web page to find the right method to calculate the integral for each object.

http://root.cern.ch/root/html/TF1.html

 

Question C1:     What is the value of ratio = fint/hint?

Extras

Question C2:     Fit the histogram hfs from the file Fits.root with a sinus function and draw the fit parameters (final look)

Question C3:     Fit the histogram fsomme from the file Fits.root with the sum of a 2nd degree polynomial, of a gaussian and of an exponential. Draw the result of fitting with the sum as well as each of the indivial functions. (final look)

Question C4:     Fit the histogram fSignalFond from the file Fits.root with the sum of a background and a signal Draw the histogram with the global fit and the 2 individual functions and with a legend for each of the functions. (final look)

 Background(x) = ( Ax / (1 + exp[ (x-B) / C ] ) 

 Signal(x) = if x <= x0 then A*exp ( -0.5 * ( (x-x0)/sigG )^2 )
             if x >  x0 then A*exp ( -0.5 * ( (x-x0)/sigD )^2 )