Using ROOT

To access the data and analyze it, you will need to utilize a program run with ROOT.  ROOT is a C++-based programming language developed at CERN specifically for the purpose of storing particle collider data.  ROOT files contain "trees" of data, which are split into various "branches," which in turn contain "leaves."  The leaves are arrays of data points.  Let's open up a tree in ROOT to explore its contents.

• First open one of the ROOT files by typing root jpmm0d_stripped.root.  This command starts up ROOT with the file "jpmm0d_stripped.root" attached and ready to be used.
   
• Now open a browser window by typing:
              TBrowser*  t = new TBrowser;
  Click on ROOT Files->jpmm0d_stripped.root->prod to explore the contents of the file. In this root file, there are three types of objects: trees, branches and leaves. A tree is is essentially a master container, it holds all the branches which in turn hold the leaves and any sub-branches. The leaves are the objects which contain the actual data. The structure of the file is that the event data is written into the "prod" tree, which is divided into branches and leaves to store different pieces of the data. Double clicking on any of the leaves (eg MOM_ntk, MOM_pt) will produce a histogram of the entries- don't worry if it is slow to load. Opening up the branches (eg. COSMICRES) will let you see what leaves they contain.
  
  
  While you can use this method to view any ROOT files you create, be aware that you will need to refresh the browser to see any changes you have made to the data (i.e. by editing and rerunning your program). It can be easier to view the file data by drawing histograms using ROOT's interactive commands, as shown below.

• We'll create a histogram of some parameter, like the muon transverse momentum (pt). You can do this quickly with this simple command, which specifies the tree to be accessed ("prod"- note that this only works because the file containing prod has already been opened) and the leaf to be drawn (MOM_pt, the muon transverse momentum):
            prod->Draw("MOM_pt");
  ROOT will automatically put the histogram inside the browser window, but you can create a separate "canvas" in which to draw the histogram.  These can be handy if you want to view multiple histograms at the same time.  You can create as many canvases as you want using this command:
            TCanvas* c1 = new TCanvas("c1","The c1 Canvas");
  Just make sure that the canvas pointers (c1 in this case) are all different!  When you enter a Draw command, the histogram will be drawn on whichever canvas is selected as the active one- the active canvas has a yellow highlighted border and you can switch to a different active canvas by selecting it with the scroll wheel of your mouse.

• You will notice that the histogram created by your Draw command isn't very pretty- a few outlying large pt values cause the majority of values to be pushed down into the zero bin.  This can be fixed by adding a cut to the data that is drawn:
            prod->Draw("MOM_pt","MOM_pt<5");