View source for HOWTO create a ROOT tree of detector hits and display event data with the tree

==Getting started==

The data in an evio file can be coverted into a ROOT tree by using the '''janaroot''' plugin.   Suppose you want to look at the CDC hits in more detail.  To do so, invoke the JANA framework using the following command line:



<font size="+1">hd_ana -PPLUGINS=janaroot -PAUTOACTIVATE=DCDCHit  file.evio</font>



The arguments are:
;'''-PPLUGINS=janaroot'''
:attach the ''janaroot'' plugin

;'''-PAUTOACTIVATE=DCDCHit'''
:tell the ''hd_ana'' program to make ''DCDCHit'' causing the ''janaroot'' plugin to put them in the output ROOT file. ( additional objects can be added after ''DCDCHit'', separated by commas. ''e.g. -PAUTOACTIVATE=DCDCHit,DFDCHit,DBCALHit'' )

;'''file.evio'''
:input file. for raw data, this might be something like ''hd_rawdata_004747_002.evio''


The '''janaroot''' plugin fills a ROOT tree for each type of object created and writes it out to a file called janaroot.root in the current working directory.

To look at the janaroot.root file, run root (''root janaroot.root'') and at the root command line invoke the browser (type ''TBrowser T'';  this will pop up another window).
In the browser window, under ROOT files select janaroot.root, which gives you a list of branches on the tree.  In the example below I have selected the distribution of the number of hits ''N'' in the CDC:

[[File:TBrowserView.png]]

In addition to the DCDCHit data, all the raw event data types (Df250PulseIntegral, etc.) are available for perusal.

==Looking at correlations==

A leaf quantity can be plotted against another leaf quantity in a given branch using the root command line interface.  For example, to look at time in the CDC as function of charge for all straws, type
<blockquote>
DCDCHit->Draw("t:q");
</blockquote>
[[File:TBrowserView_t_vs_q.png]]

You can apply cuts to the plotted distribution by using the second argument of the '''Draw''' method.  For example, to restrict the plot above to single straw in a single ring, type
<blockquote>
DCDCHit->Draw("t:q","ring==12&&straw==10","col");
</blockquote>
In this example I used the third (optional) argument of '''Draw''' to use the color drawing method.

[[File:TBrowserView_t_vs_q_with_cut.png]]

To look at correlations between variables in different branches of the tree, use the special ''event'' branch.  For example, to plot FDC TDC times vs times reported by the FADC, type
<blockquote>
event->Draw("DFDCWireDigiHit.time:DFDCCathodeDigiHit.pulse_time","DFDCCathodeDigiHit.pulse_time>0.","col");
</blockquote>
[[File:TBrowserView_event_example.png]]

Note that you must explicitly reference each branch you are using in the '''Draw''' arguments.