Walkthrough: Applying a cut
(10 minutes)
A final analysis concept
The last “trick” you need to learn is how to apply a cut in an analysis macro. Once you’ve absorbed this, you’ll know enough about ROOT to start using it for a real physics analysis.
The simplest way to apply a cut is to use the if
statement. This is
described in every introductory Python text, and I won’t go into detail
here. Instead I’ll provide an example to get you started.
Once again, let’s start with a fresh Analyze script:
%load Analyze.py
Our goal is to count the number of events for which pz
is less than
145 GeV. Since we’re going to count the events, we’re going to need
a counter. Put the following in the Set-up section:
pzCount = 0
For every event that passes the cut, we want to add one to the count. Put the following in the Loop section:
if ( pz < 145 ):
pzCount = pzCount + 1
Be careful
Remember that indentation is important. The next statement
after pzCount=pzCount+1
must not be indented the same amount, or it
will be considered part of the if
statement.
Now we have to display the value. Include the following statement in your Wrap-up section:
print ("The number of events with pz < 145 is", pzCount)
My results
When I run this macro, I get the following output:
The number of events with pz < 145 is 14962
Hopefully you’ll get the same answer.