nanoAOD-tools

Tools for working with NanoAOD (requiring only python + root, not CMSSW)

Checkout instructions: standalone

You need to setup python 2.7 and a recent ROOT version first.

git clone https://github.com/cms-nanoAOD/nanoAOD-tools.git NanoAODTools
cd NanoAODTools
bash standalone/env_standalone.sh build
source standalone/env_standalone.sh

Repeat only the last command at the beginning of every session.

Please never commit neither the build directory, nor the empty init.py files created by the script.

Checkout instructions: CMSSW

cd $CMSSW_BASE/src
git clone https://github.com/cms-nanoAOD/nanoAOD-tools.git PhysicsTools/NanoAODTools

General instructions to run the post-processing step

The script to run the post-processing step is scripts/nano_postproc.py.

The basic syntax of the command is the following:

python scripts/nano_postproc.py /path/to/output_directory /path/to/input_tree.root

Here is a summary of its features:

• the -s,--postfix option is used to specify the suffix that will be appended to the input file name to obtain the output file name. It defaults to _Friend in friend mode, _Skim in full mode.
• the -c,--cut option is used to pass a string expression (using the same syntax as in TTree::Draw) that will be used to select events. It cannot be used in friend mode.
• the -J,--json option is used to pass the name of a JSON file that will be used to select events. It cannot be used in friend mode.
• if run with the --full option (default), the output will be a full nanoAOD file. If run with the --friend option, instead, the output will be a friend tree that can be attached to the input tree. In the latter case, it is not possible to apply any kind of event selection, as the number of entries in the parent and friend tree must be the same.
• the -b,--branch-selection option is used to pass the name of a file containing directives to keep or drop branches from the output tree. The file should contain one directive among keep/drop (wildcards allowed as in TTree::SetBranchStatus) or keepmatch/dropmatch (python regexp matching the branch name) per line, as shown in the this example file.
• the --justcount option will cause the script to printout the number of selected events, without actually writing the output file.

Please run with --help for a complete list of options.

How to write and run modules

It is possible to import modules that will be run on each entry passing the event selection, and can be used to calculate new variables that will be included in the output tree (both in friend and full mode) or to apply event filter decisions.

We will use python/postprocessing/examples/mhtProducer.py as an example.

The module definition file, containing the constructor:

mht = lambda : mhtProducer( lambda j : j.pt > 40, 
                            lambda mu : mu.pt > 20 and mu.miniPFIso_all/mu.pt < 0.2,
                            lambda el : el.pt > 20 and el.miniPFIso_all/el.pt < 0.2 ) 

should be imported using the following syntax:

python scripts/nano_postproc.py -I PhysicsTools.NanoAODTools.postprocessing.examples.mhtProducer mht

Let us now examine the structure of the mhtProducer module class. All modules must inherit from PhysicsTools.NanoAODTools.postprocessing.framework.eventloop.Module.

• the __init__ constructor function should be used to set the module options.
• the beginFile function should create the branches that you want to add to the output file, calling the branch(branchname, typecode, lenVar) method of wrappedOutputTree. typecode should be the ROOT TBranch type ("F" for float, "I" for int etc.). lenVar should be the name of the variable holding the length of array branches (for instance, branch("Electron_myNewVar","F","nElectron")). If the lenVar branch does not exist already - it can happen if you create a new collection, see an example here) - it will be automatically created.
• the analyze function is called on each event. It should return True if the event is to be retained, False if it should be dropped.

The event interface, defined in PhysicsTools.NanoAODTools.postprocessing.framework.datamodule, allows to dynamically construct views of objects organized in collections, based on the branch names, for instance:

electrons = Collection(event, "Electron")
if len(electrons)>1: print electrons[0].someVar+electrons[1].someVar
electrons_highpt = filter(lambda x: x.pt>50, electrons)

and this will access the elements of the Electron_someVar, Electron_pt branch arrays. Event variables can be accessed simply by event.someVar, for instance event.rho.

The output branches should be filled calling the fillBranch(branchname, value) method of wrappedOutputTree. value should be the desired value for single-value branches, an iterable with the correct length for array branches. It is not necessary to fill the lenVar branch explicitly, as this is done automatically using the length of the passed iterable.