CMS Physics Objects

Overview

Teaching: 10 min
Exercises: min

Questions

• What are the main CMS Physics Objects and how do I access them?

Objectives

• Identify CMS physics objects

• Identify object code collections in AOD files

• Access an object code collection

CMS uses the phrase “physics objects” to speak broadly about particles that can be identified via signals from the CMS detector. A physics object typically began as a single particle, but may have showered into many particles as it traversed the detector. The principle physics objects are:

• muons
• electrons
• taus
• photons
• jets
• missing transverse momentum

Viewing modules in a data file

CMS AOD files store all physics objects of the same type in “modules”. These modules are data structures that act as containers for multiple instances of the same C++ class. The edmDumpEventContent command show below lists all the modules in a file. As an example, the “muons” module in AOD contains many reco::Muon objects (one per muon in the event).

$ edmDumpEventContent root://eospublic.cern.ch//eos/opendata/cms/MonteCarlo2012/Summer12_DR53X/TTbar_8TeV-Madspin_aMCatNLO-herwig/AODSIM/PU_S10_START53_V19-v2/00000/000A9D3F-CE4C-E311-84F8-001E673969D2.root

Type                                  Module                      Label             Process
----------------------------------------------------------------------------------------------
...
vector<reco::Muon>                    "muons"                     ""                "RECO"
vector<reco::Muon>                    "muonsFromCosmics"          ""                "RECO"
vector<reco::Muon>                    "muonsFromCosmics1Leg"      ""                "RECO"
...
vector<reco::Track>                   "cosmicMuons"               ""                "RECO"
vector<reco::Track>                   "cosmicMuons1Leg"           ""                "RECO"
vector<reco::Track>                   "globalCosmicMuons"         ""                "RECO"
vector<reco::Track>                   "globalCosmicMuons1Leg"     ""                "RECO"
vector<reco::Track>                   "globalMuons"               ""                "RECO"
vector<reco::Track>                   "refittedStandAloneMuons"   ""                "RECO"
vector<reco::Track>                   "standAloneMuons"           ""                "RECO"
vector<reco::Track>                   "refittedStandAloneMuons"   "UpdatedAtVtx"    "RECO"
vector<reco::Track>                   "standAloneMuons"           "UpdatedAtVtx"    "RECO"
vector<reco::Track>                   "tevMuons"                  "default"         "RECO"
vector<reco::Track>                   "tevMuons"                  "dyt"             "RECO"
vector<reco::Track>                   "tevMuons"                  "firstHit"        "RECO"
vector<reco::Track>                   "tevMuons"                  "picky"           "RECO"

Note that this file also contains many other muon-related modules: two modules of reco::Muon muons from cosmic-ray events, and many modules of reco::Track objects that give lower-level tracking information for muons. As you can see, the AOD file contains MANY modules, and not all of them are related directly to physics objects. Other important modules might include:

• Tracks and vertices
• Calorimeter clusters and other detector information
• Particle Flow candidates
• Triggers
• Pileup (info about multiple collisions from the same pp crossing)
• Generator-level information (in simulation)

Opening a module

Setup

The AOD2NanoAODOutreachTool repository is the example we will use for accessing information from AOD files. If you have not already done so, please check out the “dummyworkshop” branch of this repository:

$ cd ~/CMSSW_5_3_32/src/
$ cmsenv
$ mkdir workspace
$ cd workspace
$ git clone -b dummyworkshop git://github.com/jmhogan/AOD2NanoAODOutreachTool.git 
$ cd AOD2NanoAODOutreachTool
$ scram b
$ vi src/AOD2NanoAOD.cc #(or your favorite text editor)

In the source code for this tool, the definitions of the muon classes are included:

#include "DataFormats/MuonReco/interface/Muon.h"
#include "DataFormats/MuonReco/interface/MuonFwd.h"
#include "DataFormats/MuonReco/interface/MuonSelectors.h"

You learned about the EDAnalyzer class in the pre-exercises. The AOD2NanoAOD tool is an EDAnalyzer. The “analyze” function of an EDAnalyzer is performed once per event. Muons can be accessed like this:

void AOD2NanoAOD::analyze(const edm::Event &iEvent,
                          const edm::EventSetup &iSetup) {

  using namespace edm;
  using namespace reco;
  using namespace std;

  Handle<MuonCollection> muons;
  iEvent.getByLabel(InputTag("muons"), muons);

The result is an called “muons” which is a collection of all the muon objects. In the next episode we’ll look at the member functions for muons. Collection classes are generally constructed as std::vectors. We can quickly access the number of muons per event and create a loop to access individual muons:

int nMuons = muons->size();
for (auto it = muons->begin(); it != muons->end(); it++) {
    if (it->pt() > mu_min_pt) {
        // do things here, next episode!
    }
}

Key Points

• CMS physics objects include: muons, electrons, taus, photons, and jets.

• Missing transverse momentum is derived from physics objects (negative vector sum).

• Objects are stored in separate collections in the AOD files

• Objects can be accessed one-by-one via a for loop