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analysis: add initial draft for 2015 photons (#101)
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@katilp
katilp authored May 6, 2022
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Expand Up @@ -6,102 +6,166 @@ Photons are measured in the CMS experiment in the [electromagnetic calorimeter](

## Photon 4-vector information

An example of an EDAnalyzer accessing photon information is available in the [PhotonAnalyzer](https://github.com/cms-opendata-analyses/PhysObjectExtractorTool/blob/2012/PhysObjectExtractor/src/PhotonAnalyzer.cc) of the Physics Object Extractor Tool (POET). The following header files needed for accessing electron information are included:
=== "Run 1 Data"

``` cpp
//classes to extract Photon information
#include "DataFormats/EgammaCandidates/interface/Photon.h"
#include "DataFormats/EgammaCandidates/interface/PhotonFwd.h"
#include "DataFormats/GsfTrackReco/interface/GsfTrack.h"
#include "DataFormats/EgammaCandidates/interface/GsfElectron.h"
#include "RecoEgamma/EgammaTools/interface/ConversionTools.h"
#include "EgammaAnalysis/ElectronTools/interface/PFIsolationEstimator.h"
```
An example of an EDAnalyzer accessing photon information is available in the [PhotonAnalyzer](https://github.com/cms-opendata-analyses/PhysObjectExtractorTool/blob/2012/PhysObjectExtractor/src/PhotonAnalyzer.cc) of the Physics Object Extractor Tool (POET). The following header files needed for accessing electron information are included:

In [PhotonAnalyzer.cc](https://github.com/cms-opendata-analyses/PhysObjectExtractorTool/blob/2012/PhysObjectExtractor/src/PhotonAnalyzer.cc), the photon four-vector elements are accessed as shown below.
``` cpp
//classes to extract Photon information
#include "DataFormats/EgammaCandidates/interface/Photon.h"
#include "DataFormats/EgammaCandidates/interface/PhotonFwd.h"
#include "DataFormats/GsfTrackReco/interface/GsfTrack.h"
#include "DataFormats/EgammaCandidates/interface/GsfElectron.h"
#include "RecoEgamma/EgammaTools/interface/ConversionTools.h"
#include "EgammaAnalysis/ElectronTools/interface/PFIsolationEstimator.h"
```

``` cpp
Handle<reco::PhotonCollection> myphotons;
iEvent.getByLabel(photonInput, myphotons);
In [PhotonAnalyzer.cc](https://github.com/cms-opendata-analyses/PhysObjectExtractorTool/blob/2012/PhysObjectExtractor/src/PhotonAnalyzer.cc), the photon four-vector elements are accessed as shown below.

[...]
``` cpp
Handle<reco::PhotonCollection> myphotons;
iEvent.getByLabel(photonInput, myphotons);

for (reco::PhotonCollection::const_iterator itphoton=myphotons->begin(); itphoton!=myphotons->end(); ++itphoton){
[...]

[...]
for (reco::PhotonCollection::const_iterator itphoton=myphotons->begin(); itphoton!=myphotons->end(); ++itphoton){

photon_e.push_back(itphoton->energy());
photon_pt.push_back(itphoton->pt());
photon_px.push_back(itphoton->px());
photon_py.push_back(itphoton->py());
photon_pz.push_back(itphoton->pz());
photon_eta.push_back(itphoton->eta());
photon_phi.push_back(itphoton->phi());
[...]

[...]
}
```
photon_e.push_back(itphoton->energy());
photon_pt.push_back(itphoton->pt());
photon_px.push_back(itphoton->px());
photon_py.push_back(itphoton->py());
photon_pz.push_back(itphoton->pz());
photon_eta.push_back(itphoton->eta());
photon_phi.push_back(itphoton->phi());

[...]
}
```

=== "Run 2 Data"

An example of an EDAnalyzer accessing electron information is available in the [PhotonAnalyzer](https://github.com/cms-opendata-analyses/PhysObjectExtractorTool/blob/2015MiniAOD/PhysObjectExtractor/src/PhotonAnalyzer.cc) of the Physics Object Extractor Tool (POET). The following header file needed for accessing photon information is included:

``` cpp
//class to extract photon information
#include "DataFormats/PatCandidates/interface/Photon.h"
```

In [PhotonAnalyzer.cc](https://github.com/cms-opendata-analyses/PhysObjectExtractorTool/blob/2015MiniAOD/PhysObjectExtractor/src/PhotonAnalyzer.cc), the electron four-vector elements are accessed from the `pat::photon` collection as shown below.

``` cpp
Handle<pat::PhotonCollection> photons;
iEvent.getByToken(photonToken_, photons);

[...]

for (const pat::Photon &pho : *photons)
{
photon_e.push_back(pho.energy());
photon_pt.push_back(pho.pt());
photon_px.push_back(pho.px());
photon_py.push_back(pho.py());
photon_pz.push_back(pho.pz());
photon_eta.push_back(pho.eta());
photon_phi.push_back(pho.phi());

[...]
}
```

with `photonToken_` defined as a member of the `PhotonAnalyzer` class and its value read from the [configuration file](https://github.com/cms-opendata-analyses/PhysObjectExtractorTool/blob/2015MiniAOD/PhysObjectExtractor/python/poet_cfg.py).

## Photon identification

As explained in the [Physics Object page](../objects#detector-information-for-identification), a mandatory task in the physics analysis is to identify photons, i.e. to separate “real” objects from “fakes”. A large fraction of the energy deposited in the detector by all proton-proton interactions arises from photons originating in the decay of neutral mesons, and these electromagnetic showers provide a substantial background to signal photons. The identification criteria depend on the type of analysis.

The standard identification and isolation algorithm results can be accessed from the [photon object class](https://cmsdoxygen.web.cern.ch/cmsdoxygen/CMSSW_5_3_30/doc/html/d5/d35/classreco_1_1Photon.html) and the recommended working points for 2012 are implemented in the example code [PhotonAnalyzer.cc](https://github.com/cms-opendata-analyses/PhysObjectExtractorTool/blob/2012/PhysObjectExtractor/src/PhotonAnalyzer.cc).
=== "Run 1 Data"

The standard identification and isolation algorithm results can be accessed from the [photon object class](https://cmsdoxygen.web.cern.ch/cmsdoxygen/CMSSW_5_3_30/doc/html/d5/d35/classreco_1_1Photon.html) and the recommended working points for 2012 are implemented in the example code [PhotonAnalyzer.cc](https://github.com/cms-opendata-analyses/PhysObjectExtractorTool/blob/2012/PhysObjectExtractor/src/PhotonAnalyzer.cc).

Three levels of identification criteria are defined
Three levels of identification criteria are defined

``` cpp
bool isLoose = false, isMedium = false, isTight = false;
```
``` cpp
bool isLoose = false, isMedium = false, isTight = false;
```

For photons in the electromagnetic calorimeter barrel area, they are determined as follows:
For photons in the electromagnetic calorimeter barrel area, they are determined as follows:

``` cpp
if ( itphoton->eta() <= 1.479 ){
if ( ph_hOverEm<.05 && ph_sigIetaIeta<.012 &&
corrPFCHIso<2.6 && corrPFNHIso<(3.5+.04*itphoton->pt()) &&
corrPFPhIso<(1.3+.005*itphoton->pt()) && passelectronveto==true) {
isLoose = true;
``` cpp
if ( itphoton->eta() <= 1.479 ){
if ( ph_hOverEm<.05 && ph_sigIetaIeta<.012 &&
corrPFCHIso<2.6 && corrPFNHIso<(3.5+.04*itphoton->pt()) &&
corrPFPhIso<(1.3+.005*itphoton->pt()) && passelectronveto==true) {
isLoose = true;

if ( ph_sigIetaIeta<.011 && corrPFCHIso<1.5
&& corrPFNHIso<(1.0+.04*itphoton->pt())
&& corrPFPhIso<(.7+.005*itphoton->pt())){
isMedium = true;
if ( ph_sigIetaIeta<.011 && corrPFCHIso<1.5
&& corrPFNHIso<(1.0+.04*itphoton->pt())
&& corrPFPhIso<(.7+.005*itphoton->pt())){
isMedium = true;

if ( corrPFCHIso<.7 && corrPFNHIso<(.4+.04*itphoton->pt())
&& corrPFPhIso<(.5+0.005*itphoton->pt()) ){
isTight = true;
if ( corrPFCHIso<.7 && corrPFNHIso<(.4+.04*itphoton->pt())
&& corrPFPhIso<(.5+0.005*itphoton->pt()) ){
isTight = true;
}
}
}
}
}
}
```

where

- `ph_sigIetaIeta` describes the variance of the ECAL cluster in psuedorapidity ("ieta" is an integer index for this angle).
- `ph_hOverEm` describes the ratio of HCAL to ECAL energy deposits, which should be small for good quality photons.
- The electron veto `passelectronveto` is obtained from an algorithm that indicates if photons have been identified also as electrons.
- `corr...Iso` variables represent different isolation properties of the photon.

The isolation variables are defined with the `PFIsolationEstimator` class in the default cone size of 0.3 with

``` cpp
PFIsolationEstimator isolator;
isolator.initializePhotonIsolation(kTRUE);
isolator. setConeSize(0.3);
const reco::VertexRef vertex(vertices, 0);
const reco::Photon &thephoton = *itphoton;
isolator.fGetIsolation(&thephoton, pfCands.product(), vertex, vertices);
double corrPFCHIso =
std::max(isolator.getIsolationCharged() - rhoIso * aEff.CH_AEff, 0.)/itphoton->pt();
double corrPFNHIso =
std::max(isolator.getIsolationNeutral() - rhoIso * aEff.NH_AEff, 0.)/itphoton->pt();
double corrPFPhIso =
std::max(isolator.getIsolationPhoton() - rhoIso * aEff.Ph_AEff, 0.)/itphoton->pt();
```
In the endcap part of the electromagnetic calorimeter, the procedure is similar with different values.
```

where

- `ph_sigIetaIeta` describes the variance of the ECAL cluster in psuedorapidity ("ieta" is an integer index for this angle).
- `ph_hOverEm` describes the ratio of HCAL to ECAL energy deposits, which should be small for good quality photons.
- The electron veto `passelectronveto` is obtained from an algorithm that indicates if photons have been identified also as electrons.
- `corr...Iso` variables represent different isolation properties of the photon.

The isolation variables are defined with the `PFIsolationEstimator` class in the default cone size of 0.3 with

``` cpp
PFIsolationEstimator isolator;
isolator.initializePhotonIsolation(kTRUE);
isolator. setConeSize(0.3);
const reco::VertexRef vertex(vertices, 0);
const reco::Photon &thephoton = *itphoton;
isolator.fGetIsolation(&thephoton, pfCands.product(), vertex, vertices);
double corrPFCHIso =
std::max(isolator.getIsolationCharged() - rhoIso * aEff.CH_AEff, 0.)/itphoton->pt();
double corrPFNHIso =
std::max(isolator.getIsolationNeutral() - rhoIso * aEff.NH_AEff, 0.)/itphoton->pt();
double corrPFPhIso =
std::max(isolator.getIsolationPhoton() - rhoIso * aEff.Ph_AEff, 0.)/itphoton->pt();
```

In the endcap part of the electromagnetic calorimeter, the procedure is similar with different values.

!!! Note "To do"
- The isolation snippet needs more explanation

=== "Run 2 Data"

The standard identification and isolation algorithm results can be accessed from the [pat photon object class](https://cmsdoxygen.web.cern.ch/cmsdoxygen/CMSSW_7_6_7/doc/html/d4/d47/classpat_1_1Photon.html). Three levels of identification criteria are defined: loose, medium, and tight. An example selection is implemented in [PhotonAnalyzer.cc](https://github.com/cms-opendata-analyses/PhysObjectExtractorTool/blob/2015MiniAOD/PhysObjectExtractor/src/PhotonAnalyzer.cc):

``` cpp
photon_isLoose.push_back(pho.photonID("cutBasedPhotonID-PHYS14-PU20bx25-V2p1-standalone-loose"));
photon_isMedium.push_back(pho.photonID("cutBasedPhotonID-PHYS14-PU20bx25-V2p1-standalone-medium"));
photon_isTight.push_back(pho.photonID("cutBasedPhotonID-PHYS14-PU20bx25-V2p1-standalone-tight"));
```

!!! Warning
The choice of recommended photon ID criteria for 2015 data needs to be verified. In addition to `PHYS14_PU20bx25_V2` other sets, for example `Spring15_25ns_V1`, are available.

Several isolation methods are available through the class member methods, for example:

``` cpp
photon_chIso.push_back(pho.chargedHadronIso());
photon_nhIso.push_back(pho.neutralHadronIso());
photon_phIso.push_back(pho.photonIso());
```

!!! Note "To do"
- Verify the recommended isolation, many other are available in the [photon class](https://github.com/cms-sw/cmssw/blob/CMSSW_7_6_X/DataFormats/PatCandidates/interface/Photon.h)

!!! Note "To do"
- The isolation snippet needs more explanation
Add a mention on photon conversions both for Run 1 and Run 2

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