## Measurement of the ratio of cross sections for inclusive isolated-photon production in pp collisions at √s = 13 and 8 TeV with the ATLAS detector

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##### Date of Archiving

2019##### Archive

HEPData

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Dataset

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Open access

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##### Organization

High Energy Physics

Experimental High Energy Physics

##### Audience(s)

Physics

##### Key words

Hadron-Hadron scattering (experiments); Photon production; QCD; Physics##### Abstract

Measurement of the ratio of cross sections for inclusive isolated-photon production in proton-proton collisions at a centre-of-mass energy of 13 TeV over 8 TeV. Measurement is also presented of the double ratio of the ratio above to the ratio of the fiducial cross sections for Z boson production at 13 TeV and 8 TeV using the decay channels Z into an electron-positron pair and Z into a muon-antimuon pair.
The ratio of cross sections for inclusive isolated-photon production is denoted by Rgamma13/8. The ratio of fiducial cross sections for Z boson production is denoted by RZ13/8. The double ratio (Rgamma13/8)/(RZ13/8) is denoted by Dgamma/Z13/8.
The photon is required to have a transverse energy above 125 GeV and absolute value of pseudorapity |etaGamma|<2.37, excluding the region 1.37<|etaGamma|<1.56. The photon isolation is ensured by requiring the transverse energy around a cone of radius R=0.4 around the photon to be less than (4.8 + 0.0042 * ETGamma) [GeV]. At particle level it is the sum of transverse energy from all particles, except for muons, neutrinos and the photon itself. At parton level it is the sum of energy from all coloured partons.
The fiducial cross section for Z boson production at a given centre-of-mass energy is defined as the production cross section of a Z boson times the branching ratio into a lepton pair of flavour l+l- = e+e- or mu+mu- within the following phase space: the lepton transverse momentum pT> 25 GeV, the lepton pseudorapidity |eta|<2.5 and the dilepton invariant mass 66 < mll < 116 GeV. Measurements of the fiducial cross sections were made using the decay channels Z into e+e- and Z into mu+mu-, and combined for the final result. The measured RZ13/8 is 1.537 +- 0.001 (stat.) +- 0.010 (syst.) +- 0.044 (lumi.).
Predictions for Rgamma13/8 are given at NLO QCD from the program Jetphox using the MMHT2014 NLO PDF set. Predictions for Dgamma/Z13/8 are obtained from the predictions of Rgamma13/8 at NLO QCD from Jepthox using the MMHT2014 NNLO PDF set and the predictions of RZ13/8 at NNLO QCD from Dyturbo using the MMHT2014 NNLO PDF set.
Information about the bin-to-bin correlation of the systematic uncertainties of Rgamma13/8. The following uncertainties are to be treated as uncorrelated bin-to-bin: sysEnergyResolution13, sysEnergyResolution8, sysPhotonID13, sysPhotonID8, sysBackgroundIsolation13, sysBackgroundIsolation8, sysBackgroundIsolationUpperLimit13, sysBackgroundID13, sysBackgroundID8, sysIsolationMC13 and sysMCstats13. The systematic uncertainty due to the photon energy scale (sysEnergyScale) is partially correlated bin-to-bin; the table shows just the total systematic uncertainty due to the photon energy scale (separately for the up and down contributions). In order to take into account properly the correlations due to the photon energy scale, see the information provided below.
The systematic uncertainty due to the photon energy scale (GES) is decomposed into 25 independent components. Each file (see below) contains 50 lines since for each independent component there are two variations (up and down). Each variation is to be treated as fully correlated bin-to-bin. The relative changes in the ratio Rgamma13/8 of the differential cross sections due to each variation are given in the following files.
For the ratio Rgamma13/8 as a function of ETGamma for the range |etaGamma|<0.6 is in the file decompositionsystematicuncertaintygesratio1stetabin.txt
For the ratio Rgamma13/8 as a function of ETGamma for the range 0.6<|etaGamma|<1.37 is in the file decompositionsystematicuncertaintygesratio2ndetabin.txt
For the ratio Rgamma13/8 as a function of ETGamma for the range 1.56<|etaGamma|<1.81 is in the file decompositionsystematicuncertaintygesratio3rdetabin.txt
For the ratio Rgamma13/8 as a function of ETGamma for the range 1.81<|etaGamma|<2.37 is in the file decompositionsystematicuncertaintygesratio4thetabin.txt
Each file contains 50 lines and each line contains the relative changes in the ratio Rgamma13/8 of the differential cross sections for each bin of ETGamma: 14 bins for the 1st region in etaGamma, 13 bins for the 2nd region in etaGamma, 10 bins for the 3rd region in etaGamma and 10 bins for the 4th region in etaGamma. It should be noted that RELATIVE CHANGES are quoted in the files and that the relative changes are SIGNED. It should also be noted that the relative changes in the ratio Rgamma13/8 of the differential cross sections induced by the up and down variations of a given component are not necessarily symmetric. All the txt files are included in the gzipped tar file files_with_decompositionsystematicuncertaintyges.tar.gz that can be accessed in the resources.
Information about the bin-to-bin correlation of the systematic uncertainties of Dgamma/Z13/8. The following uncertainties are to be treated as uncorrelated bin-to-bin: sysEnergyResolution13, sysEnergyResolution8, sysPhotonID13, sysPhotonID8, sysBackgroundIsolation13, sysBackgroundIsolation8, sysBackgroundIsolationUpperLimit13, sysBackgroundID13, sysBackgroundID8, sysIsolationMC13 and sysMCstats13. The systematic uncertainty due to the photon energy scale (sysEnergyScale) is partially correlated bin-to-bin; the table shows just the total systematic uncertainty due to the photon energy scale (separately for the up and down contributions). In order to take into account properly the correlations due to the photon energy scale, see the information provided below.
The relative changes in the ratio Dgamma/Z13/8 of the differential cross sections due to each GES variation are given in the following files. The files are identical as those for Rgamma13/8 (the correlation between the photon energy scale and the electron energy scale can be safely neglected), the description provided above also applies and they are included in the gzipped tar file files_with_decompositionsystematicuncertaintyges.tar.gz that can be accessed in the resources.
For the double ratio Dgamma/Z13/8 as a function of ETGamma for the range |etaGamma|<0.6 is in the file decompositionsystematicuncertaintygeszratio1stetabin.txt
For the double ratio Dgamma/Z13/8 as a function of ETGamma for the range 0.6<|etaGamma|<1.37 is in the file decompositionsystematicuncertaintygeszratio2ndetabin.txt
For the double ratio Dgamma/Z13/8 as a function of ETGamma for the range 1.56<|etaGamma|<1.81 is in the file decompositionsystematicuncertaintygeszratio3rdetabin.txt
For the double ratio Dgamma/Z13/8 as a function of ETGamma for the range 1.81<|etaGamma|<2.37 is in the file decompositionsystematicuncertaintygeszratio4thetabin.txt
Regarding the correlations between 8 and 13 TeV (for both Rgamma13/8 and Dgamma/Z13/8): they are accounted for in the photon energy scale uncertainties (sysEnergyScale); the cancellations between 8 and 13 TeV are already in place. For the rest, they are quoted separately (e.g. sysPhotonID13 and sysPhotonID8) and have to be treated as uncorrelated (between 8 TeV and 13 TeV); an exception is the uncertainty due to the luminosity for which the uncertainty is quoted directly for the ratio (by the way, it was treated as uncorrelated between 8 and 13 TeV).

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