Measurement of the Differential Isolated Prompt Photon Production Cross
in pp collisions at √s = 7 TeV

A measurement of the differential cross section for the inclusive production of isolated prompt photons in proton-proton collisions at a center-of-mass energy √s=7 TeV is presented. The data sample corresponds to an integrated luminosity of 35.9 pb-1 recorded by the CMS detector at the LHC. Photons are required to have a transverse energy ET >25 GeV and pseudorapidity |η|<2.5. Photon candidates are identified by using the ratio of the transverse energy measured in the electromagnetic calorimeter to the transverse momentum measured in the tracker for converted photons or by using the isolation sum measured in the tracker and calorimeters. The measured cross section is presented as a function of ET in four pseudorapidity regions and compared with the next-to-leading-order perturbative QCD calculations.

Approved Plots from QCD-10-037 (Tarball)

Approved Plots from QCD-10-037 (click for pdf)

Figure 1 Abbreviated Caption
Measured ET/pT distributions for converted photon candidates with ET=25~30 GeV in the ECAL barrel: 0 < | η | < 0.9 and 0.9 < | η | < 1.44.
The binned extended maximum-likelihood fit result (open histogram) is overlaid in each plot.
Fitted component from background (hatched histogram) is shown separately.
Measured ET/pT distributions for converted photon candidates with ET=25~30 GeV in the ECAL endcaps: 1.57 < | η | < 2.1 and 2.1 < | η | < 2.5.
The binned extended maximum-likelihood fit result (open histogram) is overlaid in each plot.
Fitted component from background (hatched histogram) is shown separately.

Figure 2 Abbreviated Caption
Measured signal purity with the photon-conversion method in the ECAL barrel: 0 < | η | < 0.9 and 0.9 < | η | < 1.44.
The shadow bands show the statistical only and the quadratic sum of statistical and systematic uncertainties.
Measured signal purity with the photon-conversion method in the ECAL endcaps: 1.57 < | η | < 2.1 and 2.1 < | η | < 2.5.
The shadow bands show the statistical only and the quadratic sum of statistical and systematic uncertainties.

Figure 3 Abbreviated Caption
Measured combined isolation (Iso) distributions for candidates with ET=80~100 GeV in the ECAL barrel: 0 < | η | < 0.9 and 0.9 < | η | < 1.44.
The un-binned maximum-likelihood fit result (solid line) is overlaid in each plot.
The fitted signal and background components are also shown.
Imperfection of the fitting model is included as part of the systematic uncertainties.
Measured combined isolation (Iso) distributions for candidates with ET=80~100 GeV in the ECAL endcaps: 1.57 < | η | < 2.1 and 2.1 < | η | < 2.5.
The un-binned maximum-likelihood fit result (solid line) is overlaid in each plot.
The fitted signal and background components are also shown.
Imperfection of the fitting model is included as part of the systematic uncertainties.
Figure 4 Abbreviated Caption
Measured signal purity with the isolation method in the ECAL barrel: 0 < | η | < 0.9 and 0.9 < | η | < 1.44.
The shadow bands show the statistical only and the quadratic sum of statistical and systematic uncertainties.
For the highest ET bin (ET=300-400 GeV), the purity is assumed to be 100% and the data points are not shown in these figures.
Measured signal purity with the isolation method in the ECAL endcaps: 1.57 < | η | < 2.1 and 2.1 < | η | < 2.5.
The shadow bands show the statistical only and the quadratic sum of statistical and systematic uncertainties.
For the highest ET bin (ET=300-400 GeV), the purity is assumed to be 100% and the difference between 100% purity and the data points are not shown in these figures.

Figure 5 Abbreviated Caption
Measured signal efficiency for the photon-conversion (left) and isolation (right) methods in the four η regions. Data-to-simulation scaling factors have been applied. Note error bars are dominated by the systematic uncertainties from scaling factors and are 100% correlated between different ET bins.
Figure 6 Abbreviated Caption
Relative systematic uncertainties on the photon cross section measured with the photon-conversion method in the ECAL barrel: 0 < | η | < 0.9 and 0.9 < | η | < 1.44.
In the region with 0 < | η | < 0.9, the ET dependence of systematic uncertainty associated with background shape is different for the following reasons.
The size of the systematic is determined by the difference between fitting with the MC truth and data control region background templates.
The total size of the systematic is a balance between the decreasing statistics of background at high ET for this comparison (tending to make the error larger at high ET), and increasing purity (tending to make the error lower at high ET). In this η bin the first effect dominates.
Relative systematic uncertainties on the photon cross section measured with the photon-conversion method in the ECAL endcaps: 1.57 < | η | < 2.1 and 2.1 < | η | < 2.5.

Figure 7 Abbreviated Caption
Relative systematic uncertainties on the photon cross section measured with the isolation method in the ECAL barrel: 0 < | η | < 0.9 and 0.9 < | η | < 1.44.
For the highest ET bin (ET=300-400 GeV), the systematic uncertainty is assigned differently from the other ET bins.
In this bin, the purity is assumed to be 100% and the difference between 100% purity and the purity at ET=200-300 GeV is taken as the systematic uncertainty.
The following text discusses the fluctuation of systematic uncertainty associated with the background shapes. Overall the uncertainty falls with photon ET, which is expected due to the increase of photon purity.
However, the difference between simulated and observed ISO distribution increases with ET, since data with larger instantaneous luminosity and larger number of pile-up events are used for higher ET bins. Therefore, a discontinuity in systematic uncertainty is observed at ET=55 GeV.
In addition, due to a lack of events, background-enriched sample with photon ET= 80~100 GeV is used to derive the data-to-simulation scaling factors for all the ET bins above 80 GeV. Therefore, for three out of four η regions, one could see a discontinuity in systematic uncertainty at 100 GeV.
Relative systematic uncertainties on the photon cross section measured with the isolation method in the ECAL endcaps: 1.57 < | η | < 2.1 and 2.1 < | η | < 2.5.
For the highest ET bin (ET=300-400 GeV), the systematic uncertainty is assigned differently from the other ET bins.
In this bin, the purity is assumed to be 100% and the difference between 100% purity and the purity at ET=200-300 GeV is taken as the systematic uncertainty.
The following text discusses the fluctuation of systematic uncertainty associated with the background shapes. Overall the uncertainty falls with photon ET, which is expected due to the increase of photon purity.
However, the difference between simulated and observed ISO distribution increases with ET, since data with larger instantaneous luminosity and larger number of pile-up events are used for higher ET bins. Therefore, a discontinuity in systematic uncertainty is observed at ET=55 GeV.
In addition, due to a lack of events, background-enriched sample with photon ET= 80~100 GeV is used to derive the data-to-simulation scaling factors for all the ET bins above 80 GeV. Therefore, for three out of four η regions, one could see a discontinuity in systematic uncertainty at 100 GeV.

Figure 8 Abbreviated Caption
  Measured differential isolated prompt photon cross sections (markers) as a function of ET in the four pseudorapidity regions and the predictions from JETPHOX (histograms).
The cross sections are scaled by the factors shown in the legend for easier viewing.

Figure 9 Abbreviated Caption
Ratios of the measured isolated prompt photon differential cross section to the NLO pQCD predictions. The vertical error bars show the statistical uncertainties, while the shaded areas show the statistical and systematic uncertainties added in quadrature.
The 4% luminosity uncertainty on the data is not included.
The two sets of curves show the uncertainties on the theoretical predictions due to their dependency on the renormalization, factorization, and fragmentation scales, and on the variation of αs and PDFs.
A correction to account for extra activity C=0.975 +- 0.006 is applied to the theoretical predictions, as explained in the text. The ECAL barrel region is shown: 0 < | η | < 0.9 and 0.9 < | η | < 1.44.
Ratios of the measured isolated prompt photon differential cross section to the NLO pQCD predictions. The vertical error bars show the statistical uncertainties, while the shaded areas show the statistical and systematic uncertainties added in quadrature.
The 4% luminosity uncertainty on the data is not included.
The two sets of curves show the uncertainties on the theoretical predictions due to their dependency on the renormalization, factorization, and fragmentation scales, and on the variation of αs and PDFs.
A correction to account for extra activity C=0.975 +- 0.006 is applied to the theoretical predictions, as explained in the text. The ECAL endcap region is shown: 1.57 < | η | < 2.1 and 2.1 < | η | < 2.5.

Link to paper on arXiv

  • Pre-print:

-- ShinShanYu - 31-May-2011

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