Measurement of the differential cross section for isolated prompt photon production in $pp$ collisions at 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 of 7 TeV is presented. The data sample corresponds to an integrated luminosity of $36{\mathrm{pb}}^{-1}$ recorded by the CMS detector at the LHC. The measurement covers the pseudorapidity range $|\eta |<2.5$ and the transverse energy range $25<E_{\mathrm{T}}<400\mathrm{GeV}$, corresponding to the kinematic region $0.007<x_{\mathrm{T}}<0.114$. Photon candidates are identified with two complementary methods, one based on photon conversions in the silicon tracker and the other on isolated energy deposits in the electromagnetic calorimeter. The measured cross section is presented as a function of $E_{\mathrm{T}}$ in four pseudorapidity regions. The next-to-leading-order perturbative QCD calculations are consistent with the measured cross section.

Figure 1

Measured $E_{\mathrm{T}}/p_{\mathrm{T}}$ distributions for converted-photon candidates with $E_{\mathrm{T}}=25–30\mathrm{GeV}$, in the four $\eta$ regions considered. The binned extended maximum likelihood fit result (open histogram) is overlaid in each plot. The fitted component from background is shown by hatched histograms.

Figure 2

Measured signal purity for $0<E_{\mathrm{T}}/p_{\mathrm{T}}<3$ with the photon conversion method in the four $\eta$ regions. The vertical error bars show the statistical uncertainties, while the shaded areas show the statistical and systematic uncertainties added in quadrature. Estimation of the systematic uncertainties is discussed in Sec. 8.

Figure 3

Measured ISO distributions for candidates with $E_{\mathrm{T}}=80–100\mathrm{GeV}$. The unbinned maximum likelihood fit result (solid line) is overlaid in each plot. The fitted signal and background components are also shown. Imperfections of the fitting model are included as part of the systematic uncertainties.

Figure 4

Measured signal purity for $\mathrm{ISO}<5\mathrm{GeV}$ with the isolation method in the four $\eta$ regions. The purity for the bin $E_{\mathrm{T}}=300–400\mathrm{GeV}$ is not shown. The vertical error bars show the statistical uncertainties, while the shaded areas show the statistical and systematic uncertainties added in quadrature. Estimation of the systematic uncertainties is discussed in Sec. 8.

Figure 5

Measured signal efficiency $ϵ$ for the (a) photon conversion and (b) isolation methods in the four $\eta$ regions. Data-to-simulation scaling factors have been applied. The error bars are dominated by the systematic uncertainties and are 100% correlated between different $E_{\mathrm{T}}$ bins.

Figure 6

Relative systematic uncertainties on the photon cross section measured with the photon conversion method in the four $\eta$ regions. Systematic uncertainties due to the uncertainties on the fit bias, energy scale, selection efficiency, unfolding correction factors, and signal and background shapes are shown, as well as their total quadrature sum (upper curve).

Figure 7

Relative systematic uncertainties on the photon cross section measured with the isolation method in the four $\eta$ regions. Systematic uncertainties due to the uncertainties on the fit bias, energy scale, selection efficiency, unfolding correction factors, and signal and background shapes are shown, as well as their total quadrature sum (upper curve).

Figure 8

Measured isolated prompt photon differential cross sections (markers) as a function of transverse energy in the four pseudorapidity regions and the predictions from Jetphox 1.3.0 using the CT10 PDFs (histograms). The error bars are the quadrature sums of statistical and systematic uncertainties on the measurements. The cross sections are scaled by the factors shown in the legend for easier viewing.

Figure 9

Ratios of the measured isolated prompt photon differential cross section to the NLO pQCD predictions from Jetphox 1.3.0 using the CT10 PDFs. 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 dependence on the renormalization, factorization, and fragmentation scales, and on the variation of CT10 ${\alpha }_S$ and PDFs. A correction to account for extra activity ($\overline{C}=0.975\pm 0.006$) is applied to the theoretical predictions, as explained in the text.