Measurement of the cross section for prompt isolated diphoton production in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$

This article reports a measurement of the production cross section of prompt isolated photon pairs in proton-antiproton collisions at $\sqrt{s}=1.96\mathrm{TeV}$ using the CDF II detector at the Fermilab Tevatron collider. The data correspond to an integrated luminosity of $5.36{\mathrm{fb}}^{-1}$. The cross section is presented as a function of kinematic variables sensitive to the reaction mechanisms. The results are compared with three perturbative QCD calculations: (1) a leading-order parton shower Monte Carlo, (2) a fixed next-to-leading-order calculation and (3) a next-to-leading-order/next-to-next-to-leading-log resummed calculation. The comparisons show that, within their known limitations, all calculations predict the main features of the data, but no calculation adequately describes all aspects of the data.

Figure 1

Basic diagrams for prompt diphoton production: (a)-(b) direct, (c)-(d) one-photon radiation from an initial- (ISR) or final-state quark (FSR), (e) fragmentation where one photon is emitted along the direction of a final-state quark taking almost all of its energy. The symbol ⊗ denotes the nonperturbative mechanism of the fragmentation process (FRAG).

Figure 2

The track-ISO distribution in signal (solid line) and background (dashed line) events.

Figure 3

Signal (left) and background (right) efficiencies for $\mathrm{\text{track-ISO}}<1\mathrm{GeV}$. The shaded area is the total systematic uncertainty.

Figure 4

Relative systematic uncertainties on the signal (left) and background (right) efficiencies for $\mathrm{\text{track-ISO}}<1\mathrm{GeV}$.

Figure 5

Comparison of $\mathrm{\text{track-ISO}}<1\mathrm{GeV}$ efficiencies in complementary cones from data (squares), signal MC (triangles), and background MC (circles).

Figure 6

The track-ISO distribution in hard scattering and ISR (dashed line) and FSR (solid line) events.

Figure 7

The estimated signal fraction in the inclusive photon data. The shaded area is the total systematic uncertainty in the signal fraction.

Figure 8

The estimated signal fraction in the diphoton data as a function of several kinematic variables. The shaded area is the total systematic uncertainty in the signal fraction.

Figure 9

The estimated efficiency as a function of several kinematic variables. The shaded area is the total systematic uncertainty in the efficiency.

Figure 10

The estimated systematic uncertainties in the cross section as a function of several kinematic variables.

Figure 11

The individual contributions to the cross section from events where both photons are generated according to the pythia diphoton matrix element and from events where one photon originates from initial- or final-state radiation, as functions of the diphoton mass (top), transverse momentum (middle), and azimuthal difference (bottom).

Figure 12

The measured cross section as a function of the diphoton mass in comparison with the diphox predictions without and with the NLO $gg\to \gamma \gamma$ correction, calculated by the gamma2mc program. The shaded area is the total systematic uncertainty in the data.

Figure 13

The cross section as a function of the diphoton invariant mass (left) and transverse momentum (right). Top: the absolute cross section values. Bottom: the relative deviations of the data from the predictions. Note: the vertical axes scales differ between relative deviation plots. The shaded area is the total systematic uncertainty in the data.

Figure 14

The cross section as a function of the diphoton azimuthal distance (left) and of the diphoton rapidity (right). Top: the absolute cross section values. Bottom: the relative deviations of the data from the predictions. Note: the vertical axes scales differ between relative deviation plots. The shaded area is the total systematic uncertainty in the data.

Figure 15

The cross section as a function of the cosine of the polar angle in the Collins-Soper frame (left) and of the ratio of the subleading photon $E_{\mathrm{T}}$ to leading photon $E_{\mathrm{T}}$ (right). Top: the absolute cross section values. Bottom: the relative deviations of the data from the predictions. Note: the vertical axes scales differ between relative deviation plots. The shaded area is the total systematic uncertainty in the data.

Figure 16

The cross section as a function of the diphoton invariant mass (left) and transverse momentum (right) for $P_{\mathrm{T}}<M$. Top: the absolute cross section values. Bottom: the relative deviations of the data from the predictions. Note: the vertical axes scales differ between relative deviation plots. The shaded area is the total systematic uncertainty in the data.

Figure 17

The cross section as a function of the diphoton azimuthal distance (left) and of the diphoton rapidity (right) for $P_{\mathrm{T}}<M$. Top: the absolute cross section values. Bottom: the relative deviations of the data from the predictions. Note: the vertical axes scales differ between relative deviation plots. The shaded area is the total systematic uncertainty in the data.

Figure 18

The cross section as a function of the cosine of the polar angle in the Collins-Soper frame (left) and of the ratio of the subleading photon $E_{\mathrm{T}}$ to leading photon $E_{\mathrm{T}}$ (right) for $P_{\mathrm{T}}<M$. Top: the absolute cross section values. Bottom: the relative deviations of the data from the predictions. Note: the vertical axes scales differ between relative deviation plots. The shaded area is the total systematic uncertainty in the data.

Figure 19

The cross section as a function of the diphoton invariant mass (left) and transverse momentum (right) for $P_{\mathrm{T}}>M$. Top: the absolute cross section values. Bottom: the relative deviations of the data from the predictions. Note: the vertical axes scales differ between relative deviation plots. The shaded area is the total systematic uncertainty in the data.

Figure 20

The cross section as a function of the diphoton azimuthal distance (left) and of the diphoton rapidity (right) for $P_{\mathrm{T}}>M$. Top: the absolute cross section values. Bottom: the relative deviations of the data from the predictions. Note: the vertical axes scales differ between relative deviation plots. The shaded area is the total systematic uncertainty in the data.

Figure 21

The cross section as a function of the cosine of the polar angle in the Collins-Soper frame (left) and of the ratio of the subleading photon $E_{\mathrm{T}}$ to leading photon $E_{\mathrm{T}}$ (right) for $P_{\mathrm{T}}>M$. Top: the absolute cross section values. Bottom: the relative deviations of the data from the predictions. Note: the vertical axes scales differ between relative deviation plots. The shaded area is the total systematic uncertainty in the data.