Measurement of the isolated diphoton cross section in $pp$ collisions at $\sqrt{s}\mathbf{=}7\mathrm{TeV}$ with the ATLAS detector

The ATLAS experiment has measured the production cross section of events with two isolated photons in the final state, in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$. The full data set acquired in 2010 is used, corresponding to an integrated luminosity of $37{\mathrm{pb}}^{-1}$. The background, consisting of hadronic jets and isolated electrons, is estimated with fully data-driven techniques and subtracted. The differential cross sections, as functions of the di-photon mass ($m_{\gamma \gamma }$), total transverse momentum ($p_{\mathrm{T},\gamma \gamma }$), and azimuthal separation ($\Delta {\varphi }_{\gamma \gamma }$), are presented and compared to the predictions of next-to-leading-order QCD.

Figure 1

Extraction of the isolation energy ($E_{\mathrm{T}}^{\mathrm{iso}}$) distributions for signal and background. The plots are made with a “di-jet-like” Monte Carlo sample: the “signal” and “background” classifications are based on the Monte Carlo information. (a) Normalized $E_{\mathrm{T}}^{\mathrm{iso}}$ distribution for the background and for the nontight sample. (b) $E_{\mathrm{T}}^{\mathrm{iso}}$ distribution, for the tight and the nontight samples: the latter is scaled as explained in the text. (c) Normalized $E_{\mathrm{T}}^{\mathrm{iso}}$ distribution for the signal and for the tight sample, after subtracting the scaled nontight sample. In (a, c) the vertical line shows the isolation cut $E_{\mathrm{T}}^{\mathrm{iso}}<3\mathrm{GeV}$.

Figure 2

Data-driven signal isolation distributions for the leading (top) and subleading (bottom) photons obtained using the photon candidates (solid circles) or extrapolated from electrons (continuous lines).

Figure 3

Differential $\gamma \gamma$ yields in the $\mathbf{T}\mathbf{I}\mathbf{T}\mathbf{I}$ sample ($N_{\gamma \gamma }^{\mathbf{T}\mathbf{I}\mathbf{T}\mathbf{I}}$), as a function of the three observables $m_{\gamma \gamma }$, $p_{\mathrm{T},\gamma \gamma }$, $\Delta {\varphi }_{\gamma \gamma }$, obtained with the three methods. In each bin, the yield is divided by the bin width. The vertical error bars display the total errors, accounting for both the statistical uncertainties and the systematic effects. The points are artificially shifted horizontally, to better display the three results.

Figure 4

Projections of the 2-dimensional PDF fit on transverse isolation energies of the two photon candidates: leading photon (top) and subleading photon (bottom). Solid circles represent the observed data. The continuous curve is the fit result, while the dashed-dotted curve shows the $\gamma \gamma$ component. The dashed line represents the background component of the leading and subleading photon sample, respectively.

Figure 5

Schematic representation of the two-dimensional sideband method. The top plane displays the isolation (x axis) and tight identification (y axis) criteria for the classification of the leading photon candidate. When the leading photon belongs to region $A$, the same classification is applied to the subleading photon, as described by the bottom plane.

Figure 6

Electron background subtraction as a function of $m_{\gamma \gamma }$. The top plot displays the impurity, overall and for the $\gamma e$ and $ee$ separately. The bottom plot shows the di-photon yield before (open squares) and after (solid circles) the electron background subtraction. The points are artificially shifted horizontally, to better display the different values.

Figure 7

Differential cross-section $d\sigma /d{m}_{\gamma \gamma }$ of di-photon production. The solid circles display the experimental values, the hatched bands display the NLO computations by DIPHOX and ResBos. The bottom panels show the relative difference between the measurements and the NLO predictions. The data/theory point in the bin $0<m_{\gamma \gamma }<30\mathrm{GeV}$ lies above the frames.

Figure 8

Differential cross-section $d\sigma /d{p}_{\mathrm{T},\gamma \gamma }$ of di-photon production. The solid circles display the experimental values, the hatched bands display the NLO computations by DIPHOX and ResBos. The bottom panels show the relative difference between the measurements and the NLO predictions. The data point in the bin $150<p_{\mathrm{T},\gamma \gamma }<200\mathrm{GeV}$ in the main panel lies below the frame.

Figure 9

Differential cross-section $d\sigma /d\Delta {\varphi }_{\gamma \gamma }$ of di-photon production. The solid circles display the experimental values, the hatched bands display the NLO computations by DIPHOX and ResBos. The bottom panels show the relative difference between the measurements and the NLO predictions.