DOI: 10.17182/hepdata.159299.v1/t3
title: "Table 1" of "Observation of $t\bar{t}\gamma\gamma$ production at $\sqrt{s}=$13 TeV with the ATLAS detector"

description: Summary of the relative impact of all the systematic uncertainties, in percentage, on the $t\bar{t}\gamma\gamma$ fiducial inclusive cross section and $R_{t\bar{t}\gamma\gamma/t\bar{t}\gamma}$ grouped into different categories. The category ‘Jet’ corresponds to the effect of JES, jet resolution and JVT uncertainties, ‘Photon’ and ‘Leptons’ include all experimental uncertainties related to photons and leptons (including trigger uncertainties), respectively.
description: This paper presents the first observation of top-quark pair production in association with two photons ($t\bar{t}\gamma\gamma$). The measurement is performed in the single-lepton decay channel using proton–proton collision data collected by the ATLAS detector at the Large Hadron Collider. The data correspond to an integrated luminosity of 140 fb$^{-1}$ recorded during Run 2 at a centre-of-mass energy of 13 TeV. The $t\bar{t}\gamma\gamma$ production cross section, measured in a fiducial phase space based on particle-level kinematic criteria for the lepton, photons, and jets, is found to be $2.42^{+0.58}_{-0.53}\,\text{fb}$, corresponding to an observed significance of 5.2 standard deviations. Additionally, the ratio of the production cross section of $t\bar{t}\gamma\gamma$ to top-quark pair production in association with one photon is determined, yielding $(3.30^{+0.70}_{-0.65})\times 10^{-3}$.

The $t\bar{t}\gamma\gamma$ fiducial region is defined as:
- Exactly one electron or muon with $p_{\text{T}} &gt; 25$ GeV and $|\eta| &lt; 2.5$ which do not originate from hadron decays. Events are rejected if there are additional leptons with $p_{\text{T}} &gt; 7$ GeV.
- Exactly two photons with $E_{\text{T}} &gt; 20$ GeV and $|\eta| &lt; 2.37$, and the sum of transverse momenta of all charged particles surrounding the photons within $\Delta R = 0.2$ is less than 5% of their own $p_{\text{T}} $.
- $\geq$ four jets with $p_{\text{T}} &gt; 25$ GeV and $|\eta| &lt; 2.5$. Jets are clustered with the anti-$k_t$ algorithm with $R = 0.4$. All stable particles are considered in the clustering, except for the selected electrons, muons and photons, and the neutrinos originating from the top quarks.
- $\geq$ 1 $b$-tagged jet. A jet is identified as a $b$-jet if a hadron with $p_{\text{T}} &gt; 5$ GeV containing a $b$-quark is matched to the jet through a ghost-matching method.
- Muons and electrons within $\Delta R &lt; 0.4$ from a jet are excluded.
- Jets within $\Delta R = 0.4$ of an isolated photon candidate are excluded.
- Events must satisfy $\Delta R(\gamma,\ell) &gt; 0.4$ and $\Delta R(\gamma,\gamma) &gt; 0.4$.

For the ratio, the definition of the $t\bar{t}\gamma$ fiducial region is analogous to the $t\bar{t}\gamma\gamma$ one, except that exactly one photon is required. ($t\bar{t}\gamma$ HEPdata record: https://www.hepdata.net/record/146899).

The full likelihoods are provided for the cross-section and the ratio measurement in HS3 JSON format: 
https://github.com/hep-statistics-serialization-standard/hep-statistics-serialization-standard/tree/master.