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| CMS-TOP-23-002 ; CERN-EP-2025-240 | ||
| Inclusive and differential measurements of the $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross section and the $ {\mathrm{t}\overline{\mathrm{t}}} \gamma/{\mathrm{t}\overline{\mathrm{t}}} $ cross section ratio in proton-proton collisions at $ \sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| 3 November 2025 | ||
| Submitted to J. High Energy Phys. | ||
| Abstract: Inclusive and differential cross section measurements of top quark pair ($ \mathrm{t} \overline{\mathrm{t}} $) production in association with a photon ($ \gamma $) are performed as a function of lepton, photon, top quark, and $ \mathrm{t} \overline{\mathrm{t}} $ kinematic observables, using data from proton-proton collisions at $ \sqrt{s}= $ 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. Events containing two leptons (electrons or muons) and a photon in the final state are considered. The fiducial cross section of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ is measured to be 137 $ \pm $ 8 fb, in a phase space including events with a high momentum, isolated photon. The fiducial cross section of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ is also measured to be 56 $ \pm $ 5 fb when considering only events where the photon is emitted in the production part of the process. Both measurements are in agreement with the theoretical predictions, of 126 $ \pm $ 19 fb and 57 $ \pm $ 5 fb, respectively. Differential measurements are performed at the particle and parton levels. Additionally, inclusive and differential ratios between the cross sections of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ and $ \mathrm{t} \overline{\mathrm{t}} $ production are measured. The inclusive ratio is found to be 0.0133 $ \pm $ 0.0005, in agreement with the standard model prediction of 0.0127 $ \pm $ 0.0008. The top quark charge asymmetry in $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ production is also measured to be $-$0.012 $ \pm $ 0.042, compatible with both the standard model prediction and with no asymmetry. | ||
| Links: e-print arXiv:2511.01995 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Example Feynman diagrams for the production of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, where both top quarks decay leptonically. The photon can be emitted from the initial state (left), from an off-shell top quark (centre), or from a top quark decay product (right). |
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Figure 1-a:
Example Feynman diagrams for the production of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, where both top quarks decay leptonically. The photon can be emitted from the initial state (left), from an off-shell top quark (centre), or from a top quark decay product (right). |
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Figure 1-b:
Example Feynman diagrams for the production of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, where both top quarks decay leptonically. The photon can be emitted from the initial state (left), from an off-shell top quark (centre), or from a top quark decay product (right). |
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Figure 1-c:
Example Feynman diagrams for the production of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, where both top quarks decay leptonically. The photon can be emitted from the initial state (left), from an off-shell top quark (centre), or from a top quark decay product (right). |
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Figure 2:
Distributions after the event selection for:\ the $ p_{\mathrm{T}} $ of the photon (upper left), the $ p_{\mathrm{T}} $ of the lepton with the highest $ p_{\mathrm{T}} $ (upper right), the number of jets (lower left), and the minimum $\Delta R(\gamma,\ell) $ (lower right). The data and their statistical uncertainties are indicated by black points and error bars, respectively. The postfit prediction for the $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ process is shown in red and yellow, after the fit to the production component of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, described in Section 9.3. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). The black (red) hatched areas represent the post-fit (pre-fit) uncertainties. The last bin includes all events above the plotted range. |
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Figure 2-a:
Distributions after the event selection for:\ the $ p_{\mathrm{T}} $ of the photon (upper left), the $ p_{\mathrm{T}} $ of the lepton with the highest $ p_{\mathrm{T}} $ (upper right), the number of jets (lower left), and the minimum $\Delta R(\gamma,\ell) $ (lower right). The data and their statistical uncertainties are indicated by black points and error bars, respectively. The postfit prediction for the $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ process is shown in red and yellow, after the fit to the production component of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, described in Section 9.3. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). The black (red) hatched areas represent the post-fit (pre-fit) uncertainties. The last bin includes all events above the plotted range. |
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Figure 2-b:
Distributions after the event selection for:\ the $ p_{\mathrm{T}} $ of the photon (upper left), the $ p_{\mathrm{T}} $ of the lepton with the highest $ p_{\mathrm{T}} $ (upper right), the number of jets (lower left), and the minimum $\Delta R(\gamma,\ell) $ (lower right). The data and their statistical uncertainties are indicated by black points and error bars, respectively. The postfit prediction for the $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ process is shown in red and yellow, after the fit to the production component of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, described in Section 9.3. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). The black (red) hatched areas represent the post-fit (pre-fit) uncertainties. The last bin includes all events above the plotted range. |
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Figure 2-c:
Distributions after the event selection for:\ the $ p_{\mathrm{T}} $ of the photon (upper left), the $ p_{\mathrm{T}} $ of the lepton with the highest $ p_{\mathrm{T}} $ (upper right), the number of jets (lower left), and the minimum $\Delta R(\gamma,\ell) $ (lower right). The data and their statistical uncertainties are indicated by black points and error bars, respectively. The postfit prediction for the $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ process is shown in red and yellow, after the fit to the production component of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, described in Section 9.3. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). The black (red) hatched areas represent the post-fit (pre-fit) uncertainties. The last bin includes all events above the plotted range. |
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Figure 2-d:
Distributions after the event selection for:\ the $ p_{\mathrm{T}} $ of the photon (upper left), the $ p_{\mathrm{T}} $ of the lepton with the highest $ p_{\mathrm{T}} $ (upper right), the number of jets (lower left), and the minimum $\Delta R(\gamma,\ell) $ (lower right). The data and their statistical uncertainties are indicated by black points and error bars, respectively. The postfit prediction for the $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ process is shown in red and yellow, after the fit to the production component of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, described in Section 9.3. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). The black (red) hatched areas represent the post-fit (pre-fit) uncertainties. The last bin includes all events above the plotted range. |
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Figure 3:
The $ p_{\mathrm{T}}(\mathrm{t}_{1}) $ (upper left), $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper right), and $ \Delta|y|(\mathrm{t},\overline{\mathrm{t}}) $ (lower) distributions in data and simulation, after the fit to the production component of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, described in Section 9.3. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). The black (red) hatched areas represent the post-fit (pre-fit) uncertainties. The last bin includes all events above the plotted range and, where applicable, the first bin includes all events below the plotted range. |
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Figure 3-a:
The $ p_{\mathrm{T}}(\mathrm{t}_{1}) $ (upper left), $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper right), and $ \Delta|y|(\mathrm{t},\overline{\mathrm{t}}) $ (lower) distributions in data and simulation, after the fit to the production component of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, described in Section 9.3. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). The black (red) hatched areas represent the post-fit (pre-fit) uncertainties. The last bin includes all events above the plotted range and, where applicable, the first bin includes all events below the plotted range. |
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Figure 3-b:
The $ p_{\mathrm{T}}(\mathrm{t}_{1}) $ (upper left), $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper right), and $ \Delta|y|(\mathrm{t},\overline{\mathrm{t}}) $ (lower) distributions in data and simulation, after the fit to the production component of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, described in Section 9.3. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). The black (red) hatched areas represent the post-fit (pre-fit) uncertainties. The last bin includes all events above the plotted range and, where applicable, the first bin includes all events below the plotted range. |
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Figure 3-c:
The $ p_{\mathrm{T}}(\mathrm{t}_{1}) $ (upper left), $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper right), and $ \Delta|y|(\mathrm{t},\overline{\mathrm{t}}) $ (lower) distributions in data and simulation, after the fit to the production component of $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $, described in Section 9.3. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). The black (red) hatched areas represent the post-fit (pre-fit) uncertainties. The last bin includes all events above the plotted range and, where applicable, the first bin includes all events below the plotted range. |
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Figure 4:
Distributions of the invariant mass of the two leptons (left) and two leptons + photon (right) system, for events with two same-flavour leptons, as estimated by the simulation before the fit. These distributions are shown after requiring that the events pass the full event selection, but excluding the requirement that both invariant masses are reconstructed with a value not compatible with $ m_{\mathrm{Z}} $, within 15 GeV. The data and their statistical uncertainties are indicated by the black points and error bars, respectively. The hatched area indicates the total pre-fit uncertainty in the prediction. The last bin includes all events above the plotted range. |
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Figure 4-a:
Distributions of the invariant mass of the two leptons (left) and two leptons + photon (right) system, for events with two same-flavour leptons, as estimated by the simulation before the fit. These distributions are shown after requiring that the events pass the full event selection, but excluding the requirement that both invariant masses are reconstructed with a value not compatible with $ m_{\mathrm{Z}} $, within 15 GeV. The data and their statistical uncertainties are indicated by the black points and error bars, respectively. The hatched area indicates the total pre-fit uncertainty in the prediction. The last bin includes all events above the plotted range. |
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Figure 4-b:
Distributions of the invariant mass of the two leptons (left) and two leptons + photon (right) system, for events with two same-flavour leptons, as estimated by the simulation before the fit. These distributions are shown after requiring that the events pass the full event selection, but excluding the requirement that both invariant masses are reconstructed with a value not compatible with $ m_{\mathrm{Z}} $, within 15 GeV. The data and their statistical uncertainties are indicated by the black points and error bars, respectively. The hatched area indicates the total pre-fit uncertainty in the prediction. The last bin includes all events above the plotted range. |
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Figure 5:
Schematic representation of the regions used to estimate the contribution from events with a nonprompt photon in the SR. The blue (light red) areas represent the fraction of events with a nonprompt (prompt) photon in each region. The area shaded in grey represents a gap between the regions, and events falling in that gap are excluded. The numbers in black on the axes represent the selections applied to separate the regions for events with a photon reconstructed in the ECAL barrel, while those in pink represent the selections applied to events with a photon reconstructed in the endcaps. |
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Figure 7:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the parton level as a function of $ p_{\mathrm{T}}(\mathrm{t}_{1}) $ (upper) and $ \Delta R(\gamma,{\mathrm{t}\overline{\mathrm{t}}} ) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 7-a:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the parton level as a function of $ p_{\mathrm{T}}(\mathrm{t}_{1}) $ (upper) and $ \Delta R(\gamma,{\mathrm{t}\overline{\mathrm{t}}} ) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 7-b:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the parton level as a function of $ p_{\mathrm{T}}(\mathrm{t}_{1}) $ (upper) and $ \Delta R(\gamma,{\mathrm{t}\overline{\mathrm{t}}} ) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 7-c:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the parton level as a function of $ p_{\mathrm{T}}(\mathrm{t}_{1}) $ (upper) and $ \Delta R(\gamma,{\mathrm{t}\overline{\mathrm{t}}} ) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 7-d:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the parton level as a function of $ p_{\mathrm{T}}(\mathrm{t}_{1}) $ (upper) and $ \Delta R(\gamma,{\mathrm{t}\overline{\mathrm{t}}} ) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 8:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the parton level as a function of the min. $\Delta R(\gamma,\mathrm{t}) $ (upper) and $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 8-a:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the parton level as a function of the min. $\Delta R(\gamma,\mathrm{t}) $ (upper) and $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 8-b:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the parton level as a function of the min. $\Delta R(\gamma,\mathrm{t}) $ (upper) and $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 8-c:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the parton level as a function of the min. $\Delta R(\gamma,\mathrm{t}) $ (upper) and $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 8-d:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the parton level as a function of the min. $\Delta R(\gamma,\mathrm{t}) $ (upper) and $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 9:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the particle level as a function of $ p_{\mathrm{T}}(\ell_{1}) $ (upper) and $ p_{\mathrm{T}}(\gamma) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainty in the predictions. The gray lines and bands represent the fixed-order prediction and their respective uncertainty. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainty includes the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 9-a:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the particle level as a function of $ p_{\mathrm{T}}(\ell_{1}) $ (upper) and $ p_{\mathrm{T}}(\gamma) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainty in the predictions. The gray lines and bands represent the fixed-order prediction and their respective uncertainty. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainty includes the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 9-b:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the particle level as a function of $ p_{\mathrm{T}}(\ell_{1}) $ (upper) and $ p_{\mathrm{T}}(\gamma) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainty in the predictions. The gray lines and bands represent the fixed-order prediction and their respective uncertainty. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainty includes the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 9-c:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the particle level as a function of $ p_{\mathrm{T}}(\ell_{1}) $ (upper) and $ p_{\mathrm{T}}(\gamma) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainty in the predictions. The gray lines and bands represent the fixed-order prediction and their respective uncertainty. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainty includes the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 9-d:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the particle level as a function of $ p_{\mathrm{T}}(\ell_{1}) $ (upper) and $ p_{\mathrm{T}}(\gamma) $ (lower). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainty in the predictions. The gray lines and bands represent the fixed-order prediction and their respective uncertainty. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainty includes the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 10:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the particle level as a function of the $ \Delta\phi(\ell,\ell) $. The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainty in the predictions. The gray lines and bands represent the fixed-order prediction and their respective uncertainty. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainty include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 10-a:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the particle level as a function of the $ \Delta\phi(\ell,\ell) $. The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainty in the predictions. The gray lines and bands represent the fixed-order prediction and their respective uncertainty. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainty include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 10-b:
Absolute (left) and normalized (right) differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross sections at the particle level as a function of the $ \Delta\phi(\ell,\ell) $. The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainty in the predictions. The gray lines and bands represent the fixed-order prediction and their respective uncertainty. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainty include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
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Figure 11:
Distribution of the $ p_{\mathrm{T}} $ of the leading lepton for the ``$ {\mathrm{t}\overline{\mathrm{t}}}, 0 \gamma $'' region (upper left) and the ``$ {\mathrm{t}\overline{\mathrm{t}}}, 1 \gamma $'' SR (upper right), and the number of jets for the DY$+$jets (lower left) and $ \mathrm{Z}/\gamma{+}$jets (lower right) CRs after the fit. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). |
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Figure 11-a:
Distribution of the $ p_{\mathrm{T}} $ of the leading lepton for the ``$ {\mathrm{t}\overline{\mathrm{t}}}, 0 \gamma $'' region (upper left) and the ``$ {\mathrm{t}\overline{\mathrm{t}}}, 1 \gamma $'' SR (upper right), and the number of jets for the DY$+$jets (lower left) and $ \mathrm{Z}/\gamma{+}$jets (lower right) CRs after the fit. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). |
png pdf |
Figure 11-b:
Distribution of the $ p_{\mathrm{T}} $ of the leading lepton for the ``$ {\mathrm{t}\overline{\mathrm{t}}}, 0 \gamma $'' region (upper left) and the ``$ {\mathrm{t}\overline{\mathrm{t}}}, 1 \gamma $'' SR (upper right), and the number of jets for the DY$+$jets (lower left) and $ \mathrm{Z}/\gamma{+}$jets (lower right) CRs after the fit. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). |
png pdf |
Figure 11-c:
Distribution of the $ p_{\mathrm{T}} $ of the leading lepton for the ``$ {\mathrm{t}\overline{\mathrm{t}}}, 0 \gamma $'' region (upper left) and the ``$ {\mathrm{t}\overline{\mathrm{t}}}, 1 \gamma $'' SR (upper right), and the number of jets for the DY$+$jets (lower left) and $ \mathrm{Z}/\gamma{+}$jets (lower right) CRs after the fit. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). |
png pdf |
Figure 11-d:
Distribution of the $ p_{\mathrm{T}} $ of the leading lepton for the ``$ {\mathrm{t}\overline{\mathrm{t}}}, 0 \gamma $'' region (upper left) and the ``$ {\mathrm{t}\overline{\mathrm{t}}}, 1 \gamma $'' SR (upper right), and the number of jets for the DY$+$jets (lower left) and $ \mathrm{Z}/\gamma{+}$jets (lower right) CRs after the fit. The hatched area indicates the total uncertainty in the prediction. The lower panels show the ratio of the data to the sum of the postfit predictions (points) and the ratio of the data to the sum of the prefit predictions (red line). |
png pdf |
Figure 12:
Absolute differential measurements of $ R_{\gamma} $ as a function of $ p_{\mathrm{T}}(\mathrm{t}_{1}) $ at the parton level (left) and $ p_{\mathrm{T}}(\ell_{1}) $ at the particle level (right). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. The gray lines and bands represent the fixed-order predictions and their respective uncertainties. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
png pdf |
Figure 12-a:
Absolute differential measurements of $ R_{\gamma} $ as a function of $ p_{\mathrm{T}}(\mathrm{t}_{1}) $ at the parton level (left) and $ p_{\mathrm{T}}(\ell_{1}) $ at the particle level (right). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. The gray lines and bands represent the fixed-order predictions and their respective uncertainties. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
png pdf |
Figure 12-b:
Absolute differential measurements of $ R_{\gamma} $ as a function of $ p_{\mathrm{T}}(\mathrm{t}_{1}) $ at the parton level (left) and $ p_{\mathrm{T}}(\ell_{1}) $ at the particle level (right). The purple (blue) lines show the predictions from the nominal (alternative) simulation, and the lighter purple (blue) shaded areas represent the theoretical uncertainties in the predictions. The gray lines and bands represent the fixed-order predictions and their respective uncertainties. In the legends, ``MG5'' refers to MadGraph-5_aMC@NLO, while ``PH+Py8'' refers to POWHEG and PYTHIA. The theoretical uncertainties include the choice of $ \mu_{\mathrm{R}} $ and $ \mu_{\mathrm{F}} $ and PDFs, including $ \alpha_\mathrm{S} $ variations. The black points represent the measured values, with the total uncertainty, while the red error bar shows the results considering only the statistical uncertainty. |
| Tables | |
png pdf |
Table 1:
Summary of the systematic uncertainty sources in the inclusive and differential $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ cross section, $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $/ $ \mathrm{t} \overline{\mathrm{t}} $ ratio, and charge asymmetry measurements. The first column lists the source of the uncertainty, while the second (third) column indicates the treatment of correlations of the uncertainties between different data-taking periods (processes), where $ \checkmark $ means fully correlated, $ \sim $ means partially correlated (i.e., contains sub-sources that are either fully correlated or uncorrelated), $ \times $ means uncorrelated, and $ \text{---} $ means not applicable. |
png pdf |
Table 2:
Definition of the fiducial phase space. |
| Summary |
| A comprehensive study of the top quark pair ($ \mathrm{t} \overline{\mathrm{t}} $) production in association with a photon ($ \gamma $) at the LHC is presented, using data collected by the CMS experiment in 2016-2018 at a centre-of-mass energy of 13 TeV, and corresponding to an integrated luminosity of 138 fb$^{-1}$. Inclusive and differential measurements are performed in the dilepton decay channels, in a fiducial region at the particle level including events with photon transverse momentum larger than 20 GeV. The inclusive fiducial cross section for $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ with a photon radiated at any stage of the process is 137 $ \pm $ 3 (stat) $ \pm $ 7 (syst) fb, while the cross section for events with a photon radiated at the production stage of the process is 56 $ \pm $ 2 (stat) $ \pm $ 4 (syst) fb. The measured cross sections agree with the predictions from the standard model (SM) for the combined $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ process and for the $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ process with photons from the production stage. The cross section is also measured differentially, in bins of seven different observables, related to the kinematic properties and topology of the photon, the leptons, and the top quarks reconstructed in the event. The predictions from simulation accurately describe the shape of the measured cross sections. The $ {\mathrm{t}\overline{\mathrm{t}}} \gamma / {\mathrm{t}\overline{\mathrm{t}}} $ cross section ratio is measured for the first time, inclusively and differentially. The inclusive ratio is found to be 0.0133 $ \pm $ 0.0002 (stat) $ \pm $ 0.0005 (syst), in agreement with the nominal predictions from simulation. The differential ratios are well described by the predictions, within the total uncertainty. The top quark charge asymmetry in $ {\mathrm{t}\overline{\mathrm{t}}} \gamma $ events is also measured to be $- $1.2 $ \pm $ 4.1 (stat) $ \pm $ 0.9 (syst)%, compatible with both the SM prediction at next-to-leading order in quantum chromodynamics and with no asymmetry. |
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