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| CMS-TOP-18-010 ; CERN-EP-2021-117 | ||
| Measurement of the inclusive and differential $\mathrm{t\bar{t}}\gamma$ cross sections in the single-lepton channel and EFT interpretation at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 3 July 2021 | ||
| JHEP 12 (2021) 180 | ||
| Abstract: The production cross section of a top quark pair in association with a photon is measured in proton-proton collisions at a center-of-mass energy of 13 TeV. The data set, corresponding to an integrated luminosity of 137 fb$^{-1}$, was recorded by the CMS experiment during the 2016-2018 data taking of the LHC. The measurements are performed in a fiducial volume defined at the particle level. Events with an isolated, highly energetic lepton, at least three jets from the hadronization of quarks, among which at least one is b tagged, and one isolated photon are selected. The inclusive fiducial ${\mathrm{t\bar{t}}\gamma}$ cross section, for a photon with transverse momentum greater than 20 GeV and pseudorapidity ${|\eta|} < $ 1.4442, is measured to be 800 $\pm$ 7 (stat) $\pm$ 46 (syst) fb, in goo d agreement with the prediction from the standard model at next-to-leading order in quantum chromodynamics. The differential cross sections are also measured as a function of several kinematic observables and interpreted in the framework of the standard model effective field theory (EFT), leading to the most stringent direct limits to date on anomalous electromagnetic dipole moment interactions of the top quark and the photon. | ||
| Links: e-print arXiv:2107.01508 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Representative LO Feynman diagrams for the ${{\mathrm{t} \mathrm{\bar{t}}} \gamma}$ signal process in the single-lepton channel, where the highly energetic photon originates from the top quark (left), or is emitted from a lepton (right). The ${{\mathrm{t} \mathrm{\bar{t}}} \gamma}$ interaction vertex is indicated by a circle. |
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Figure 1-a:
LO Feynman diagram for the ${{\mathrm{t} \mathrm{\bar{t}}} \gamma}$ signal process in the single-lepton channel, where the highly energetic photon originates from the top quark. The ${{\mathrm{t} \mathrm{\bar{t}}} \gamma}$ interaction vertex is indicated by a circle. |
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Figure 1-b:
LO Feynman diagram for the ${{\mathrm{t} \mathrm{\bar{t}}} \gamma}$ signal process in the single-lepton channel, where the highly energetic photon is emitted from a lepton. |
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Figure 2:
Distribution of ${{p_{\mathrm {T}}} (\gamma)}$, the transverse mass ${{m_{\mathrm {T}}} (\mathrm{W})}$ of the W boson candidate, the three-jet invariant mass ${M_3}$ (upper row); the invariant mass ${m(\ell,\gamma)}$ of the lepton and the photon, the angular separation ${\Delta R(\ell,\gamma)}$ of the lepton and the photon, and the minimal angular separation $\textrm {min}\, {\Delta R}(j, \gamma)$ of the photon and all jets (lower row) in the SR3p region. The backgrounds are normalized according to the methods described in Sec. 7. The systematic uncertainties are shown as a hatched band. |
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Figure 2-a:
Distribution of ${{p_{\mathrm {T}}} (\gamma)}$, in the SR3p region. The backgrounds are normalized according to the methods described in Sec. 7. The systematic uncertainties are shown as a hatched band. |
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Figure 2-b:
Distribution of the transverse mass ${{m_{\mathrm {T}}} (\mathrm{W})}$ of the W boson candidate, in the SR3p region. The backgrounds are normalized according to the methods described in Sec. 7. The systematic uncertainties are shown as a hatched band. |
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Figure 2-c:
Distribution of the three-jet invariant mass ${M_3}$, in the SR3p region. The backgrounds are normalized according to the methods described in Sec. 7. The systematic uncertainties are shown as a hatched band. |
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Figure 2-d:
Distribution of the invariant mass ${m(\ell,\gamma)}$ of the lepton and the photon, in the SR3p region. The backgrounds are normalized according to the methods described in Sec. 7. The systematic uncertainties are shown as a hatched band. |
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Figure 2-e:
Distribution of the angular separation ${\Delta R(\ell,\gamma)}$ of the lepton and the photon, in the SR3p region. The backgrounds are normalized according to the methods described in Sec. 7. The systematic uncertainties are shown as a hatched band. |
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Figure 2-f:
Distribution of the minimal angular separation $\textrm {min}\, {\Delta R}(j, \gamma)$ of the photon and all jets, in the SR3p region. The backgrounds are normalized according to the methods described in Sec. 7. The systematic uncertainties are shown as a hatched band. |
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Figure 3:
Fit result of the ${{m_{\mathrm {T}}} (\mathrm{W})}$ multijet distribution in the selection with $ {N_\text {j}} =$ 2, $ {N_\mathrm{b}} =$ 0, and tightly isolated electrons (left) and muons (right). The template obtained from the selection with loosely isolated leptons (green) and the total normalization of the electroweak background are floating in the fit. The lower panels show the ratio of the observed to the predicted event yields. The systematic uncertainties are shown as a hatched band. |
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Figure 3-a:
Fit result of the ${{m_{\mathrm {T}}} (\mathrm{W})}$ multijet distribution in the selection with $ {N_\text {j}} =$ 2, $ {N_\mathrm{b}} =$ 0, and tightly isolated electrons. The template obtained from the selection with loosely isolated leptons (green) and the total normalization of the electroweak background are floating in the fit. The lower panel shows the ratio of the observed to the predicted event yields. The systematic uncertainties are shown as a hatched band. |
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Figure 3-b:
Fit result of the ${{m_{\mathrm {T}}} (\mathrm{W})}$ multijet distribution in the selection with $ {N_\text {j}} =$ 2, $ {N_\mathrm{b}} =$ 0, and tightly isolated muons. The template obtained from the selection with loosely isolated leptons (green) and the total normalization of the electroweak background are floating in the fit. The lower panel shows the ratio of the observed to the predicted event yields. The systematic uncertainties are shown as a hatched band. |
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Figure 4:
Distribution of the invariant mass of the lepton and the photon, ${m(\ell,\gamma)}$, in the ${N_\text {j}} \geq $ 3, ${N_\mathrm{b}} =$ 0 selection for the e channel (left) and the $\mu$ channel (right). The genuine photon contributions of W$\gamma$ and Z$\gamma$ are visualized separately. The lower panels show the ratio of the observed to the predicted event yields. The systematic uncertainties are shown as a hatched band. |
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Figure 4-a:
Distribution of the invariant mass of the lepton and the photon, ${m(\ell,\gamma)}$, in the ${N_\text {j}} \geq $ 3, ${N_\mathrm{b}} =$ 0 selection for the e channel. The genuine photon contributions of W$\gamma$ and Z$\gamma$ are visualized separately. The lower panels show the ratio of the observed to the predicted event yields. The systematic uncertainties are shown as a hatched band. |
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Figure 4-b:
Distribution of the invariant mass of the lepton and the photon, ${m(\ell,\gamma)}$, in the ${N_\text {j}} \geq $ 3, ${N_\mathrm{b}} =$ 0 selection for the $\mu$ channel. The genuine photon contributions of W$\gamma$ and Z$\gamma$ are visualized separately. The lower panels show the ratio of the observed to the predicted event yields. The systematic uncertainties are shown as a hatched band. |
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Figure 5:
Fitted and observed yields in the LM3, LM4p, HM3, HM4p, misDY3, and misDY4p control regions using the post-fit values of the nuisance parameters. The lower panel shows the ratio of the observed to the predicted event yields. The hatched band shows the systematic uncertainty in the background prediction. |
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Figure 6:
Fitted and observed yields in the SR3 and SR4p signal regions using the post-fit values of the nuisance parameters. The lower panel shows the ratio of the observed to the predicted event yields. The systematic uncertainties are shown as a hatched band. |
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Figure 7 :
Ranking of the systematic uncertainties from the profile likelihood fit used in the inclusive cross section measurement. For each uncertainty, the red and blue bands indicate the post-fit impact on the fit result. The black dots indicate the post-fit values of the nuisance parameters and the numerical values provide the extracted SFs for the misidentified electron background and the normalization of the W$\gamma$ process. The black lines represent the post-fit uncertainties normalized to the pre-fit uncertainties (constraints). |
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Figure 8:
Summary of the measured cross section ratios with respect to the NLO cross section prediction for $ {N_\text {j}} =$ 3, $\geq $4, and combined signal regions in the electron channel, muon channel, and the combined single-lepton channel. The orange band indicates the theory uncertainty in the prediction. |
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Figure 9:
The unfolded differential cross sections for ${{p_{\mathrm {T}}} (\gamma)}$ (upper left), ${| \eta (\gamma) |}$ (upper right), and ${\Delta R(\ell,\gamma)}$ (lower) compared with simulation obtained from the MadGraph 5_aMC@NLO event generator interfaced to PYTHIA (red, solid), HERWIG 7 (black, dashed) and HERWIG++ (green, dotted) for the parton shower and hadronization. For ${{p_{\mathrm {T}}} (\gamma)}$ and ${\Delta R(\ell,\gamma)}$, the last bin includes the overflow. The lower panel displays the ratio of simulation to the observation. The inner and outer bands show the statistical and total uncertainties, respectively. Photons radiated from leptons and satisfying $ {\Delta R(\ell,\gamma)} > $ 0.4 are included in the signal and contribute significantly to the first bin of the differential ${\Delta R(\ell,\gamma)}$ cross section. |
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Figure 9-a:
The unfolded differential cross section for ${{p_{\mathrm {T}}} (\gamma)}$ compared with simulation obtained from the MadGraph 5_aMC@NLO event generator interfaced to PYTHIA (red, solid), HERWIG 7 (black, dashed) and HERWIG++ (green, dotted) for the parton shower and hadronization. For ${{p_{\mathrm {T}}} (\gamma)}$ and ${\Delta R(\ell,\gamma)}$, the last bin includes the overflow. The lower panel displays the ratio of simulation to the observation. The inner and outer bands show the statistical and total uncertainties, respectively. Photons radiated from leptons and satisfying $ {\Delta R(\ell,\gamma)} > $ 0.4 are included in the signal. |
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Figure 9-b:
The unfolded differential cross section for ${| \eta (\gamma) |}$ compared with simulation obtained from the MadGraph 5_aMC@NLO event generator interfaced to PYTHIA (red, solid), HERWIG 7 (black, dashed) and HERWIG++ (green, dotted) for the parton shower and hadronization. For ${{p_{\mathrm {T}}} (\gamma)}$ and ${\Delta R(\ell,\gamma)}$, the last bin includes the overflow. The lower panel displays the ratio of simulation to the observation. The inner and outer bands show the statistical and total uncertainties, respectively. Photons radiated from leptons and satisfying $ {\Delta R(\ell,\gamma)} > $ 0.4 are included in the signal. |
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Figure 9-c:
The unfolded differential cross section for ${\Delta R(\ell,\gamma)}$ compared with simulation obtained from the MadGraph 5_aMC@NLO event generator interfaced to PYTHIA (red, solid), HERWIG 7 (black, dashed) and HERWIG++ (green, dotted) for the parton shower and hadronization. For ${{p_{\mathrm {T}}} (\gamma)}$ and ${\Delta R(\ell,\gamma)}$, the last bin includes the overflow. The lower panel displays the ratio of simulation to the observation. The inner and outer bands show the statistical and total uncertainties, respectively. Photons radiated from leptons and satisfying $ {\Delta R(\ell,\gamma)} > $ 0.4 are included in the signal and contribute significantly to the first bin of the differential cross section. |
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Figure 10:
The correlation matrices of systematic uncertainties for the unfolded differential measurement for ${{p_{\mathrm {T}}} (\gamma)}$ (upper left), ${| \eta (\gamma) |}$ (upper right), and ${\Delta R(\ell,\gamma)}$ (lower). |
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Figure 10-a:
The correlation matrix of systematic uncertainties for the unfolded differential measurement for ${{p_{\mathrm {T}}} (\gamma)}$. |
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Figure 10-b:
The correlation matrix of systematic uncertainties for the unfolded differential measurement for ${| \eta (\gamma) |}$. |
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Figure 10-c:
The correlation matrix of systematic uncertainties for the unfolded differential measurement for ${\Delta R(\ell,\gamma)}$. |
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Figure 11:
The observed (points) and predicted (shaded histograms) post-fit yields for the combined Run 2 data set in the SR3 (upper) and SR4p (lower) signal regions for the electron (left) and muon channel (right). The vertical bars on the points give the statistical uncertainties in the data. The lower panel displays the ratio of the data to the predictions and the hatched regions show the total uncertainty. The solid line shows the SM-EFT best fit prediction and the dashed lines show different predictions for non-zero Wilson coefficients, $ {c_{\mathrm{t} \mathrm{Z}}} =0.45$ (light blue), $ {c_{\mathrm{t} \mathrm{Z}}^\mathrm {I}} =0.45$ (green), and $ {c_{\mathrm{t} \mathrm{Z}}} =-0.45$ (dark blue), where $\Lambda $ is set to 1 TeV. |
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Figure 11-a:
The observed (points) and predicted (shaded histograms) post-fit yields for the combined Run 2 data set in the SR3 signal region for the electron channel. The vertical bars on the points give the statistical uncertainties in the data. The lower panel displays the ratio of the data to the predictions and the hatched regions show the total uncertainty. The solid line shows the SM-EFT best fit prediction and the dashed lines show different predictions for non-zero Wilson coefficients, $ {c_{\mathrm{t} \mathrm{Z}}} =0.45$ (light blue), $ {c_{\mathrm{t} \mathrm{Z}}^\mathrm {I}} =0.45$ (green), and $ {c_{\mathrm{t} \mathrm{Z}}} =-0.45$ (dark blue), where $\Lambda $ is set to 1 TeV. |
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Figure 11-b:
The observed (points) and predicted (shaded histograms) post-fit yields for the combined Run 2 data set in the SR3 signal region for the muon channel. The vertical bars on the points give the statistical uncertainties in the data. The lower panel displays the ratio of the data to the predictions and the hatched regions show the total uncertainty. The solid line shows the SM-EFT best fit prediction and the dashed lines show different predictions for non-zero Wilson coefficients, $ {c_{\mathrm{t} \mathrm{Z}}} =0.45$ (light blue), $ {c_{\mathrm{t} \mathrm{Z}}^\mathrm {I}} =0.45$ (green), and $ {c_{\mathrm{t} \mathrm{Z}}} =-0.45$ (dark blue), where $\Lambda $ is set to 1 TeV. |
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Figure 11-c:
The observed (points) and predicted (shaded histograms) post-fit yields for the combined Run 2 data set in the SR4p signal region for the electron channel. The vertical bars on the points give the statistical uncertainties in the data. The lower panel displays the ratio of the data to the predictions and the hatched regions show the total uncertainty. The solid line shows the SM-EFT best fit prediction and the dashed lines show different predictions for non-zero Wilson coefficients, $ {c_{\mathrm{t} \mathrm{Z}}} =0.45$ (light blue), $ {c_{\mathrm{t} \mathrm{Z}}^\mathrm {I}} =0.45$ (green), and $ {c_{\mathrm{t} \mathrm{Z}}} =-0.45$ (dark blue), where $\Lambda $ is set to 1 TeV. |
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Figure 11-d:
The observed (points) and predicted (shaded histograms) post-fit yields for the combined Run 2 data set in the SR4p signal region for the muon channel. The vertical bars on the points give the statistical uncertainties in the data. The lower panel displays the ratio of the data to the predictions and the hatched regions show the total uncertainty. The solid line shows the SM-EFT best fit prediction and the dashed lines show different predictions for non-zero Wilson coefficients, $ {c_{\mathrm{t} \mathrm{Z}}} =0.45$ (light blue), $ {c_{\mathrm{t} \mathrm{Z}}^\mathrm {I}} =0.45$ (green), and $ {c_{\mathrm{t} \mathrm{Z}}} =-0.45$ (dark blue), where $\Lambda $ is set to 1 TeV. |
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Figure 12:
Results of the one-dimensional scans of the Wilson coefficients ${c_{\mathrm{t} \mathrm{Z}}}$ (left) and ${c_{\mathrm{t} \mathrm{Z}}^\mathrm {I}}$ (right). In the upper row, the other Wilson coefficient is profiled, while in the lower row it is set to zero. The green and orange bands indicate the 68 and 95% CL contours on the Wilson coefficients, respectively. |
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Figure 12-a:
Results of the one-dimensional scans of the Wilson coefficient ${c_{\mathrm{t} \mathrm{Z}}}$. The other Wilson coefficient is profiled. The green and orange bands indicate the 68 and 95% CL contours on the Wilson coefficients, respectively. |
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Figure 12-b:
Results of the one-dimensional scans of the Wilson coefficient ${c_{\mathrm{t} \mathrm{Z}}^\mathrm {I}}$. The other Wilson coefficient is profiled. The green and orange bands indicate the 68 and 95% CL contours on the Wilson coefficients, respectively. |
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Figure 12-c:
Results of the one-dimensional scans of the Wilson coefficient ${c_{\mathrm{t} \mathrm{Z}}}$. The other Wilson coefficient is set to zero. The green and orange bands indicate the 68 and 95% CL contours on the Wilson coefficients, respectively. |
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Figure 12-d:
Results of the one-dimensional scans of the Wilson coefficient ${c_{\mathrm{t} \mathrm{Z}}^\mathrm {I}}$. The other Wilson coefficient is set to zero. The green and orange bands indicate the 68 and 95% CL contours on the Wilson coefficients, respectively. |
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Figure 13:
Result of the two-dimensional scan of the Wilson coefficients ${c_{\mathrm{t} \mathrm{Z}}}$ and ${c_{\mathrm{t} \mathrm{Z}}^\mathrm {I}}$. The shading quantified by the color scale on the right reflects the negative log-likelihood ratio with respect to the best fit value that is designated by the star. The green and orange lines indicate the 68 and 95% CL contours from the fit, respectively. The allowed areas are those between the two green contours and that inside the orange contour. The dot shows the SM prediction. |
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Figure 14:
The observed 95% CL intervals for the Wilson coefficients from this measurement with the other Wilson coefficient set to zero, the previous CMS results based on the inclusive [97] and differential [95] ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{Z}}$ cross section measurement, a CMS result based on ${\mathrm{t} \mathrm{\bar{t}}}$ in final states with additional leptons [94], and the most recent ATLAS result [96]. The result of a global SM-EFT analysis, including results from Ref. [95], is also shown [98]. The vertical line displays the SM prediction. |
| Tables | |
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Table 1:
Event generator, perturbative order in QCD of the simulation, and perturbative order of the cross section normalization for each process. |
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Table 2:
Overview of the definition of fiducial regions for various objects at particle level. A photon is isolated, if there are no stable particles (except neutrinos) with $ {p_{\mathrm {T}}} > $ 5 GeV within a cone of $ {\Delta R}=$ 0.1. |
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Table 3:
Overview of signal and control regions. |
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Table 4:
Extracted SFs for the contribution from misidentified electrons for the three data-taking periods and for the normalization of the Z$\gamma$ and W$\gamma$ background components obtained from the likelihood fit. |
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Table 5:
Breakdown of the total uncertainty in its statistical and systematic components. The first column indicates the source of the uncertainty. The second column shows the correlation between the data-taking periods. The third column shows the typical pre-fit uncertainties in the total simulated yields in the signal region. The last column gives the corresponding systematic uncertainty in the ${{\mathrm{t} \mathrm{\bar{t}}} \gamma}$ cross section from the fit to the data. |
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Table 6 :
The observed number of events for the SR3 and SR4p signal regions in the e and $\mu$ channels, and the predicted yields and total post-fit uncertainties in each background component. |
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Table 7:
Binning choices in the differential measurements at the reconstruction level. |
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Table 8:
Summary of the one-dimensional intervals at 68 and 95% CL. |
| Summary |
|
A measurement of the cross section for the top quark pair production in association with a photon using a data sample of proton-proton collisions at $\sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 137 fb$^{-1}$, collected with the CMS detector at the LHC has been presented. It is the first result of the CMS Collaboration on measurements in the ${\mathrm{t\bar{t}}\gamma}$ final state using 13 TeV data. The analysis has been performed in the single-lepton channel with events with exactly three and four or more jets among which at least one is b tagged. Background components with misidentified electrons, photons originating in the hadronization of jets, the multijet component, and prompt photons from the W$\gamma$ and Z$\gamma$ processes are estimated from data. The measured inclusive cross section in a fiducial region with photon transverse momentum ${{p_{\mathrm {T}}} (\gamma)} > $ 20 GeV and jet multiplicity greater than 3 is measured to be 800 $\pm$ 7 (stat) $\pm$ 46 (syst) fb, in good agreement with the standard model prediction at next-to-leading order in quantum chromodynamics. Differential cross sections for ${{p_{\mathrm {T}}} (\gamma)}$ and absolute value of the photon pseudorapidity, as well as for the angular separation of the lepton and the photon, have been measured and unfolded to particle level in the same fiducial volume. The comparison to simulation was performed using different showering algorithms. The measurements are also interpreted in terms of limits on the Wilson coefficients in the context of the standard model effective field theory. The confidence intervals for the Wilson coefficients ${c_{\mathrm{t}\mathrm{Z}}} $ and ${c_{\mathrm{t}\mathrm{Z}}^\mathrm{I}}$ are the most stringent to date. |
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