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| CMS-PAS-EXO-21-017 | ||
| Search for a resonance decaying to a W boson and a photon in proton-proton collisions at $ \sqrt{s}= $ 13 TeV using leptonic W boson decays | ||
| CMS Collaboration | ||
| 26 March 2024 | ||
| Abstract: A search for a new charged particle X with mass between 0.3-2.0 TeV decaying to a W boson and a photon using proton-proton collision data at a center-of-mass energy of 13 TeV collected by the CMS experiment and corresponding to an integrated luminosity of 138 fb$ ^{-1} $ is presented. The search is performed using the electron and muon decays of the W boson. No significant excess above the predicted background is observed. The upper limit at the 95% confidence level on the product of the X production cross section and its branching fraction to a W boson and a photon is found to be 94 (137) fb for a 0.3 TeV resonance and 0.75 (0.81) fb for a 2.0 TeV resonance, for an X width to mass ratio of 0.01% (5%). | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Leading order Feynman diagram for heavy particle X decaying to a photon and a W boson that decays leptonically. |
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Figure 2:
The red, blue, and yellow curves are the efficiencies of different particle mass assumptions$-$300, 1000, and 2000 GeV, respectively$-$to pass sequential requirements. The first bin is reconstruction, which indicates that exactly one basic lepton and one photon are selected, as described in Table 1. All other selections are summarized in Table 2. The left (right) plot is for the electron (muon) channel. |
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Figure 2-a:
The red, blue, and yellow curves are the efficiencies of different particle mass assumptions$-$300, 1000, and 2000 GeV, respectively$-$to pass sequential requirements. The first bin is reconstruction, which indicates that exactly one basic lepton and one photon are selected, as described in Table 1. All other selections are summarized in Table 2. The left (right) plot is for the electron (muon) channel. |
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Figure 2-b:
The red, blue, and yellow curves are the efficiencies of different particle mass assumptions$-$300, 1000, and 2000 GeV, respectively$-$to pass sequential requirements. The first bin is reconstruction, which indicates that exactly one basic lepton and one photon are selected, as described in Table 1. All other selections are summarized in Table 2. The left (right) plot is for the electron (muon) channel. |
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Figure 3:
Signal efficiencies in the electron (left) and muon (right) channel for the broad width ($ \Gamma_{X}/m_{X}=$ 5%) hypotheses as a function of the mass of particle X. The narrow width signals have similar efficiencies as the broad width signals. Three analysis requirements are applied consecutively: event reconstruction, HLT, and final signal selection. The absolute efficiencies (detector acceptance $ \times $ analysis selections) are shown at each stage in red, blue, and yellow, respectively. |
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Figure 3-a:
Signal efficiencies in the electron (left) and muon (right) channel for the broad width ($ \Gamma_{X}/m_{X}=$ 5%) hypotheses as a function of the mass of particle X. The narrow width signals have similar efficiencies as the broad width signals. Three analysis requirements are applied consecutively: event reconstruction, HLT, and final signal selection. The absolute efficiencies (detector acceptance $ \times $ analysis selections) are shown at each stage in red, blue, and yellow, respectively. |
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Figure 3-b:
Signal efficiencies in the electron (left) and muon (right) channel for the broad width ($ \Gamma_{X}/m_{X}=$ 5%) hypotheses as a function of the mass of particle X. The narrow width signals have similar efficiencies as the broad width signals. Three analysis requirements are applied consecutively: event reconstruction, HLT, and final signal selection. The absolute efficiencies (detector acceptance $ \times $ analysis selections) are shown at each stage in red, blue, and yellow, respectively. |
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Figure 4:
Distributions of $ m_{\mathrm{T}} $ (left) and $ p_{\mathrm{T}}\left(\gamma\right) $ (right) from simulation (stacked histograms) and data events (black points) passing all analysis selections. The number of events is divided by the width of each individual bin. The simulation distributions agree with data within statistical precision. Four signal distributions, with two mass assumptions and two width assumptions, are also overlaid (dashed and solid lines). Each signal is plotted with a total cross section of 3 fb. Because of the limited MC statistics in the high resonance mass region, a progressively larger bin size is used. The last bin also includes all events with values beyond right-most edge of the histogram. |
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Figure 4-a:
Distributions of $ m_{\mathrm{T}} $ (left) and $ p_{\mathrm{T}}\left(\gamma\right) $ (right) from simulation (stacked histograms) and data events (black points) passing all analysis selections. The number of events is divided by the width of each individual bin. The simulation distributions agree with data within statistical precision. Four signal distributions, with two mass assumptions and two width assumptions, are also overlaid (dashed and solid lines). Each signal is plotted with a total cross section of 3 fb. Because of the limited MC statistics in the high resonance mass region, a progressively larger bin size is used. The last bin also includes all events with values beyond right-most edge of the histogram. |
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Figure 4-b:
Distributions of $ m_{\mathrm{T}} $ (left) and $ p_{\mathrm{T}}\left(\gamma\right) $ (right) from simulation (stacked histograms) and data events (black points) passing all analysis selections. The number of events is divided by the width of each individual bin. The simulation distributions agree with data within statistical precision. Four signal distributions, with two mass assumptions and two width assumptions, are also overlaid (dashed and solid lines). Each signal is plotted with a total cross section of 3 fb. Because of the limited MC statistics in the high resonance mass region, a progressively larger bin size is used. The last bin also includes all events with values beyond right-most edge of the histogram. |
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Figure 5:
Background-only fit to data (black points) with the fitted background model shown as a blue line. The green (inner) and yellow (outer) bands show, respectively, the 68% and 95% CL statistical uncertainties in the fitted model. Four signal models are also overlaid. Their total cross sections are set to the 95% confidence level expected limit, correspondingly. There are two mass assumptions$-$500 and 1400 GeV, in red and magenta, respectively$-$and two width assumptions$-$narrow (solid curves) and broad (dashed curves). The lower panel contains the pull distribution, defined as the difference between the data yield and the background prediction divided by their combined uncertainty. The left (right) panel is the electron (muon) channel with all three years' data combined. |
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Figure 5-a:
Background-only fit to data (black points) with the fitted background model shown as a blue line. The green (inner) and yellow (outer) bands show, respectively, the 68% and 95% CL statistical uncertainties in the fitted model. Four signal models are also overlaid. Their total cross sections are set to the 95% confidence level expected limit, correspondingly. There are two mass assumptions$-$500 and 1400 GeV, in red and magenta, respectively$-$and two width assumptions$-$narrow (solid curves) and broad (dashed curves). The lower panel contains the pull distribution, defined as the difference between the data yield and the background prediction divided by their combined uncertainty. The left (right) panel is the electron (muon) channel with all three years' data combined. |
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Figure 5-b:
Background-only fit to data (black points) with the fitted background model shown as a blue line. The green (inner) and yellow (outer) bands show, respectively, the 68% and 95% CL statistical uncertainties in the fitted model. Four signal models are also overlaid. Their total cross sections are set to the 95% confidence level expected limit, correspondingly. There are two mass assumptions$-$500 and 1400 GeV, in red and magenta, respectively$-$and two width assumptions$-$narrow (solid curves) and broad (dashed curves). The lower panel contains the pull distribution, defined as the difference between the data yield and the background prediction divided by their combined uncertainty. The left (right) panel is the electron (muon) channel with all three years' data combined. |
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Figure 6:
The systematic uncertainties affecting signal normalization in the electron (upper) and muon (lower) channel for the broad resonance are shown. Error bars represent the asymmetric uncertainties (downward and upward variations), with the central value representing the average uncertainty. The total systematic uncertainty (black) increases steadily with the resonant mass assumption. The uncertainty is decomposed into five parts: photon-related (brown), lepton-related (yellow), integrated luminosity (green), theoretical (red), and all other (blue) uncertainties. The total uncertainty ranges from 3.2 (2.9) to 8.3 (6.7)% in the electron (muon) channel. The uncertainties for the narrow width case are nearly identical. |
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Figure 6-a:
The systematic uncertainties affecting signal normalization in the electron (upper) and muon (lower) channel for the broad resonance are shown. Error bars represent the asymmetric uncertainties (downward and upward variations), with the central value representing the average uncertainty. The total systematic uncertainty (black) increases steadily with the resonant mass assumption. The uncertainty is decomposed into five parts: photon-related (brown), lepton-related (yellow), integrated luminosity (green), theoretical (red), and all other (blue) uncertainties. The total uncertainty ranges from 3.2 (2.9) to 8.3 (6.7)% in the electron (muon) channel. The uncertainties for the narrow width case are nearly identical. |
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Figure 6-b:
The systematic uncertainties affecting signal normalization in the electron (upper) and muon (lower) channel for the broad resonance are shown. Error bars represent the asymmetric uncertainties (downward and upward variations), with the central value representing the average uncertainty. The total systematic uncertainty (black) increases steadily with the resonant mass assumption. The uncertainty is decomposed into five parts: photon-related (brown), lepton-related (yellow), integrated luminosity (green), theoretical (red), and all other (blue) uncertainties. The total uncertainty ranges from 3.2 (2.9) to 8.3 (6.7)% in the electron (muon) channel. The uncertainties for the narrow width case are nearly identical. |
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Figure 7:
The 95% CL expected and observed limits on $ \sigma\mathcal{B}(X\rightarrow \mathrm{W}\gamma) $ from events with leptonic decays of the W boson as a function of the X resonant mass. The results for the narrow (broad) width assumption are shown on the left (right). |
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Figure 7-a:
The 95% CL expected and observed limits on $ \sigma\mathcal{B}(X\rightarrow \mathrm{W}\gamma) $ from events with leptonic decays of the W boson as a function of the X resonant mass. The results for the narrow (broad) width assumption are shown on the left (right). |
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Figure 7-b:
The 95% CL expected and observed limits on $ \sigma\mathcal{B}(X\rightarrow \mathrm{W}\gamma) $ from events with leptonic decays of the W boson as a function of the X resonant mass. The results for the narrow (broad) width assumption are shown on the left (right). |
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Figure 8:
The 95% CL expected and observed limits on $ \sigma\mathcal{B}(X\rightarrow \mathrm{W}\gamma) $ utilizing both hadronic and leptonic W decays as a function of the X resonant mass. The results for the narrow (broad) width assumption are shown on the left (right). |
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Figure 8-a:
The 95% CL expected and observed limits on $ \sigma\mathcal{B}(X\rightarrow \mathrm{W}\gamma) $ utilizing both hadronic and leptonic W decays as a function of the X resonant mass. The results for the narrow (broad) width assumption are shown on the left (right). |
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Figure 8-b:
The 95% CL expected and observed limits on $ \sigma\mathcal{B}(X\rightarrow \mathrm{W}\gamma) $ utilizing both hadronic and leptonic W decays as a function of the X resonant mass. The results for the narrow (broad) width assumption are shown on the left (right). |
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Figure 9:
The observed local $ p $-values for narrow (left) and broad (right) resonance width hypotheses in the leptonic (yellow) and hadronic (red) channel. The blue line shows the observed local $ p $-values after combination. In the hadronic channel, the largest excess corresponds to a local significance of 2.8 (3.1) standard deviations (s.d.) for narrow (broad) signals. After combining with the leptonic channel, the largest excess is 2.7 (2.5) s.d. |
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Figure 9-a:
The observed local $ p $-values for narrow (left) and broad (right) resonance width hypotheses in the leptonic (yellow) and hadronic (red) channel. The blue line shows the observed local $ p $-values after combination. In the hadronic channel, the largest excess corresponds to a local significance of 2.8 (3.1) standard deviations (s.d.) for narrow (broad) signals. After combining with the leptonic channel, the largest excess is 2.7 (2.5) s.d. |
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Figure 9-b:
The observed local $ p $-values for narrow (left) and broad (right) resonance width hypotheses in the leptonic (yellow) and hadronic (red) channel. The blue line shows the observed local $ p $-values after combination. In the hadronic channel, the largest excess corresponds to a local significance of 2.8 (3.1) standard deviations (s.d.) for narrow (broad) signals. After combining with the leptonic channel, the largest excess is 2.7 (2.5) s.d. |
| Tables | |
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Table 1:
Basic object selection requirements. Definitions are described in more detail in the text. |
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Table 2:
Event selection requirements for the electron and the muon channels. Definitions are described in more detail in the text. |
| Summary |
| This study presents a comprehensive search for a new particle X decaying into a W boson and a photon with mass hypotheses from 0.3 to 2.0 TeV. Events with a muon or an electron, large $ p_{\mathrm{T}}^\text{miss} $, and a high-$ p_{\mathrm{T}} $ photon are analyzed. The transverse mass of the lepton, photon, and $ p_{\mathrm{T}}^\text{miss} $ is the primary kinematic observable. The search utilizes proton-proton collision data collected at a center-of-mass energy of 13 TeV with the CMS detector at the LHC throughout 2016--2018, corresponding to an integrated luminosity of 138 fb$^{-1}$. This search reveals no statistically significant excess of events above the smoothly decreasing background. Upper limits at the 95% confidence level on the product of the cross section and branching fraction for W$ \gamma $ resonances are set. These limits span a range from 94 (137) to 0.75 (0.81) fb for the narrow (broad) resonance hypothesis. These findings represent the most stringent constraints to date on the existence of such resonances across the probed mass range. This search complements an earlier study involving the hadronic decay mode of the W boson using the full 13 TeV data sample [7]. By combining searches for both leptonic and hadronic decays of W bosons, the largest local excess seen in the hadronic channel is reduced from 3.1 to 2.5 standard deviations for the broad signal width hypothesis. Upper limits at the 95% confidence level at 2 TeV is reduced from 0.75 (0.81) to 0.50 (0.63) fb for the narrow (broad) resonance hypothesis. |
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