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| CMS-EXO-21-017 ; CERN-EP-2024-143 | ||
| 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 | ||
| 7 June 2024 | ||
| Submitted to J. High Energy Phys. | ||
| Abstract: A search for a new charged particle X with mass between 0.3 and 2.0 TeV decaying to a W boson and a photon is presented, 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} $. Particle X has electric charge $ \pm $1 and is assumed to have spin 0. 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 95% confidence level on the product of the production cross section of the X 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%). This search presents the most stringent constraints to date on the existence of such resonances across the probed mass range. A statistical combination with an earlier study based on the hadronic decay mode of the W boson is also performed, and the upper limit at 95% confidence level for a 2.0 TeV resonance is reduced to 0.50 (0.63) fb for an X width-to-mass ratio of 0.01% (5%). | ||
| Links: e-print arXiv:2406.05737 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Leading order Feynman diagram for a heavy particle X decaying to a W boson and a photon; the W boson subsequentially decays leptonically. |
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Figure 2:
Product of detector acceptance and analysis selections efficiency for different particle mass assumptions---300, 1000, and 2000 GeV, in red, blue, and orange, respectively---to pass sequential requirements in the broad-width case. The narrow-width case is similar. The first bin represents selecting events with exactly one reconstructed basic electron or muon and one photon, satisfying the selection criteria reported in Table 1. The next bin contains events satisfying the HLT, and the subsequent bins have the selections listed in Table 2 applied sequentially. The left (right) plot is for the electron (muon) channel. |
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Figure 2-a:
Product of detector acceptance and analysis selections efficiency for different particle mass assumptions---300, 1000, and 2000 GeV, in red, blue, and orange, respectively---to pass sequential requirements in the broad-width case. The narrow-width case is similar. The first bin represents selecting events with exactly one reconstructed basic electron or muon and one photon, satisfying the selection criteria reported in Table 1. The next bin contains events satisfying the HLT, and the subsequent bins have the selections listed in Table 2 applied sequentially. The left (right) plot is for the electron (muon) channel. |
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Figure 2-b:
Product of detector acceptance and analysis selections efficiency for different particle mass assumptions---300, 1000, and 2000 GeV, in red, blue, and orange, respectively---to pass sequential requirements in the broad-width case. The narrow-width case is similar. The first bin represents selecting events with exactly one reconstructed basic electron or muon and one photon, satisfying the selection criteria reported in Table 1. The next bin contains events satisfying the HLT, and the subsequent bins have the selections listed in Table 2 applied sequentially. The left (right) plot is for the electron (muon) channel. |
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Figure 3:
Product of detector acceptance and analysis selection efficiency in the electron (left) and muon (right) channel as functions of the particle X mass in the broad-width case. The narrow-width case is similar. Three analysis requirements are applied consecutively: event reconstruction, HLT, and final signal selection. The product of detector acceptance and analysis selection efficiencies are shown at each stage in red, blue, and orange, respectively. |
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Figure 3-a:
Product of detector acceptance and analysis selection efficiency in the electron (left) and muon (right) channel as functions of the particle X mass in the broad-width case. The narrow-width case is similar. Three analysis requirements are applied consecutively: event reconstruction, HLT, and final signal selection. The product of detector acceptance and analysis selection efficiencies are shown at each stage in red, blue, and orange, respectively. |
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Figure 3-b:
Product of detector acceptance and analysis selection efficiency in the electron (left) and muon (right) channel as functions of the particle X mass in the broad-width case. The narrow-width case is similar. Three analysis requirements are applied consecutively: event reconstruction, HLT, and final signal selection. The product of detector acceptance and analysis selection efficiencies are shown at each stage in red, blue, and orange, 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 uncertainty. 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 number of simulated events, in the high mass region, progressively larger bin sizes are used. The last bin includes the overflow events. The lower panel shows the ratio of data to simulation. |
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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 uncertainty. 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 number of simulated events, in the high mass region, progressively larger bin sizes are used. The last bin includes the overflow events. The lower panel shows the ratio of data to simulation. |
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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 uncertainty. 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 number of simulated events, in the high mass region, progressively larger bin sizes are used. The last bin includes the overflow events. The lower panel shows the ratio of data to simulation. |
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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% confidence level statistical uncertainties in the fitted model. Four signal models are also overlaid. Their total cross sections are set to the expected limits at 95% confidence level from this search with leptonic decays of the W boson. 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% confidence level statistical uncertainties in the fitted model. Four signal models are also overlaid. Their total cross sections are set to the expected limits at 95% confidence level from this search with leptonic decays of the W boson. 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. |
png pdf |
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% confidence level statistical uncertainties in the fitted model. Four signal models are also overlaid. Their total cross sections are set to the expected limits at 95% confidence level from this search with leptonic decays of the W boson. 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:
Systematic uncertainties affecting the signal normalization in the electron (upper) and muon (lower) channels 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 uncertainties for the narrow-width case are nearly identical. |
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Figure 6-a:
Systematic uncertainties affecting the signal normalization in the electron (upper) and muon (lower) channels 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 uncertainties for the narrow-width case are nearly identical. |
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Figure 6-b:
Systematic uncertainties affecting the signal normalization in the electron (upper) and muon (lower) channels 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 uncertainties for the narrow-width case are nearly identical. |
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Figure 7:
Expected and observed limits at 95% CL on $ \sigma\mathcal{B}(X\to\mathrm{W}\gamma) $ from events with leptonic decays (solid black lines) of the W boson as a function of the X resonant mass. The limits from the hadronic decays (dashed red lines) of the W boson are taken from [7] and included for comparison. The results for the narrow (broad) width assumption are shown on the left (right). |
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Figure 7-a:
Expected and observed limits at 95% CL on $ \sigma\mathcal{B}(X\to\mathrm{W}\gamma) $ from events with leptonic decays (solid black lines) of the W boson as a function of the X resonant mass. The limits from the hadronic decays (dashed red lines) of the W boson are taken from [7] and included for comparison. The results for the narrow (broad) width assumption are shown on the left (right). |
png pdf |
Figure 7-b:
Expected and observed limits at 95% CL on $ \sigma\mathcal{B}(X\to\mathrm{W}\gamma) $ from events with leptonic decays (solid black lines) of the W boson as a function of the X resonant mass. The limits from the hadronic decays (dashed red lines) of the W boson are taken from [7] and included for comparison. The results for the narrow (broad) width assumption are shown on the left (right). |
png pdf |
Figure 8:
Expected and observed limits at 95% CL on $ \sigma\mathcal{B}(X\to\mathrm{W}\gamma) $ utilizing both hadronic (from [7]) and leptonic (this analysis) W decays as a function of the X resonant mass. The results for the narrow (broad) width assumption are shown on the left (right). |
png pdf |
Figure 8-a:
Expected and observed limits at 95% CL on $ \sigma\mathcal{B}(X\to\mathrm{W}\gamma) $ utilizing both hadronic (from [7]) and leptonic (this analysis) W decays as a function of the X resonant mass. The results for the narrow (broad) width assumption are shown on the left (right). |
png pdf |
Figure 8-b:
Expected and observed limits at 95% CL on $ \sigma\mathcal{B}(X\to\mathrm{W}\gamma) $ utilizing both hadronic (from [7]) and leptonic (this analysis) W decays as a function of the X resonant mass. The results for the narrow (broad) width assumption are shown on the left (right). |
png pdf |
Figure 9:
Observed local $ p $-values for narrow (left) and broad (right) resonance width hypotheses with the background-only fit in the hadronic (from [7]) and leptonic (this analysis) channels. The blue line shows the observed local $ \it{p} $-values after their combination. In the hadronic channel (violet line), the largest excess corresponds to a local significance of 2.8 (3.1) standard deviations (s.d.) for narrow (broad) signals. In the leptonic channel (orange line), the largest local significance is 1.7 (1.6) standard deviations. After combining with the leptonic channel, the largest excess is 2.7 (2.5) standard deviations. |
png pdf |
Figure 9-a:
Observed local $ p $-values for narrow (left) and broad (right) resonance width hypotheses with the background-only fit in the hadronic (from [7]) and leptonic (this analysis) channels. The blue line shows the observed local $ \it{p} $-values after their combination. In the hadronic channel (violet line), the largest excess corresponds to a local significance of 2.8 (3.1) standard deviations (s.d.) for narrow (broad) signals. In the leptonic channel (orange line), the largest local significance is 1.7 (1.6) standard deviations. After combining with the leptonic channel, the largest excess is 2.7 (2.5) standard deviations. |
png pdf |
Figure 9-b:
Observed local $ p $-values for narrow (left) and broad (right) resonance width hypotheses with the background-only fit in the hadronic (from [7]) and leptonic (this analysis) channels. The blue line shows the observed local $ \it{p} $-values after their combination. In the hadronic channel (violet line), the largest excess corresponds to a local significance of 2.8 (3.1) standard deviations (s.d.) for narrow (broad) signals. In the leptonic channel (orange line), the largest local significance is 1.7 (1.6) standard deviations. After combining with the leptonic channel, the largest excess is 2.7 (2.5) standard deviations. |
| 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 search for a new particle X decaying into a W boson and a photon with mass hypotheses from 0.3 to 2.0 TeV and X width-to-mass ratio hypotheses of 0.01% (narrow) and 5% (broad). 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} $. This search reveals no statistically significant excess of events above the background. Upper limits at the 95% confidence level on the product of the cross section and branching fraction for W$ \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 based on 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. The upper limit 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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