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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
JHEP 09 (2024) 186
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%).
Figures & Tables Summary References CMS Publications
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.

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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% 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).

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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).

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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).

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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).

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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).

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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.

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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.

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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.
References
1 N. Arkani-Hamed, R. T. D'Agnolo, M. Low, and D. Pinner Unification and new particles at the LHC JHEP 11 (2016) 082 1608.01675
2 H. E. Logan and Y. Wu Searching for the $ \mathrm{W}\gamma $ decay of a charged Higgs boson JHEP 11 (2018) 121 1809.09127
3 G. Burdman et al. Colorless top partners, a 125 GeV Higgs boson, and the limits on naturalness PRD 91 (2015) 055007 1411.3310
4 ATLAS Collaboration Search for new resonances in $ \mathrm{W}\gamma $ and $ \mathrm{Z}\gamma $ final states in $ pp $ collisions at $ \sqrt s= $ 8 TeV with the ATLAS detector PLB 738 (2014) 428 1407.8150
5 ATLAS Collaboration Search for heavy resonances decaying to a photon and a hadronically decaying $ \mathrm{Z}/\mathrm{W}/\mathrm{H} $ boson in $ pp $ collisions at $ \sqrt{s}= $ 13 TeV with the ATLAS detector PRD 98 (2018) 032015 1805.01908
6 ATLAS Collaboration Search for high-mass $ \mathrm{W}\gamma $ and $ \mathrm{Z}\gamma $ resonances using hadronic $ \mathrm{W}/\mathrm{Z} $ boson decays from 139 fb$ ^{-1} $ of pp collisions at $ \sqrt{s} $ = 13 TeV with the ATLAS detector JHEP 07 (2023) 125 2304.11962
7 CMS Collaboration Search for $ \mathrm{W}\gamma $ resonances in proton-proton collisions at $ \sqrt{s} = $ 13 TeV using hadronic decays of Lorentz-boosted W bosons PLB 826 (2022) 136888 CMS-EXO-20-001
2106.10509
8 Particle Data Group , R. L. Workman et al. Review of Particle Physics PTEP 2022 (2022) 083C01
9 CMSnoop HEPData record for this analysis \href,, 2024
link
10 CMS Collaboration The CMS experiment at the CERN LHC JINST 3 (2008) S08004
11 CMS Collaboration Development of the CMS detector for the CERN LHC Run 3 Accepted by JINST, 2023 CMS-PRF-21-001
2309.05466
12 CMS Collaboration The CMS trigger system JINST 12 (2017) P01020 CMS-TRG-12-001
1609.02366
13 CMS Collaboration Performance of the CMS level-1 trigger in proton-proton collisions at $ \sqrt{s} = $ 13 TeV JINST 15 (2020) P10017 CMS-TRG-17-001
2006.10165
14 CMS Collaboration Electron and photon reconstruction and identification with the CMS experiment at the CERN LHC JINST 16 (2021) P05014 CMS-EGM-17-001
2012.06888
15 CMS Collaboration Performance of the CMS muon detector and muon reconstruction with proton-proton collisions at $ \sqrt{s}= $ 13 TeV JINST 13 (2018) P06015 CMS-MUO-16-001
1804.04528
16 CMS Collaboration Description and performance of track and primary-vertex reconstruction with the CMS tracker JINST 9 (2014) P10009 CMS-TRK-11-001
1405.6569
17 CMS Collaboration Precision luminosity measurement in proton-proton collisions at $ \sqrt{s} = $ 13 TeV in 2015 and 2016 at CMS EPJC 81 (2021) 800 CMS-LUM-17-003
2104.01927
18 CMS Collaboration CMS luminosity measurement for the 2017 data-taking period at $ \sqrt{s} $ = 13 TeV CMS Physics Analysis Summary, 2018
link
CMS-PAS-LUM-17-004
19 CMS Collaboration CMS luminosity measurement for the 2018 data-taking period at $ \sqrt{s} $ = 13 TeV CMS Physics Analysis Summary, 2019
link
CMS-PAS-LUM-18-002
20 P. Nason A new method for combining NLO QCD with shower Monte Carlo algorithms JHEP 11 (2004) 040 hep-ph/0409146
21 S. Frixione, P. Nason, and C. Oleari Matching NLO QCD computations with parton shower simulations: the POWHEG method JHEP 11 (2007) 070 0709.2092
22 S. Alioli, P. Nason, C. Oleari, and E. Re A general framework for implementing NLO calculations in shower Monte Carlo programs: the POWHEG \textscbox JHEP 06 (2010) 043 1002.2581
23 T. Je \v z o and P. Nason On the treatment of resonances in next-to-leading order calculations matched to a parton shower JHEP 12 (2015) 065 1509.09071
24 J. Alwall et al. The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations JHEP 07 (2014) 079 1405.0301
25 NNPDF Collaboration Parton distributions for the LHC Run II JHEP 04 (2015) 040 1410.8849
26 NNPDF Collaboration Parton distributions from high-precision collider data EPJC 77 (2017) 663 1706.00428
27 T. Sjöstrand et al. An introduction to PYTHIA8.2 Comput. Phys. Commun. 191 (2015) 159 1410.3012
28 P. Skands, S. Carrazza, and J. Rojo Tuning PYTHIA 8.1: the Monash 2013 tune EPJC 74 (2014) 3024 1404.5630
29 CMS Collaboration Event generator tunes obtained from underlying event and multiparton scattering measurements EPJC 76 (2016) 155 CMS-GEN-14-001
1512.00815
30 CMS Collaboration Extraction and validation of a new set of CMS PYTHIA8 tunes from underlying-event measurements EPJC 80 (2020) 4 CMS-GEN-17-001
1903.12179
31 GEANT4 Collaboration GEANT 4---a simulation toolkit NIM A 506 (2003) 250
32 CMS Collaboration Measurement of the inelastic proton-proton cross section at $ \sqrt{s}= $ 13 TeV JHEP 07 (2018) 161 CMS-FSQ-15-005
1802.02613
33 CMS Collaboration Particle-flow reconstruction and global event description with the CMS detector JINST 12 (2017) P10003 CMS-PRF-14-001
1706.04965
34 CMS Collaboration Performance of photon reconstruction and identification with the CMS detector in proton-proton collisions at $ \sqrt{s} = $ 8 TeV JINST 10 (2015) P08010 CMS-EGM-14-001
1502.02702
35 M. Cacciari, G. P. Salam, and G. Soyez The anti-$ k_{\mathrm{T}} $ jet clustering algorithm JHEP 04 (2008) 063 0802.1189
36 M. Cacciari, G. P. Salam, and G. Soyez FastJet user manual EPJC 72 (2012) 1896 1111.6097
37 E. Bols et al. Jet flavour classification using DeepJet JINST 15 (2020) P12012 2008.10519
38 CMS Collaboration Performance summary of AK4 jet b tagging with data from proton-proton collisions at 13 TeV with the CMS detector CMS Detector Performance Note CMS-DP-2023-005, 2023
CDS
39 CMS Collaboration Performance of missing transverse momentum reconstruction in proton-proton collisions at $ \sqrt{s} = $ 13 TeV using the CMS detector JINST 14 (2019) P07004 CMS-JME-17-001
1903.06078
40 CMS Collaboration Technical proposal for the Phase-II upgrade of the Compact Muon Solenoid CMS Technical Proposal CERN-LHCC-2015-010, CMS-TDR-15-02, 2015
CDS
41 M. J. Oreglia A study of the reactions $ {\psi^\prime\to\gamma\gamma\psi} $ PhD thesis, Stanford University, . SLAC-R-236, 1980
link
42 J. E. Gaiser Charmonium Spectroscopy from Radiative Decays of the $ \mathrm{J}/\psi $ and $ {\psi^\prime} $ PhD thesis, Stanford University, . SLAC-R-255, 1982
link
43 R. A. Fisher On the interpretation of $ \chi^{2} $ from contingency tables, and the calculation of P J. R. Stat. Soc. 85 (1922) 87
44 P. D. Dauncey, M. Kenzie, N. Wardle, and G. J. Davies Handling uncertainties in background shapes: the discrete profiling method JINST 10 (2015) P04015 1408.6865
45 T. Junk Confidence level computation for combining searches with small statistics NIM A 434 (1999) 435 hep-ex/9902006
46 A. L. Read Presentation of search results: the $ \text{CL}_\text{s} $ technique JPG 28 (2002) 2693
47 ATLAS and CMS Collaborations, and LHC Higgs Combination Group Procedure for the LHC Higgs boson search combination in Summer 2011 Technical Report CMS-NOTE-2011-005, ATL-PHYS-PUB-2011-11, 2011
48 G. Cowan, K. Cranmer, E. Gross, and O. Vitells Asymptotic formulae for likelihood-based tests of new physics EPJC 71 (2011) 1554 1007.1727
49 CMS Collaboration The CMS statistical analysis and combination tool: \textscCombine Submitted to Comput. Softw. Big Sci, 2024 CMS-CAT-23-001
2404.06614
Compact Muon Solenoid
LHC, CERN