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| CMS-PAS-EXO-24-027 | ||
| Search for low-mass resonances decaying into a quark-antiquark pair produced with an initial state radiation photon in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 2026-05-18 | ||
| Abstract: A search for low-mass resonances decaying to a quark-antiquark pair in the mass range of 10 to 150 GeV is presented. This search uses proton-proton collision data at $ \sqrt{s} = $ 13 TeV, collected by the CMS detector at the CERN LHC in 2016--2018, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The analysis strategy makes use of an initial-state radiation photon recoiling against the resonance. As a result, the resonance is produced with high transverse momentum, and the quark-antiquark decay products are reconstructed as a single large-radius jet with an internal two-pronged structure. A machine learning algorithm is used to distinguish such jets from the background. The results are interpreted in the context of a spin-1, leptophobic Z$ ^{\prime} $ boson, which is a dark matter mediator candidate. No significant excess is observed when searching for this signature above the standard model background in the jet mass spectrum. Upper limits at the 95% confidence level are set on the coupling strength of the Z$ ^{\prime} $ to quark-antiquark pairs. This represents the most sensitive search in the mass range of 10 to 50 GeV for resonances decaying to quark-antiquark pairs to date. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Leading-order Feynman diagrams for models used in this analysis: signal production of a $ \mathrm{Z}^{'} $ decaying to $ \mathrm{q} \overline{\mathrm{q}} $ produced in association with an ISR photon (left), the nonresonant background of multijet QCD events produced in association with a high-$ p_{\mathrm{T}} $ photon $ \gamma+\text{jets} $ (center), and the primary resonant background, $ \mathrm{W}/\mathrm{Z}+\gamma $ (right). |
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Figure 1-a:
Leading-order Feynman diagrams for models used in this analysis: signal production of a $ \mathrm{Z}^{'} $ decaying to $ \mathrm{q} \overline{\mathrm{q}} $ produced in association with an ISR photon (left), the nonresonant background of multijet QCD events produced in association with a high-$ p_{\mathrm{T}} $ photon $ \gamma+\text{jets} $ (center), and the primary resonant background, $ \mathrm{W}/\mathrm{Z}+\gamma $ (right). |
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Figure 1-b:
Leading-order Feynman diagrams for models used in this analysis: signal production of a $ \mathrm{Z}^{'} $ decaying to $ \mathrm{q} \overline{\mathrm{q}} $ produced in association with an ISR photon (left), the nonresonant background of multijet QCD events produced in association with a high-$ p_{\mathrm{T}} $ photon $ \gamma+\text{jets} $ (center), and the primary resonant background, $ \mathrm{W}/\mathrm{Z}+\gamma $ (right). |
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Figure 1-c:
Leading-order Feynman diagrams for models used in this analysis: signal production of a $ \mathrm{Z}^{'} $ decaying to $ \mathrm{q} \overline{\mathrm{q}} $ produced in association with an ISR photon (left), the nonresonant background of multijet QCD events produced in association with a high-$ p_{\mathrm{T}} $ photon $ \gamma+\text{jets} $ (center), and the primary resonant background, $ \mathrm{W}/\mathrm{Z}+\gamma $ (right). |
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Figure 2:
The $ p_{\text{sig}} $ scores for simulated background events, including $ \gamma+\text{jets} $ (yellow), $ \mathrm{W}+\gamma $ (purple), and $ \mathrm{Z}+\gamma $ (cyan), and representative $ \mathrm{Z}^{'} $ signal events with masses of 10 (black), 25 (red), 50 (green), and 75 (blue) GeV. The primary nonresonant background process, $ \gamma+\text{jets} $, peaks at a value of zero, while the resonant backgrounds, $ \mathrm{W}+\gamma $ and $ \mathrm{Z}+\gamma $, and the $ \mathrm{Z}^{'} $ signal processes peak at one. The integral of each distribution has been normalized to unity. |
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Figure 3:
Jet $ m_{\mathrm{SD}} $ distributions fitted using a background-only hypothesis for all data-taking periods. Top row (2016), left to right: jet $ p_{\mathrm{T}} $ in the ranges 200--240, 240--275, 275--300, and 300-700 GeV. Middle row (2017), left to right: jet $ p_{\mathrm{T}} $ in the ranges 220--260, 260--300, 300--360, and 360--700 GeV. Bottom row (2018), left to right: jet $ p_{\mathrm{T}} $ in the ranges 120--160, 160--200, 200-255, adn 255--700 GeV. The data (black dots), nonresonant background (cyan), resonant backgrounds (purple), and total background (blue) are shown. Representative signals for $ \mathrm{Z}^{'} $ masses of 25, 50, and 100 GeV are also shown, scaled to their fitted yields in the signal-plus-background fit. The lower panel shows the residual difference between data and the total background divided by the statistical uncertainty in the data, and the gray band represents the background uncertainty divided by the data statistical uncertainty. Due to the jet $ \rho $ selection applied, high masses are restricted in the lower jet $ p_{\mathrm{T}} $ bins, leading to no events above a certain mass value in a given jet $ p_{\mathrm{T}} $ bin. |
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Figure 4:
A three-dimensional event display of an SR candidate event from 2018 data. The event display shows energy from the ECAL in red, energy from the HCAL in blue, and charged particle tracks in green. A cone around the energy depositions reconstructed as a jet is shown in yellow. For the photon $ p_{\mathrm{T}} $, $ \eta $, and $ \phi $ values are shown in red text. For the jet $ m_{SD} $, $ p_{\mathrm{T}} $, $ \eta $, $ \phi $, $ p_{\text{sig}} $, and $ \Delta R $ separation between the photon and jet are shown in blue text. |
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Figure 5:
Upper limits at 95%CL on the coupling strength $ g_{\mathrm{q}} $ between $ \mathrm{Z}^{'} $ and quarks. The $ \mathrm{Z}^{'} $ is assumed to decay to all quark flavors equally, and the branching ratio for decay to quark-antiquark pairs is assumed to be 100%. The observed limit is shown as a solid black line. The dashed black line and the green and yellow bands represent the expected limits and its variation of one and two standard deviations, respectively. Indirect constraints from Z and $ \Upsilon $ decay widths are also shown [61]. |
| Tables | |
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Table 1:
Summary of event selection criteria applied in the analysis. |
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Table 2:
Summary of corrections to the simulation of resonant signal and background contributions derived from the $ \mathrm{t} \overline{\mathrm{t}} $ CR. |
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Table 3:
Summary of all systematic uncertainties applied to signal and resonant background simulations. The names, types (normalization and/or shape), and values for all data-taking periods are shown. Shape systematics affect the shape of the $ m_{\mathrm{SD}} $ distribution, normalization systematics affect the yield of the $ m_{\mathrm{SD}} $ distribution in a given jet $ p_{\mathrm{T}} $ bin. |
| Summary |
| A search for narrow quark-antiquark resonances in the mass range of 10 to 150 GeV has been presented, using proton-proton collision data at $ \sqrt{s}= $ 13 TeV, collected by the CMS detector at the CERN LHC in 2016--2018, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The analysis strategy targets a final state where an initial-state radiation photon recoils against the resonance. As a result, the resonance is produced with high transverse momentum, and the quark-antiquark decay products are reconstructed as a single large-radius jet with an internal two-pronged structure. The resonances are distinguished from background using the machine learning algorithm called PARTICLENET. A portion of the data uses a trigger with a lower threshold on the photon transverse momentum, allowing the analysis to further probe the lower mass resonance region. The results are interpreted in the context of a new spin-1 vector boson, a leptophobic $ \mathrm{Z}^{'} $, which is a dark matter mediator candidate. Searching for this signature as a localized excess above the standard model background in the jet soft-drop mass spectrum reveals no significant excess. Upper limits at the 95% confidence level are set on the coupling strength of the $ \mathrm{Z}^{'} $ decaying to quark-antiquark pairs. This represents the most sensitive search in the mass range of 10 to 50 GeV for resonances decaying into quark-antiquark pairs to date. |
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