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| CMS-PAS-EXO-25-004 | ||
| Search for resonant production of pairs of dijet resonances in proton-proton collisions at $ \sqrt{s} = $ 13.6 TeV | ||
| CMS Collaboration | ||
| 2025-10-27 | ||
| Abstract: A search for a resonance, decaying to pairs of dijet resonances with the same mass, is conducted in final states with at least four individually resolved jets and four-jet masses above 1.6 TeV. The search is conducted using data collected by the CMS experiment, during the year 2024, in pp collisions at a center-of-mass energy of 13.6 TeV. Our benchmark model is a diquark, decaying to a pair of vector-like quarks, which in turn decay to a pair of jets. This model probes resonant production in the four-jet and dijet mass distributions. Both upper limits at 95% confidence level and significances are reported on the production cross section of new resonances as a function of their masses. This search does not observe any events with four-jet mass around 8 TeV, where in a previous analysis at $ \sqrt{s}= $ 13 TeV an excess with a local (global) significance of 3.9 (1.6) standard deviations was observed. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Resonant production via a particle, Y, of pairs of dijet resonances, X. |
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Figure 2:
Number of observed events (color scale) within bins of the four-jet mass and the average mass of the two dijets. The dotted and dashed curves show the 68% and 95% probability contours, respectively, from a signal simulation of a diquark with a mass of 8.4 TeV, decaying to a pair of vector-like quarks, each with a mass of 2.1 TeV. |
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Figure 3:
(Left) Number of observed events (color scale) within bins of the four-jet mass and the ratio $ \alpha $, which is the average mass of the two dijets divided by the four-jet mass. (Right) Number of events predicted in the same bins by a simulation of a diquark with a mass of 8.4 TeV, decaying to a pair of vector-like quarks, each with a mass of 2.1 TeV. Both distributions also show the thirteen $ \alpha $ bins used to define the four-jet mass distributions used in the search (dashed lines). |
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Figure 3-a:
(Left) Number of observed events (color scale) within bins of the four-jet mass and the ratio $ \alpha $, which is the average mass of the two dijets divided by the four-jet mass. (Right) Number of events predicted in the same bins by a simulation of a diquark with a mass of 8.4 TeV, decaying to a pair of vector-like quarks, each with a mass of 2.1 TeV. Both distributions also show the thirteen $ \alpha $ bins used to define the four-jet mass distributions used in the search (dashed lines). |
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Figure 3-b:
(Left) Number of observed events (color scale) within bins of the four-jet mass and the ratio $ \alpha $, which is the average mass of the two dijets divided by the four-jet mass. (Right) Number of events predicted in the same bins by a simulation of a diquark with a mass of 8.4 TeV, decaying to a pair of vector-like quarks, each with a mass of 2.1 TeV. Both distributions also show the thirteen $ \alpha $ bins used to define the four-jet mass distributions used in the search (dashed lines). |
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Figure 4:
Signal differential distributions as functions of four-jet mass for various resonance masses, shown for $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.19 (left) in the reconstructed 0.18 $ < \alpha < $ 0.20 bin and for $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.25 (right) in the reconstructed 0.24 $ < \alpha < $ 0.26 bin. These reconstructed $ \alpha $ bins contain the majority of the signal in each case. The integral of each distribution has been normalized to unity. |
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Figure 4-a:
Signal differential distributions as functions of four-jet mass for various resonance masses, shown for $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.19 (left) in the reconstructed 0.18 $ < \alpha < $ 0.20 bin and for $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.25 (right) in the reconstructed 0.24 $ < \alpha < $ 0.26 bin. These reconstructed $ \alpha $ bins contain the majority of the signal in each case. The integral of each distribution has been normalized to unity. |
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Figure 4-b:
Signal differential distributions as functions of four-jet mass for various resonance masses, shown for $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.19 (left) in the reconstructed 0.18 $ < \alpha < $ 0.20 bin and for $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.25 (right) in the reconstructed 0.24 $ < \alpha < $ 0.26 bin. These reconstructed $ \alpha $ bins contain the majority of the signal in each case. The integral of each distribution has been normalized to unity. |
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Figure 5:
The products of acceptance and efficiency of a resonant signal vs. the diquark mass inclusively, i.e., for all $ \alpha $ values, and for the three $ \alpha $ bins that contain the majority ($ \geq $85%) of the signal for signals with $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.19 (left) and $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.25 (right). The case when the efficiency of the mass selection is unity is also shown as a solid line. |
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Figure 5-a:
The products of acceptance and efficiency of a resonant signal vs. the diquark mass inclusively, i.e., for all $ \alpha $ values, and for the three $ \alpha $ bins that contain the majority ($ \geq $85%) of the signal for signals with $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.19 (left) and $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.25 (right). The case when the efficiency of the mass selection is unity is also shown as a solid line. |
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Figure 5-b:
The products of acceptance and efficiency of a resonant signal vs. the diquark mass inclusively, i.e., for all $ \alpha $ values, and for the three $ \alpha $ bins that contain the majority ($ \geq $85%) of the signal for signals with $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.19 (left) and $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.25 (right). The case when the efficiency of the mass selection is unity is also shown as a solid line. |
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Figure 6:
The four-jet mass distributions of the data (points), within six of the thirteen $ \alpha $ bins, compared with the simulated LO QCD background distribution normalized to the data (green histogram) and fitted with three functions: a power-law times an exponential (magenta dotted), the dijet function (black dashed), and the modified dijet function (red solid), each function with three free parameters. Examples of predicted diquark signals are shown, with cross sections equal to the observed upper limits at 95% confidence level, for low (blue dotted), medium (blue dashed) and high (blue solid) resonance masses. The lower panels show the pulls from the fit of the modified dijet function to the data, calculated using the statistical uncertainty of the data. |
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Figure 6-a:
The four-jet mass distributions of the data (points), within six of the thirteen $ \alpha $ bins, compared with the simulated LO QCD background distribution normalized to the data (green histogram) and fitted with three functions: a power-law times an exponential (magenta dotted), the dijet function (black dashed), and the modified dijet function (red solid), each function with three free parameters. Examples of predicted diquark signals are shown, with cross sections equal to the observed upper limits at 95% confidence level, for low (blue dotted), medium (blue dashed) and high (blue solid) resonance masses. The lower panels show the pulls from the fit of the modified dijet function to the data, calculated using the statistical uncertainty of the data. |
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Figure 6-b:
The four-jet mass distributions of the data (points), within six of the thirteen $ \alpha $ bins, compared with the simulated LO QCD background distribution normalized to the data (green histogram) and fitted with three functions: a power-law times an exponential (magenta dotted), the dijet function (black dashed), and the modified dijet function (red solid), each function with three free parameters. Examples of predicted diquark signals are shown, with cross sections equal to the observed upper limits at 95% confidence level, for low (blue dotted), medium (blue dashed) and high (blue solid) resonance masses. The lower panels show the pulls from the fit of the modified dijet function to the data, calculated using the statistical uncertainty of the data. |
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Figure 7:
The pulls from the fit of the modified dijet function to the four-jet mass distribution, and the reduced chi-squared of the fit $ \chi^2 $/NDF, for all thirteen $ \alpha $ bins of the search. The pulls are calculated using the statistical uncertainty of the data. |
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Figure 8:
The observed 95% CL upper limits (black lines with points) on the product of the cross section, branching fraction, and acceptance for resonant production of paired dijet resonances decaying to a quark-gluon pair, with the values of $ \alpha_{\mathrm{true}} = M(\mathrm{X})/M(\mathrm{Y}) $ shown. The corresponding expected limits (dashed lines) and their variations at the 1 and 2 standard deviation levels (shaded bands) are also shown. Limits are compared to predictions for a scalar diquark [3] (dot-dashed line) with couplings to pairs of up quarks, $ y_{uu} = $ 0.4, and to pairs of vector-like quarks, $ y_{\chi} = $ 0.6. |
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Figure 9:
The observed 95% CL upper limits (black lines with points) on the product of the cross section, branching fraction, and acceptance for resonant production of paired dijet resonances decaying to a quark-gluon pair, with $ \alpha_{\mathrm{true}}=M(\mathrm{X})/M(\mathrm{Y})= $ 0.25. The corresponding expected limits (dashed lines) and their variations at the 1 and 2 standard deviation levels (shaded bands) are also shown. Limits are compared to predictions for a scalar diquark [3] (dot-dashed line) with couplings to pairs of up quarks, $ y_{uu} = $ 0.4, and to pairs of vector-like quarks, $ y_{\chi} = $ 0.6. |
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Figure 10:
Observed $ p $-values (points) for a four-jet resonance, Y, decaying to a pair of dijet resonances, $ \mathrm{X} $, with $ \alpha_\textrm{true} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.19 (left) and $ \alpha_\textrm{true} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.25 (right). The vertical scales indicate the local $ p $-value for a signal over all $ \alpha $ bins. Also shown are corresponding levels of local significance in units of standard deviation ($ \sigma $). |
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Figure 10-a:
Observed $ p $-values (points) for a four-jet resonance, Y, decaying to a pair of dijet resonances, $ \mathrm{X} $, with $ \alpha_\textrm{true} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.19 (left) and $ \alpha_\textrm{true} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.25 (right). The vertical scales indicate the local $ p $-value for a signal over all $ \alpha $ bins. Also shown are corresponding levels of local significance in units of standard deviation ($ \sigma $). |
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Figure 10-b:
Observed $ p $-values (points) for a four-jet resonance, Y, decaying to a pair of dijet resonances, $ \mathrm{X} $, with $ \alpha_\textrm{true} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.19 (left) and $ \alpha_\textrm{true} = M(\mathrm{X})/M(\mathrm{Y}) = $ 0.25 (right). The vertical scales indicate the local $ p $-value for a signal over all $ \alpha $ bins. Also shown are corresponding levels of local significance in units of standard deviation ($ \sigma $). |
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Figure 11:
Three--dimensional display of the event with the high four-jet mass of 6.21 TeV. The display shows the energy deposited in the electromagnetic (red) and hadronic (blue) calorimeters and the reconstructed tracks of charged particles (green). The grouping of the four observed jets into two dijet pairs (purple box) is discussed in the text. |
| Summary |
| A search for paired dijet resonances has been presented in final states with at least four jets, probing four-jet masses above 1.6 TeV. Data from proton-proton collisions at $ \sqrt{s}= $ 13.6 TeV were used in this search, collected by the CMS experiment at the LHC during 2024, corresponding to an integrated luminosity of 90 fb$^{-1}$. Empirical functions that model the background, and simulated shapes of resonance signals, are fitted to the observed four-jet mass distributions. The maximum significance is observed at a four-jet resonance mass of 4.7 TeV and a dijet resonance mass of 0.89 TeV, corresponding to 3.3 s.d. of local and 1.0 s.d. of global significance. Model-independent upper limits at 95% confidence level (CL) are presented on the production cross section times branching fraction and acceptance. The limits are presented as a function of the four-jet resonance mass between 2 and 9 TeV, for all accessible values of the ratio of the dijet to four-jet resonance masses. Limits are compared to a model [3] of diquarks, that decay to pairs of vector-like quarks, which in-turn decay to a quark and a gluon. Mass limits for all accessible values of the ratio of the vector-like quark to diquark masses, for a benchmark scenario where the diquark couplings to pairs of up quarks is 0.4, and the diquark couplings to pairs of vector-like quarks is 0.6 are presented. An interesting high four-jet mass event is observed in this search at 6.2 TeV four-jet and 2.0 TeV average dijet mass, similar to one of the highest four-jet mass CMS events and the hightest four-jet mass ATLAS event of Run 2. However, no additional events have been observed yet around 8 TeV in this search, but we do not exclude a four-jet resonance signal hypothesis for the Run 2 events. More data are needed to establish if these Run 2 events were the first hints of a signal or a statistical fluctuation of the QCD background. |
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