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Search for resonant production of pairs of dijet resonances through broad mediators in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Using 138 fb $^{-1}$ of 13 TeV proton-proton collision data from the CMS experiment, this study reinterprets a prior narrow-resonance search to constrain broad mediators decaying into dijet pairs, revealing local excesses with significances up to 3.9 standard deviations at four-jet masses of 8.6 TeV and 3.6 TeV that remain robust across varying resonance widths.

Original authors: CMS Collaboration

Published 2026-10-28

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CC BY 4.0

Original authors: CMS Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The universe is built on a foundation of particles and forces that physicists have spent decades mapping out. This map, known as the Standard Model, explains how matter behaves and interacts with incredible precision. Yet, scientists know this map is incomplete. It does not explain gravity, nor does it account for the invisible matter that holds galaxies together. To find the missing pieces, researchers smash protons together at nearly the speed of light, creating a shower of new particles that fly out in all directions. Most of the time, these collisions produce familiar debris, but occasionally, they might reveal something entirely new: a heavy, short-lived particle that decays instantly into a spray of jets, or streams of particles, that detectors can record. The challenge is to find a faint signal of new physics hidden within a massive ocean of ordinary background noise.

In a recent analysis, the CMS collaboration at CERN's Large Hadron Collider revisited a specific type of collision to see if they could spot a different kind of new particle. Previous searches had focused on finding very sharp, narrow peaks in the data, looking for new particles that appear and disappear almost instantly with a very specific mass. However, some theories suggest that new particles could be much "broader," meaning they have a wider range of possible masses and a more spread-out signature. The researchers asked a simple question: if a new, heavy particle exists but is broad rather than sharp, would the data look different? They took a large dataset of proton collisions, equivalent to 138 inverse femtobarns of integrated luminosity, and re-examined events where four distinct jets of particles were produced. They specifically looked for a pattern where two pairs of jets formed, suggesting that a heavy parent particle had split into two intermediate particles, which then each split into two jets.

The team focused their search on events where the combined mass of the four jets was greater than 1.6 TeV, and where the two pairs of jets each had a mass greater than 0.2 TeV. They compared the actual data against computer simulations of what the background noise should look like, as well as simulations of what a new, broad particle would look like. They tested models where the new particle's width ranged from 1.5% to 10% of its total mass. In the data, they found three specific events that stood out. Two of these events had a total mass of about 8 TeV, and a third had a mass of 5.8 TeV. All three events shared a common feature: the two pairs of jets within each event had an average mass of about 2 TeV.

When the researchers analyzed these events assuming the new particle was narrow, as in previous studies, the two 8 TeV events showed a local significance of 3.9 standard deviations, which is a statistical measure of how unlikely the result is to be a random fluke. However, the third event at 5.8 TeV did not fit the narrow model well. When they tested the hypothesis that the new particle was broad, the picture changed. The broad model accommodated all three events much better. As the assumed width of the particle increased from 1.5% to 10%, the statistical significance of the 8.6 TeV signal remained high, staying between 3.6 and 3.9 standard deviations locally. This suggests that the excess of events is not just a statistical fluctuation of a narrow particle, but could equally well be explained by a broad resonance. The presence of the 5.8 TeV event, which was poorly explained by narrow models, became a strong piece of evidence supporting the broad resonance hypothesis.

The researchers also identified a second potential signal at a lower mass of 3.6 TeV. In this case, the significance of the excess increased as the width of the hypothetical particle increased, reaching a local significance of 3.9 standard deviations for a 10% width. This second excess was consistent with a structure previously noted in other searches. While these findings are intriguing, they do not yet constitute a discovery. The global significance, which accounts for the fact that the researchers looked at many different mass ranges and widths, was lower, ranging from 1.4 to 2.2 standard deviations. This means that while the data shows interesting deviations from the expected background, it is still possible that these are random fluctuations.

To be thorough, the team calculated upper limits on how often such new particles could be produced. They determined that if these broad particles exist, their production rate must be below a certain threshold for masses between 2 and 10 TeV. They compared these limits to specific theoretical models involving diquarks, which are hypothetical particles made of two quarks. For a specific model with a particle width of 10%, the data ruled out the existence of a scalar diquark with a mass of 8.6 TeV, but a different type of diquark with the same mass remained a viable possibility. The study concludes that the data contains an excess that is equally well described by a broad resonance as by a narrow one, and that the broad interpretation is supported by the specific characteristics of the events observed. The results provide a new way to interpret the data, keeping the door open for new physics that might be hiding in the broader, more diffuse signals of the universe.

Technical Summary: Search for Resonant Production of Pairs of Dijet Resonances through Broad Mediators

Problem and Motivation
This paper presents a reinterpretation of a prior search for narrow resonances conducted by the CMS Collaboration using proton-proton collision data at $\sqrt{s} = 13$ TeV. While previous searches focused on narrow mediators (natural widths $<0.5\%$ of their mass), this analysis investigates the resonant production of pairs of dijet resonances via broad mediators. The motivation stems from specific events observed in the data that were previously interpreted as potential narrow resonance candidates but may also be consistent with a broad resonance hypothesis.

Specifically, the analysis targets events with four resolved jets ( $pp \to Y \to XX \to (jj)(jj)$ ), where $Y$ is a massive intermediate particle decaying into two identical dijet resonances $X$ . The study focuses on a scenario where $Y$ has a natural width comparable to or larger than the experimental resolution (broad resonance), while $X$ remains narrow. This reinterpretation is driven by the observation of three candidate events: two with a four-jet mass ( $m_{4j}$ ) of 8 TeV and an average dijet mass ( $m_{2j}$ ) of 2 TeV, and a third event with $m_{4j} = 5.8$ TeV and $m_{2j} = 2.0$ TeV. While the first two events drove the significance in the narrow resonance search, the third event, which contributes minimally to a narrow resonance fit, is notably compatible with a broad resonance interpretation. Additionally, an event reported by ATLAS ( $m_{4j} = 6.6$ TeV, $m_{2j} = 2.2$ TeV) falls within the 68% probability contour of the broad resonance hypothesis.

Methodology
The analysis utilizes a dataset corresponding to an integrated luminosity of 138 fb $^{-1}$ collected by the CMS experiment. The methodology extends the previous narrow-resonance search (Ref. [1]) by modifying the signal simulation to include broad mediators.

Signal Model: The benchmark model is a diquark model where a color sextet diquark ( $S_{uu}$ or $S_{dd}$ ) is produced and decays into a pair of vector-like quarks ( $\chi$ or $\omega$ ), which subsequently decay into a quark and a gluon. The diquark mass ( $M_S$ ) ranges from 2 to 10 TeV, and the diquark width ( $\Gamma/M_S$ ) is varied between 1.5%, 5%, and 10%. The vector-like quark mass ( $M_X$ ) is defined by the ratio $\alpha_{true} = M_X/M_S$ .
Event Selection: Events are required to have four resolved jets with dijet invariant masses $m_{2j} > 0.2$ TeV and four-jet invariant masses $m_{4j} > 1.6$ TeV. The analysis bins the data in the ratio $\alpha = m_{2j}/m_{4j}$ (13 bins) and performs a simultaneous fit across these bins.
Background Estimation: The background is modeled using empirical parametric functions fitted directly to the observed data, avoiding reliance on perturbative QCD simulations which carry large theoretical uncertainties. The primary background function is a modified dijet function with three parameters. Systematic uncertainties regarding the background shape are estimated using the discrete profiling method with two alternative functional forms.
Statistical Analysis: A multibin counting-experiment likelihood is used to fit the signal plus background model to the data. Upper limits on the production cross section are set at 95% confidence level (CL) using the modified frequentist criterion ( $CL_s$ ). Local and global significances are calculated, accounting for the look-elsewhere effect across the 2D plane of resonance mass and $\alpha_{true}$ .

Key Contributions

Broad Resonance Reinterpretation: The paper provides the first comprehensive reinterpretation of the CMS dijet pair search specifically for broad mediators, demonstrating that the signal shapes for widths of 1.5%, 5%, and 10% significantly alter the expected kinematic distributions compared to narrow resonances.
Inclusion of the Third Candidate Event: The analysis explicitly incorporates the $m_{4j} = 5.8$ TeV event, showing that while this event has minimal impact on narrow resonance fits, it is well-described by broad resonance models, thereby strengthening the case for a broad mediator interpretation of the excess.
Model-Independent Limits: The study establishes model-independent 95% CL upper limits on the product of the cross section, branching fraction, and acceptance ( $\sigma \mathcal{B} \mathcal{A}$ ) as a function of the four-jet resonance mass and width.

Results

Significance of the 8.6 TeV Excess: For a four-jet resonance mass of 8.6 TeV, the local significance remains high across different widths. It ranges from 3.9 standard deviations (s.d.) for a 1.5% width to 3.6 s.d. for a 10% width. The global significance is 1.6 s.d. (1.5% width) and 1.4 s.d. (10% width). The author notes that the significance is relatively insensitive to the choice of width, indicating that a broad resonance is an equally valid interpretation of the excess as a narrow one.
Second Excess at 3.6 TeV: A second excess is observed at a four-jet resonance mass of 3.6 TeV with $\alpha_{true} = 0.29$ . For a 10% width, this excess has a local significance of 3.9 s.d. and a global significance of 2.2 s.d. The significance for this excess increases with the resonance width.
Exclusion Limits: The analysis sets exclusion limits on scalar diquark models. For a width of 10%, the $S_{uu}$ diquark model is excluded up to a mass of 8.8 TeV, while the $S_{dd}$ model is excluded up to 7.3 TeV (for $\alpha_{true} = 0.25$ ). Notably, for the 8.6 TeV resonance with a 10% width, the $S_{uu}$ model is excluded, but the $S_{dd}$ model remains a viable hypothesis for the observed events.

Significance and Claims
The paper claims that the reinterpretation of the data supports the hypothesis of a broad resonance. The relative insensitivity of the local significance to the resonance width (remaining above 3.6 s.d. even for the broadest case) suggests that a broad mediator is a valid explanation for the observed excess. Furthermore, the presence of the 5.8 TeV event, which is naturally accommodated by the broad resonance hypothesis but not the narrow one, serves as a key piece of evidence supporting this interpretation. The author concludes that the data does not rule out the broad resonance hypothesis and that it provides an equally valid description of the observed events as the narrow resonance hypothesis. The results are presented as limits and significances for specific benchmark models, without claiming the discovery of new physics.

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