⚛️ phenomenology

Evidence for Kaluza-Klein excitations of the graviton, gluon and quark at LHC

This paper interprets CMS Run 2 and Run 3 four-jet excesses as evidence for Kaluza-Klein excitations of gluons and quarks, proposing a decay chain involving heavy KK gluons (3.6 TeV and 8.6 TeV) into lighter KK quarks (1 TeV and 2.1 TeV) while challenging existing exclusion limits based on assumed decay branching ratios.

Original authors: Alain Le Yaouanc, François Richard

Published 2026-10-23

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Alain Le Yaouanc, François Richard

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

Deep within the subatomic world, physicists have long suspected that our familiar universe is not the only layer of reality. Theories suggest that hidden dimensions, curled up so tightly we cannot see them, might hold the key to some of nature's most puzzling mysteries. In this hidden landscape, particles we know well—like the heavy top quark or the force-carrying gluon—might have heavier, excited versions, much like a musical note that can be played in higher octaves. These heavier versions are called Kaluza-Klein excitations. If they exist, they would be massive, unstable particles that decay rapidly into the familiar particles we detect in our experiments. Finding them would be a monumental discovery, confirming that extra dimensions are real and reshaping our understanding of the universe's fundamental structure.

A team of researchers from France has recently revisited data from the Large Hadron Collider to see if these hidden echoes might already be hiding in plain sight. They focused on a specific type of collision event where four distinct sprays of particles, known as jets, appear in the detector. While previous studies had tried to explain these four-jet events using other theories, this team proposed a different story: that the events could come from the decay of heavy, hidden particles into pairs of even heavier hidden particles, which then break apart into the jets we see. Their analysis suggests that the data contains signals that could correspond to two very heavy particles, one weighing about 3.6 trillion electron volts and another at 8.6 trillion electron volts. These heavy particles appear to potentially decay into pairs of lighter, but still massive, partners weighing roughly 1 trillion and 2.1 trillion electron volts, respectively.

The researchers argue that these heavy particles are likely excited versions of the gluon, the particle that holds atomic nuclei together, and their partners are excited versions of quarks. A major hurdle in accepting this idea has been that standard searches for such particles usually assume they decay primarily into pairs of top quarks. However, the authors point out that in their previous work on similar heavy particles called gravitons, the decay into top quarks was surprisingly suppressed. If the same suppression applies here, the heavy gluons would instead decay into the pairs of excited quarks that the team has proposed. This shift in expectation changes the picture significantly, allowing the observed signals to fit the model without contradicting existing limits, although the authors acknowledge that current limits on the 3.6 TeV signal appear to exclude it under standard assumptions.

The evidence they present is not yet a definitive discovery, but it is a compelling suggestion. The data shows local statistical significance approaching four times the level of random chance, a threshold that often hints at a real physical phenomenon. The team notes that the masses of these particles follow a specific mathematical pattern predicted by the Randall-Sundrum model, a popular theory describing how extra dimensions might be shaped. The ratio between the mass of the heavy gluon and the lighter quark matches the theoretical prediction closely. Furthermore, the researchers suggest that the current limits set by other experiments, which claim to have ruled out these particles, may be based on the incorrect assumption that the particles decay into top quarks. If that assumption is wrong, the particles could still be hiding in the data, waiting to be recognized.

Looking ahead, the team is optimistic that more data from the collider's ongoing runs will clarify the picture. They calculate that with the additional statistics expected from the latest data collection, the significance of these signals could rise to a level that would constitute a formal discovery. They also predict that if this interpretation is correct, there should be a third, even heavier excited quark that can be produced in the decay of the 8.6 trillion electron volt particle. Finding this specific combination of particles would provide a crucial confirmation of the model. Additionally, the study hints at the possible existence of a heavy version of the Z boson, another fundamental particle, which might appear in the data around 3 trillion electron volts. While this signal is currently weaker, it offers another potential avenue for discovery. The authors conclude that if their interpretation holds, the Large Hadron Collider is on the verge of revealing a fundamental new layer of reality, turning a theoretical possibility into a concrete observation of the universe's hidden dimensions.

Technical Summary: Evidence for Kaluza-Klein Excitations at the LHC

Problem Statement
The paper addresses the interpretation of massive four-jet excesses observed by the CMS collaboration in Run 2 and Run 3 data. While previous interpretations attributed these excesses to R-parity violating (RPV) squark pairs or diquark pairs, the authors propose an alternative explanation within the framework of the Randall-Sundrum (RS) model. Specifically, they investigate whether these excesses correspond to the decay of heavy Kaluza-Klein (KK) gluons ( $g_{KK}$ ) into pairs of lighter KK quarks ( $q_{KK}$ ). The study also revisits the authors' prior findings regarding a sequence of KK graviton resonances ( $G_{KK}$ ) to establish a consistent mass hierarchy and decay phenomenology across different particle sectors.

Methodology
The authors employ a phenomenological analysis of CMS data, focusing on the $Y \to X + X \to JJ + JJ$ topology (where $Y$ is a heavy resonance decaying into two intermediate resonances $X$ , each decaying into two jets). The methodology involves:

Mass Spectrum Analysis: Comparing observed resonance masses against the quantized mass predictions of the RS model, which are proportional to the zeros of the Bessel function $J_1(x)$ .
Decay Channel Re-evaluation: Challenging standard assumptions regarding the dominant decay modes of KK particles. Specifically, the authors argue that, similar to their findings for KK gravitons, the branching ratio (BR) of KK gluons into top quark pairs ( $t\bar{t}$ ) may be suppressed. This suppression would alter the total width and production limits derived from standard LHC searches.
Cross-Section and Width Calculations: Utilizing RS model predictions to calculate production cross-sections and decay widths for $g_{KK} \to q_{KK}q_{KK}$ and $G_{KK} \to G_{KK}G_{KK}$ , comparing these theoretical values against CMS observed significances and background estimates.
Statistical Interpretation: Analyzing local $p$ -values and significance levels (in standard deviations) for specific mass hypotheses ( $M_Y \approx 3.6$ TeV and $8.6 $TeV;$ M_X \approx 1.0$ TeV and $2.1$ TeV), accounting for varying mass ratios ( $M_X/M_Y$ ) and resonance widths.

Key Contributions

Reinterpretation of CMS Excesses: The paper proposes that the CMS four-jet excesses at $M_Y \approx 3.6$ TeV and $8.6$ TeV are not non-resonant or squark-related, but rather the result of $g_{KK} \to q_{KK}q_{KK}$ decays.
Challenge to Standard Limits: The authors highlight that current LHC exclusion limits on KK gluons assume a dominant decay into top pairs. They argue that if the BR into $t\bar{t}$ is suppressed (as observed in their KK graviton studies), the total width decreases, and the existing limits on $g_{KK}(3.6 \text{ TeV})$ are overly optimistic and potentially invalid.
Mass Hierarchy Consistency: The paper demonstrates that the observed mass ratios between the first two generations of KK gluons ( $8.6/3.6 \approx 2.38$ ) and KK quarks ( $2.15/1.04 \approx 2.07$ ) align well with RS model predictions (theoretical ratios of $\approx 2.27$ and $2$, respectively).
Prediction of New Channels: The authors predict a specific, background-free decay channel: $g_{KK}(8.6 \text{ TeV}) \to q_{KK}(3.1 \text{ TeV}) + q_{KK}(3.1 \text{ TeV})$ , which would serve as a crucial confirmation of the model.
Z' and Z_KK Indications: The paper extends the analysis to the KK excitation of the Z boson ( $Z_{KK}$ ), suggesting an indication at 3 TeV in hadronic $VV$ modes and linking this to the potential existence of a $Z'$ resonance required to satisfy LEP precision measurements.

Results

Significance Levels: The analysis identifies local significances approaching $4\sigma$ for the four-jet resonances. Specifically, the $8.6$ TeV candidate corresponds to a KK quark mass of $2.15$ TeV, and the $3.6$ TeV candidate corresponds to a KK quark mass of $1.04$ TeV.
Branching Ratios: To reconcile the observed event counts with the predicted cross-sections, the authors estimate a required branching ratio for $g_{KK} \to q_{KK}q_{KK}$ of approximately $10\%$ (assuming an acceptance of $\sim 30\%$ ). This is deemed "not unlikely" given the potential suppression of the $t\bar{t}$ mode.
Width Discrepancies: The paper notes a significant discrepancy in the width of the 3.6 TeV resonance: the measured width is of the order of 200 GeV, whereas standard models assuming a dominant $t\bar{t}$ decay (as depicted in Figure 2 of the paper) predict a width of approximately 700 GeV. This large difference supports the authors' hypothesis of a reduced coupling to top pairs.
Future Prospects: The authors project that with full Run 3 data, the significance of these four-jet events could reach $6\sigma$ across three distinct channels, potentially leading to a fundamental discovery.

Claimed Significance
The paper claims to provide a "plausible explanation" for multiple CMS excesses that fits within the Randall-Sundrum framework, provided that the branching ratios of KK gluons into top pairs are suppressed. The authors assert that the mass spectrum of the observed resonances follows the RS predictions with high fidelity. They conclude that if their interpretation is correct, the LHC is on the verge of a fundamental discovery, with the $g_{KK}(8.6) \to q_{KK}(3.1)q_{KK}(3.1)$ channel offering a "spectacular" and "background-free" confirmation. Additionally, they suggest that the observation of a $Z_{KK}$ at 3 TeV would imply the existence of a $Z'$ resonance, testable via precision measurements at future $e^+e^-$ colliders.

Technical Summary: Evidence for Kaluza-Klein Excitations at the LHC

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