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Search for high-mass resonances in photon-jet final states using 140 fb $^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

Using 140 fb $^{-1}$ of 13 TeV proton-proton collision data, the ATLAS experiment performed a search for high-mass resonances in photon-jet final states, finding no significant deviations from the Standard Model and setting stringent exclusion limits on excited quark masses up to 6.0 TeV and quantum black hole thresholds up to 7.5 TeV.

Original authors: ATLAS Collaboration

Published 2026-08-28

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CC BY 4.0

Original authors: ATLAS 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 from a set of fundamental rules that physicists have spent decades trying to map out. This map, known as the Standard Model, successfully describes the tiny particles that make up everything we see and how they interact. However, the map is incomplete. It leaves out deep questions about why particles have the masses they do, why there are exactly three families of matter, and how gravity fits into the picture. To fill these gaps, scientists propose new ideas, suggesting that the particles we know might actually be made of smaller, more fundamental pieces, or that hidden dimensions of space exist just beyond our perception. These theories predict the existence of heavy, short-lived particles that should appear if we smash protons together with enough energy. Finding them would rewrite our understanding of reality, while failing to find them helps scientists narrow down where the true laws of nature might be hiding.

At the Large Hadron Collider, a massive ring buried beneath the border of France and Switzerland, protons are accelerated to nearly the speed of light and collided. The ATLAS experiment, one of the giant detectors surrounding the collision points, acts as a high-speed camera capturing the debris from these crashes. In a recent analysis, the ATLAS team sifted through a vast collection of data corresponding to 140 fb⁻¹ of integrated luminosity, looking for a very specific signature: a high-energy photon, a particle of light, flying out alongside a jet of particles. This combination is a clean signal that could reveal the decay of a new, heavy particle. The researchers were hunting for two main types of hypothetical objects: excited quarks, which would be heavier versions of the building blocks of matter, and quantum black holes, microscopic versions of the cosmic giants that form from collapsed stars.

The team examined the mass of the photon and jet pairs in every recorded event. If a new heavy particle existed, it would appear as a sudden spike, or a "bump," in the data at a specific mass value, rising above the smooth, predictable background of known physics. The researchers looked carefully across a wide range of energies, from the lowest masses they could reliably measure up to the highest energies the machine can produce. They also divided their search into different categories based on the type of heavy particle that might have been produced, checking for signs of light quarks, as well as heavier, less common varieties like those containing charm or bottom quarks. They even looked for the specific patterns expected if microscopic black holes were forming and evaporating instantly.

The result was a clear picture of what is not there. The data showed no significant spikes or unexpected excesses. The distribution of events matched the predictions of the Standard Model perfectly, with no evidence of new heavy particles hiding in the noise. Because no signal was found, the team was able to set strict limits on what could exist. They determined that if excited quarks do exist, they must be heavier than 6.0 TeV for light versions, 3.5 TeV for charm versions, and 2.6 TeV for bottom versions. Similarly, they ruled out the existence of quantum black holes with mass thresholds below 5.3 TeV in one theoretical model and 7.5 TeV in another. These limits are the most stringent constraints to date for these specific scenarios in this type of collision.

This work represents a significant step forward in the search for new physics, utilizing the full dataset collected during the second run of the Large Hadron Collider. By combining a larger volume of data with refined techniques for identifying particles and filtering out background noise, the ATLAS collaboration has pushed the boundaries of what we know. While the search did not uncover the new particles some theories predicted, the absence of a signal is a powerful scientific result. It tells theorists that if these particles exist, they are too heavy to be produced at current energy levels, forcing the scientific community to refine their models and look elsewhere. The search continues, but for now, the universe remains consistent with the known laws, and the hunt for the next layer of reality moves to higher energies and new frontiers.

Technical Summary: Search for High-Mass Resonances in Photon-Jet Final States with ATLAS

Problem and Motivation
This paper presents a search for new high-mass phenomena in proton–proton ($pp$) collisions at a centre-of-mass energy of $\sqrt{s} = 13$ TeV, utilizing data collected by the ATLAS detector during LHC Run 2. The analysis targets final states consisting of a high-transverse-momentum photon and a jet ( $\gamma$ –jet). Such signatures are predicted by various Beyond the Standard Model (SM) scenarios, including quark compositeness models which introduce excited quark states ( $q^*$ ), and theories involving extra spatial dimensions (Randall-Sundrum and Arkani-Hamed-Dimopoulos-Dvali models) which predict the production of microscopic quantum black holes (QBHs). The study aims to identify localized excesses in the $\gamma$ –jet invariant mass ( $m_{\gamma j}$ ) spectrum that would indicate resonant production of these new particles, thereby addressing unresolved SM issues such as the fermion family structure and the hierarchy problem.

Methodology
The analysis is based on a dataset corresponding to an integrated luminosity of 140 fb $^{-1}$ . The event selection requires a high- $p_T$ photon ( $p_T > 150$ GeV) and a jet ( $p_T > 150$ GeV) within the central detector region ( $|\eta| < 1.37$ ), with an invariant mass $m_{\gamma j} > 500$ GeV. To suppress backgrounds from bremsstrahlung and $t$ -channel production, kinematic constraints are applied, including a requirement on the momentum balance between the photon and jet and a restriction on their pseudorapidity difference ( $|\Delta\eta| < 1.6$ ).

A key methodological advancement in this analysis is the utilization of the GN2 tagger, a transformer-based algorithm, to discriminate between light-flavour, $c$ -, and $b$ -jets. This allows the definition of three exclusive signal regions:

SR (Inclusive): Contains all selected events.
SRB: Requires the leading jet to be tagged as a $b$ -jet.
SRC: Requires the leading jet to be tagged as a $c$ -jet.
SRL: Contains events where the jet fails both $b$ - and $c$ -tagging requirements.

Signal modelling for excited quarks and QBHs is performed using Monte Carlo simulations processed with fast detector simulation (AtlFast3) or full Geant4 simulation, depending on the sample. Background modelling relies on a functional fit to the data using smoothly falling functions (e.g., $f_b(x) = n_{bkg}(1-x)^{p_0}x^{-\sum p_i(\ln x)^{i-1}}$ ). Systematic uncertainties, including those from luminosity, object reconstruction, pile-up, and background modelling (quantified via "spurious signal" tests), are incorporated into a binned likelihood fit. The statistical analysis employs the $CL_s$ method to set exclusion limits and the BumpHunter algorithm to scan for localised excesses.

Key Contributions

First Direct Constraints on $c^*$ : This work provides the first direct constraints on the production of excited charm quarks ( $c^*$ ) in the $\gamma$ –jet final state at the LHC. By performing a simultaneous fit across the SRL, SRB, and SRC regions, the analysis improves sensitivity to $c^*$ signals, which populate multiple flavour-tagged categories.
Heavy-Flavour Excited Quarks: The study extends the search for excited quarks to include heavy-flavour states ( $b^*$ and $c^*$ ) in addition to light-flavour states ( $q^*_L$ ), exploring the dependence of the cross-section on the coupling strength $f$ .
Model-Independent Limits: In addition to specific benchmark models, the analysis sets limits on the visible cross-section for generic Gaussian-shaped resonances with widths ranging from 2% to 15% of the mass, facilitating reinterpretation in other theoretical frameworks.
Improved Background Modelling: The analysis utilizes an improved background modelling technique and a larger dataset (140 fb $^{-1}$ ) compared to previous ATLAS searches (36.7 fb $^{-1}$ ), enhancing the sensitivity to high-mass resonances.

Results
No significant deviation from the Standard Model prediction was observed in the $m_{\gamma j}$ spectrum across any of the signal regions. The data are well-described by the background-only hypothesis, with $p$ -values ranging from 0.11 to 0.62 depending on the region. The BumpHunter algorithm identified no statistically significant localised excesses.

Consequently, stringent upper limits were set at the 95% confidence level (CL) on the production cross-sections for various new physics scenarios:

Excited Quarks ( $f=1$ ): Masses up to 6.0 TeV are excluded for light-flavour $q^*_L$ ( $u^*/d^*$ ), 3.5 TeV for $c^*$ , and 2.6 TeV for $b^*$ .
Quantum Black Holes: Mass thresholds are excluded up to 5.3 TeV for the Randall-Sundrum (RS1, $n=1$ ) model and 7.5 TeV for the ADD model ( $n=6$ ).
Generic Resonances: Limits on the visible cross-section were established for Gaussian resonances across the explored mass and width ranges.

Significance
The author states that these results constitute the most stringent constraints to date in the $\gamma$ –jet final state. The exclusion of $c^*$ production represents a novel contribution to the field, as no previous LHC results had directly constrained these processes in this channel. The analysis demonstrates the capability of the ATLAS detector to probe high-mass scales using the full Run 2 dataset, leveraging advanced flavour-tagging techniques and refined statistical methods to constrain theoretical frameworks addressing the hierarchy problem and fermion compositeness.

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