⚛️ high-energy experiments

Projected sensitivity to the ADD large extra dimensions model in the monojet plus missing transverse momentum final state at HL-LHC, HE-LHC, and FCC-hh

This paper presents a unified projection of the sensitivity to the ADD large extra dimensions model via the monojet plus missing transverse momentum channel at the HL-LHC, HE-LHC, and FCC-hh, utilizing a novel energy-scaling method for backgrounds and a transparent calibration against ATLAS Run-2 data to establish improved 95% CL exclusion and 5 $\sigma$ discovery reach in the fundamental scale $M_D$ .

Original authors: Emine Gurpinar Guler (Konya Technical University)

Published 2026-10-28

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Emine Gurpinar Guler (Konya Technical University)

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 operates on a set of fundamental rules that govern how matter and energy interact, yet one of the most profound mysteries in modern physics remains the vast difference between the strength of gravity and the other forces of nature. Gravity, the force that keeps our feet on the ground and the planets in orbit, is incredibly weak compared to the forces that hold atoms together. For decades, scientists have struggled to explain why gravity is so feeble, a puzzle known as the hierarchy problem. One compelling solution suggests that gravity might not be weak at all, but rather that it is leaking into hidden, extra dimensions of space that we cannot see. In this scenario, the familiar three dimensions of space and one of time are just a slice of a much larger reality. If these extra dimensions exist, they would allow gravity to spread out and dilute, making it appear weak to us while remaining powerful in the full, higher-dimensional space.

To test this idea, physicists look for signs of these hidden dimensions in high-energy particle collisions. When particles smash together at incredible speeds, they can produce tiny ripples in the fabric of space-time called gravitons. If extra dimensions exist, these gravitons could escape our visible world and vanish into the hidden dimensions, taking energy with them. Since we cannot see the gravitons, we would only notice their absence as a sudden imbalance in the energy of the collision, appearing as a single jet of particles recoiling against nothing. This "monojet" signature, accompanied by missing energy, is the primary way scientists search for evidence of these extra dimensions.

A recent study by Emine Gülpınar Güler at Konya Technical University takes a fresh look at how well future particle accelerators could find these extra dimensions. The research focuses on three upcoming machines: the High-Luminosity Large Hadron Collider, the proposed High-Energy LHC, and the Future Circular Collider. These facilities are designed to smash protons together with far greater energy and intensity than current machines, offering a much deeper look into the subatomic world. The study does not just guess at what these machines might find; instead, it builds a unified projection using real data from current experiments to simulate how the search would play out at these future frontiers.

The researcher's approach is distinct because it avoids relying on theoretical guesses for the background noise that obscures the signal. Instead of simulating every possible interaction from scratch, the study uses actual data released by the ATLAS and CMS experiments from their recent runs. By taking the measured background yields—the expected number of events from known physics—and carefully adjusting them for the higher energies of the future colliders, the study creates a more reliable prediction. A crucial part of this work involves measuring how the background changes as the collision energy increases. Rather than assuming the background grows at a steady, predictable rate, the study measured this growth specifically for different energy levels. This detailed measurement revealed that the background increases more sharply at higher energies than previously thought, a finding that significantly tempers the expected reach of the new machines.

The results show that while future colliders will certainly extend our reach, the improvements are more modest than some earlier, less detailed estimates suggested. For the scenario with two extra dimensions, the current limit on the scale of these dimensions is set at 11.2 tera-electronvolts. The study projects that the High-Luminosity LHC could push this limit to 12.5 tera-electronvolts. The High-Energy LHC would extend it further to 15.2 tera-electronvolts, and the massive Future Circular Collider could reach up to 22.0 tera-electronvolts. These numbers represent a doubling of the current sensitivity, a substantial leap that would allow scientists to probe a much wider range of possibilities. However, the study also notes that if the number of extra dimensions is higher, the reach decreases, making the search more difficult.

The study also calculated the potential for a definitive discovery, where the evidence is so strong that it cannot be dismissed as a statistical fluke. For two extra dimensions, the discovery reach is projected to be 9.8 tera-electronvolts at the High-Luminosity LHC, 12.0 tera-electronvolts at the High-Energy LHC, and 17.0 tera-electronvolts at the Future Circular Collider. These figures indicate that while the next generation of machines will not immediately solve the mystery of extra dimensions, they will dramatically narrow the field of possibilities. The research underscores that the path forward requires not just more powerful machines, but also a more precise understanding of how the background of known physics behaves at extreme energies. By anchoring their projections to real, published data and carefully measuring the energy scaling of background events, the study provides a realistic and rigorous roadmap for the next chapter in the search for the hidden dimensions of our universe.

Technical Summary: Projected Sensitivity to the ADD Model at Future Hadron Colliders

Problem and Motivation
The hierarchy problem—the vast disparity between the electroweak scale ( $M_{EW} \sim 10^2$ GeV) and the Planck scale ( $M_{Pl} \sim 10^{19}$ GeV)—remains a central challenge in fundamental physics. The Arkani-Hamed–Dimopoulos–Dvali (ADD) model proposes that gravity appears weak because it propagates through $n$ compactified extra spatial dimensions, while Standard Model fields are confined to a 4D brane. This framework lowers the fundamental gravity scale ( $M_D$ ) to the TeV range. At hadron colliders, the primary signature of this model is the production of Kaluza–Klein (KK) gravitons that escape into the bulk, manifesting as missing transverse momentum ( $E_T^{miss}$ ) recoiling against a single jet (monojet).

While current LHC Run-2 analyses (ATLAS and CMS) have set limits on $M_D$ (e.g., $M_D > 11.2$ TeV for $n=2$ ), future colliders—the High-Luminosity LHC (HL-LHC), High-Energy LHC (HE-LHC), and Future Circular Collider (FCC-hh)—promise significantly higher energies and luminosities. However, previous projections often relied on generator-level simulations for backgrounds and assumed constant energy-scaling factors, potentially overestimating sensitivity. This work aims to provide a unified, internally consistent projection of the ADD reach across these three future stages, addressing these methodological limitations.

Methodology
The study employs a hybrid approach combining parton-shower-level signal generation with experimentally constrained background data:

Signal Generation: Signal samples for $n \in \{2, 3, 4, 6\}$ are generated using Pythia 8 (v8.317) with the GRW convention for KK graviton towers. A kinematic truncation prescription ( $\hat{s} > M_D^2$ suppressed) is applied for future collider projections to ensure validity, while untruncated samples are used for calibration anchors.
Background Estimation: Instead of simulating Standard Model (SM) backgrounds, the analysis utilizes post-fit SM background yields released by ATLAS and CMS on HEPData. This incorporates full detector response, control-region constraints, and higher-order corrections directly.
Energy Scaling: A critical innovation is the measurement of the background energy scaling factor ( $r_\sigma$ ) as a function of $E_T^{miss}$ using dedicated $Z(\nu\bar{\nu})$ +jets and $W(\ell\nu)$ +jets samples at $\sqrt{s} = 13, 14, 27,$ and $100$ TeV. This replaces the assumption of a constant scaling factor. The scaling is found to be $E_T^{miss}$ -dependent, rising significantly at high $E_T^{miss}$ due to small- $x$ PDF growth.
Calibration: The framework is anchored to published ATLAS Run-2 expected 95% CL limits via an $n$ -dependent calibration factor ( $C_n$ ). This factor bridges the gap between the parton-shower-level analysis and full detector simulation. The factors are determined directly for $n=2,3,4,5,6$ and found to be consistent with unity ( $1.001 \pm 1.4\%$ ), serving primarily as a consistency check.
Significance Calculation: Expected significance is computed by combining thirteen exclusive $E_T^{miss}$ bins using an Asimov estimator, matching the binning of the ATLAS likelihood fit. Uncertainties are propagated conservatively, assuming fixed relative uncertainties from Run-2.

Key Contributions
The paper distinguishes itself through four specific methodological contributions:

Unified Framework: It provides the first unified projection of ADD monojet sensitivity across HL-LHC, HE-LHC, and FCC-hh within a single, consistent framework.
Data-Driven Backgrounds: It utilizes post-fit HEPData yields rather than generator-level multi-process simulations, ensuring backgrounds are experimentally constrained.
Measured Energy Scaling: It measures the background energy scaling as a function of $E_T^{miss}$ rather than assuming a constant factor. This measurement reduces the projected FCC-hh reach by 39% relative to constant-factor treatments, highlighting the importance of accurate scaling.
$n$ -Dependent Calibration: It introduces a transparent calibration scheme determined directly for all values of $n$ , validated against published ATLAS cutflows, providing a template for future collider phenomenology.

Results
The projected 95% CL exclusion and 5 $\sigma$ discovery reach for $M_D$ are summarized below:

Collider	$\sqrt{s}$ / Luminosity	$n=2$ (95% CL)	$n=2$ (5 $\sigma$ )	$n=6$ (95% CL)
HL-LHC	14 TeV / 3 ab $^{-1}$	12.5 TeV	9.8 TeV	6.3 TeV
HE-LHC	27 TeV / 15 ab $^{-1}$	15.2 TeV	12.0 TeV	9.0 TeV
FCC-hh	100 TeV / 30 ab $^{-1}$	22.0 TeV	17.0 TeV	17.3 TeV

Extension of Reach: For $n=2$ , the FCC-hh extends the current ATLAS Run-2 bound ($11.2$ TeV) by a factor of 2.0.
Impact of Scaling: The use of measured $E_T^{miss}$ -dependent scaling lowers the FCC-hh reach from a hypothetical 36.2 TeV (using constant factors) to 22.0 TeV.
Systematic Uncertainties: The total experimental projection uncertainty is estimated at $\pm 4.5\%$ . An assumed $\pm 30\%$ variation in signal yield (representing un-evaluated theory uncertainties) shifts the reach by $\lesssim 9\%$ .

Significance and Claims
The paper claims to offer a robust, reproducible template for future-collider studies that avoids reliance on fast detector simulations whose applicability to future environments requires validation. By anchoring projections to existing Run-2 data and measuring background scaling rather than assuming it, the study provides a more conservative and realistic assessment of the ADD model's testability. The results indicate that while the HL-LHC offers a modest improvement, the HE-LHC and especially the FCC-hh will significantly probe the parameter space of large extra dimensions, with the FCC-hh capable of probing $M_D$ up to 22 TeV for $n=2$ . The author emphasizes that their calibration scheme and background handling strategy are generalizable to other Beyond the Standard Model (BSM) searches at future colliders.

Technical Summary: Projected Sensitivity to the ADD Model at Future Hadron Colliders

More like this