The story begins with the measurement of the total proton-proton cross-section, a fundamental quantity representing the probability of an interaction occurring between two colliding protons. Both ATLAS and TOTEM, operating with unparalleled precision, have independently measured this cross-section at the LHC. While their results are remarkably consistent overall, a closer look at the data, particularly across a range of collision energies and kinematic conditions, reveals a slight, persistent deviation. This deviation, seemingly minor to the uninitiated, is precisely the kind of subtle clue that seasoned particle physicists pore over, as it often signifies the presence of physical processes not fully accounted for in current theoretical frameworks. Grafström and Staszewski’s work focuses on this very discrepancy, positing that it is not an experimental artifact but rather a signature of underexplored diffractive events, particularly those involving the creation of low-mass systems.

Diffractive scattering, in the context of high-energy proton collisions, is a peculiar phenomenon. Unlike “inelastic” collisions where protons shatter into a shower of new particles, in diffractive events, the protons themselves, or at least their fundamental constituents, emerge from the collision largely unscathed. However, they have exchanged energy and momentum, a bit like a glancing blow. This energy exchange can lead to the formation of new, typically less massive, particles in the “gap” between the scattered protons, which continue on their original trajectories. The challenge has always been to precisely isolate and quantify these diffractive events, especially those producing very light systems, which can easily be overwhelmed by the sheer volume of other interaction types.

The brilliance of Grafström and Staszewski’s analysis lies in their innovative methodology. Instead of trying to directly observe these elusive low-mass diffractive systems, which are incredibly difficult to disentangle from background noise, they have adopted an indirect approach. They reasoned that if these specific diffractive processes are indeed occurring and contributing to the overall interaction rate, then their absence or underestimation in the theoretical modeling used to interpret the experimental data should manifest as a discrepancy in the measured total cross-section. Therefore, by precisely quantifying the observed difference between the experimental measurements and the theoretical predictions that do not explicitly account for these low-mass diffractive contributions, they can essentially “extract” the missing cross-section, thereby inferring the strength and characteristics of these hidden interactions.

This method is akin to deducing the presence of a hidden suspect in a crime scene by observing what is missing from the overall picture or slightly out of place. The researchers meticulously compared the combined results of ATLAS and TOTEM, which represent a particularly sensitive probe of the total cross-section, with theoretical predictions that primarily focused on non-diffractive and more massive diffractive channels. The residual difference, the amount by which the experimental data exceeds the sum of the accounted-for processes, is then attributed to the precisely defined yet challenging-to-observe low-mass diffractive contribution. This sophisticated statistical sleight of hand is what allows them to put a number on something that is otherwise incredibly difficult to see directly.

The implications of this finding are profound. The strong nuclear force, mediated by gluons, is notoriously complex to model, especially at the low momentum transfer characteristic of diffractive interactions. Low-mass diffractive systems are thought to be governed by dynamics that are particularly sensitive to the behavior of gluons, the force-carrying particles of the strong force. Understanding how these gluons combine and interact to produce these light systems can provide crucial empirical data to test and refine theoretical models of quantum chromodynamics (QCD), the theory of the strong interaction. This could lead to a more unified understanding of how protons are structured internally and how they interact at high energies.

Furthermore, the research opens up new avenues for future experimental searches. Armed with the knowledge of the typical magnitude and kinematic distributions of these low-mass diffractive cross-sections, physicists at the LHC, and potentially future colliders, can design experiments and analysis techniques specifically optimized to detect these signals more directly. Currently, these events are statistical whispers lost in the cacophony of high-energy collisions. Grafström and Staszewski’s work provides a roadmap, a set of predictions, that can guide future efforts to turn these whispers into clear, undeniable signals, shedding more direct light on the processes at play.

The paper, published in the esteemed European Physical Journal C, represents a significant synthesis of two major experimental efforts. ATLAS, a general-purpose detector, captures a wide array of particles produced in collisions, while TOTEM specializes in measuring protons that scatter at very small angles, the very protons crucial for understanding diffractive processes. By combining the strengths and sensitivities of both collaborations, Grafström and Staszewski are able to leverage the most comprehensive data set available, allowing for the fine-grained analysis required to pinpoint such subtle effects. This collaborative spirit, essential in big science, is what pushes the boundaries of discovery.

The research delves into the theoretical underpinnings of diffractive scattering, examining various models that predict the production of low-mass systems. These models often involve concepts like Regge theory, Pomeron exchange, and saturation effects, which describe how the strong force behaves at high energies and low momentum transfers. The discrepancy they identify suggests that current standard models might be underestimating the contribution of certain types of diffractive exchange, possibly related to the interplay between perturbative and non-perturbative QCD phenomena. Effectively, the invisible is being made visible through the careful accounting of what is visible.

This discovery has the potential to resonate far beyond the immediate confines of high-energy physics. Our fundamental understanding of matter and energy is built upon the bedrock of particle physics. Any refinement or enhancement of our knowledge of fundamental forces, such as the strong nuclear force, has the capacity to influence technological advancements and our conceptualization of the universe. For instance, a deeper understanding of QCD can have indirect implications in fields ranging from nuclear engineering to astrophysics, where the behavior of matter under extreme conditions is paramount.

The scientific community is abuzz with the implications of this meticulous work. It’s a testament to the power of theoretical insight and experimental precision working in tandem. The very act of identifying a discrepancy and interpreting it as a signal of new physics rather than an error is a hallmark of truly innovative research. Grafström and Staszewski have demonstrated that even in the mature field of proton-proton scattering at the LHC, there are still profound mysteries waiting to be uncovered, simply by looking at the data with a fresh perspective and a refined theoretical lens.

The path forward involves further validation and refinement of these findings. Future LHC runs with even higher luminosity and potentially different collision energies will provide more precise data points. Alongside this, ongoing theoretical work to develop more sophisticated models of low-mass diffractive phenomena will be crucial in interpreting these new measurements. The dialogue between experiment and theory is more critical than ever, ensuring that observations are robustly explained and that theoretical advancements are grounded in empirical reality, creating a virtuous cycle of discovery.

This research is a powerful reminder that the universe is full of complexity and that our current understanding, while impressive, is always a work in progress. The LHC is a phenomenal tool, but it is the curiosity and ingenuity of physicists like Grafström and Staszewski that truly unlock its secrets. They have managed to find a significant scientific message in what might otherwise have been dismissed as statistical noise, transforming an anomaly into a beacon for future exploration and potentially rewriting parts of our textbooks on the fundamental interactions governing the cosmos.

Subject of Research: The differential cross-section of low-mass diffractive phenomena in proton-proton collisions at the Large Hadron Collider, extracted from discrepancies in total cross-section measurements.

Article Title: Extraction of low-mass diffractive cross section from the discrepancy between ATLAS and TOTEM total cross sections.

Article References:

Grafström, P., Staszewski, R. Extraction of low-mass diffractive cross section from the discrepancy between ATLAS and TOTEM total cross sections.
Eur. Phys. J. C 85, 873 (2025). https://doi.org/10.1140/epjc/s10052-025-14602-2

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-025-14602-2

Keywords: Diffractive scattering, Total cross-section, Proton-proton collisions, Large Hadron Collider, ATLAS, TOTEM, Quantum Chromodynamics, Strong interaction, Low-mass systems, Particle physics, High-energy physics, Pomeron exchange.

The ATLAS detector stands out as one of the most intricate and massive scientific instruments ever built, measuring over 40 meters in length and approximately 25 meters in height. Its primary goal is to probe the fundamental components of matter and the fundamental forces shaping the cosmos. This sophisticated apparatus employs state-of-the-art technology to track particles emerging from high-energy collisions, resulting in groundbreaking discoveries such as the Higgs boson, which has fundamentally altered our understanding of particle mass and the universe itself.

The Breakthrough Prize specifically commends the remarkable contributions made by the ATLAS Collaboration to the field of particle physics. This recognition underscores the collaboration’s advanced techniques in detailing the properties of the Higgs boson, investigating rare particle interactions, and exploring the delicate balance between matter and antimatter—a fundamental aspect of our understanding of the universe. The profound implications of these studies contribute significantly to the ongoing quest for knowledge in the realm of theoretical physics.

Stephane Willocq, ATLAS Spokesperson, expressed that this award reflects the hard work and creativity of thousands of collaborators who strive daily to push the boundaries of scientific inquiry. The acclaim awarded to the ATLAS team is not just a recognition of individual efforts but a celebration of the collective achievement of thousands who have dedicated their careers to exploring the depths of fundamental physics.

Fabiola Gianotti, Director-General of CERN, conveyed her pride in the accomplishments of the LHC collaborations. She emphasized that the honor symbolizes the extraordinary commitment, expertise, and determination demonstrated by researchers worldwide. This collaborative spirit embodies the essence of scientific discovery, elevating humanity’s understanding of the physical laws governing our universe.

The Mainz group, one of the largest university contributors to the ATLAS Collaboration, has been instrumental in the ongoing success of the project. Researchers from JGU have been deeply involved in various aspects of the ATLAS endeavor, including the construction, upgrade, and operation of critical systems. Their significant contributions encompass the design and construction of advanced high-speed electronics for the detector’s trigger system, which plays a vital role in efficiently capturing collision events.

During the second operational phase of the LHC, spanning from 2015 to 2018, the Mainz team spearheaded numerous impactful studies. They engaged in detailed measurements assessing the interaction strength between the Higgs boson and other fundamental particles, leading to groundbreaking conclusions about particle mass generation. The Mainz group’s leadership in these investigations underscores their crucial role in propelling forward the understanding of particle physics and the universe’s fundamental workings.

Volker Büscher, a professor at JGU and a former spokesperson for ATLAS Germany, articulated the excitement within the Mainz group regarding the wealth of scientific results obtained through the ATLAS detector. He shared a forward-looking perspective, expressing anticipation for future investigations that will delve deeper into the fundamental elements of the universe as further data and advancements in detector technology become available.

The successes achieved during Run 2 have showcased the ingenuity inherent within the ATLAS Collaboration. Beyond simply collecting data with unparalleled precision, the team has consistently pursued a deeper understanding of the results generated, demonstrating a relentless drive to decode the complexities of the universe and the behaviors of fundamental particles.

While the accolade from the Breakthrough Prize is a moment of celebration for the ATLAS Collaboration, the focus remains steadfastly on future endeavors. Currently, the third operational phase of the LHC is in progress, alongside rapid preparations for the High-Luminosity LHC upgrade. This ambitious project will significantly enhance the collider’s collision rates and data collection capabilities, paving the way for more exciting discoveries in the coming years.

The Mainz ATLAS team, composed of over 50 passionate physicists and engineers, is heavily involved in optimizing the experiment for the coming chapter of discovery. They lead the development of cutting-edge trigger electronics and contribute to constructing a new high-granularity timing detector for the High-Luminosity LHC. This future upgrade will provide unprecedented collision rates when operational in 2030, unlocking new opportunities for scientific breakthroughs.

As these advancements unfold, it becomes increasingly clear that the ATLAS detector will play a crucial role in harnessing the data produced by high-energy collisions to further push the frontiers of knowledge in particle physics. Willocq concluded by emphasizing the goal of preparing future ATLAS detectors to efficiently analyze the expansive data that will emerge from ongoing and upcoming experiments, reiterating the unwavering commitment of the collaboration to unravel the fundamental building blocks of our universe.

Through concerted efforts and collaborative ingenuity, the ATLAS Collaboration continues to lead the charge in the exploration of fundamental physics, driving the quest for knowledge forward in extraordinary ways.

Subject of Research: Investigation of Fundamental Physics through the ATLAS Experiment at CERN
Article Title: ATLAS Collaboration Honored with 2025 Breakthrough Prize in Fundamental Physics
News Publication Date: April 5, 2025
Web References: CERN News, Breakthrough Prize
References: ATLAS Collaboration Publications, CERN Annual Reports
Image Credits: M. Struik/CERN

Keywords

Particle physics, ATLAS Collaboration, Higgs boson, CERN, breakthrough prize, fundamental forces, universe, scientific discovery, high-luminosity LHC.