CT25: the new generation of CTEQ-TEA PDFs
This paper introduces CT25, a new generation of CTEQ-TEA parton distribution functions released in 2025 that offers diverse, optimized PDF ensembles with advanced uncertainty quantification to support precision QCD calculations for HL-LHC and EIC studies.
Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). 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
Inside the heart of every atom, protons and neutrons are not solid, unchanging spheres but rather chaotic, seething clouds of smaller particles called quarks and gluons. These fundamental building blocks are constantly swapping energy and changing their behavior depending on how hard they are hit. To understand the universe, from the formation of stars to the collisions in massive particle accelerators, scientists must know exactly how these particles are arranged and how they move. This arrangement is described by something called a parton distribution function, which acts like a detailed map showing the probability of finding a specific type of particle at a certain speed and location inside a proton. Without an accurate map, predictions about what happens when particles smash together at near-light speeds become guesswork, leaving gaps in our understanding of the fundamental laws of nature.
A team of physicists known as the CTEQ-TEA group has just released a new, highly refined version of this map, called CT25. This update is designed to meet the extreme demands of modern experiments, particularly those planned for the High-Luminosity Large Hadron Collider, where collisions will occur with unprecedented frequency and precision. The researchers did not simply tweak their old calculations; they built a vast digital library containing hundreds of different versions of the proton map. Each version in this library, which they call the Parton Cloud, was created using slightly different mathematical shapes, different sets of experimental data, and different assumptions about how the particles behave. By exploring this entire cloud of possibilities, the team could identify which maps fit the real-world data best and, more importantly, understand exactly where their knowledge is still uncertain.
The core achievement of this work is the creation of a new family of maps that come with a much clearer picture of their own reliability. In the past, scientists often relied on a single best guess for the proton's structure, but the CT25 team realized that the choice of mathematical formula used to draw the map could significantly change the results, especially for particles moving at very high speeds. To address this, they developed several distinct sets of maps. One set serves as the standard reference for most general calculations, while others are specialized tools for specific questions. For instance, some maps were created using only data from collisions involving protons, deliberately ignoring data from heavier atomic nuclei to see if those heavier atoms were skewing the results. Other maps were designed to be extra cautious, allowing for a wider range of possibilities to ensure that no potential discovery is missed because the initial guess was too narrow.
A major finding of this study is that the uncertainty in these maps is larger than previously thought when looking at the most extreme conditions. When particles carry a very large fraction of the proton's total momentum, the data becomes sparse, and the mathematical models have more freedom to vary. The CT25 team found that by including a wider variety of mathematical shapes and by carefully weighing conflicting data from different experiments, the uncertainty in these high-speed regions grows. This is not a failure of the science but a necessary honesty; it tells future researchers exactly how much they can trust a prediction in these difficult zones. The team also discovered that the strength of the force holding quarks together, a value known as the strong coupling constant, is best determined when the tensions between different experimental measurements are acknowledged and adjusted for, rather than ignored.
The researchers also used these new maps to simultaneously solve for three critical pieces of the puzzle: the mass of the top quark, the strength of the strong force, and the behavior of the gluon inside the proton. They found that these three values are deeply interconnected; changing one affects the others. Their analysis suggests a specific mass for the top quark and a specific strength for the strong force that aligns well with global averages, but only when the full complexity of the proton's internal structure is taken into account. This interdependence highlights that scientists cannot treat these fundamental properties as isolated numbers; they must be understood as part of a single, complex system.
By releasing this comprehensive collection of maps and the methods used to create them, the CTEQ-TEA group has provided the scientific community with a flexible toolkit. Instead of forcing everyone to use a single, rigid definition of the proton, they offer a range of options that can be chosen based on the specific question being asked. This approach allows experimentalists to test how sensitive their results are to the underlying assumptions about the proton's structure. If a new discovery depends heavily on a specific detail of the proton's map, researchers can now check if that detail holds up across the entire Parton Cloud or if it is an artifact of a particular choice. This rigorous, transparent approach ensures that as we push the boundaries of physics with more powerful machines, our understanding of the building blocks of the universe remains grounded in a realistic and thorough assessment of what we know and what we do not.
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