The most precise measurement yet of the force binding nuclear matter

Modern particle physics faces the frustrating paradox that, while the Standard Model describes the fundamental forces pretty well overall, one of its main components, the strong nuclear force, is notoriously hard to measure precisely.

For the first time, a team has reported direct parameters αs, determined without using data to infer anything (a model-free determination), with unprecedented precision relative to previous conjectures. This parameter controls the strength of interactions between quarks and gluons, the fundamental constituents of nuclear matter. But their research, published in the journal Nature, provides a clearer framework for understanding what happens during a typical collision at one of the world’s largest particle accelerators.

Quantum chromodynamics (QCD) describes the strong force that binds quarks in protons and neutrons. The coupling strength, αs, behaves strangely. It weakens at high energies due to asymptotic freedom, but strengthens at low energies to hold quarks inside hadrons. This energy dependence makes αs important, but also hard to understand.

Trinity’s Prof. Stefan Sint from Trinity’s School of Mathematics said, “It binds quarks together via exchange of gluons, and unlike other forces, becomes stronger with distance. This effect, known as confinement, prevents quarks from existing in isolation and makes precise calculations extremely difficult. While LHC experiments at CERN, such as ATLAS and CMS, can estimate the strong coupling constant, their accuracy is limited by confinement models.”

A precise knowledge of αs is essential for a rigorous interpretation of proton–proton collisions at CERN’s Large Hadron Collider (LHC), since even very small uncertainties can hide New Physics signals. Most earlier determinations of αs relied on well-known phenomenological models that had large systematic uncertainties.

To address this, the researchers used an unprecedented combination of low-energy experimental data and large-scale lattice QCD, a pathway to first-principles numerical simulations of QCD mapped onto a space–time grid. In particular, this method completely sidesteps the need for additional model assumptions.

The result has an uncertainty about half that of the combined uncertainties of all other measurements, and the remaining uncertainty primarily arises from statistical Monte Carlo assessment. This uncertainty (mostly) has a strict probabilistic interpretation and makes the final value much more robust.

This new determination connects two distinct physical regimes. QCD at low (lattice) and high energy scattering processes where perturbative expansions in αs remain qualitatively effective.

It provides a way of removing one significant source of theoretical uncertainty in the assessment of high-energy collision events. Such additional sensitivity is important for discovering small deviations from the predictions of new physics models beyond the Standard Model.

Physicists just made the most precise measurement ever of Gravity’s strength

It is not merely a technical update. It sets a new precision frontier in particle physics, one that we expect to:

Make precise predictions for LHC experiments; enable stringent tests of the Standard Model; and increase the likelihood of discovering small effects from unknown physics.

In the words of the researchers, their estimate of αs ‘could enable markedly improved analyses of many high-energy experiments,’ opening the door to discoveries that may reshape our understanding of the universe.

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