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Deep inside the Large Hadron Collider, lead nuclei are accelerated to nearly the speed of light and smashed together. In such collisions, a multitude of particles are born, including kaons, particles containing a strange quark, and deuterons, the nuclei of heavy hydrogen. Physicists from the ALICE collaboration set out to determine how these particles interact with each other, using a method akin to eavesdropping on conversations at a noisy party.

When two particles emerge from the collision point close together, their mutual attraction or repulsion slightly alters the probability of finding them nearby. By measuring how often a kaon and a deuteron appear together, one can deduce the strength of their interaction. This is called femtoscopy — a technique that allows us to peer into a world measured in femtometers (millionths of a nanometer). For the first time, such measurements have been performed for K⁻d and K⁺d pairs in lead–lead collisions at an energy of 5.02 teraelectronvolts per nucleon pair.

The results came as a surprise. For K⁻d, the scattering length (a measure of interaction strength) turned out to be negative with a significant imaginary part, indicating absorption of the kaon by the deuteron. For K⁺d, the value is also negative but smaller in magnitude. These numbers are the first experimental benchmarks for theories describing the strong interaction at low energies. Previously, physicists had to rely solely on calculations; now they have a reference point.

The measured scattering lengths will help refine models of the strong interaction, especially in the realm of strange quarks. This is important for understanding how nuclear matter behaves under extreme conditions, such as in neutron stars, where kaons may play a role.

Original abstract

as written by the authors arXiv:2602.22833

Charged kaon--deuteron interactions offer a sensitive probe of the strangeness sector of QCD at low energy, particularly through their scattering parameters. Despite available theoretical predictions, experimental constraints remain absent, particularly for the $$\rm K^{-}d$$ system. The first measurement of femtoscopic correlation functions for $$\rm K^{-}d \oplus K^{+}\overline{d}$$ and $$\rm K^{+}d \oplus K^{-}\overline{d}$$ particle pairs in Pb--Pb collisions at $$\sqrt{s_{\rm NN}}=5.02$$ TeV recorded by ALICE at the LHC is presented. Correlation functions for both systems are analyzed in three centrality classes and fitted using the Lednick\'y--Lyuboshitz model to extract the source size and strong interaction scattering lengths. The $$\rm K^{-}d$$ scattering length is found to be $$\Re f_0 = -1.44 \pm 0.15\text{ (stat.)}^{+0.10}_{-0.10}$$ (syst.) fm and $$\Im f_0 = 1.34 \pm 0.33$$ (stat.)$$^{+0.21}_{-0.15}$$ (syst.) fm, while for $$\rm K^{+}d$$ the value $$\Re f_0 = -0.68 \pm 0.16$$ (stat.)$$^{+0.09}_{-0.09}$$ (syst.) fm is obtained. These results provide the first constraints on the $$\rm K^{\pm} d$$ interaction modeling at low energy, delivering a long-awaited experimental benchmark for testing chiral QCD dynamics.