Three Protons Detected Together for the First Time express
Three billiard balls scatter across the table after a strike: their trajectories depend only on the initial push. But in the world of atomic nuclei, things are more complex: particles sense each other on the fly, even if they are not bound together. For a long time, physicists could only measure pairwise interactions, while what happens when three meet remained behind the scenes.
In the ALICE experiment at the Large Hadron Collider, protons were collided at an energy of 13.6 TeV, and for the first time, a signal from three protons produced together was isolated. To do this, a new data-cleaning method was applied, similar to picking out the conversation of three specific people from the noise of a crowd. The measured correlation function showed that the nuclear interaction works even at high angular momenta and parity, creating an effective attraction at long distances.
The result turns three-body femtoscopy into a full-fledged tool for studying three-body forces. And since modern colliders produce many hyperons (particles with a strange quark), the method can be extended to systems involving strangeness. There is a chance to peek into three-body dynamics where there were only guesses before.
Original abstract
as written by the authors arXiv:2608.05708v1
Scattering experiments have long been the gold standard for constraining hadron$$-$$hadron interactions, providing direct information on the angular momentum and spin dependence over a wide range of kinematic configurations. However, experimental constraints on three-body dynamics remain limited, specifically for unbound systems and systems involving short-lived hadrons. In this work, the three-proton correlation function is measured in pp collisions at $$\sqrt{s}=13.6$$ TeV with ALICE at the LHC and presented as a novel approach to access hadronic interactions in three-body systems. A new analysis strategy is employed to isolate the p$$-$$p$$-$$p contribution to the correlation function by correcting for background channels and experimental effects, and enabling a direct comparison with state-of-the-art three-body continuum calculations. The extracted correlation function provides the first direct access to the isospin $$3/2$$ three-body system. The measured observable is found to be sensitive to the partial-wave structure of the nucleon$$-$$nucleon interaction and indicates that the nuclear interaction acts even at high angular momentum and parity states of the three-body system, revealing an effective long-range attractive component, observed experimentally for the first time in a three-proton continuum system. Hence, three-hadron femtoscopy emerges as an effective 3$$\rightarrow$$3 scattering experiment with three unbound hadrons in initial and final states. The copious production of hyperons at the modern high-energy colliders ensures the possibility of extending such measurements beyond nucleons, opening a new avenue for future precision studies of three-body dynamics in the strangeness sector.
The three protons studied in the experiment were never part of a single nucleus: they were born in a collision and existed as a free trio for only an instant.
In Liu Cixin's novel "The Three-Body Problem," the chaotic motion of three stars makes a planet uninhabitable, but in the quantum world, trios of particles can exhibit unexpected orderliness.