Three-body nuclear systems are key to many aspects of modern nuclear physics, such as understanding the equation of state of high-density nuclear matter and the composition of neutron star cores. In particular, scattering data from deuterons (bound proton-neutron pairs) and hadrons offer important ingredients for constraining parameters of nuclear interactions. Physicists with the ALICE Collaboration show that such three-body nuclear reactions can be investigated via hadron-deuteron correlations in momentum space at CERN’s Large Hadron Collider (LHC).
An illustration of the strong interaction in the proton-deuteron system produced in proton-proton collisions at CERN’s Large Hadron Collider. Image credit: ALICE / CERN.
A fundamental force is typically described as an interaction between two objects. Extending this to more complicated systems is not always trivial.
The description of strongly interacting three-hadron systems is key to understanding many phenomena in modern nuclear physics, such as the structure of nuclei, properties of high-density nuclear matter and the composition of neutron star cores.
Proton-proton collisions at the LHC produce a large number of particles that are emitted very close to each other, at distances of about 10-15 m (a femtometer).
It is interesting to explore whether they influence each other in any way before spraying off in all directions.
If two particles are produced close to each other and with similar momenta and direction, the pair can be subject to quantum statistics, Coulomb force and strong interaction.
If one of the pair is a deuteron, then a system with a deuteron and another hadron, like a proton or a kaon, is effectively a three-body system.
Thus, the measurement of correlations between deuterons and kaons or protons is expected to reveal the interactions of three-body systems.
The ALICE Collaboration utilises its excellent particle identification capabilities to study these correlations in high-multiplicity proton-proton collisions at a centre-of-mass energy of 13 TeV.
The result is a correlation function that measures how the probability of finding two particles with certain relative momenta differs from what would be expected if their momenta were completely independent or uncorrelated.
In the absence of correlation, the value of the function is unity.
A value above one indicates attractive interaction, whereas a value below one indicates repulsive interaction.
The correlation functions for both the kaon-deuteron and proton-deuteron systems are below unity for low relative transverse momenta, indicating an overall repulsive interaction.
The analysis of the kaon-deuteron correlation shows that the relative distances at which deuterons and protons or kaons are produced are quite small, around 2 fm.
The kaon-deuteron correlations are well described with an effective two-body model that incorporates both the Coulomb interaction and strong interaction between the kaon and the deuteron.
In contrast, the same effective two-body approach fails to describe the proton-deuteron correlations, necessitating a full three-body calculation that accounts for the structure of the deuteron.
An excellent data description is achieved using theoretical calculations that account for both two- and three-body strong interactions.
This demonstrates the sensitivity of the correlation function to the short-range dynamics of the three-nucleon system.
The correlation measurements at short distances constitute an innovative method to study three-body systems at the LHC, with the potential to extend such studies to other hadrons.
“Our result establishes a new experimental method to study the dynamics and forces of three-body nuclear systems with precision,” the physicists said.
“Indeed, given the copious production of strange and charm particles at the nuclear collisions at the LHC, the presented method sets the stage for the study of three-body forces in systems with strangeness and charm in a direct way.”
Their paper was published in the journal Physical Review X.
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S. Acharya et al. (ALICE Collaboration). 2024. Exploring the Strong Interaction of Three-Body Systems at the LHC. Phys. Rev. X 14 (3): 031051; doi: 10.1103/PhysRevX.14.031051
