In a paper in Nature, the ALICE Collaboration1 reports that data from high-energy collisions between protons can be used to investigate the little-understood nuclear forces between protons and subatomic particles called hyperons. The measurements have comparable precision to state-of-the-art numerical calculations of the forces, thereby allowing conclusive quantitative comparisons of experimental data with theory. Accurate knowledge of these forces is needed for various aspects of physics research, for example in efforts to understand the stability of neutron stars.

The nuclear force between neutrons and protons (which are known collectively as nucleons) is a residual effect of the strong interaction that acts between their elementary constituents (quarks and gluons). First-principles calculations of the nuclear force have been challenging because of the peculiarities of the strong interaction. Our knowledge of this force is, therefore, based largely on simplified models and theories2, guided by experimental data3. The strong interaction between hadrons (subatomic particles, such as nucleons, that consist of two or more quarks bound together by the strong interaction) at low energies is therefore often referred to as the final frontier of the standard model of particle physics.

The interaction between nucleons has been measured with high accuracy3, but the interaction of nucleons with their heavier siblings, the hyperons, is less well assessed. Hyperons consist of three quarks, at least one of which must be a type (flavour) known as a strange quark; the other quarks can be up or down, the two lightest quark flavours. Hyperons are not present in the everyday matter that surrounds us on Earth, but — depending on their interactions with nucleons — might affect the compressibility of nuclear matter at high densities. This means they could be relevant to the stability of neutron stars4. Precise knowledge of hyperon–nucleon interactions is therefore of great importance not only for nuclear physics, but also for astrophysics. However, measurements of these interactions are difficult to make in conventional experiments involving direct particle collisions in accelerators, because hyperons are short-lived (their lifetimes are about 10−10 s5) and fly only a few centimetres, on average, before they decay.

The ALICE Collaboration now reports that proton–hyperon interactions can be investigated using high-energy collisions between protons carried out at the Large Hadron Collider (LHC) at CERN, Europe’s particle physics laboratory near Geneva, Switzerland. The technique depends on measurements of correlations between the momenta of protons and hyperons produced in the collisions.

The process studied in the experiments involves three steps (Fig. 1). First, protons are collided at extremely high energies, taking advantage of the fact that the LHC produces higher collision energies than any other accelerator. Second, hadrons are emitted by a ‘source’ produced by the collision — a volume of space in which quarks and gluons that originally came from the protons interact and become confined within new hadrons. The source emits various types of hadron, including protons and hyperons, some of which form proton–hyperon pairs. Finally, the proton and hyperon in each of these pairs interact with each other in ways that alter the momentum of the paired system. This momentum is measured by a detector and used to determine the momentum correlations.

Figure 1 | Investigating the proton–hyperon interaction. a, The ALICE Collaboration1 smashed together high-energy protons in CERN’s Large Hadron Collider. b, The collisions generate a ‘particle source’ — a volume of space in which components of the colliding protons interact and become confined within new particles. These new particles are emitted from the source, and include protons that pair up with heavier particles known as hyperons. c, The paired-up protons and hyperons interact with each other in a way that alters the relative momentum of the system, which is then measured by a detector. These measurements are then used to determine the nuclear force between the proton–hyperon pair.

The momentum correlations reflect the size of the hadron source and the properties of the interaction between the produced proton–hyperon pairs. Such correlation analyses were originally used to determine the source size in collisions of heavy ions6, but in the new work, they are instead used to investigate the interaction between the particles of interest. This approach to studying particle interactions was pioneered by the HADES Collaboration7 at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, and was further developed by the ALICE collaboration8 at the LHC. The current work depends on the fact that the extremely high-energy proton–proton collisions carried out at the LHC produce a high abundance of hyperons from small-volume hadron sources. The authors used this method to measure the strong force between protons and Ω– hyperons (which consist of three strange quarks) and between protons and Ξ hyperons (which consist of two strange quarks and one up or down quark).

The ALICE Collaboration’s findings open up a new ‘laboratory’ for investigating other nucleon–hyperon interactions, including the little-explored interactions with hyperons that contain two or three strange quarks. This will aid our understanding of metastable states of hyperon pairs or of the compressibility of nuclear matter at high densities. The latter is relevant not only for the stability of neutron stars, but also for neutron-star mergers and heavy-ion collisions.

In a lucky coincidence, recent developments in theoretical physics9,10 allow nuclear forces to be calculated from first principles so that the results can be compared with experimental findings. The precision with which nucleon–nucleon interactions can be determined from experimental data is still superior to that obtained from these calculations, but the ALICE Collaboration’s measurements of the proton–hyperon interactions almost exactly match those obtained from theory.

A wealth of high-precision measurements of proton–hyperon interactions is expected from the LHC in the next decade, following on from its recent upgrade. Moreover, various other facilities that will study particle collisions at lower energies than those produced at the LHC are expected to go into full operation in the coming years, including NICA in Russia, J-PARC in Japan and FAIR in Germany. Although fewer proton–hyperon pairs are generated per collision in lower-energy collisions, a greater proportion of those pairs will be emitted at low momenta — which might turn out to be advantageous, because more data are needed to reduce the statistical errors in measurements of low-momentum systems. Increases in computing power should also substantially reduce the uncertainties of first-principles calculations of nuclear forces. Taken together, these developments bode well for future research into the final frontier of the standard model of particle physics.