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ALICE takes the next step in understanding the interaction between hadrons

The ALICE collaboration has for the first time observed the residual strong interaction between protons and phi mesons

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Proton-phi interaction
An artistic representation of the interaction between a proton (consisting of two up quarks and a down quark) and a ɸ meson (consisting of a strange-antistrange quark pair) as they emerge from a proton-proton collision at the LHC with a distance between them of the order of a femtometer. (Image: ALICE collaboration)

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In an article recently published in Physical Review Letters, the ALICE collaboration has used a method known as femtoscopy to study the residual interaction between two-quark and three-quark particles. Through this measurement, an interaction between the ɸ (phi) meson (strange-antistrange quarks) and a proton (two up quarks and one down quark) has been observed for the first time.

Since the ɸ meson is not electrically charged, an interaction between the proton and the ɸ cannot be of electromagnetic origin and can only be attributed to the residual strong interaction. The strong interaction is what holds quarks together inside hadrons (such as the proton and the ɸ meson), while the residual strong interaction is the force that acts between hadrons. This is the interaction that holds protons and neutrons together in the form of atomic nuclei.

Unlike the residual strong interaction between protons and neutrons, which can be studied in stable bound states like the nuclei, the interaction between unstable hadrons produced in particle collisions is very difficult to observe. It was found to be possible in the LHC using an approach known as femtoscopy. Hadrons in the LHC collisions are produced very close to each other, at distances of about 10^-15 m (a unit known as a femtometer, hence the name femtoscopy). This scale matches the range of the residual strong force, giving the hadrons a brief chance to interact before flying away. As a result, pairs of hadrons that experience an attractive interaction will move slightly closer to each other, while, for a repulsive interaction, the opposite occurs. Both effects can be clearly observed through detailed analysis of the measured relative velocities of the particles.

The knowledge of the p-ɸ (proton-ɸ meson) interaction is of twofold interest in nuclear physics. First, this interaction is an anchor point for searches for the partial restoration of chiral symmetry. The left- and right-handed (chiral) symmetry that characterises the strong interaction is found to be broken in Nature, and this effect is responsible for the much larger mass of hadrons, such as the proton and the neutron, with respect to the masses of the quarks that make them up. Hence, chiral symmetry is linked to the origin of mass itself! A possible way of searching for the restoration of chiral symmetry and shedding light on the mechanism that generates mass is to study modifications of the properties of ɸ mesons within dense nuclear matter formed in collisions at the LHC. However, for this purpose, it is essential that the simple two-body p-ɸ interaction in vacuum is first understood.

The second point of interest is that, due to its strange-antistrange quark content, the ɸ meson is regarded as a possible vehicle of the interaction among baryons (hadrons consisting of three quarks) that contain one or more strange quarks, called hyperons (Y). Depending on the strength of this interaction, hyperons may form the core of neutron stars, which are among the densest and least understood astrophysical objects. Direct measurement of the Y-ɸ interaction strength, although feasible, has not yet been carried out, but already today this quantity can be estimated on the basis of the p-ɸ findings via fundamental symmetries. Therefore, measuring p-ɸ interaction provides indirect access to the Y-Y interaction in neutron stars.

The moderate interaction strength measured by ALICE provides a quantitative reference for further studies of the ɸ properties within the nuclear medium and also translates into a negligible interaction among hyperons in neutron stars. More accurate measurements will follow during the upcoming LHC Runs 3 and 4, allowing the precision of the extracted parameters to be significantly improved and also making it possible to pin down the Y-ɸ interaction directly.