Unlike the recurring strong interaction in between neutrons and protons, which can be studied in stable bound states like the nuclei, the interaction in between unsteady hadrons produced in particle accidents is extremely challenging to observe. As an outcome, sets of hadrons that experience an appealing interaction will move slightly closer to each other, while, for a repulsive interaction, the opposite happens. The understanding of the p-ɸ (proton-ɸ meson) interaction is of twofold interest in nuclear physics. The left- and right-handed (chiral) balance that identifies 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 neutron and the proton, with respect to the masses of the quarks that make them up. Determining p-ɸ interaction supplies indirect access to the Y-Y interaction in neutron stars.
An artistic representation of the interaction between a proton (including 2 up quarks and a down quark) and a ɸ meson (consisting of a strange-antistrange quark pair) as they emerge from a proton-proton accident at the LHC with a range between them of the order of a femtometer. Credit: ALICE partnership The ALICE partnership has for the very first time observed the residual strong interaction in between protons and phi mesons.
In a short article just recently released in Physical Review Letters, the ALICE cooperation has actually used an approach called femtoscopy to study the recurring interaction between three-quark and two-quark particles. Through this measurement, an interaction in between the ɸ ( phi) meson (strange-antistrange quarks) and a proton (2 up quarks and one down quark) has actually been observed for the very first time.
Because the ɸ meson is not electrically charged, an interaction in between the ɸ and the proton can not be of electromagnetic origin and can just be associated 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 kind of atomic nuclei.
A Large Ion Collider Experiment. Credit: CERN
Unlike the residual strong interaction between neutrons and protons, which can be studied in stable bound states like the nuclei, the interaction in between unstable hadrons produced in particle collisions is very challenging to observe. It was discovered to be possible in the LHC using a technique known as femtoscopy. Hadrons in the LHC crashes are produced really near to each other, at ranges of about 10 ^ -15 m (a system called a femtometer, for this reason the name femtoscopy). This scale matches the variety of the recurring strong force, providing the hadrons a short chance to engage before flying away. As an outcome, sets of hadrons that experience an appealing interaction will move slightly closer to each other, while, for a repulsive interaction, the opposite occurs. Both effects can be plainly observed through comprehensive 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. Initially, this interaction is an anchor point for searches for the partial remediation of chiral balance. The left- and right-handed (chiral) balance that defines the strong interaction is discovered to be broken in Nature, and this effect is accountable for the much larger mass of hadrons, such as the neutron and the proton, with regard to the masses of the quarks that make them up. For this reason, chiral proportion is linked to the origin of mass itself! A possible method of looking for the repair of chiral proportion and clarifying the system that generates mass is to study modifications of the properties of ɸ mesons within thick nuclear matter formed in collisions at the LHC. However, for this purpose, it is necessary that the basic two-body p-ɸ interaction in vacuum is first understood.
ALICE ITS Outer Barrel Installation. Credit: CERN
The 2nd point of interest is that, due to its strange-antistrange quark material, the ɸ meson is considered as a possible vehicle of the interaction amongst baryons (hadrons consisting of three quarks) that include one or more odd quarks, called hyperons (Y). Depending upon the strength of this interaction, hyperons might form the core of neutron stars, which are among the densest and least comprehended astrophysical things. Direct measurement of the Y-ɸ interaction strength, although possible, has not yet been brought out, but already today this quantity can be approximated on the basis of the p-ɸ findings via basic balances. Therefore, determining p-ɸ interaction offers indirect access to the Y-Y interaction in neutron stars.
The moderate interaction strength measured by ALICE provides a quantitative recommendation for further studies of the ɸ properties within the nuclear medium and also equates into a minimal interaction amongst hyperons in neutron stars. More accurate measurements will follow throughout the upcoming LHC Runs 3 and 4, enabling the accuracy of the extracted parameters to be significantly improved and also making it possible to select the Y-ɸ interaction straight.
Reference: “Experimental Evidence for an Attractive p − ϕ Interaction” by S. Acharya et al. (ALICE Collaboration), 14 September 2021, Physical Review Letters.DOI: 10.1103/ PhysRevLett.127.172301.