In a recently published article in Physical Review Letters, the ALICE
collaboration has used a method called femtoscopy to study the residual
interaction between two-quark and three-quark particles. Through this
measurement, an interaction between the ɸ meson (strange-antistrange quarks)
and a proton (two up and one down quarks) was unveiled 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 together quarks inside hadrons (like 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.
But unlike the residual strong interaction between protons and neutrons,
that 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 called
femtoscopy. Hadrons in the LHC collisions are produced very close to each
other, at distances of about 10-15 m (femtometre, hence the name
femtoscopy). This scale matches the range of the residual strong force,
giving them 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
contrary 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
of the partial restoration of chiral symmetry. The left- and right-handed
(chiral) symmetry that characterizes the strong interaction is found to be
broken in nature and this effect is responsible for the much larger mass of
hadrons, like the proton and neutron, with respect to the masses of the
quarks that constitute them. Hence, chiral symmetry is connected to the
origin of mass itself! A possible way to search for restoration of chiral
symmetry and shed light on the mechanism that generates mass is by studying
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 understood first.
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, 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 related to 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 Run 3
and Run 4 allowing to significantly improve the precision of the extracted
parameters and also to pin down the Y-ɸ interaction directly.
Reference:
S. Acharya et al, Experimental Evidence for an Attractive p−Ï• Interaction,
Physical Review Letters (2021).
DOI: 10.1103/PhysRevLett.127.172301
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Physics