Balancing matter and antimatter in Pb–Pb collisions

When lead ions collide head-on at the LHC they deposit most of their kinetic energy in the collision zone, forming new matter at extremely high temperatures and energy densities. The hot and dense zone quickly expands and cools down, leading to the production of approximately equal numbers of particles and antiparticles at mid-rapidity. However, in reality the balance between matter and antimatter can be slightly distorted.

The collision starts with matter only, i.e. protons and neutrons from the incoming beam. During the collision process, incoming lead nuclei interact while penetrating each other, and most of their quantum numbers are carried away by particles travelling close to the beam direction. Due to strong interactions among the quarks and gluons, quantum numbers of the colliding ions are transported to mid-rapidity rather than to the ions themselves. This leads to an imbalance of baryons originating from the initial state, which has more baryons than antibaryons.

This matter–antimatter imbalance can be quantified by determining two global system properties: the chemical potentials associated with the electric charge and baryon number (denoted μ_Q and μ_B, respectively). In a thermodynamic description, the chemical potentials determine the net electric-charge and baryon-number densities of the system. Thus, μ_B measures the imbalance between matter and antimatter, with a vanishing value indicating a perfect balance.

In a new, high-precision measurement, the ALICE collaboration reports the most precise characterisation so far of the imbalance between matter and antimatter in collisions between lead nuclei at a centre-of-mass energy per nucleon pair of 5.02 TeV. The study was carried out by measuring the antiparticle-to-particle yield ratios of light-flavour hadrons, which make up the bulk of particles produced in heavy-ion collisions. The measurement using the ALICE central barrel detectors included identified charged pions, protons and multi- strange Ω^– baryons, in addition to light nuclei, ³He, triton and the hypertriton (a bound state of a proton, a neutron and a Λ-baryon). The larger baryon content of these light nuclei makes them more sensitive to baryon-asymmetry effects.

The medium created in lead–lead collisions at the LHC is nearly electrically neutral and baryon-number-free at mid-rapidity

The analysis reveals that in head-on lead–ion collisions, for every 1000 produced protons, approximately 986 ± 6 antiprotons are produced. The chemical potentials extracted from the experimental data are μ_Q = -0.18 ± 0.90 MeV and μ_B = 0.71 ± 0.45 MeV. These values are compatible with zero, showing that the medium created in lead–lead collisions at the LHC is nearly electrically neutral and baryon-number-free at mid-rapidity. This observation holds for the full centrality range, from collisions where the incoming ions peripherally interact with each other up to the most violent head-on processes, indicating that quantum-number transport at the LHC is independent of the size of the system formed.

The values of μ_B are shown in figure 1 as a function of the centre-of-mass energy of the colliding nuclei, along with lower-energy measurements at other facilities. The recent ALICE result is indicated by the red solid circle, along with a phenom­enological parametrisation of μ_B. The decreasing trend of μ_B observed as a function of increasing collision energy indicates that different net-baryon-number density conditions can be explored by varying the beam energy, reaching almost vanishing net-baryon content at the LHC. The inset gives the μ_B values extracted at two LHC energies. It shows that the new ALICE result is almost one order of magnitude more precise than the previous estimate (violet), thanks to a more refined study of systematic uncertainties.

The present study with improved precision characterises the vanishing baryon-asymmetry at the LHC, posing stringent limits to models describing baryon-number transport effects. Using the data samples collected in LHC Run 3, these studies will be extended to the strangeness sectors, enabling a full characterisation of quantum-number transport at the LHC.