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Description
The composition of the fireball produced in high-energy Pb-Pb collisions at the LHC can be described by the Grand Canonical Statistical Hadronisation Model (SHM). One of the parameters of the model is the baryon chemical potential $\mu_B$, which determines the fraction of antimatter and matter present in the gas. The hypothesis that at the LHC $\mu_{B}=0$, i.e. antimatter and matter are perfectly balanced, was tested by a study published in Nature in 2018, where it was found that $\mu_B=0.7\pm3.8\ \mathrm{MeV}$. The goal of this work is to improve the precision on $\mu_B$ by measuring the antimatter-over-matter ratios of protons, $^3\mathrm{He}$ and hypertriton ($^3_\Lambda\mathrm{H}$) reconstructed by ALICE in the LHC Run 2 data. In the SHM the antimatter-over-matter ratios are connected to $\mu_B$ as:
$\bar{h}/h\propto \exp\left[-2\left(B+S/3\right)\mu_{B}/T\right]$,
where $B$ is the baryon number and $S$ the strangeness number of the considered particle species $h$. The analysed species are the most sensitive to the value of $\mu_B$, as they are characterised by large $B$. The techniques used for the measurements of the analysed particles include standard analyses for the proton and helium, which are directly tracked by the ALICE detectors, while the $^3_\Lambda\mathrm{H}$ analysis is based on a Boosted Decision Tree selection applied to the candidates, which are reconstructed via the two-body charged-$\pi$ mesonic decay of the hypertriton. The obtained $\mu_B$ represents the most precise measurement ever.
