Measuring $\mu B$ at the LHC with ALICE via antiparticle-over-particle ratios.

Ciacco M. per la ALICE Collaboration
  Venerdì 16/09   09:00 - 13:30   Aula B - Maria Goeppert-Mayer   I - Fisica nucleare e subnucleare   Presentazione
The baryon chemical potential $\mu B$ is a fundamental parameter for the statistical mechanical description of particle production in heavy-ion collisions: its value is connected to the asymmetry between the produced matter and antimatter. The first $\mu B$ measurement in high-energy Pb-Pb collisions at the LHC was published in 2018 in $Nature$, and it was found that $\mu B=0.7 \pm 3.8$ MeV. In the Statistical Hadronisation Model, antimatter-over-matter ratios $R$ are connected to $\mu B$ as $R\alpha \exp[-2(B+S/3) \mu B/T-2I_{3} \mu I_{3}/T]$ where $B, S$ and $I_{3}$ are the baryon number, strangeness and third isospin component of the considered particle species, respectively. In this contribution, an improved $\mu B$ measurement based on the study of such ratios for protons, $^{3}He$ and hypertriton $(\Lambda {}^{3}H)$ using the data collected by ALICE in Run 2 of the LHC, is presented. The isospin chemical potential is also determined via the $\pi^{-}/\pi^{+}$ ratio. The study of the pion, proton and helium ratios is based on standard analyses, while the selection of $\Lambda {}^{3}H$ candidates, which are reconstructed via the two-body charged-$\pi$ mesonic decay of the hypertriton, is performed using Boosted Decision Trees. The obtained $\mu B$ represents the most precise measurement.