A report from the ALICE experiment.
Despite two decades of extensive studies, the production of antinuclei in heavy-ion collisions is not yet fully understood. Antinuclei production is usually modelled by two conceptually different theoretical models, the statistical hadronisation model (SHM) and coalescence models. In the SHM, deuteron antinuclei are produced from a locally thermally equilibrated source, while antinuclei are formed from the binding of constituent nucleons, which are close in momentum and position phase space in the coalescence model. Both models predict very similar production yields of, for example, deuteron antinuclei, bound states of an antiproton and an antineutron. This calls for new experimental observables that discern different production models.
Measuring higher moments of the multiplicity distribution of antinuclei as well as the correlation with antinucleons produced in the collision have been recently proposed as sensitive variables to antinucleosynthesis processes in heavy-ion collisions. The first measurement of the variance to mean ratio of the multiplicity distribution of antideuterons is compared to the predictions of the SHM and coalescence models (figure 1). The coalescence model fails to describe the observed ratio of the variance and mean of the multiplicity distribution of antideuterons. The measurements are consistent with the statistical baseline, a Poissonian distribution, as well as with the SHM in the presence of baryon number conservation. However, this observable proves insensitive to the size of the correlation volume used in the SHM to conserve the baryon number.
The Pearson correlation coefficient between the number of produced antideuterons and antiprotons constrains the latter effectively. The small negative correlation reflects that there are less protons observed in events with at least one deuteron than in an average event (figure 1). The coalescence model does not reproduce the measurement, whereas it is possible to fit the measurement to extract the correlation volume out of the SHM. The obtained correlation volume is 1.6 times the volume of the fireball per unit of rapidity, which is smaller compared to those describing proton yields and a similar measurement of net-proton number fluctuations. These findings point to a later formation of the correlation among protons and deuterons compared to that among antiprotons and protons.
Overall, these results present a severe challenge to the current understanding of antinuclei production in heavy-ion collisions at the LHC energies. With the LHC Run 3 data it will be possible to extend these measurements to heavier antinuclei and to higher order correlation coefficients and moments of the antinuclei multiplicity distribution that are even more sensitive to details of the nucleosynthesis process in heavy-ion collisions.
ALICE Collab. 2022 arXiv:2204.10166.