Shedding Light on (Anti-)nuclei Production with Pion-Nucleus Femtoscopy

Abstract

High-energy nuclear collisions provide a unique environment for synthesizing both nuclei and antinuclei (such as $\bar{d}$ and $\overline{^4\text{He}}$) at temperatures ($k_BT\sim100$ MeV) nearly two orders of magnitude above their binding energies of a few MeV. The underlying production mechanism, whether through statistical hadronization, nucleon coalescence, or dynamical regeneration and disintegration, remains unsettled. Here we address this question using a novel tool of pion-nucleus femtoscopy. By solving relativistic kinetic equations for pion-catalyzed reactions ($πNN \leftrightarrow πd$) for deuteron production and including a $70~\mathrm{MeV}/c^2$ downward shift of the in-medium $Δ(1232)$ mass, we successfully reproduce the resonance peaks observed by the ALICE Collaboration in both $π^+-p$ and $π^+-d$ femtoscopic correlation functions in high-multiplicity $pp$ collisions at $\sqrt{s} = 13~\mathrm{TeV}$. We further find that the nucleon coalescence model reproduces only about half of the observed peak strength, while the statistical hadronization model predicts no resonance feature. These results provide compelling evidence that pion-catalyzed reactions play a dominant role in the production of light (anti-)nuclei in high-energy nuclear collisions and cosmic rays.

Figures (3)

• Figure 1: Scenarios of deuteron production in high-energy nuclear collisions, including the statistical hadronization of QGP (a), the nucleon coalescence at the kinetic freeze-out (b), and the pion-catalyzed reactions (e.g. $\pi^+np\leftrightarrow \pi^+d$) with the assistance of intermediate $\Delta$ resonance during the hadronic matter expansion (c).
• Figure 2: Correlation function $C(k^*)$ of $\pi^+-p$ (a) and $\pi^+-d$ (b) in high-multiplicity $pp$ collisions at $\sqrt{s}=13$ TeV. Theoretical results with nominal and reduced $\Delta$ masses are shown by green and red bands, respectively. Experimental data with combined statistical and systematic uncertainties taken from the ALICE Collaboration ALICE:2025aurALICE:2025byl are denoted by filled symbols. Following the experimental kinematic cuts, particle rapidity is restricted to $|\eta| \leq 0.8$. The transverse momentum ($p_T$) is required to be within the range $0.14 < p_T < 4.0$ GeV/$c$ for pions and $0.5 < p_T < 2.4$ GeV/$c$ for deuterons. For $\pi^+-p$ correlations, additional cut of pair transverse momentum, defined by $m_T=\sqrt{(m_p+m_\pi)^2+(\mathbf{k}_p+\mathbf{k}_\pi)^2}/2$, of $m_T\in$ [0.54 0.75) GeV/$c^2$ is imposed ALICE:2025aur.
• Figure 3: Correlation function $C(k^*)$ of $\pi^+-d$ in different production scenarios for $pp$ collisions at $\sqrt{s}=13$ TeV. Experimental data shown as filled symbols are taken from the ALICE Collaboration ALICE:2025byl, and theoretical predictions are represented by colored bands.