The formation of light (anti)nuclei with mass number A of a few units (e.g., d, $^3$He, and $^4$He) in high-energy hadronic collisions presents a longstanding mystery in nuclear physics [1,2]. It is not clear how nuclei bound by a few MeV can emerge in environments characterized by temperatures above 100 MeV [3-5], about 100,000 times hotter than the center of the Sun. Despite extensive studies, this question remained unanswered. The ALICE Collaboration now addresses it with a novel approach using deuteron-pion momentum correlations in proton-proton (pp) collisions at the Large Hadron Collider (LHC). Our results provide model-independent evidence that about 80% of the observed (anti)deuterons are produced in nuclear fusion reactions [6] following the decay of short-lived resonances, such as the $\Delta (1232)$. These findings resolve a crucial gap in our understanding of nucleosynthesis in hadronic collisions. Beyond answering the fundamental question on how nuclei are formed in hadronic collisions, the results can be employed in the modeling of the production of light and heavy nuclei in cosmic rays [7] and dark matter decays [8,9].
Submitted to: OTHERS
e-Print: arXiv:2504.02393 | PDF | inSPIRE
CERN-EP-2025-081
Figure group
Figure 1
Illustration of three scenarios for deuteron production and interaction with pions (left) and the resulting \piPMd correlation functions (right). All scenarios include Coulomb attraction between $\pi^-$--d (green curves) and Coulomb repulsion between the $\pi^+$--d (red curves). The dashed lines always show the correlation function using Coulomb interaction. Scenarios (i) and (ii) represent thermally-produced deuterons with only Coulomb and Coulomb+elastic+inelastic interactions, respectively. Scenario (iii) depicts deuteron formation via nuclear fusion following $\Delta$-resonance decays. All the simulations include the charge conjugates ($\piPd \equiv \piPd\oplus \piMdbar$ and $\piMd \equiv \piMd\oplus\piPdbar$). The bands width corresponds to the statistical uncertainties of the models. |  |
Figure 2
Measured \piPd (left) and \piMd (right) correlation functions fitted with model calculations (upper panels) and the number of standard deviations (lower panels). The brown crosshatched bands in both panels correspond to contributions by the $\Delta$ resonance, blue bands represent the Coulomb interactions, teal diagonally hatched bands the residual background and the magenta bands the total fit function. The lower panels display comparisons between the data points and the fit in terms of the number of standard deviations $n\sigma$, where statistically 95\% of points are expected to lie within $\pm2\sigma$. |   |
Figure A.1
The extracted kinetic decoupling temperature is derived from $\pi^{\pm}$--p and \piPMd correlation functions. The bands correspondto the uncertainties obtained by fits to the correlation functions, incorporating systematic uncertainties on the measured data, as well as those arising from variations in the source size and the $\lambda$ parameter for the genuine interaction. |  |