Femtoscopy studies of pion radiation in heavy-ion collisions have been conducted extensively at all available collider energies, both theoretically and experimentally. In all these studies special attention is given to $m_T$ dependency of pion femtoscopy radii, usually approximated by a power-law function at transverse momenta above $200\mathrm{MeV}/c$. However, the radii behavior has been much less explored for the ultrasoft pions, possessing transverse momentum comparable to or lower than the pion mass. For many experimental setups this region is difficult to measure. In this work we present theoretical calculations of pion emission in the ultrasoft region in the two hybrid models iHKM and $\text{LHYQUID}+\mathrm{th}\text{ERMINATOR2}$. Along with the particle transverse momentum spectra, we present the calculated femtoscopy radii, in both one-dimensional and three-dimensional representations. We investigate the radii dependence on pair $m_T$ and observe, in particular, a departure from the power-law behavior at ultrasoft momenta, potentially reflecting a decoupling of such slow pions from the rest of collectively expanding system. We provide the theoretical interpretation of this result and discuss its significance, in particular, for the ongoing nonidentical particle femtoscopy analysis for pairs consisting of a pion and a baryon (or of a pion and a charmed meson).

• Received 7 March 2024
• Revised 20 June 2024
• Accepted 31 July 2024

DOI:https://doi.org/10.1103/PhysRevC.110.034904

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Published by the American Physical Society