COALA : COALescence Afterburner

This repository provides transport-model independent C++/ROOT afterburners for forming deuterons, tritons, and helium-3 nuclei from a normalized nucleon event record. It contains two alternative coalescence implementations:

Package	Coalescence model	Cluster criterion
coala_box	Minimum-spanning-tree (box) coalescence	Hard relative-position and relative-momentum limits
coala_wigner	Wigner-function coalescence	Probabilistic phase-space overlap with spin-isospin statistical factors

The packages share the same event-processing workflow and output structure, while their underlying coalescence mechanisms remain distinct.

Input

Both afterburners read a text file containing paths to ROOT input files. The ROOT files must expose the normalized event and particle branches documented in each package README. The input can originate from any event generator or transport model, provided it is converted to this interface; it is not limited to AMPT.

See coala_box/README.md and coala_wigner/README.md for the complete input schema, parameter definitions, and execution examples.

Requirements

• A C++ compiler compatible with the installed ROOT build
• ROOT with root-config available on PATH
• GNU Make

The Makefiles inherit ROOT's compiler and linker flags and use the C++ standard reported by root-config. C++17 is used as a fallback.

Build

Build either package from its directory:

cd coala_box       # or coala_wigner
make clean
make

If root-config is not on PATH, specify it explicitly:

make ROOT_CONFIG=/path/to/root/bin/root-config

The C++ standard can also be overridden when required by the ROOT installation:

make CXXSTD=c++20

Citation

If you use this software in scientific work, please cite the paper associated with the coalescence implementation:

• Box coalescence: Y. Bailung, N. Shah, and A. Roy, “Exploring light nuclei production at RHIC and LHC energies with A Multi-Phase Transport model and a coalescence afterburner,” Nuclear Physics A 1037, 122701 (2023), 10.1016/j.nuclphysa.2023.122701.
• Wigner coalescence: Y. Bailung, S. P. Rode, N. Shah, and A. Roy, “Probing coalescence of light nuclei via femtoscopy and azimuthal anisotropies,” Physical Review C 111, 014911 (2025), 10.1103/PhysRevC.111.014911.