EEXICC: An event generator for doubly heavy baryon production at e+e− colliders

We present EEXICC, a Monte Carlo event generator designed to simulate the production of doubly heavy baryons (Ξ cc , Ξ bc , and Ξ bb ) via 𝑒 + ⁢ 𝑒 − annihilation. Based on nonrelativistic QCD effective theory, the generator calculates the process 𝑒 + + 𝑒 − → Ξ 𝑄 ⁢ 𝑄 ′ + ̅ ̅ ̅ ̅ ̅ ̅ 𝑄 ′ + ̅ ̅ ̅ ̅ ̅ ̅ 𝑄 using an improved trace technique at the amplitude level, which greatly improves numerical efficiency compared with traditional squared-amplitude methods. EEXICC uses a modular Fortran implementation for hard-process generation and a C++ interface to the bundled PYTHIA 8.317 library, enabling convenient integration into complete event simulation workflows. The program supports both weighted and unweighted event generation, and its numerical reliability has been verified against existing theoretical results. EEXICC provides a flexible and robust tool for studying the properties of doubly heavy baryons at future high-luminosity and high-energy 𝑒 + ⁢ 𝑒 − colliders such as the CEPC and FCC-ee. PROGRAM SUMMARY Program title : EEXICC v1.0 CPC Library link to program files : https://doi.org/10.17632/95r3grx4cv.1 Licensing provisions: MIT Programming language : Fortran 77/90, C++. External routines/libraries : PYTHIA 8.317 (required by the current build); GNU Fortran, a C++11 compiler and GNU Make. Nature of problem : The program simulates the production of doubly heavy baryons (Ξ cc , Ξ bc , and Ξ bb ) (In this paper, we use Ξ 𝑄 ⁢ 𝑄 ′ to denote a generic doubly heavy baryon containing two heavy quarks, Q and Q ′.) via 𝑒 + ⁢ 𝑒 − annihilation at high-energy colliders. It is designed to investigate the properties and production rates of these baryons for future experiments. Solution method : The simulation is based on the nonrelativistic QCD factorization framework. The program calculates the production amplitudes using an improved trace technique, which enhances numerical stability and speed. Both color ̅ ̅ ̅ ̅ ̅ 𝟑 and color 6 S -wave ( QQ ′) intermediate states are included. The generator offers options for both weighted and unweighted events, where unweighted event generation is optimized using an improved hit-and-miss algorithm to maximize efficiency. Additional comments including restrictions and unusual features : The current version implements a leading-order calculation in both the strong coupling constant and the typical heavy-quark velocity in the baryon rest frame, based on the nonrelativistic QCD framework; consequently, neither QCD radiative corrections nor relativistic corrections are included. Furthermore, the generator focuses exclusively on the 𝑒 + ⁢ 𝑒 − annihilation channel. Contributions from photon-photon scattering processes, which become significant at collision energies far above the Z -boson resonance, are not currently implemented.