Computer Physics Communications (2021) Contents lists available at ScienceDirect Computer Physics Communications journal homepage: www.elsevier.com/locate/cpc QTRAJ 1.0: A Lindblad equation solver for heavy-quarkonium dynamics Hisham Ba Omar a , Miguel ´ Angel Escobedo b , Ajaharul Islam a , Michael Strickland a,∗ , Sabin Thapa a , Peter Vander Griend c , Johannes Heinrich Weber d a Department of Physics, Kent State University, Kent, OH 44242, United States b Instituto Galego de F´ ısica de Altas Enerx´ ıas (IGFAE), Universidade de Santiago de Compostela. E-15782, Galicia, Spain c Physik-Department, Technische Universit¨ at M¨ unchen, James-Franck-Str. 1, 85748 Garching, Germany d Institut f¨ ur Physik, Humboldt-Universit¨ at zu Berlin & IRIS Adlershof, D-12489 Berlin, Germany ARTICLE INFO Article history: Communicated by M. Strickland 2000 MSC: 81V05, 46N50, 37N20, 82D10 Keywords: Heavy Quarkonium, Open Quantum Systems, Lindblad Equation, Quantum Trajectories ABSTRACT We introduce an open-source package called QTraj that solves the Lindblad equation for heavy-quarkonium dynamics using the quantum trajectories al- gorithm. The package allows users to simulate the suppression of heavy- quarkonium states using externally-supplied input from 3+1D hydrodynamics simulations. The code uses a split-step pseudo-spectral method for updat- ing the wave-function between jumps, which is implemented using the open- source multi-threaded FFTW3 package. This allows one to have manifestly unitary evolution when using real-valued potentials. In this paper, we provide detailed documentation of QTraj 1.0, installation instructions, and present var- ious tests and benchmarks of the code. © 2021 Elsevier Inc. All rights reserved. 1. Introduction Quantum Chromodynamics (QCD) is the quantum field theory that describes the strong nuclear force. The degrees of freedom contained in the QCD Lagrangian are quarks and gluons, which are collectively called partons. At low temperatures (T  2 × 10 12 K), partons are confined inside hadrons such as protons and neutrons, and, as a result, they are never observed in isolation. At temperatures higher than this threshold or, alternatively, at sufficiently high baryon density, QCD predicts a phase transition to a deconfined quark-gluon plasma (QGP) [1–3]. The QGP can be studied on Earth by colliding ultrarelativistic heavy ions, such as lead or gold, with ongoing experiments being performed at both the Relativistic Heavy Ion Collider at Brookhaven National Laboratory and the Large Hadron Collider at the European Organization for Nuclear Research (CERN) [4, 5]. In practice, it is very challenging to extract dynamical information about the QGP from experimental data since this new state of matter exists for only a very short time after the collision, t QGP  15 fm/c. It was theorized in [6, 7] that heavy quarkonium, a colorless bound state consisting of a heavy quark and a heavy anti-quark, melts inside of the QGP and, therefore, quarkonium suppression in heavy-ion collisions could be used as an important Preprint numbers: TUM-EFT 142/21; HU-EP-21/17-RTG ∗ Corresponding author: mstrick6@kent.edu arXiv:2107.06147v1 [physics.comp-ph] 13 Jul 2021