The Parton-Hadron-Quantum-Molecular Dynamics (PHQMD) transport approach is designed to provide a microscopic description of nuclear cluster and hypernucleus formation, as well as general particle production in heavy-ion reactions at relativistic energies.
PHQMD is a microscopic n-body transport approach that combines baryon propagation from the Quantum Molecular Dynamics (QMD) model with the dynamical properties and interactions of hadronic and partonic degrees of freedom from the established Parton-Hadron-String Dynamics (PHSD) approach.
In contrast to coalescence or statistical models often used for cluster formation, PHQMD forms clusters dynamically through baryon interactions within QMD, allowing the propagation of n-body Wigner density and n-body phase-space correlations, which are essential for cluster formation.
Clusters are identified by the MST (Minimum Spanning Tree) or SACA (Simulated Annealing Cluster Algorithm), which search for the most bound nucleon-cluster configurations. Collisions among hadrons, Quark-Gluon-Plasma formation, and parton dynamics are treated in PHQMD in the same way as in PHSD.
Cluster Production in PHQMD
Clusters can be identified in PHQMD using three different algorithms:
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Potential mechanism: The attractive potential between baryons with small relative momentum can form bound nucleon groups. In the MST clusterization algorithm, nucleons i and j are considered bound when
|ri* - rj*| < rclus, where positions are boosted to the pair center-of-mass frame andrclus = 4 fm(approximately the range of the attractive NN potential). Additionally, clusters must satisfy negative binding energy,EB < 0. MST is used as a cluster recognition tool (applied perturbatively), while QMD propagates baryons rather than pre-formed clusters. -
Kinetic mechanism: Deuterons are created through catalytic hadronic reactions
πNN ↔ πdandNNN ↔ Ndin different isospin channels. Quantum effects are included via excluded volume and projection onto the deuteron wave function in momentum space, which reduces production especially at target/projectile rapidities. -
Coalescence mechanism: A proton and neutron form a deuteron if their phase-space distance at freeze-out satisfies
|r1 - r2| ≤ 3.575 fmand|p1 - p2| ≤ 285 MeV/c. In PHQMD this method is used for model studies and comparison purposes.