arXiv subjects

Iouri Vassiliev

Publications and source records attributed to Iouri Vassiliev.

3 recordsLinked to original sources

arXiv2026

Resolving the $ϕ$-meson directed-flow puzzle by multi-step meson--baryon dynamics

Recent STAR measurements at fixed-target Beam Energy Scan energies have revealed an unexpectedly large directed flow of $ϕ$ mesons in Au+Au collisions, comparable to that of protons and $Λ$ baryons and much stronger than that of light strange mesons. Since the $ϕ$ is a hidden-strangeness meson with relatively weak interactions with non-strange hadrons, this observation has been interpreted as a possible signal of unconventional baryonic dynamics or exotic baryonic resonances coupled to the $ϕ$ channel. Within the framework of the Parton-Hadron-Quantum-Molecular-Dynamics(PHQMD) model, we demonstrate that in the high baryon density region, $ϕ$ mesons are produced predominantly through multi-step meson--baryon and meson--hyperon reactions, whose transition amplitudes are constrained by a coupled-channel $T$-matrix calculation based on an extended SU(6) chiral effective Lagrangian. Together with the in-medium broadening of the $ϕ$ spectral function, these baryon-driven production channels enhance near-threshold $ϕ$ production and imprint the collective motion of the baryon-rich source on the produced $ϕ$ mesons.

nucl-th↗

arXiv2026

The High Level Trigger and Express Data Production at STAR

To meet the demands of the Beam Energy Scan phase-II (BES-II) program, the STAR experiment at RHIC developed a dual real-time framework consisting of a High Level Trigger (HLT) and an Express Data Production system (xProduction). The HLT operates online within the Data Acquisition (DAQ) chain on a multicore CPU cluster, with optional acceleration using Xeon Phi coprocessors. It employs parallelized algorithms, such as the Cellular Automaton track finder, for fast tracking, vertexing, and event filtering, enabling real-time event selection and detector monitoring. In parallel, xProduction runs independently of the DAQ loop and performs near offline-quality calibration and reconstruction within hours. Using the express data stream, enhanced by HLT selections, and the STAR calibration framework, it enables early physics analysis and provides collaboration-wide access to analysis-ready datasets. Together, HLT and xProduction form a complementary system combining real-time selection with rapid high-quality reconstruction. This framework has enabled prompt reconstruction of the ${}^5_Λ\mathrm{He}$ hypernucleus and efficient processing of large datasets, demonstrating scalability for future high-luminosity experiments.

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arXiv2025

Probing of EoS with clusters and hypernuclei

The study of the nuclear equation-of-state (EoS) is a one of the primary goals of experimental and theoretical heavy-ion physics. The comparison of recent high statistics data from the STAR Collaboration with transport models provides a unique possibility to address this topic in a yet unexplored energy domain. Employing the microscopic N-body Parton-Hadron-Quantum-Molecular Dynamics (PHQMD) transport approach, which allows to describe the propagation and interactions of hadronic and partonic degrees of freedom including cluster and hyper-nucleus formation and dynamics, we investigate the influence of different EoS on bulk observables, the multiplicity, $p_T$ and rapidity distributions of protons, $Λ$s and clusters up to A=4 as well as their influence on the collective flow. We explore three different EoS: two static EoS, dubbed 'soft' and 'hard', which differ in the compressibility modulus, as well as a soft momentum dependent EoS. We find that a soft momentum dependent EoS reproduces most baryon and cluster observables, including the flow observables, quantitatively, however, hard EOS show a similar trend.

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