New model of quark-gluon plasma resolves long-standing discrepancy between theory and data

Basic research has revealed the existence of quark-gluon plasma (QGP) – a newly identified state of matter – as part of the early Universe. Essentially a soup of quarks and gluons, the QGP known to have existed a microsecond after the Big Bang cooled over time to form hadrons like protons and neutrons — the building blocks of all matter.

One way to reproduce the extreme conditions that existed in the days of QGP is through relativistic heavy-ion collisions. In this regard, particle accelerator facilities such as the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider have enhanced our understanding of QGP with experimental data on such collisions.

Meanwhile, theoretical physicists have used multilevel relativistic hydrodynamic models to explain the data, as the QGP behaves very much like a perfect liquid. However, there was a serious ongoing disagreement between these models and data in the low transverse momentum regime, where both the conventional and hybrid models failed to explain the particle yields observed in the experiments.

With this in mind, a team of researchers from Japan, led by theoretical physicist Professor Tetsufumi Hirano of Sophia University, conducted an investigation to explain the missing particle yields in the relativistic hydrodynamic models.

In their recent work, they proposed a novel “Dynamic Core-Corona Initialization Framework” to comprehensively describe high-energy core collisions. Their results were published in the journal Physical Check C and contributing contributions by Dr. Yuuka Kanakubo, PhD student at Sophia University, (current affiliation: Postdoctoral Research Fellow at the University of Jyväskylä, Finland) and Assistant Professor Yasuki Tachibana from Akita International University, Japan.

“To find a mechanism that can explain the discrepancy between theoretical modeling and experimental data, we used a dynamic core-corona initialization (DCCI2) framework, in which the particles generated in high-energy nuclear collisions are described with two components: the The core, or matter in equilibrium, and the corona, or matter that is not in equilibrium,” explains Prof. Hirano. “This image allows us to study the contributions of the core and corona components to hadron production in the low transverse momentum region.”

Researchers performed heavy ion Pb-Pb collision simulations on PYTHIA (a computer simulation program) at an energy of 2.76 TeV to test their DCCI2 framework. Dynamic initialization of the QGP fluids enabled the separation of core and corona components, which underwent hadronization by “hypersurface switching” and “string fragmentation” respectively. These hadrons were then subjected to resonance decays to obtain the transverse momentum (p_T) spectra.

“We turned off hadronic scattering and only performed resonance decays to see a breakdown of the total yield into core and corona components, since hadronic scattering mixes the two components late in the reaction,” explains Dr. Kanakubo.

The researchers then examined the proportion of core and corona components in the P_T Spectra of charged pions, charged kaons and protons and antiprotons for collisions at 2.76 TeV. Next, they compared these spectra with those obtained from experimental data (from the ALICE detector at the LHC for Pb-Pb collisions at 2.76 TeV) to quantify contributions from corona components. Finally, they studied the effects of contributions from corona components on the flow variables.

They found a relative increase in corona contributions in the spectral range of about 1 GeV for both the 0-5% and 40-60% centrality classes. While this was true for all hadrons, they found almost 50% corona contribution to particle production in the spectra of protons and antiprotons in the very low p range_T (≈ 0 GeV) .

In addition, results from full DCCI2 simulations showed better agreement with ALICE’s experimental data when only core components with hadronic scattering (ignoring corona components) were compared. The corona contribution was found to be responsible for the dilution of the four-particle cumulants (an observable flux) obtained solely from nuclear contributions, indicating more permutations of corona-contributing particles.

“These results imply that the non-equilibrium corona components contribute to particle production in the very low transverse spectra region,” says Prof. Hirano. “This explains the missing yields in hydrodynamic models that only extract the equilibrated core components from experimental data. This clearly shows that for a more detailed understanding of the properties of QGP it is necessary to also extract the non-equilibrated components.”