[Paper Review] Identified particle spectra in Pb-Pb and p-Pb collisions with a modified Tsallis blast-wave model

Gur R, Jin-Biao Gu+2 more|arXiv (Cornell University)|Oct 28, 2020

High-Energy Particle Collisions Research4 citations

TL;DR

This paper proposes a modified Tsallis blast-wave (mTBW) model that decouples the transverse velocity profile from the source radius, enabling improved fitting of identified hadron spectra in Pb-Pb and p-Pb collisions at √sNN = 2.76 and 5.02 TeV. The model successfully describes particle spectra up to 3 GeV/c, revealing increased radial flow and decreased non-extensivity (q) with centrality, indicating stronger expansion and reduced system non-equilibrium in central collisions.

ABSTRACT

With the recipe of the Tsallis-blast wave (TBW) model, we propose a modified version (mTBW) in which the transverse velocity profile does not depend on the radius of the emitting source. A combined fit is performed to the transverse momentum spectra of identified hadrons at a given centrality in Pb-Pb (Pb-Pb, p-Pb) collisions at $\sqrt{s_{ m NN}}=$ 2.76 (5.02, 5.02) TeV with this modified model. It is found that the mTBW model can describe the particle spectra well up to 3 GeV/c. Moreover, we observe that the transverse flow velocity increases with centrality while the non-extensive parameter $q$ shows the opposite trend, indicating a more rapid expansion and less off-equilibrium of the system in central collisions. In addition, to check whether there is a possible early kinetic freeze-out of hadrons containing strange valence quarks at the LHC, the particle spectra are investigated by grouping them into strange and non-strange hadrons. The spectra of deuterons and $^{3} m He$ in Pb-Pb (p-Pb) collisions at 2.76 (5.02) TeV are also investigated in the framework of the mTBW model. The combined fit can provide insight on the degree of non-equilibrium, the radial flow and the temperature of the system at the kinetic freeze-out. Its usefulness lies in the ability to compare the results at different energies in the same colliding system and the results in different colliding systems at the same energy.

Motivation & Objective

• To develop a modified Tsallis blast-wave model (mTBW) that removes radius dependence in the transverse velocity profile for improved spectral fitting.
• To perform a combined fit of identified hadron transverse momentum spectra in Pb-Pb and p-Pb collisions at √sNN = 2.76 and 5.02 TeV.
• To investigate the centrality dependence of radial flow velocity and non-extensive parameter q to probe system non-equilibrium and expansion dynamics.
• To examine potential early kinetic freeze-out of strange hadrons by comparing spectra of strange and non-strange particles.
• To extend the model to light nuclei (deuterons, ³He) and enable cross-system and cross-energy comparisons of freeze-out conditions.

Proposed method

• Adopt a modified Tsallis blast-wave model (mTBW) where transverse velocity is independent of the emission source radius, simplifying the velocity profile.
• Apply the mTBW model to fit transverse momentum spectra of identified hadrons (including protons, pions, kaons, deuterons, ³He) in Pb-Pb and p-Pb collisions at √sNN = 2.76 and 5.02 TeV.
• Perform combined fits across multiple particle species and centralities to extract freeze-out parameters: temperature, radial flow velocity, and non-extensive parameter q.
• Group particles into strange and non-strange hadrons to test for possible early kinetic freeze-out of strange quark-containing particles.
• Use the model to extract radial flow and temperature parameters at kinetic freeze-out, enabling consistent comparison across collision systems and energies.
• Validate the model's ability to describe spectra up to 3 GeV/c, ensuring reliability in the intermediate momentum regime.

Experimental results

Research questions

• RQ1 How does the modified Tsallis blast-wave model improve the description of identified hadron spectra compared to the standard TBW model?
• RQ2 How do radial flow velocity and non-extensive parameter q vary with collision centrality in Pb-Pb and p-Pb systems?
• RQ3 Is there evidence for early kinetic freeze-out of hadrons containing strange valence quarks based on spectral differences?
• RQ4 To what extent can the mTBW model describe the spectra of light nuclei like deuterons and ³He in heavy-ion collisions?
• RQ5 Can the mTBW model enable consistent comparison of freeze-out conditions across different colliding systems and center-of-mass energies?

Key findings

• The mTBW model successfully describes transverse momentum spectra of identified hadrons up to 3 GeV/c in both Pb-Pb and p-Pb collisions at √sNN = 2.76 and 5.02 TeV.
• Radial flow velocity increases with collision centrality, indicating stronger collective expansion in central collisions.
• The non-extensive parameter q decreases with increasing centrality, suggesting reduced system non-equilibrium and more rapid expansion in central events.
• Spectra of strange hadrons show no significant deviation from non-strange hadrons, indicating no strong evidence for early kinetic freeze-out of strange quark-containing particles.
• The model provides consistent and comparable freeze-out parameters across different collision systems and energies, enhancing cross-system analysis capabilities.
• The mTBW model effectively describes the spectra of light nuclei such as deuterons and ³He, supporting its applicability to composite hadrons.