Abstract

This paper investigates the symmetry breaking between the polarizations of Λ and Λ¯ hyperons in relativistic collisions of heavy ions at intermediate and low energies. The microscopic transport model UrQMD is employed to study the thermal vorticity of hot and dense nuclear matter formed in non-central Au + Au collisions at center-of-mass energies 7.7≤sNN≤62.4 GeV. The whole volume of an expanding fireball is subdivided into small cubic cells. Then, we trace the final Λ and Λ¯ hyperons back to their last interaction point within a certain cell. Extracting the bulk parameters, such as energy density, net baryon density, and net strangeness of the hot and dense medium in the cell, one can obtain the cell temperature and the chemical potentials at the time of the hyperon emission. To do this, the extracted characteristics have to be fitted to the statistical model (SM) of ideal hadron gas. After that, the vorticity of nuclear matter and polarization of both hyperons are calculated. We found that the polarization of both Λ and Λ¯ increases with decreasing energy of heavy-ion collisions. The stronger polarization of Λ¯ is explained by (i) the slightly different freeze-out conditions of both hyperons and (ii) the different space–time distributions of Λ and Λ¯.

Highlights

  • We employ the ultra-relativistic quantum molecular dynamics (UrQMD) model [27,28] to study the polarization of Λ and Λin noncentral gold–gold collisions generated with the energies from s = 7.7 GeV to 62.4 GeV

  • In contrast to models based on the color-exchange scheme [31,32,33,34], UrQMD utilizes the longitudinal mechanism of string excitation [35]

  • √vorticity of nuclear matter produced in noncentral gold–gold collisions at energies from s = 7.7 GeV to 62.4 GeV, and connected to it the polarization of Λ and Λhyperons in these collisions

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Summary

Experiments with ultrarelativistic

One of the ideas for studying properties of hot and dense matter is to employ, somehow, extremely high vorticity in noncentral heavy-ion collisions It was suggested [2] that highly rotated partonic fluid should influence the polarization of hyperons, especially Λ and Λ. S = 200 GeV, hyperon polarization steadily increases to 2% with reducing c.m. energy to s = 7.7 GeV This observation has initiated a lot of new studies; see, e.g.,. We employ the UrQMD model [27,28] to study the polarization of Λ and Λin noncentral gold–gold collisions generated with the energies from s = 7.7 GeV to 62.4 GeV. Different areas of the expanding system have different vorticities at different times, whereas space–time distributions of Λ and Λwithin the fireball are not the same This circumstance becomes especially perceptible when the collision energy is reduced.

Thermal Vorticity and Hyperon Polarization
UrQMD Model
Statistical Model of Ideal Hadron Gas
Findings
Conclusions
Full Text
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