Polarization In Collisions Research Articles

Λ and Λ¯ Freeze-Out Distributions and Global Polarizations in Au+Au Collisions

The gold–gold collisions at sNN=7.7 and 11.5 GeV are simulated within the PHSD transport model. In each collision event, the spectator nucleons are separated and the fluidization procedure for the participants is performed. The local velocities are determined in the Landau frame and the kinematic and thermal vorticity fields are evaluated. We analyze the thermodynamic properties of the cells where Λs and Λ¯s were born or had their last interaction. Such cells contribute to the formation of the observed global polarization of hyperons induced by the thermal vorticity of the medium. The Λ¯ polarization signal is found to be mainly determined by hot, dense, and highly vortical cells at the earlier stage of the collision, whereas the Λ polarization signal is accumulated over the longer time and includes cells with lower vorticity. The calculated global polarizations for both Λs and Λ¯s agree well with the experimental finding by the STAR collaboration at energy sNN=11.5 GeV. For collisions at sNN=7.7 GeV, we can reproduce the STAR data for Λ hyperons, but significantly underpredict the observed global polarization of Λ¯. Furthermore, we consider the centrality dependence of the hyperon polarization in collisions at 7.7 GeV. It increases with an increase of centrality, reaches a maximum at 65–75% and then starts decreasing rapidly for peripheral collisions.

Rapidity dependence of global polarization in heavy ion collisions * *ZTL and QW are supported in part by the National Natural Science Foundation of China (NSFC) (11890713, 11535012). JS is supported in part by Shandong Province Natural Science Foundation (ZR2019YQ06)

We use a geometric model for hadron polarization in heavy ion collisions with an emphasis on the rapidity dependence. The model is based on the model of Brodsky, Gunion, and Kuhn, as well as the Bjorken scaling model. We make predictions regarding the rapidity dependence of global polarization in the collision energy range of 7.7-200 GeV by assuming the rapidity dependence of two parameters, and . The predictions can be tested by future beam-energy-scan experiments at the Relativistic Heavy Ion Collider of Brookhaven National Lab.

Lambda hyperon production and polarization in collisions of p(3.5 GeV)+Nb

Results on $ \Lambda$ hyperon production are reported for collisions of p(3.5 GeV) + Nb , studied with the High-Acceptance Di-Electron Spectrometer (HADES) at SIS18 at GSI Helmholtzzentrum for Heavy-Ion Research, Darmstadt. The transverse mass distributions in rapidity bins are well described by Boltzmann shapes with a maximum inverse slope parameter of about 90 MeV at a rapidity of y = 1.0, i.e. slightly below the center-of-mass rapidity for nucleon-nucleon collisions, $ y_{cm}=1.12$ . The rapidity density decreases monotonically with increasing rapidity within a rapidity window ranging from 0.3 to 1.3. The $ \Lambda$ phase-space distribution is compared with results of other experiments and with predictions of two transport approaches which are available publicly. None of the present versions of the employed models is able to fully reproduce the experimental distributions, i.e. in absolute yield and in shape. Presumably, this finding results from an insufficient modelling in the transport models of the elementary processes being relevant for $ \Lambda$ production, rescattering and absorption. The present high-statistics data allow for a genuine two-dimensional investigation as a function of phase space of the self-analyzing $ \Lambda$ polarization in the weak decay $ \Lambda\rightarrow p\pi^{-}$ . Finite negative values of the polarization in the order of 5-20% are observed over the entire phase space studied. The absolute value of the polarization increases almost linearly with increasing transverse momentum for p t > 300 MeV/c and increases with decreasing rapidity for y < 0.8 .

Closed-form description of polarization in light and heavy ion scattering: Elastic scattering

We present a closed-form description of the vector and tensor polarization in the elastic scattering of strongly absorbed particles of general spin s by spin-zero nuclei. This description generalizes the closed formalism developed previously for the elastic scattering of spinless particles by including spin-orbit interaction in the partial-wave scattering matrix. The resulting expressions are used to analyze the recent data of the Birmingham group on the elastic scattering of polarized 3He by medium-mass nuclei ( 26Mg, 27Al, 58Ni), and those of the Heidelberg group for polarized 6Li ions scattered by 28Si and 58Ni. Our results confirm earlier predictions that the polarization in collisions of strongly absorbed particles is dominated by diffractive processes. We are able to draw some general conclusions on the l-space structure of the spin-orbit interaction, and find that this has an appreciable imaginary part. A comparison of 3He and 6Li scattering indicates that the spin-orbit coupling is of similar strength for light and heavy ions.