Measurement of global polarization of Λ hyperons in few-GeV heavy-ion collisions
nucl-ex
/ Authors
R. Abou Yassine, J. Adamczewski-Musch, C. Asal, M. Becker, A. Belounnas, A. Blanco, C. Blume, L. Chlad, P. Chudoba, I. Ciepal
and 98 more authors
M. Cordts, J. Dreyer, W. A. Esmail, L. Fabbietti, H. Floersheimer, P. Fonte, J. Friese, I. Fröhlich, J. Förtsch, T. Galatyuk, T Gniazdowski, R. Greifenhagen, M. Grunwald, M. Gumberidze, S. Harabasz, T. Heinz, C. Höhne, F. Hojeij, R. Holzmann, H. Huck, M. Idzik, B. Kämpfer, K-H. Kampert, B. Kardan
/ Abstract
The global polarization of Λ hyperons along the total orbital angular momentum of a relativistic heavy-ion collision is presented based on the high statistics data samples collected in Au+Au collisions at \sqrt{s_{NN}} = 2.4 GeV and Ag+Ag at 2.55 GeV with the High-Acceptance Di-Electron Spectrometer (HADES) at GSI, Darmstadt. This is the first measurement below the strangeness production threshold in nucleon-nucleon collisions. Results are reported as a function of the collision centrality as well as a function of the hyperon transverse momentum (p_T) and rapidity (y_{CM}) for the range of centrality 0--40%. We observe a strong centrality dependence of the polarization with an increasing signal towards peripheral collisions. For mid-central (20--40%) collisions the polarization magnitudes are <P_Λ>(%) = 6.8 \pm 1.3 (stat.) \pm 2.1 (syst.) for Au+Au and <P_Λ>(%) = 6.2 \pm 0.4 (stat.) \pm 0.6 (syst.) for Ag+Ag, which are the largest values observed so far. This observation thus provides a continuation of the increasing trend previously observed by STAR and contrasts expectations from recent theoretical calculations predicting a maximum in the region of collision energies about 3 GeV. The observed polarization is of a similar magnitude as predicted by 3D fluid dynamics and the UrQMD plus thermal vorticity model and significantly above results from the AMPT model.