Measurement of global polarization of Λ hyperons in few-GeV heavy-ion collisions
R. Abou Yassine, J. Adamczewski-Musch, C. Asal, M. Becker, A. Belounnas, A. Blanco, C. Blume, L. Chlad, P. Chudoba, I. Ciepał, Malte Cordts, J. Dreyer, Waleed Esmail, L. Fabbietti, Henrik 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, Fatima Hojeij, R. Holzmann, Holger Huck, M. Idzik, B. Kämpfer, K.‐H. Kampert, B. Kardan, V. Kedych, I. Köenig, Wolfgang Köenig, M. Kohls, J. Kolas, G. Kornakov, R. Kotte, I. Kres, W. Krueger, A. Kugler, R. Lalik, S. Lebedev, S. Linev, F. Linz, L. Lopes, M. Lorenz, A. Malige, J. Markert, T. Matulewicz, S. Maurus, V. Metag, J. Michel, A. Molenda, C. Müntz, M. Nabroth, L. Naumann, K. Nowakowski, J. Orliński, J.–H. Otto, M. Parschau, C. Pauly, V. Pechenov, O. Pechenova, D. Pfeifer, K. Piasecki, J. Pietraszko, T. Povar, A. Prozorov, Witold Wojciech Przygoda, K. Pysz, B. Ramstein, N. Rathod, J. Ritman, P. Rodriguez-Ramos, A. Rost, A. Rustamov, P. Salabura, J. G. Saraiva, Niklas Schild, E. Schwab, F. Scozzi, F. Seck, I. Selyuzhenkov, U. Singh, L. Skorpil, J. Smyrski, S. Spies, M. Stefaniak, H. Ströbele, J. Stroth, K. Sumara, O. Svoboda, M. Szala, P. Tlusty, M. Traxler, V. Wagner
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 sNN=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's transverse momentum (pT) and rapidity (yCM) 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 〈PΛ〉(%)=6.8±1.3(stat.)±2.1(syst.) for Au+Au and 〈PΛ〉(%)=6.2±0.4(stat.)±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.