Global polarization of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi mathvariant="normal">Λ</mml:mi></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mover accent="true"><mml:mi mathvariant="normal">Λ</mml:mi><mml:mo>¯</mml:mo></mml:mover></mml:math> hyperons in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Au</mml:mi><mml:mo>+</mml:mo><mml:mi>Au</mml:mi></mml:mrow></mml:math> collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:msub><mml:mi>s</mml:mi><mml:mrow><mml:mi>N</mml:mi><mml:mi>N</mml:mi></mml:mrow></mml:msub></mml:msqrt><mml:mo>=</mml:mo><mml:mn>19.6</mml:mn></mml:mrow></mml:math> and 27 GeV
M. I. Abdulhamid, B. E. Aboona, J. Adam, L. Adamczyk, J. R. Adams, Y. Hu, M. M. Aggarwal, Z. Ahammed, E. Alpatov, D. M. Anderson, E. C. Aschenauer, S. Aslam, J. Atchison, V. Bairathi, W. Baker, J. Ball, K. N. Barish, R. Bellwied, P. Bhagat, A. Bhasin, S. Bhatta, J. Bielčík, J. Bielčíková, J. D. Brandenburg, X. Z. Cai, H. Caines, M. Calderón de la Barca Sánchez, D. Cebra, J. Ceska, I. Chakaberia, P. Chaloupka, B. K. Chan, Z. Chang, A. Chatterjee, D. Chen, J. Chen, J. H. Chen, Z. Chen, J. Cheng, Y. Cheng, S. Choudhury, W. Christie, X. Chu, H. J. Crawford, M. Csanád, G. Dale-Gau, Arpita Das, M. Daugherity, I. M. Deppner, A. Dhamija, L. Di Carlo, L. Didenko, P. Dixit, X. Dong, J. L. Drachenberg, E. Duckworth, J. C. Dunlop, J. Engelage, G. Eppley, S. Esumi, O. Evdokimov, A. Ewigleben, O. Eyser, R. Fatemi, S. Fazio, C. Feng, Y. Feng, E. Finch, Y. Fisyak, F. Flor, C. Fu, C. A. Gagliardi, T. Galatyuk, F. J. M. Geurts, N. Ghimire, S. M. Gibson, K. Gopal, X. Gou, D. Grosnick, A. Gupta, W. Guryn, A. Hamed, Y. Han, S. Harabasz, M. D. Harasty, J. W. Harris, H. Harrison-Smith, W. He, X. H. He, Y. He, N. Herrmann, L. Holub, C. Hu, Q. Hu, Y. Hu, H. Z. Huang, H. Z. Huang, S. Huang, T. Huang, Xu-Guang Huang
Abstract
In relativistic heavy-ion collisions, a global spin polarization, ${P}_{\mathrm{H}}$, of $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ hyperons along the direction of the system angular momentum was discovered and measured across a broad range of collision energies and demonstrated a trend of increasing ${P}_{\mathrm{H}}$ with decreasing $\sqrt{{s}_{NN}}$. A splitting between $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ polarization may be possible due to their different magnetic moments in a late-stage magnetic field sustained by the quark-gluon plasma which is formed in the collision. The results presented in this study find no significant splitting at the collision energies of $\sqrt{{s}_{NN}}=19.6$ and 27 GeV in the BNL Relativistic Heavy Ion Collisions Beam Energy Scan Phase II using the STAR detector, with an upper limit of ${P}_{\overline{\mathrm{\ensuremath{\Lambda}}}}\ensuremath{-}{P}_{\mathrm{\ensuremath{\Lambda}}}<0.24$% and ${P}_{\overline{\mathrm{\ensuremath{\Lambda}}}}\ensuremath{-}{P}_{\mathrm{\ensuremath{\Lambda}}}<0.35$%, respectively, at a 95% confidence level. We derive an upper limit on the naive extraction of the late-stage magnetic field of $B<9.4\ifmmode\times\else\texttimes\fi{}{10}^{12}$ T and $B<1.4\ifmmode\times\else\texttimes\fi{}{10}^{13}$ T at $\sqrt{{s}_{NN}}=19.6$ and 27 GeV, respectively, although more thorough derivations are needed. Differential measurements of ${P}_{\mathrm{H}}$ were performed with respect to collision centrality, transverse momentum, and rapidity. With our current acceptance of $|y|<1$ and uncertainties, we observe no dependence on transverse momentum and rapidity in this analysis. These results challenge multiple existing model calculations following a variety of different assumptions which have each predicted a strong dependence on rapidity in this collision-energy range.