Open Access

Genie Jhang, Justin Estee, Jonathan Barney, Giordano Cerizza, Masanori Kaneko, Jung Woo Lee, William G. Lynch, Tadaaki Isobe, Mizuki Kurata-Nishimura, Tetsuya Murakami, Chun Yuen Tsang, Betty Tsang, Rensheng Wang, Deuk Soon Ahn, Leyla Atar, Thomas Aumann, Hidetada Baba, Konstanze Boretzky, Janusz Brzychczyk, Nobuyuki Chiga, Naoki Fukuda, Igor Gašparić, Byungsik Hong, Andrea Horvat, Kazuo Ieki, Naohito Inabe, Y.J. Kim, Taizo Kobayashi, Yosuke Kondo, Pawel Lasko, H.S. Lee, Yvonne Leifels, Jerzy Łukasik, Juan Manfredi, Alan B. McIntosh, Pierre Morfouace, Takashi Nakamura, N. Nakatsuka, Shunji Nishimura, Robert Olsen, Hideaki Otsu, Piotr Pawłowski, Krzysztof Pelczar, Dominic M. Rossi, Hiroyoshi Sakurai, Clementine Santamaria, Hiromi Sato, Heiko Scheit, Rebecca Shane, Yohei Shimizu, Haik Simon, Aleksandra Snoch, Anna Sochocka, Zbigniew Sosin, Toshiyuki Sumikama, Hiroshi Suzuki, Daisuke Suzuki, Hiroshi Takeda, Suwat Tangwancharoen, Hans Toshihide Törnqvist, Yasuhiro Togano, Zhi-Gang Xiao, Sherry J. Yennello, John Yurkon, Youhong Zhang, Maria Colonna, Dan Cozma, Paweł Danielewicz, Hannah Elfner, Natsumi Ikeno, Che Ming Ko, Justin Mohs, Dmytro Oliinychenko, Akira Ono, Jun Su, Yong Jia Wang, Hermann Wolter, Jun Xu, Ying-Xun Zhang, Zhen Zhang

• In the past two decades, pions created in the high density regions of heavy ion collisions have been predicted to be sensitive at high densities to the symmetry energy term in the nuclear equation of state, a property that is key to our understanding of neutron stars. In a new experiment designed to study the symmetry energy, the multiplicities of negatively and positively charged pions have been measured with high accuracy for central 132Sn+124Sn, 112Sn+124Sn, and 108Sn+112Sn collisions at E/A = 270 MeV with the SπRIT Time Projection Chamber. While individual pion multiplicities are measured to 4% accuracy, those of the charged pion multiplicity ratios are measured to 2% accuracy. We compare these data to predictions from seven major transport models. The calculations reproduce qualitatively the dependence of the multiplicities and their ratios on the total neutron and proton number in the colliding systems. However, the predictions of the transport models from different codes differ too much to allow extraction of reliable constraints on the symmetry energy from the data. This finding may explain previous contradictory conclusions on symmetry energy constraints obtained from pion data in Au+Au system. These new results call for still better understanding of the differences among transport codes, and new observables that are more sensitive to the density dependence of the symmetry energy.