II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Symmetry energy investigation with pion production from Sn + Snsystems†
G. Jhang,∗1,∗2,∗3 J. Estee,∗1,∗2,∗3 J. Barney,∗1,∗2,∗3 G. Cerizza,∗1,∗2,∗3 M. Kaneko,∗1,∗4 J. W. Lee,∗1,∗5W. G. Lynch,∗2,∗3 T. Isobe,∗1 M. Kurata-Nishimura,∗1 T. Murakami,∗1,∗4 C. Y. Tsang,∗2,∗3 M. B. Tsang,∗2,∗3R. Wang,∗2 H. Baba,∗1 N. Fukuda,∗1 B. Hong,∗5 T. Kobayashi,∗6 P. Lasko,∗7 J. Łukasik,∗7 A. B. McIntosh,∗8H. Otsu,∗1 P. Pawłowski,∗7 H. Sakurai,∗1 C. Santamaria,∗1,∗2 R. Shane,∗2 D. Suzuki,∗1 S. Tangwancharoen,∗2Z. G. Xiao,∗9 S. J. Yennello,∗8,∗10 Y. Zhang∗9 for the SπRIT CollaborationandM. Colonna,∗11 P. Danielewicz,∗2,∗3 C.-M. Ko,∗8,∗12 A. Ono,∗6 H. Wolter∗13 for the TMEP collaboration
In the last couple of decades, pions produced in
the high density regions of heavy ion collisions havebeen considered to be one of sensitive probes to investigate the symmetry energy term in the nuclearequation of state at high densities, a key property
to understand neutron stars. In our new experiment
designed to study the symmetry energy, the multiplicities of negatively and positively charged pions
have been measured with high accuracy for central
132
Sn + 124 Sn, 112 Sn + 124 Sn, and 108 Sn + 112 Sn collisions at E/A = 270 MeV with the SπRIT Time Projection Chamber2,3) placed inside the SAMURAI spectrometer4) at RIBF. 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 which have taken part in theTransport Model Evaluation Project (TMEP).5–7) The
calculations reproduce qualitatively the dependence ofthe multiplicities and their ratios on the total neutron
and proton number in the colliding systems.
As shown in Fig. 1, however, the predictions of thetransport models from diﬀerent codes diﬀer too muchto allow extraction of reliable constraints on the symmetry energy from the data even using the double pion
ratio. This ﬁnding may explain previous contradictory conclusions on symmetry energy constraints obtained from pion data in Au + Au system.8–12) 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 ofthe symmetry energy.

Fig. 1. (Left panel) Experimental charged pion yield ratios as a function of N/Z together with the results ofseven transport-model predictions for the soft and stiﬀ
symmetry energies (the diﬀerence of predictions are presented by the height of colored boxes). The dashed blueline is a power-law ﬁt with the function (N/Z)3.6 , while
the dotted blue line represents (N/Z)2 of the system.
(Right panel) Double pion yield ratios for 132 Sn + 124 Sn
and 108 Sn + 112 Sn. The data and their uncertainty are
given by the red horizontal bar and the results of thetransport models are shown by the colored boxes, in asimilar way as in the left panel. Taken from Ref. 1).

References

1) G. Jhang et al., Phys. Lett. B 813, 136016 (2021).
2) J. Barney et al., arXiv:2005.10806.
3) R. Shane et al., Nucl. Instrum. Methods Phys. Res. A784, 513 (2015).
4) H. Otsu et al., Nucl. Instrum. Methods Phys. Res. B
376, 175 (2016).
5) A. Ono et al., Phys. Rev. C 100, 044617 (2019).
6) J. Xu et al., Phys. Rev. C 93, 044609 (2016).
7) Y.-X. Zhang et al., Phys. Rev. C 97, 034625 (2018).
8) J. Hong, P. Danielewicz, Phys. Rev. C 90, 024605(2014).
9) B. -A. Li, Phys. Rev. Lett. 88, 192701 (2002).
10) Z. -Q. Feng, G .-M. Jin, Phys. Lett. B 683, 140 (2010).
11) Z. Xiao et al., Phys. Rev. Lett. 102, 062502 (2009).
12) W. -J. Xie, J. Su, L. Zhu, F. -S. Zhang, Phys. Lett. B
718, 1510 (2013).

Condensed from the article in Phys. Lett. B 813, 136016(2021)1)∗1 RIKEN Nishina Center∗2 National Superconducting Cyclotron Laboratory, Michigan
State University∗3 Department of Physics, Michigan State University∗4 Department of Physics, Kyoto University∗5 Department of Physics, Korea University∗6 Department of Physics, Tohoku University∗7 Institute of Nuclear Physics PAN∗8 Cyclotron Institute, Texas A&M University∗9 Department of Physics, Tsinghua University∗10 Department of Chemistry, Texas A&M University∗11 INFN-LNS, Laboratori Nazionali del Sud∗12 Department of Physics and Astronomy, Texas A&M University∗13 Faculty of Physics, Ludwig-Maximilians-University of Munich†- 15 -1