Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Status of collective flow analysis for SπRIT-TPC experiment
M. Kurata-Nishimura,∗1 J. Barney,2∗,1∗ G. Cerizza,2∗,1∗ J. Estee,2∗,1∗ B. Hong,∗3 T. Isobe,∗1 G. Jhang,3∗,1∗
M. Kaneko,4∗,1∗ H. S. Lee,∗7 J. W. Lee,3∗,1∗ J. L
� ukasik,∗5 W. G. Lynch,∗2 A. B. McIntosh,∗8 T. Murakami,4∗,1∗
5∗,1∗∗6∗1P. Paw�lowski,
K. Pelczar, H. Sakurai, C. Santamaria,2∗,1∗ R. Shane,∗2 M. B. Tsang,∗2 S. J. Yennello,∗8and for SπRIT Collaboration
The successful observation of gravitational waves
from a neutron star merger1) highlights the importanceof the nuclear Equation of State (EoS). Heavy ion collisions are an appropriate tool to evaluate the nuclear
EoS at supra-saturation. In nuclear EoS at a density
more than that of normal nuclear matter (ρ > 2ρ0 ),the isospin symmetry energy term includes large uncertainly in theory, because of the lack of experimental
data. In a previous work, the π − /π + production ratio was a super soft EoS;2) however, the proton andneutron collective ﬂow analysis3) was inconsistent.
The SAMURAI Pion-Reconstruction and IonTracker-Time-Projection Chamber (SπRIT-TPC)
project was proposed to constrain the EoS using diﬀerent isospin asymmetry systems with 132 Sn and 108 Sn
beams at 270 MeV/u on 112 Sn and 124 Sn targets at
SAMURAI in RIBF. Multiple observations, such as
π − /π + production ratio, proton and neutron collectiveﬂow, and H3 /He3 production ratio, will be obtained tostudy the EoS for heavy ion collisions.
The collective ﬂow of neutron and proton is expected
to be sensitive to the isospin symmetry potential because it could minimize the inﬂuence of the isoscalarpotential.4) In this paper, recent results for the collective ﬂow analysis will be discussed.
The SπRIT-TPC is described in Ref. 5) NeuLAND
was installed 8.8 m downstream from the target to detect neutrons emitted around the mid-rapidity region.
Trigger devices, KATANA array6) and Multiplicity array7) were installed surrounding the SπRIT-TPC.
The strength of the collective ﬂow is analyzed fromthe azimuthal distribution with respect to a reaction
plane. The reaction plane orientation angle, Ψ , is determined event by event. The azimuthal angle of thereaction plane is deﬁned as the sum of the transverse
momentum unit vector.
∑mi ωi sin(nϕi )Ψ = ∑m(1)
i ωi cos(nϕi )∆Ψsub = ΨA − ΨB
(2)

The geometrical acceptance of the SπRIT-TPC islimited and asymmetric in the azimuthal angle, so it
was necessary to apply a ﬂattening correction.8) Tracks
were randomly selected from independent events tocreate “mixed” events. To check the feasibility of determining the reaction plane with this ﬂattening correction applied, two sub-events of equal multiplicity
were formed event by event. The reaction planes ΨAand ΨB were measured from the sub-events, and theopening angle of two sub-events, ∆Ψsub , is plotted inFig. 1. The real events are plotted as red circles, whichshow an enhancement at ∆Ψsub = 0 indicating theability of determining the reaction plane from the measurements. The mixed events are plotted as green circles, which show a ﬂat distribution indicating that thedetector bias has been removed. It was conﬁrmed thatthe reaction plane could be determined using sub-event
correlations with SπRIT-TPC. More detailed analysis
is on going.
counts23000∗2∗3∗4∗5∗6∗7∗8REAL

22000

MIXed

21000

20000

19000

18000

17000

−3−2−10123∆ ΨsubFig. 1. Opening angle of reaction planes determined by twosub-events Red and green circles show real and mixed
events, respectively.

References
1) B. P. Abbot et al., Phys. Rev. Lett. 119, 161101 (2017).
2) Bao-An Li et al., Phys. Rev. C 71, 014608 (2005).
3) P. Russotto et al., Phys. Lett. B 471, (2011).
4) B.-A. Li et al., Phys. Rev. Lett. 85, 4221 (2000).
5) R. Shane et al., Nucl. Instrum. Methods A 784, 513(2015).
6) P. Lasko et al., Accel. Prog. Rep. 50, 173 (2017).
7) M. Kaneko et al., Accel. Prog. Rep. 50, 172 (2017).
8) J.-Y. Ollitrault et al., Nucl. Phys. 638, 195c (1998).

The coeﬃcient ω is 1 if rapidity is larger than the centerof rapidity, otherwise it is −1.
∗1Mid-Central

RIKEN Nishina Center
NSCL and Dept. of Phys. & Ast., Michigan State University
Department of Physics, Korea University
Department of Physics, Kyoto University
IFJ PAN, Kraków
Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University, Kraków
Rare Isotope Science Project, Institute for Basic Science
Cyclotron Institute, Texas A&M University- 42 -