Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Study on the impact parameter dependence
on the trigger eﬃciency for the SπRIT experiment
M. Kaneko,∗1,∗2 J. Barney,∗2,∗3 G. Cerizza,∗2,∗3 J. Estee,∗2,∗3 W. G. Lynch,∗3 T. Isobe,∗2 G. Jhang,∗4
M. Kurata-Nishimura,∗2 P. Lasko,∗5,∗2 J. L� ukasik,∗5 T. Murakami,∗1,∗2 P. Paw�lowski∗5,∗2 C. Santamaria,∗2,∗3∗3∗4,∗2M. B. Tsang, J. W. Lee,and Y. Zhang∗6,∗2 for the SπRIT collaboration
2.880 and UrQMD8) Ver. 3.4 were used to reproduce132
Sn+124 Sn reactions at 270 MeV/nucleon. Figure 1
shows the trigger eﬃciency curve as a function of theimpact parameter. For both models, about 100% trigger eﬃciency was obtained for central collisions with
impact parameters of 0–2 fm. However, signiﬁcantmodel dependence is found in semi-central collisions
with impact parameters of 5–9 fm. For a more proper
model parametrization, it would be necessary to compare the experimental observables with further simulations.

The main objective of the SAMURAI PionReconstruction and Ion-Tracker (SπRIT) project is toplace a constraint on the density dependence of thenuclear equation of state (EOS). In particular, theisospin asymmetric term of the EOS, which is called
symmetry energy, plays an important role not only in
unstable nuclei but also in neutron stars. Currently,
the density dependence of symmetry energy is poorly
constrained at around twice the saturation density.1)
It is proposed that charged π mesons from heavyion (HI) collisions at energies of several hundreds of
MeV/nucleon could be a useful probe for the symmetryenergy at supra-saturation densities.2) At intermediate
energies, π mesons are produced from the decay of ∆resonance states, which are excited from the nucleonnucleon (NN) scattering in HI collisions. The π meson
production cross-section depends on the NN scatteringcross-section, which depends on the impact parameter.
Therefore, central collisions will produce higher statistics of π mesons, and will be a region of interest in thisexperiment.
In the spring of 2016, we performed an experiment at RIBF with collisions between various Sn isotopes at 270 MeV/nucleon.3) The SπRIT-TPC4) inside
the SAMURAI spectrometer was used to detect thecharged particles. To provide a trigger signal focusing
on the central collision, a combination of two kinds ofdetectors—KATANA veto5) and Kyoto multiplicity array6) —was utilized. It consisted of an array of plastic
paddles and Multi-Pixel Photon Counter for use in themagnetic ﬁeld of 0.5 T. The KATANA veto was placeddownstream from the SπRIT-TPC, enabling us to vetoperipheral collisions, which involve high-Z spectator
particles. The Kyoto multiplicity array covered bothsides of the SπRIT-TPC to detect central collisions bysetting a threshold on the sideward-moving charged
particle multiplicity. For regular data acquisition runs,the trigger condition required Z less than 20 in theKATANA veto and a multiplicity greater than four inthe Kyoto multiplicity array.
The dependece of impact parameter on the trigger
eﬃciency in regular runs has been studied by using
Monte Carlo simulation with event generators. Asa HI collision generator, JQMD-2.0 in PHITS7) Ver.
∗1
∗2
∗3
∗4
∗5∗6132

Sn+124Sn at 270 MeV/ATrigger efficiecy (%)

100

UrQMD
PHITS

80604020002468Impact parameter (fm)

1012Fig. 1. Trigger eﬃciency curves as a function of impact
parameter in the regular runs.

Further analysis on the impact parameter determination using the reconstructed track in SπRIT-TPC is
now under way.
This work is supported by the U.S. DOE underGrant Nos. DE-SC0004835, DE-SC0014530, and DENA0002923, the US NSF Grant No. PHY-1565546, theJapanese MEXT KAKENHI grant No. 24105004, andthe Polish NSC Grant Nos. UMO-2013/09/B/ST2/04064 and UMO-2013/10/M/ST2/00624.
References
1) M. B. Tsang et al., Phys. Rev. C 86, 015803 (2012).
2) B. A. Li et al., Phys. Rev. C 71, 014608 (2005).
3) J. Barney et al., RIKEN Accel. Prog. Rep. 50, 49(2017).
4) R. Shane et al., Nucl. Instrum. Methods A 784, 513(2015).
5) P. Lasko et al., Nucl. Instrum. Methods A 856, 92(2017).
6) M. Kaneko et al., RIKEN Accel. Prog. Rep. 49, 163(2016).
7) T. Sato et al., J. Nucl. Sci. Technol., (2018).
8) M. Breicher et al., J. Phys. G 25, 1859 (1999).

Department of Physics, Kyoto University
RIKEN Nishina Center
NSCL and Dept. of Phys. & Ast., Michigan State University
Department of Physics, Korea University
Institute of Nuclear Physics, PAN, Kraków, Poland
Department of Physics, Tsinghua University- 41 -