II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Proton eﬃciency function for high-multiplicity events in theSπRIT-TPC investigated by the embedding technique
M. Kaneko,∗1,∗2 J. Barney,∗2,∗3 G. Cerizza,∗2,∗3 J. Estee,∗2,∗3 T. Isobe,∗2 G. Jhang,∗2,∗3 M. Kurata-Nishimura,∗2P. Lasko,∗5,∗2 J. W. Lee,∗4,∗2 J. Łukasik,∗5 W. G. Lynch,∗3 T. Murakami,∗1,∗2 P. Pawłowski,∗5,∗2C. Santamaria,∗2,∗7 C. Y. Tsang,∗2,∗3 M. B. Tsang∗3 for the SπRIT Collaboration∗1∗2∗3∗4∗5∗6∗7

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
Department of Physics, Tsinghua University
Lawrence Berkeley National Laboratory, UC Berkeley

dN/dy020Raw data
IterNo. 1
IterNo. 2
IterNo. 3

Proton yieldin central 132Sn+124Sn

151051.2
10.8
0.6

Eff.

The SπRIT-time projection chamber (TPC)1) projectaims to constrain the high-density nuclear symmetry energy by using heavy-ion collisions. We measured lightcharged particles emitted from central Sn + Sn collisions at 270 MeV/nucleon, which have been predicted
to provide sensitive probes of the symmetry energy
at supra-saturation densities.2) In this report, we describe the reconstruction eﬃciency of protons in central
132
Sn + 124 Sn collisions.
The Sn target is placed approximately 1 cm upstream
from the TPC; hence, the charged particles emitted inforward angles are mainly observed. In central Sn + Sncollisions producing high charged-particle multiplicity,
signals induced by charged particles can interfere witheach other, causing the tracks to be merged and/or broken. As it would be diﬃcult to fully simulate realistic
situations, the embedding technique3) is applied. The
digitized signal of a single proton traversing the TPC
is generated based on a Monte Carlo simulation. Subsequently, the generated signal is embedded into real
events, which are analyzed by the same track reconstruction algorithm as for the physical tracks. The proton’sreconstruction eﬃciency with a given track multiplicity
can be estimated by comparing the embedded tracks andthe reconstructed tracks.
The spectrum of transverse momentum pT vs. scaledc.m.
− 1 of protons is corrected by therapidity y0 = y/yNNc.m.
are the rapidity
following procedure, where y and yNNof the detected proton and the center-of-mass rapidityof the nucleon-nucleon system, respectively. It is notedthat the momentum resolution of the TPC can distort
the measured spectrum. To consider this bin-smearing
eﬀect in the eﬃciency calculation, the distribution ofembedded protons needs to be properly weighted. First,
the measured pT -y0 spectrum is used as the weightingfactor. The eﬃciency as a function of pT -y0 is obtained
as the ratio of reconstructed protons to embedded ones.
Subsequently, the corrected pT -y0 spectrum of protons isobtained as the raw spectrum divided by the eﬃciency.
In the next iteration, the corrected spectrum is used as
the weighting factor. This process is iterated until thecorrection converges.
As shown in Fig. 1, the correction converged after twoiterations. Since we limited the azimuthal angle of tracks−2−1012y0 = y/ yc.m.
-1
NNFig. 1. The top panel shows pT -integrated dN/dy0 spectraof protons in central 132 Sn + 124 Sn collisions with andwithout eﬃciency corrections (colored and black markers, respectively). The bottom panel shows the eﬃciency
obtained with respect to their ratios.

to −30◦ ≤ ϕ ≤ 20◦ in the analysis, which is expected
to be the eﬃcient region of the TPC, the reconstruction
eﬃciency of protons is estimated to be approximately
90%. The eﬃciency exceeds 100% at y0 > 1.8 because
the original statistics of protons is quite low at such ahigh y0 and overcounting occurs owing to the smearing of
the proton rapidity at a lower y0 . In addtion to protons,
the pT -y0 spectra of deuterons and tritons are also corrected, which are being compared with theoretical predictions to extract physics statements for publicationsin the future.
This work is supported by the Japanese MEXT
KAKENHI under grant No. 24105004; the U.S. DOE
under Grant Nos. DE-SC0004835, DE-SC0014530,
and DE-NA0002923; the U.S. NSF under Grant
No. PHY-1565546; and the Polish NSC underGrant Nos. UMO-2013/09/B/ST2/04064 and UMO2013/10/M/ST2/00624. The computing resources wereprovided by the HOKUSAI system at RIKEN.
References

1) R. Shane et al., Nucl. Instrum. Methods Phys. Res. A
784, 513 (2015).
2) M. B. Tsang et al., Phys. Rev. C 95, 044614 (2017).
3) M. Anderson et al., Nucl. Instrum. Methods Phys. Res.
A 499, 659 (2003).

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