Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Study of azimuthal anisotropy of charged particles in Au + Au√
collisions at sN N = 200 GeV at RHIC-PHENIX

It has been established that the high temperatureand high dense matter, quark gluon plasma (QGP),
was produced in the gold-gold collision by the relativistic heavy ion collider (RHIC) at the Brookhaven National Laboratory (BNL) in the United States. QGP isa form of matter in which quarks and gluons stronglyinteract with each other. It was found that it is a “perfect ﬂuid” having almost no viscosity. The quantitative
research on the QGP properties is progressing. One ofthe discoveries that was made at the RHIC is a strong
elliptical ﬂow, in which the particles generated by nuclear collision come out with anisotropy, not uniformly
with respect to the reaction plane. In this report, we
describe the analysis of the azimuthal anisotropy parameter (v2 ), which represents the diﬀerence in yield
between the in and out of planes, in order to investigate
the properties of the energy loss mechanism resulting
from the diﬀerent interacting lengths with QGP whenhigh transverse momentum particles are emitted.1,2)
The parameter v2 is calculated from the azimuthal
angle Φ of the reaction plane using the south and northof the Beam-Beam Counter (BBC) and the azimuthal
angle Ψ of the emitted particles. This is called the “reaction plane method.”3) In this method, v2 is quantiﬁed by the Fourier coeﬃcient in the following equation:dN/d(Φ − Ψ ) ∝ 1 + 2v2 cos[2(Φ − Ψ )](1)

The angle Φ is determined from the density of the azimuthal angle of the particles detected in the forward
and backward BBC for each event. In addition, since
the value of v2measured observed in the experiment isinﬂuenced by the detector, we can obtain v2true fromthe following formula with the correction factor Creso .
v2true = v2measured /Creso(2)

The detectors used for the measurement of the planein this analysis are the south and north of the BBC
and Central Arm Detectors (CNT). The resolution of
the reaction plane Creso is obtained using the three
sub method, which uses these three detectors. The
result of the resolution of the reaction plane measured
in step of 5% is shown in Fig. 1, which shows that theresolution is at most 20–25%. When the resolution
is large, it is possible to measure the reaction planewith good performance. On the other hand, for lowcentrality (0∼10%), the resolution is small, because
the eccentricity of the reaction region is small. For
high centrality (30%∼), the resolution decreases dueto the reduction in the number of particles.
∗1∗2RIKEN Nishina Center
Department of Physics, Nara Women’s University- 110 -Resolution

R. Nishitani∗1,∗2 for the PHENIX Collaboration
Resolution vs. CentralityFig. 1. Resolution of the reaction plane to centrality.4)

The following event selections are required for thisanalysis.
• Collision vertex is within ±10 cm from the origin
in the beam axis direction,• One or more hits in both BBC South and BBC
North. In addition, the following track selections
are required,• 5 or 6 hits with Drift Chamber (DC) and Pad
Chamber 1,• Transverse momentum pT > 0.5 GeV/c• The hit position in the beam axis direction at DCis ±75 cm,• χ square cut on the tracking• Position matching cut (< 3σ)• E/p cut (0.2 < E/p < 0.8)
I will remove the background even more by turning
the above cuts with a detailed study, especially about
the E/p cut.
References
1) K. Adcox et al., (PHENIX Collaboration), Nucl. Phys.
757, 1–2, 8, 184–283 (2005).
2) Y. Akiba, Physics of Quark Gluon Plasma Recreating
Hot Matter in Early Universe in Laboratory (Kyoritsu
Pub. Co., Ltd., Tokyo, 2014, in Japanese).
3) S. S. Adler et al., (PHENIX Collaboration), Phys. Rev.
Lett. 91, 182301 (2003).
4) A. Takeda, Master thesis, 2017, Nara Women’s University.