Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

RIKEN Accel. Prog. Rep. 00 (0)Status of CuAu flow measurement
H.Nakagomi ∗1 for the PHENIX Collaboration
relation between true vntr , Ψtrn that could not be measured experimentally and observed vn , Ψn is needed.
vntr = vnob / < cos(n[Ψn − Ψtr(2)n ]) >,tr
where < cos(n[Ψn − Ψn ]) > correspond to the eventplane resolution. The event plane is determined by∑∑Qxn =wi cos(nφi ), Qyn =wi sin(nφi ), (3)The quark-gluon plasma(QGP) is a phase of matter in quantum chromodynamics (QCD). This phase ispredicted to exist at high temperature and high density. Currently at RHIC and LHC, QGP is created bycolliding nuclei. In the heavy-ion collistions, azimuthal
anisotropy of produced particle emmission exists. Collectively, this anisotropy is a quite important probe tounderstand the properties of QGP because this collectivity is sensitive to initial collision geometry andearly time evolution. The strength of anisotropic ﬂow
is expressed as vn (n = 1, 2, 3) and the azimuthal distribution of emitted particles dN/dφ is expressed asfollows using vn .
∑dN
∝1+2vn cos(n[φ − Ψn ]),(1)
dφn=1

i=1

(4)

where Ψn is the measure of the event plane and Qx(y)nis the projection of Ψn to the x(y) axis. wi is theweight and φi is the angle of a particle.
In this analysis, the event plane is determined bybeam beam counter(BBC) and a forward silicon vertex detector(FVTX). These detectors are located at
foward/backward rapidity. In order to measure vn
precisely, there should be rapidity gap between theregions of measurement of vn and Ψn because if are
not separeted these regions, vn would include a nonﬂow contribution. This non-ﬂow is a correlation thatdose not originate from the event plane. Thus, it isbetter to choose the detector that is located at forward/backward rapidity as the event plane measrement detector.
Figure1 shows the event plane resoluiton of BBC andFVTX for Ψ2 . In the central region (0 − 20%), the resolution of FVTX South resolution is larger than thatof FVTX North. This behavior is also found in BBC.
This behavior originate from the multiplicity that isused to measure the event plane and strength of v2 .
Currently, I have calibrated Ψ3 and am working oncalculating Ψ3 resolution.
Resolution

where vn =< cos(n[φ − Ψn ]) > with n = 1, 2, 3..., φ is
the transverse angle of an emitted particle and Ψn isan event plane. The event plane is deﬁned as the average angle of all emmitted particles that are detected.
Thus even-order ﬂow(v2 ) which is called elliptic ﬂow
has been studied. These studies provide initial spatial conditions and the information of speciﬁc viscosity
η/s of QGP in the hydrodynamic. The anisotropic
ﬂow is originated from initial spatial anisotropy. The
initial spatial anisotropy lead to anisotropic collectivity in momentumu space. However, the hydrodynamic
model does not completely agree with experimental
data completly. There is still uncertainty in the theoretical model. Recently, the ﬂuctuation of initial spatial anisotropy was focused upon. The ﬂuctuation of
eccentricity can lead to initial spatial triangularity.
The initial spatial triangularity from the ﬂuctuationis the origin of v3 which is triangular ﬂow strength.
This Fourier coeﬃcient is important to determine theinitial state anisotropy and η/s.
In 2012, Cu+Au collisions were investigated atRHIC. Such asymmetric collisions of heavy nuclei canprovide diﬀerent participant proﬁles through symmetric collisions of heavy nuclei such as Au+Au andCu+Cu because of uniqe initial geometry. In symmetric collisions, initial geometry ﬂuctuations lead to
odd harmonics. However in Cu+Au collisions such anuniqe initial geometry could lead to non-zero odd harmonics. Cu+Au v3 could come from such a initial geometrical triangularity, rather than ﬂuctuation. Therefore, the measurement of Cu+Au non-zero harmonics
is quite important to determine initial conditions.
In this paper, we report the current status of v2 ,v3
measurement at midrapidity in Cu+Au collisions. Inorder to measure v2 , v3 , an event-plane method is applied. To apply the event-plane method, the following∗1i=1

Ψn = tan−1 (Qyn /Qxn )/n,0.5

BBC S
BBC N
FVTX|η|>1 S
FVTX|η|>1 N

0.4

0.3

0.2

0.1

00102030405060708090100
centrality[%]Fig. 1. FVTX/BBC Ψ2 resolution as a function of centrality.

References
1) PRL 107, 252301 40, L1299 (2011).
2) arXiv:1111.5095v1 Lett. 20113) arXiv:1210.5570v2 [nucl-ex] 8 Nov 2012

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