RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-4. Hadron Physics

Measurement of neutral pion v2 in Pb-Pb collisions atTeV by the ALICE experiment√sN N = 2.76T. Tsuji,∗1,∗2 H. Hamagaki,∗2 T. Gunji,∗2 for the ALICE collaboration√In central Pb-Pb collisions at sN N = 2.76 TeV at
the Large Hadron Collider (LHC) in CERN, the yieldof particles at high transverse momentum (pT ) is observed to be strongly suppressed as compared with theexpected yield from p+p collisions scaled by the average number of the binary collisions. This suppression isexplained by the energy loss of hard scattered partonsin the medium created in heavy ion collisions. This
suppression is called jet quenching. This suppression
is quantified by the nuclear modification factor (RAA ),which is the ratio of the yield in Pb-Pb collisions to theyield in p-p collisions scaled by the nuclear thickness
function TAA  as follows:
RAA (pT ) =π 0 ’s are identified by the invariant mass between
two photons with a Photon Spectrometer (PHOS).2)
Figure 1 shows the v2 of π 0 values as a function of1d2 NAA /dpT dy.
< TAA > d2 σpp /dpT dy

RAA (pT ) is well described by many models that employ different approaches for the calculation of partonenergy loss. For a better understanding of the energy
loss mechanisms, measurement of the path length dependence of the energy loss is crucial.1) Since the path
length is highly correlated with the azimuthal angle
with respect to the reaction plane (∆φ), RAA is measured as a function of (∆φ).
The RAA (pT , cent, ∆φ) is expressed asRAA (pT , cent, ∆φ) = F (∆φ, pT ) · RAA (pT , cent),
where F (∆φ, pT ) is the ratio of the relative yield asgiven byF (∆φ, pT ) = N (∆φ, pT ),dφN (∆φ, pT )and N (∆φ, pT ) can be expressed in terms of a Fourier
expansion with ∆φ.
N (∆φ, pT ) ∝ 1 + 2infFig. 1. v2 of π 0 values as a function of Npart . The data
points represent v2 of π 0 measured by the LHC-ALICEexperiment. Solid and dashed lines indicate model calculations using pQCD and Ads/CFT, respectively.

Npart at 6 GeV < pT < 20 GeV. The analysis is performed by using semi-central triggered data recorded
in 2011 Pb-Pb collisions. Solid and dashed lines represent model calculations from pQCD-based (weaklycoupled medium) and AdS/CFT-based (strongly coupled medium) energy loss models, respectively. In thisplot, the v2 of π 0 values shows the same tendency as
the v2 values of the charged particles qualitatively.3)
Comparison of the model calculations and experimental results for v2 of π 0 is currently in progress.
References(vn cos(n∆φ)),n=1

where vn is the magnitude of the n-th order harmonic.
The second harmonic, v2 , represents the strength ofelliptic azimuthal anisotropy. Since high pT particles
are dominated by the jet fragmentation, the v2 of jets
is induced by the path length dependence of the energyloss.
The v2 is obtained by fitting the azimuthal angular
distribution of π 0 with

1) A. Majumber: J. Phys. G. Nucl. Part. Phys. 34, S377
(2007).
2) K. Aamodt et al., for the ALICE Collaboration: JINST
3, S08002 (2008).
3) B. Abelev et al., for the ALICE Collaboration: Phys.
Lett. B719, 18-28 (2013).

N (∆φ, pT ) = N (1 + 2v2 cos(2∆φ)).
∗1∗2RIKEN Nishina CenterCenter for Nuclear Study, Graduate School of Science, University of Tokyo- 118 -完全版2014_本文.indd 118

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