Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

RIKEN Accel. Prog. Rep. 47 (2014)

Elliptic flow of neutral pion in Pb+Pb collisions at
TeV by ALICE experiment√sN N = 2.76

T. Tsuji,∗1,∗2 H. Hamagaki,∗2 T. Gunji,∗2 H. Torii,∗1 for the ALICE collaboration
where vn is the magnitude of the n-th order harmonic.
The second harmonic, v2 , represents the strength ofelliptic azimuthal anisotropy. The anisotropy v2 at alow pT is caused by the collective flow, which gives riseto the background in the measurement of RAA (pT , ∆φ)for investigating energy loss.
The values of π 0 v2 were calculated. π 0 v2 was extracted by using the dN/dφ method. In this method,v2 is obtained by fitting the azimuthal angular distribution of π 0 with

It has been observed in central Pb+Pb collisions√at ( sN N ) = 2.76 TeV at the Large Hadron Collider(LHC) facility at CERN that the yield of charged particles at a high transverse momentum (pT ) is strongly
suppressed compared with the expected yield from p+pcollisions, assuming scaling with the number of binary
collisions. This suppression is attributed to the energy loss of hard-scattered partons within quark-gluon
plasma (QGP) created in heavy ion collisions. Thisphenomenon known as jet quenching. A useful way toquantify the suppression of high-pT hadrons is to introduce the nuclear modification factor (RAA ), where thep + p cross section is scaled with the thickness function
TAA  of the two nuclei
RAA (pT ) =N (∆φ, pT ) = N (1 + 2v2 cos(2∆φ)).
π 0 values are reconstructed by the invariant massmethod with reconstructed energy obtained using aphoton spectrometer (PHOS) in the ALICE experiment.2) Fig.1 shows π 0 v2 values as a function of pT . In1evt 2)d NAA /dpT dy(1/NAA.
< TAA >
d2 σpp /dpT dy

Experimental data can be well reproduced by using
multiple models employing different approaches thatare used to calculate the energy loss of hard-scatteredpartons as they traverse the dense medium. To compare these models, improved experimental control ofthe path length L is required because the energy lossof a high-pT parton increases rapidly with increase of
the the distance traveled through the medium.1) Thus,
the measurement of the energy loss with respect to thepath length is expected to provide detailed information
about the mechanism of the energy loss of the parton.
If RAA is measured as a function of centrality (cent)and the azimuthal angle (∆φ) with respect to the event
plane, RAA (L) can be determined. Therefore, the differential observable RAA (∆φ) directly probes the pathlength dependence of the energy loss.
The RAA (pT , cent, ∆φ) with respect to the azimuthal angle is factorized asFig. 1. π 0 v2 values as a function of pT . Bars indicate theamplitude of statistical errors estimated from all data
for semi-central triggered events in 2011.

this figure, all data for semi-central triggered events in2011 are analyzed. Centrality is defined by V0 detectors, which are scintillation detectors, and covers therange from -3.7 to -1.7 and from 2.8 to 5.1 in pseudo
rapidity. In this plot, π 0 v2 values denote the same tendency of the v2 values of the charged particles qualitatively.3) Calculations of π 0 v2 are presently ongoing.

RAA (pT , cent, ∆φ) = F (∆φ, pT ) · RAA (pT , cent),where F (∆φ, pT ) is the ratio of the relative yield, givenasF (∆φ, pT ) = N (∆φ, pT ),dφN (∆φ, pT )References

and N (∆φ, pT ) can be expressed in terms of a Fourier
expansion with ∆φ.
N (∆φ, pT ) ∝ 1 + 2∗1∗2inf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).

(vn cos(n∆φ)),n=1

RIKEN Nishina Center
Center for Nuclear Study, Graduate School of Science, University of Tokyo- 98 -1