Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Neutral pion production with respect to centrality and reaction√plane in Au+Au collisions at sN N = 200 GeV†
Y. Aramaki∗1 for the PHENIX Collaboration
lider (LHC) (ALICE experiment)3) , the two data setsfor RHIC and LHC appear to be similar for the entire pT range of 5–20 GeV/c. However, the RHIC andLHC are different in terms of colliding energy, resulting in an approximate increase by a factor of 2 increasein the parton density at the LHC4) . On the basis ofthe slope of the pT distribution and RAA , the average fractional momentum loss (Sloss ) of π 0 is deduced.
If one assumes that the fragmentation function of theparton after energy loss is unchanged, the Sloss can beinterpreted as the average fractional energy loss of theinitial parton. The calculated Sloss shows a decrease
with increasing pT at central collisions. In comparison with the Sloss value of the ALICE charged hadronmeasurement (Sloss ∼ 0.3), the Sloss at the PHENIX π 0measurement below 10 GeV/c (Sloss ∼ 0.21) is about
30% lower value.
To study the path-length dependence of the suppression, the π 0 yield is also measured at different azimuthal angles with respect to the event plane; a strong
azimuthal-angle dependence of the π 0 RAA is observed.
The data are compared to theoretical models of partonenergy loss as a function of the path length L in thecreated medium. While all models considered describe
the φ-integrated RAA adequately, the pQCD-based calculations, in which the energy loss depends on the path
length as L2 , fail to describe the differential RAA (∆φ).
The data obtained using a hybrid model5) that utilizes pQCD for hard interactions and anti-de-Sitter
space/conformal field theory6) (AdS/CFT) for soft interactions is also compared to the measured data, andwere able to obtain an adequate fit. Since the energy
loss in this model is proportional to L3 , the data require an energy loss with a power greater than 2, asgiven by models in which the soft interactions with themedium are strongly coupled. Therefore, one is led tothe tentative conclusion that strong coupling plays animportant role in parton energy loss in the medium.

The suppression of high transverse momentum (pT )hadrons in relativistic heavy ion collisions was observedat the Relativistic Heavy Ion Collider (RHIC). The
phenomenon is interpreted as the energy loss of a hardscattered parton in the hot, dense, strongly interacting
quark–gluon plasma (QGP) formed in the collisions.
The suppression patterns are quantified by the nuclear
modification factor RAA of the neutral pion as
RAA (pT ) =evt 21/NAAd /dpT dy,
π 0 /dp dy�TAB � d2 σppT(1)0πis the production cross section of π 0 inwhere σpp
inelp + p collisions, �TAB � = �Ncoll � /σpp
is the nuclear
overlap function averaged over the relevant range ofimpact parameters, and �Ncoll � is the number of biinel.
nary nucleon–nucleon collisions computed using σpp
In this paper, the results of π 0 production and RAA
and its azimuthal angular dependence are presented.
The results are based on the data collected in the 2007
RHIC run. The data sample is four times larger thanthat of Ref.1) . Furthermore, the reaction plane detector installed in 2007 improved event-plane resolution.
Firstly, the measured π 0 invariant yields of Au+Au
collisions for all centralities and for minimum bias data
have been reported. These results are compared tothe published η yields2) . The measured η/π 0 ratiosfrom minimum bias collisions for various data sets andcolliding systems are compared. Although the uncertainties vary, the ratios of η/π 0 are consistent withpreviously published ones and are also consistent withthe overlaid PYTHIA-6.131 p + p calculation. The
production rate of η/π 0 , 0.46±0.01(stat)±0.01(syst),
is constant with the centralities at pT >2 GeV/c forthe same collision energy. These observations indicatethat at high pT , the fragmentation occurs outside themedium and the ratio is governed by vacuum fragmentation.
Secondly, the nuclear modification factor RAA of π 0
is compared to the previous result and the charged
hadron RAA at the LHC energy. The yields of π 0 are
suppressed by a factor of 5, as in earlier measurements;
however, with the improved statistical and systematic
uncertainties, the significant rise of RAA as a function of pT with a slope dRAA /dpT of 0.0106±0.0034
0.0029(GeV/c)−1 in central collisions has been observed forthe first time at the RHIC energy. In comparison√
with the charged hadron RAA observed in sN N =2.76 TeV Pb+Pb collisions at the Large Hadron Col†
∗1References
1) A. Adare et al. (PHENIX Collaboration): Phys. Rev.
Lett. 101, 232301 (2008).
2) A. Adare et al. (PHENIX Collaboration): Phys. Rev. C
82, 011902 (2010).
3) K. Aamodt et al. (ALICE Collaboration): Phys. Lett.
B 696, 30 (2011).
4) W.A. Horowitz and M. Gyulassy: Nucl. Phys. A 872,265 (2011).
5) C. Marquet and T. Renk: Phys. Lett. B 685, 270(2010).
6) J.M. Maldacena: Adv. Theor. Math. Phys. 2, 231(1998).

Condensed from the article in Phys.Rev.C 87, 034911 (2013)
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