Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 49 (2016)√0Final results of AπLL measurement at s = 510 GeV at mid-rapidity
through a PHENIX experiment†
I. Yoon∗1,∗2 for the PHENIX collaboration
One of the important functions of the relativistic
heavy ion collider (RHIC) longitudinally polarized proton program is to constrain the gluon-spin componentof proton (∆G) by measuring the double helicity asym0metry (ALL ) of π 0 production (AπLL ) and jet production (AJetLL ). Based on the results of deep inelastic scattering experiments, the quark-spin component of theproton is only 0.330 ± 0.011(T heo.) ± 0.025(Exp.) ±0.028(Evol.)1) . The remaining spin might be carried
by gluons or orbital momentum.
π0Jet√ Studies on the measurement of ALL and ALL at
s = 200 GeV have been successfully published, andthey have contributed to constrain ∆G2,3) . Consequently, a positive ∆g(x) has been observed in themeasured x region4) . However, the uncertainty of ∆Gis still significant owing to the uncertainty at lower theBjorken x region, which has not been accessed so far4) .
To explore the lower x region, where uncertainty isdominant, longitudinally polarized proton-protoncol√lisions with increased energy, i.e., s = 510 GeV,
were successfully carried out in 2012 (Run12) and 2013
√0(Run13). With the data, AπLL at s = 510 GeV wasmeasured and published recently. Because of√ the in0
creased energy, the measurement of AπLL at s = 510
GeV could reach a lower x range, 0.01 < x, √while the0ats = 200
previous measurement of AπLL and AJetLLGeV could reach 0.02 < x and 0.05 < x, respectively.
0AπLL can be defined in terms of differences in crosssections as0d∆σ ππ0
ALL =π0dσ0

A LL

where PB(Y ) is the polarization of RHIC’s “Blue (Yellow)” beam, N++(+−) is the yield of the π 0 candidatefrom the same (opposite) helicity collisions, and R is
the relative luminosity of same and opposite helicity
collisions.
π 0 s are reconstructed by photon pairs detected byPHENIX mid-rapidity electromagnetic calorimeters.
Sophisticated cuts are applied to suppress combinatorial background. The relative luminosity is fully corrected for multiple collisions and detector resolutioneffects.
π0√ Fig. 1 shows the result of ALL measurement ats = 510 GeV. The world’s first non-zero ALL inhadron production is observed. The results agree withDSSV14 calculation based on a global fit of world ALL
data and supports positive ∆G. It thus supports positive ∆G in the previously accessed x region and extends the probed x region down to x ∼ 0.01. Therefore
it provides an additional constraint on ∆G5,6) This isa crucial step toward world-wide efforts to extract ∆G.

0.02

†∗1
∗2

s=510 GeVRel. lum. uncertainty
6.5% pol. scale uncertainty not shown
DSSV’14 with 90% CL band0.01
PHENIX00πdσ π − dσ+−= ++0π + dσ π 0dσ+++−(1)01 N++ − RN+−L++,R =PB PY N++ + RN+−
L+−

51015p (GeV/c)
T

where σ++(+−) denotes the π 0 cross-section for proton collisions with the same(opposite) helicity. Theπ 0 cross-section can be decomposed into parton distribution functions, partonic reaction cross-sections, andfragmentation functions. Because most of π 0 s are fromquark-gluon or gluon-gluon scattering at mid-rapidity
0region, ∆g(x) is accessible by measuring AπLL . This description is verified by comparing the π 0 cross-section
between theoretical and experimental data. Equation
1 can be rewritten in terms of experimental observablesas
ALL =pp → π0+X |η|<0.35√
0Fig. 1. Result of AπLL measurement at s = 510 GeV.
The red error bars represent combined statistical andpoint-to-point systematic uncertainties. The yellow box
represents systematic uncertainty from the relative luminosity. The theoretical curve with 90% C.L. band is
obtained from the DSSV14 calculation4) .

References
1) A. Airapetian et al., Phys. Rev. D 75, 012007 (2007).
2) A. Adare et al., Phys. Rev. D 90, 012007 (2014).
3) L. Adamczyk et al., Phys. Rev. Lett. 115, 092002(2015).
4) D. de Floran, R. Sassot, M. Stratmann and W. Vogelsang: Phys. Rev. Lett. 113. 012001 (2014)5) R. Sassot (DSSV Group), private communication.
6) E. Nocera (NNPDF Group), private communication.

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Condensed from the article in Phys. Rev. D. 93, 011501(2016)RIKEN Nishina Center
The Research Institute of Basic Science, Seoul National University-9-