Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Status of double helicity asymmetries (ALL ) in π ± production at
mid-rapidity in longitudinally polarized p + p collisions withPHENIX experiment
T. Moon∗1,∗2 for the PHENIX collaboration

σ++ − σ+−ALL =,σ++ + σ+−

∗20.1
-

Projected A/ @ 510 GeV
LL+

Projected A/ @ 510 GeV
LL

0.08

-

A/LL @ 200 GeV (PRD91,032001)+

A/LL @ 200 GeV (PRD91,032001)

0.06

DSSV14 (6G > 0) for /-DSSV14 (6G > 0) for /+

0.04

0.02

0-0.02-0.04

567891011121314p [GeV/c]
T√Fig. 1. Projected ALL in π ± production at s = 510√GeV and published ALL at s = 200 GeV along withDSSV14 curve.

ALL =1 N++ − RN+−,PB PY N++ + RN+−

(2)

where N++ (N+− ) denotes the yield of π ± candidates
from the collisions in the same (opposite) helicity. R is
the relative luminosity between colliding bunches with
diﬀerent spin conﬁgurations:L++.
(3)
L+−Figure 1 shows √the comparison of projected ALL in±production
ats = 510 GeV and published ALL atπ√s = 200 GeV along with the DSSV14 curve with positive gluon polarization. The uncertainty of charged pions at 510 GeV was signiﬁcantly improved compared tothat at 200 GeV. As a complementary probe, charged
pions might help in directly inferring the sign of thegluon polarization.
In summary, charged pion analysis as a complementary probe is ongoing with improved statistics and isexpected to double-check the sign of the gluon polarization.
R=(1)where, σ++ (σ+− ) denotes the π ± cross section froma collision√ with the same (opposite) helicity.
At s = 510 GeV in 2013, PHENIX recorded a total integrated luminosity of 108 pb−1 within a 30 cm
vertex region and average polarization PB (PY ) = 0.55
± 0.02 (0.56 ± 0.02), where PB (PY ) is the polarizationof RHIC’s blue (yellow) beam.
π ± are reconstructed with the PHENIX tracking detectors, the drift chamber (DC) and pad chambers(PC), covering the pseudo-rapidity range |η| < 0.35
and the azimuthal angle Δφ = π, and are separated
from backgrounds, electrons, kaons, protons and so onby using a ring-imaging Cherenkov detector (RICH)
and electromagnetic calorimeter (EMCal)
Experimentally, ALL can be measured as∗1

A LL in /( production at s = 200 and 510 GeVA LLThe spin of the proton, 12 h̄, is explained by the
angular momentum sum rule in terms of quark andgluon components. The result of EMC experimentand others suggested that the spin of the quarks andanti-quarks only contributes about 30% of the proton
spin.1) This intrigued us to explore how much of therest of the proton spin originates from the gluons andthe orbital angular momenta of quarks and gluons.
The Relativistic Heavy Ion Collider (RHIC) provides
polarized proton beams to determine the gluon helicity
contribution to the proton spin as one of the main goalsof the RHIC spin program.
In perturbative quantum chromodynomics (pQCD),
pion and high pT jet productions are sensitive to thegluon helicity distribution (Δg) in the proton where
their production is dominated by gluon-related interactions, for example, q-g and g-g scatterings.
The RHIC double helicity asymmetries(ALL ) result√in inclusive π 0 production at s = 510 GeV with theRHIC 2012 and 2013 data sets extending the x range
down to x ∼ 0.01 and conﬁrming non-zero ΔG.2)
For complementary information, the ordering of theasymmetries among three π meson species, π 0 and π ± ,can directly reveal the sign of the gluon spin contribution to the proton spin.
The ALL in π ± production in longitudinally polarized p+p collisions can access the gluon helicity distribution (Δg) and is deﬁned as

RIKEN Nishina Center
Department of Physics, Yonsei University- 111 -ReferenceT1) J. Ashman et al. (EMC Collaboration), Nucl. Phys.
B328, 167 1 (1989)
2) A. Adare et al. (PHENIX Collaboration), Phys. Rev.
D93, 011501 (2016), 1510.02317