Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Status of longitudinal double helicity asymmetries (ALL ) in π ±√productions in s = 510 GeV polarized p + p collisions inRHIC-PHENIX experiment
T. Moon∗1,∗2 for the PHENIX collaboration
Uncorrected π± p spectra

The spin of the proton is 1/2 which is explained by
the angular momentum sum rule in terms of quark andgluon components. Results of EMC experiments andothers suggested that the quarks and anti-quarks contribute to only about 30% of the proton spin. Gluons
and the orbital angular momentum of quarks and gluons probably account for the rest of the proton spin.
Spin analysis using π ± production is particularly interesting in investigating the contribution of the gluons tothe proton spin.
The Relativistic Heavy Ion Collider (RHIC) is aunique facility providing longitudinally polarized p + p
collisions. It enables us to expore the role of gluons in
the intrinsic properties of the proton.
In perturbative Quantum Chromodynomics, pion
production is sensitive to the gluon helicity distributions (∆g) in the proton because their production isdominated by gluon-related interactions (q-g and g-gscatterings) in the pT region probed in PHENIX, 4.8
GeV/c < pT < 16.5 GeV/c. The gluon helicity distribution (∆g) can be constrained by measuring thedouble helicity asymmetries (ALL ) in π 0 and π ± production in longitudinally polarized p+p collisions. Theasymmetry is defined asσ++ − σ+−(1)ALL (pT ) =σ++ + σ+−

π ± Yield [a.u]

T

∗2π+

103

102

24681012141618pT [GeV/c]

Fig. 1. pT spectra of π ±Charged particles including π ± are reconstructed using the drift chamber (DC), pad chambers (PC) covering the pseudo-rapidity range |η| < 0.35, and azimuthal angle ∆φ = π. When a charged particle passes
through a di-electric medium, CO2 gas in the RICH,
at a speed faster than the phase velocity of light in themedium, it emits the Cerenkov light which then fires
the array of photomultiplier tubes (PMT). In the momentum region of interest, only e± and π ± emit lightand can thus be preselected by RICH activity. e± deposits most of its energy in the EM showers. On theother hand π ± on average deposits only a third of theirenergy in the EMCal. Therefore, π ± is separated fromother particles using the RICH and the EMCal. In
addition to the cuts mentioned above, the acceptancecut and the track matching with hit on the EMCal andPCs were required. Fig.1 shows π ± pT spectra after
applying the cuts. A turn on curve is cleary observed
at a pT of 4.8 GeV/c because π ± survives from the cut
requirement. However, some background may remain,
as mentioned.
(1) conversion electron reconstructed with wrong pT(2) π ± originating from hadronic decay
In the PHENIX tracking algorithm, the tracks are
not propagated to the decay vertex but to the eventvertex. So the tracks for types (1) and (2) may not
be reconstructed correctly and cuts are carefully being applied to the tracks at present. After the cut
optimization, both the cross section and ALL will bemeasured.
√In summary, π ± analysis was started at s = 510
GeV. With more statistical data, a more precise measurement of ALL in π ± production is expected.

where, σ++(+−) denotes the π ± cross section from acollision with the same (opposite) helicity.
Yet, unlike the π 0 which can be triggered via photons in an Electro-Magnetic Calorimeter (EMCal) andRing Imaging Cerenkov Detector (RICH), no dedicated charged hadron trigger exists and thus the π ±statistics are less than for π 0 .
However, π ± analysis at mid-rapidity has the advantage of confirming the sign of gluon polarization (∆G)
in the proton simply by ordering the longitudinal double helicity asymmetries (ALL ) of the pion charges.
√The result of π ± analysis with data taken at s =200 GeV has already been published in 20091) . In 2013,PHENIX√recorded a total integrated luminosity of 145
pb−1 at s = 510 GeV within a 30 cm vertex region
which is 10 times
 higher than the previous one. The
figure of merit ( LPB2 PY2 dt) considering beam polarization (PB and PY ) was also higher. Therefore, theresult of the ongoing analysis is expected to help determine the sign of the gluon polarization (∆G) witha higher precision.
∗1π104Reference1) A. Adare et al. (PHENIX Collaboration), Phys. Rev. D
91, 032001 (2015).

RIKEN Nishina Center
Department of Physics, Yonsei University- 95 -