RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-4. Hadron Physics

Single transverse spin asymmetry (AN) via charged pion production inpolarized p + Au collisions at ξ࢙ = 200 GeV
1,䠆2for the PHENIX collaboration
J.H.Yoo,Yoo䠆*1,*2
J.H.
for the PHENIX collaboration

A proton is basically composed of many sea quarks andgluons in addition to the three valence quarks. According to
the early EMC (European Muon Collaboration) data in the80s, the contribution of the quarks and antiquarks to theentire proton’s spin value (1/2) is less than 30%. As a result,
the rest should come from the gluon spin and the orbital
motions of quarks, antiquarks and gluons. The detailed spin
structure of the proton can be revealed by investigating thelongitudinal and transverse components. In particular, thetransverse spin structure of the proton can provide someinsight into the angular momentum component of the partonsin the proton.
Transverse single spin asymmetries (AN) are relevant to
the transverse spin structure of the proton. Initially, AN of
hadrons produced in the transversely polarized pp collision
was expected to be small, but experiments instead measured
large asymmetries of up to AN §To better describe thelarge AN measurements, the theoretical framework has beenextended to include transverse momentum dependent (TMD)
distributions and multiparton dynamics (higher twisteffects).1) At least two TMD effects have been proposed toexplain the observed nonzero asymmetries. The first of these,
known as the Sivers effect, correlates the proton spin with thepartonic transverse momentum kT.1, 2) The second TMD
effect, known as the Collins effect, describes the coupling ofa transverse quark polarization (transversity) and a transverse
spin dependent fragmentation from a struck quark into ahadron.1, 3)
On the other hand, a pA collision gives a parton distribution and transverse momentum distribution in the nucleus
by comparing the nucleon PDF. The generally accepted cold
nuclear matter (CNM) effects are “Nuclear shadowing,”“Gluon saturation,” “Radiative energy loss,” and the “Cronineffect.” Nuclear shadowing means modification of the parton
distribution functions within a nucleus. Gluon saturation
signifies saturation of the gluon distribution function.
Radiative energy loss means modification of the momentum
fraction of partons due to multiple soft scattering. Finally, theCronin effect implies broadening of the transverse momentum distribution due to multiple scattering of incident partons.
Until recently, the transverse spin structure and CNM had
been studied separately. The RHIC Run15 experiment was
the first high-energy polarized proton and nuclear collisionin the world. This unique collision experiment allows us toexplore the spin degree of freedom in CNM effects.
AN of neutral pions and inclusive charged hadrons havepreviously been measured with the PHENIX midrapidity
䠆1
䠆2

RIKEN Nishina Center
Department of Physics, Korea UniversityFig. 1. AN measured at midrapidity (ȁߟȁ < 0.35) as a functionof pT for (a) ߨ଴ and (b) Ʉ mesons.1)
spectrometer. These asymmetries have been found to beconsistent with zero.4) These precise results are all consistent
with zero over the observed pT range,1) as shown in Fig. 1. In
addition to a pp collision, AN of neutral pions in a pA
collision has already been analyzed. The analysis of AN ofcharged pions using central arm detectors in the midrapidity
region in a polarized pA collision is in progress. Wide A(nucleon number) coverage of AN measurements from apolarized pp to pA collision will allow us to study the A
dependence of AN and the CNM effects in the system oftransverse polarized protons colliding with nuclei.
References
1)(PHENIX Collaboration):
Phys. Rev. D[1] A. Adare
Adare et al.
al. (PHENIX
Collaboration): Phys.
D 90,012006
012006(2014).
(2014).
[2] D.
2)
D. W.
W. Sivers:Sivers: Phys.
Phys. Rev.
Rev. DD 41,41, 8383 (1990).
(1990).
[3] J.J. C.
3)
C. Collins:Collins: Nucl.
Nucl. Phys.
Phys. B396,
B396, 161
161 (1993).
(1993).
[4] N.
4)
N. Novitzky,
Novitzky, SPIN
SPIN 2016.
2016.

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