Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Single transverse spin asymmetry (AN ) in polarized p + Au collisions√
at sNN = 200 GeV
J. H. Yoo∗1,∗2 for the PHENIX collaboration
A proton is basically composed of many sea quarks
and gluons in addition to the three valence quarks.
According to the early European Muon Collaboration(EMC) data in the 80 s, the contribution of quarks
and antiquarks to the entire proton’s spin value (1/2)
is less than 30%. As a result, the rest should comefrom the gluon spin and the orbital motions of quarks,antiquarks and gluons. The detailed spin structure of
the proton can be revealed by investigating the longitudinal and transverse components. In particular,
the transverse spin structure of the proton can provide some insight into the orbital angular momentum
component of the partons (quarks and gluons) in theproton.
Transverse single spin asymmetries (AN ) are relevant to the transverse spin structure of the proton. Initially, the AN of hadrons produced in the transversely
polarized p+p collision was expected to be small; however, experiments instead measured large asymmetries
of up to AN ≈ 40% in the forward direction. To better describe the large AN measurements, the theoretical framework has been extended to include transverse
momentum dependent (TMD) distributions and multiparton dynamics (higher twist eﬀects).1) At least two
TMD eﬀects have been proposed to explain the observed nonzero asymmetries. The ﬁrst eﬀect, known
as the Sivers eﬀect, correlates the proton spin with thepartonic transverse momentum kT .1,2) The second effect, known as the Collins eﬀect, describes the couplingof a transverse quark polarization (transversity) and atransverse spin dependent fragmentation from a struck
quark into a hadron.1,3) A proton + nucleus (p + A)
collision gives a parton distribution and transverse momentum distribution in the nucleus by comparing itwith the nucleon parton distribution function (PDF).
Generally accepted cold nuclear matter (CNM) eﬀects
are “nuclear shadowing,” “gluon saturation,” “radiative energy loss,” and the “Cronin eﬀect.” Nuclear
shadowing implies the modiﬁcation of the parton distribution functions within a nucleus. The gluon saturation signiﬁes the saturation of the gluon distributionfunction. The radiative energy loss implies the modiﬁcation of the momentum fraction of partons due tomultiple soft scattering. Finally, the Cronin 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 exper∗1∗2

RIKEN Nishina Center
Department of Physics, Korea University- 111 -Fig. 1. AN measured at both forward and backward rapidity in p + p, p + Al and p + Au collision for J/Ψ
mesons.1)

iment was the ﬁrst high-energy transversely polarized
proton (p ↑) and nuclear collision in the world. This
unique collision experiment allows us to explore thespin degree of freedom in the CNM eﬀects.
The results for AN in forward and backward J/Ψ
production in p ↑ +p, p ↑ +Al, p ↑ +Au collisions
is shown in in Fig. 1. These results indicate that thenuclear dependence of AN for J/Ψ production is notsmall. Compared to the AN of p ↑ +p collisions, theobserved asymmetry in p ↑ +Au collisions is larger
in absolute value and has opposite sign. Moreover,
the AN of neutral pions and inclusive charged hadrons
in p ↑ +p have previously been measured with thePHENIX midrapidity spectrometer. These asymmetries have been found to be consistent with zero. In
addition to a p ↑ +p collision, the AN of neutral pions
in a p ↑ +A collision has already been analyzed. The
analysis of AN of charged pions in the midrapidity region in a polarized p + A collision is in progress. AN
measurements with two species of nuclei (Al and Au)
will allow us to study the A dependence of AN and theCNM eﬀects in the system of transversely polarized
protons colliding with nuclei.
References
1) PHENIX Collaboration, arXiv:1805.01491v1.
2) D. W. Sivers, Phys. Rev. D 41, 83 (1990).
3) J. C. Collins, Nucl. Phys. 396, 161 (1993).