Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Studies on transverse spin properties of nucleons at PHENIX
Y. Goto∗1 for the PHENIX Collaboration
For understanding the polarized structure of the nucleon, we have performed measurements with polarized proton collisions at RHIC using the PHENIX detector. A 3-D picture including the transverse structure of the nucleon provides a conclusive understanding of the nucleon structure beyond the simple parton
picture. It shows many-body correlation of partonsand presents the orbital motion inside the nucleon. It
is described by extending or generalizing the pictureof the 1-D parton distribution, and measured withtransversely polarized proton collisions. Experimentally, it is measured as single transverse-spin asymmetry AN = (dσ ↑ −dσ ↓)/(dσ ↑ +dσ ↓), where dσ ↑(dσ ↓) is the production cross section when the protons are polarized up (down).
AN has shown unexpected large asymmetry in
the forward-rapidity region and required development of many models based on perturbative QCD.
There are two theory frameworks. One is the socalled Sivers effect in the initial state. It is described by Sivers distribution function, which represents transverse-momentum dependence of partonsinside the transversely-polarized nucleon, or highertwist distribution function, which shows quark-gluon
and multi-gluon correlations. The other is the socalled Collins effect with transversity distribution in
the initial state combined with final-state effect. The
transversity distribution represents a correlation between the transversely polarized nucleon and the transversely polarized partons inside. The final-state effect is described by Collins fragmentation function or
higher-twist fragmentation function.
In 2006, an electromagnetic calorimeter systemcalled Muon Piston Calorimeter (MPC) was installed
in a small cylindrical hole in the muon magnet piston.
It consists of lead tungstate crystals and covers theforward and backward rapidity region, 3.1 < |η| < 3.9.
Measurements of AN in forward electromagnetic clusters 1) and π 0 and η mesons2) with the MPC showedlarge forward asymmetry increasing with Feynman x(xF ) as shown in the previous measurements. In frontof the MPC, preshower detectors3) were installed in2014. We will measure prompt photon asymmetry,
which is sensitive to the Sivers effect, and π 0 correlations with jet-like clusters, which are sensitive to theCollins effect.
We also measured asymmetry of open heavy-flavor
production, or charmed meson production. Since
heavy-flavor production is dominated by the gluonfusion process and has no final-state effect, it is sensitive to the gluon inside the nucleon. We have already∗1Fig. 1. AN of the single-muon from charmed-meson decay;(left) xF dependence, (center) pT dependence of theforward muon, xF > 0, (right) pT dependence of thebackward muon, xF < 0.

had a good knowledge on the quark sector where twist3 quark-gluon correlation functions contribute to theAN , however the gluon sector where twist-3 tri-gluon
correlation functions contribute is largely unknown .
Figure 1 shows our preliminary results for AN measurement of the single-muon from charmed-meson decay in 2012. Much improved results are expected to beobtained in 2015 through the use of the forward siliconvertex detector (FVTX) installed in 2012. The FVTX
can identify secondary decay vertex of the charmed meson by measuring the distance of the closest approachof the muon track from the initial collision vertex. ANof µ+ and µ− will be measured at an error level of 1%.
In theoretical calculations, they are expected to be different so that charm and anti-charm contributions canbe distinguished. Asymmetry of heavy-flavor production has also been measured in J/ψ channel4) .
After the 2016 RHIC run, the PHENIX detector will
be upgraded by replacing the present central magnet
with a solenoid, which will make the forward direction
available. We have proposed a forward spectrometerto cover rapidity region 1 < η < 4, with a capabilityof measuring hadrons, photons, electrons, muons andjets5) . The forward upgrades will extend our ability toinvestigate the Sivers and Collins effects by performing
full jet reconstruction; the Sivers effect through theobservation of the AN of the final-state jet itself, andthe Collins effect through an azimuthal anisotropy inthe distribution of hadrons in the jet.
References
1) A. Adare et al. [PHENIX Collaboration], Phys. Rev. D
90, no. 1, 012006 (2014).
2) A. Adare et al. [PHENIX Collaboration], Phys. Rev. D
90, no. 7, 072008 (2014).
3) S. Campbell et al. [PHENIX Collaboration], arXiv:1301.
1096 [nucl-ex].
4) X. Wang for the PHENIX Collaboration: in this report.
5) Y. Goto for the PHENIX Collaboration: RIKEN Accel.
Prog. Rep., Vol.47 (2014), p.93.

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