RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-4. Hadron Physics

Spin Physics in RHICf Experiment∗1∗2J.S. ParkforPark,∗1,∗2for RHICf
RHICf collaborationcollaboration

Transverse single spin asymmetry (AN ) is deﬁned asAN =σ↑ − σ↓σ↑ + σ↓(1)where ↑ and ↓ denote up and down spins, respectively, of the incident beam. Since the discovery ofan unexpectdly large AN in 1991 from the E704 experiment.1) AN values measured from subsequent experiments2–4) have provided important information toexpand QCD application. When we categorize QCD
into perturbative QCD (pQCD) and non-perturbative
QCD (non-pQCD) according to Q2 , the AN in thepQCD region can be explained by two kinds of theoretical framworks: twist-3 perturbative QCD andthe transverse-momentum-dependent parton distribution function. On the other hand, no general physicalframework to calculate AN in the region of non-pQCD
exists. In the case of the inclusive neutron production
process, however, AN can be calculated by the spinﬂip one pion exchange (OPE) model and spin-non-ﬂip
model.5)5).
Reggeon exchange model
At the Relativistic Heavy Ion Collider (RHIC), theﬁrst polarized proton-proton collider, AN measure√ment with forward neutrons(8.8> η >6.9) at s =62
GeV, 200 GeV and 500 GeV has been performed.6,7)
These data have played an important role in testing
the explanation based on the OPE model and Regge
theory. Nevertheless, more AN measurements of neutrons with high pseudorapidity and with higher pT areneeded for a better understanding of the mechanism
in inclusive neutron production√ and clariﬁcation of therelation among AN , PT and s.
In the RHICf experiment, we will measure very forward neutral particles (η > 6.9) by using STAR ZDC
and the RHICf detector. ZDC is a sampling typehadron calorimeter that has an energy resolution of20% for 100 GeV neutrons with a position resolutionof 1 cm. The RHICf detector is a sampling type EMcalorimeter that has an energy resolution of 40% for350GeV neutrons with a position resolution of 1 mm.8)
In addition to the improvement of the position resolution of ZDC and the RHICf detector, we are planningto manipulate the position of the RHICf detector vertically as shown Fig. 1 and use a radial polarized beamduring the dedicated beam time for the RHICf experiment.9) These works will allow more precise measurement and higher pT (up to 1.5 GeV/C) data. Anotherchallenge in the RHICf experiment is the measurementof AN with forward π 0 ; the RHICf detector consists of2 towers for reconstruction from the detection of 2γ.
∗1∗2Department of Physics and Astronomy, Seoul National University
RIKEN Nishina Center- 108 -Fig. 1. Various positions of the RHICf detector. The black
cross indicates the beam center.

Fig. 2. Relationship between AN of forward neutrons andpT at various collision energies. Red eclipses show theresults expected from RHICf experiment.

References
1) D. L. Adams, et al., Fermilab E704 Collaboration, Nucl.
Phys. B 510 3 (1998).
2) S. S. Adler, et al., PHENIX Collaboration, Phys. Rev.
Lett. 95 202001 (2005).
3) A. Airapetian, et al., HERMES Collaboration, Phys.
Rev. D 64 097101 (2001).
4) V. Y. Alexakhin, et al., COMPASS Collaboration,Phys. Rev. Lett. 94 202002 (2005).
5) B. Kopeliovich, et al., Physical Review D 84 114012(2011).
6) Y. Fukao et al., Phys. Lett. B 650, 325 (2007).
7) A. Adare et al., PHENIX Collaboration, Physical Review D 88 032006 (2013).
8) M. H. Kim for the RHICf Collaboration: In this report.
9) Y. Goto for the RHICf Collaboration. RIKEN Accel.
Prog. Rep. 48. (2015).