Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 53

xF dependence of the transverse single spin asymmetry of very forward
π 0 production in 510 GeV p↑ + p collisions

The transverse single spin asymmetry, AN , of veryforward (η > 6) π 0 was ﬁrst measured in the RHICf1)
in June, 2017 from polarized p + p colliexperiment√sions at s = 510 GeV. The AN is deﬁned as (dσLeft −dσRight )/(dσLeft +dσRight ), where σLeft(Right) is the particle production cross sections in the left (right) side ofthe polarization direction of the proton beam. The ANof very forward π 0 is a powerful observable to investigate the contribution from the non-perturbative process
to the non-zero AN of π 0 , which has been measured inmany experiments2–4) thus far.
In order to measure the very forward π 0 precisely, anelectromagnetic calorimeter, the RHICf detector, wasinstalled at the zero-degree area of the STAR experiment at Relativistic Heavy Ion Collider, which was 18 m
away from the beam collision point. The conﬁgurationof the RHICf detector can be found in Ref. 5). TheRHICf detector has an energy resolution of 2.5–3.5%
for 100–250 GeV π 0 and a transverse momentum (pT )resolution of 0.005–0.025 GeV/c for 0.0–0.8 GeV/c π 0 .
The experiment was conducted for 28 h with threevertical detector positions, where the beam propagatedtowards the center of the large tower, center of the small
tower, and a point 24 mm below the center of the small
tower of the detector before the collisions. With thethree detector positions, we requested radial polarization, which was the direction at an angle of 90◦ fromthe usual vertical polarization at RHIC, and a large β ∗= 8 m so as to measure the wide pT range with higherresolution. The kinematic coverage of the measured π 0is xF > 0.25 and 0.0 < pT < 1.0 GeV/c, where the nonperturbative process was expected to mainly contributeto the π 0 production.
Figure 1 shows our preliminary result for the AN ofvery forward π 0 as a function of xF for ﬁve diﬀerent
pT ranges. The line and box error bars represent statistical and systematic uncertainties, respectively. Theestimation; of the polarization; beam center, which isdeﬁned as the extrapolated zero-degree position of thebeam to the detector; and background (mostly from thetwo-photon coincidence of diﬀerent π 0 ) were the threemain systematic sources. See Ref. 6) for more details
on the analysis procedure and systematic study.
In Fig. 1, the AN s are consistent with zero at a very
low pT < 0.09R GeV/c. However, its slope with respect
to the xF -axis becomes steeper as pT increases. WhenpT > 0.36 GeV/c, the increasing AN shows the same
slope with the previous forward (2 < η < 4) π 0 measurements2–4) even in the low-pT region where only the∗1∗2ANM. H. Kim∗1,∗2 for the RHICf collaboration

0.25
0.2

RHICf Preliminaryp↑+p → π0+X at s = 510 GeV (6 < η)
0.00 < pT < 0.09 GeV/c

0.15

0.09 < pT < 0.17 GeV/c
0.17 < p < 0.36 GeV/c

0.1

0.36 < pT < 0.59 GeV/c
0.59 < p < 1.00 GeV/c

TT

0.05
0−0.05
00.2

0.4

0.6

0.8xF

Fig. 1. Preliminary result of the AN of very forward π 0 production as a function xF . The ﬁve diﬀerent colors correspond to diﬀerent pT ranges.

non-perturbative process is expected to be dominant.
It might be possible that the non-perturbative process
can contribute to the non-zero AN of π 0 at a higher pT .
The result also raises the necessity of further experimental eﬀorts to compare the roles of the perturbativeand non-perturbative processes in the AN of π 0 andthe measurement of the intermediate kinematic region
where both processes possibly contribute (0.8 < pT <2.0 GeV/c). Combined analysis with STAR forward andRoman pot7) detectors is useful to separate each process by measuring the rapidity gap or tagging a proton
scattered at a very low angle, which are the characteristics of the diﬀraction of the non-perturbative process.
References

1) RHICf Collaboration, arXiv: 1409.4860 [physics.ins-det].
2) D. L. Adams et al. (FNAL-E581 and E704 Collaborations), Phys. Lett. B 261, 201 (1991).
3) A. Adare et al. (PHENIX Collaboration), Phys. Rev. D
90, 012006 (2014).
4) B. I. Abelev et al. (STAR Collaboration), Phys. Rev. D
101, 222001 (2008).
5) M. H. Kim for the RHICf Collaboration, RIKEN Accel.
Prog. Rep. 52, 83 (2019).
6) M. H. Kim for the RHICf Collaboration, arXiv:1902.07852[hep-ex].
7) S. Bültmann et al. Nucl. Instrum. Methods Phys. Res.
535, 415 (2004).

Department of Physics, Korea University
RIKEN Nishina Center-9-