II-4. Hadron Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Transverse single-spin asymmetry for very forward neutral pion√
production in polarized p + p collisions at s = 510 GeV†

In high-energy polarized p + p collision, the left-rightcross section asymmetry, which is called as transverse
single-spin asymmetry (AN ) of very forward (η > 6)
particle production, plays an important role in understanding the spin-involved production mechanism fromthe view points of perturbative and nonperturbativeinteractions. The RHICf experiment1) measured theAN of very√forward neutral pion in polarized p + p collisions at s = 510 GeV in June, 2017 at the Relativistic Heavy Ion Collider (RHIC).
The nonzero asymmetries of the neutral pion havebeen measured by many experiments2,3) in the forward(2 < η < 4) kinematic range, and they have been
explained only by quarks and gluons’ degrees of freedom. However, a possible contribution from the nonperturbative interaction has been recently highlighted
because larger asymmetries were observed in more isolated neutral pion events4,5) that could be connected
to nonperturbative event topology. A straightforward
approach to studying the role of nonperturbative interaction is measuring the AN of the neutral pion in
the very forward kinematic area where the nonperturbative interaction is expected to dominate.
We installed an electromagnetic calorimeter (RHICfdetector) at the zero-degree area 18 m away from thebeam collision point at the STAR experiment. TheRHICf detector consists of two sampling calorimeterswith lateral dimensions of 20 mm × 20 mm and 40 mm
× 40 mm, respectively. Both calorimeters are composed of 17 tungsten absorbers, 16 GSO plates for energy measurement, and 4 layers of GSO bars for position measurement. Since the radiation length of thedetector is 44 X0 , the electromagnetic shower stops
its development in the middle of the detector. TheRHICf detector has an energy resolution of 2.5–3.5%
for 100–250 GeV π 0 s and the pT resolution of 3.0–4.5%
in 0.0 < pT < 0.8 GeV/c.
To the best of our knowledge, we observed large
asymmetry in a very forward neutral pion production
for the ﬁrst time, which means a possible contribution from the nonperturbative interaction. Comparison between RHICf and previous forward neutral pionmeasurements are depicted in Fig. 1. At very low
pT < 0.07 GeV/c, the asymmetries are consistent withzero. However, as pT increases, the asymmetries increase as a function of xF , approximately showing thesame magnitudes and tendency of the forward ones.
†∗1∗2

ANM. H. Kim∗1,∗2 for the RHICf Collaboration
0.2

p↑+p → π0+X
RHICf π0 6<η s=510 GeV 0.00<p <0.07 GeV/c
RHICf 0.07<p <0.19 GeV/c0.15TTRHICf 0.19<p <0.30 GeV/cTRHICf 0.50<p <0.69 GeV/cTPHENIX π0 3.1<η<3.8 s=62.4 GeV

0.1

0E704 πs=19.4 GeVSTAR π0 <η>=3.3 s=200 GeV

0.05
00

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8xF

Fig. 1. Forward (black) and very forward (color) neutralpion asymmetries as a function of xF in diﬀerent pT
regions.

It may be necessary to consider nonperturbative interaction to understand both forward and very forward
neutral pion asymmetries.
However, RHICf data can be aﬀected by perturbative interaction in the low pT region. The perturbativeand nonperturbative interactions may make their own
nonzero asymmetries respectively. Since the result of
this report is the one analyzed inclusively, more detailed analysis is necessary to reduce the diverse possibilities.
We have started combined analysis with STAR. Signals in the STAR detectors will identify whether theneutral pions of the nonzero asymmetries come fromperturbative or nonperturbative interaction. Further,
we are preparing a series of follow-up experiments fordeveloping a larger detector. In the near future, further analysis and experiments will provide a powerful
input to understand the origin of both forward andvery forward neutral pion asymmetries.
References

1) RHICf Collaboration, LOI, arXiv: 1409. 4860v1.
2) A. Adare et al. (PHENIX Collaboration), Phys. Rev. D
90, 012006 (2014).
3) B. I. Abelev et al. (STAR Collaboration), Phys. Rev.
Lett. 101, 222001 (2008).
4) S. Heppelmann (STAR Collaboration), Proc. Sci. 191(DIS2013), 240 (2013).
5) M. M. Mondal (STAR Collaboration), Proc. Sci. 203(DIS2014), 216 (2014).

Condensed from the article in Phys. Rev. Lett. 124, 252501(2020)RIKEN Nishina Center
Department of Physics, Korea University- 56 -1