Ⅱ-4. Hadron Physics
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Transverse single spin asymmetry for forward neutron production√in polarized p + p collisions at s = 510 GeV
M. H. Kim∗1 for the RHICf Collaboration∗1RHICf Preliminary

AN

Transverse single spin asymmetry (AN ) is deﬁned
by a left-right cross-section asymmetry with respect tothe beam polarization. In high-energy polarized p + p
collisions, AN of the forward (pseudorapidity η > 6)
particle is a unique observable for studying the spininvolved diﬀractive particle production mechanism.
AN for forward neutron production has been explained by the interference between the spin ﬂip (πexchange) and nonﬂip (a1 exchange) amplitudes witha nonzero phase shift. This π and a1 exchange model
predicted that the neutron AN would increase in magnitude with the transverse momentum (pT ) and explained the PHENIX data, which had been measured
with three diﬀerent collision energies, 62.4, 200, and500 GeV.1) RecentPHENIX results,2) which unfolded√AN on pT at s = 200 GeV, were also in good agreement with the prediction of the π and a1 exchange
model. However, the π and a1 exchange model predicted the neutron AN only in the range of pT <0.4 GeV/c.
In June 2017, the RHICf experiment3) measured ANfor forward√ neutron production in polarized p + p collisions at s = 510 GeV by installing an electromagnetic
calorimeter,4) the RHICf detector, at the zero-degreearea of the STAR experiment at the Relativistic Heavy
Ion Collider. We measured the neutron AN over awider range of 0 < pT < 1 GeV/c to compare theresults with those of PHENIX and to test the π anda1 exchange model in the higher pT region. In this article, we report the preliminary results of the neutron
AN measured by the RHICf experiment.
Neutrons were separated from the photon background using the diﬀerence between their shower developments in the detector. Once the neutron candidates
were selected, Bayesian unfolding,5) which is available
in the RooUnfold library,6) was applied to estimate
the true longitudinal momentum fraction (xF ) and pT
distributions. AN could be calculated because unfolding was applied for events from both spin-up and spindown polarizations. Because ﬁnite photon and charged
hadron backgrounds were included in the neutron candidates, the AN backgrounds were subtracted by estimating their fractions. Refer to Ref. 7) for a moredetailed analysis procedure.
Figure 1 shows the preliminary results for the forward neutron AN as a function of pT . The systematic
uncertainties due to unfolding, the beam center calculation, polarization estimation, and background subtraction are included. In the range pT < 0.2 GeV/c,the values of the forward AN measured by RHICf arep↑ + p → n + X at s = 510 GeV (6 < η)
0−0.1

−0.2

−0.3
00.28 < xF < 0.46
0.46 < xF < 0.64 RHICf at 510 GeV
0.64 < xF < 1.00
0.70 < xF < 0.85
PHENIX at 200 GeV
0.85 < xF < 1.00
0.2

0.4

0.6

0.8

1p (GeV/c)
TFig. 1. Forward neutron AN as a function of pT for diﬀerent
xF regions. The triangular and circular data points are
the RHICf and PHENIX results, respectively. The error bars indicated by lines and boxes correspond to thestatistical and systematic uncertainties, respectively.

consistent with those of PHENIX, even though the collision energies are diﬀerent. In the range xF > 0.46, AN
increases in magnitude with pT , even in the pT range
beyond where AN was calculated theoretically. Comparing the data points in xF < 0.46 and xF > 0.46,
there is a gap, which indicates that the neutron AN
is possibly dependent on xF , which has not been predicted.
In the preliminary results, large systematic uncertainties are assigned to background subtraction because an analysis of the front counter, which had been
installed in front of the RHICf detector to suppress thecharged hadron background, was not complete. However, the ﬁnal results will be released soon because thebackground study has been recently completed.
References
1) B. Z. Kopeliovich et al., Phys. Rev. D 84, 114012 (2011).
2) U. A. Acharya et al. (PHENIX Collaboration), Phys.
Rev. D 105, 032004 (2022).
3) RHICf Collaboration, arXiv:1409.4860v1.
4) K. Kawade et al., J. Instrum. 9, P03016 (2014).
5) G. D’Agostini, Nucl. Instrum. Methods Phys. Res. A
362, 487 (1995).
6) T. Adye, arXiv:1105.1160.
7) M. H. Kim for the RHICf Collaboration,
https://doi.org/10.5281/zenodo.7271694 .

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