Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 51 (2018)√Beam polarization monitor in s = 510 GeV polarized
proton-proton collisions at the RHICf experiment
M. H. Kim∗1,∗2 for the RHICf collaboration

N ↑ − RN ↓ϵN = ↑
N + RN ↓∗20.050.04
0.03
Shower Trig. at TS
Shower Trig. at TL

0.02
0.01
0−0.01
−0.02
−0.03
−0.04
−0.05

2840

2850

2860

2870

2880

2890

2900
2910
RHICf run number

Fig. 1. Measured ϵN depending on run number.

Because N is proportional to the production cross section, AN can be studied using ϵN .
Calculated ϵN with diﬀerent run numbers that lasted
longer than 20 minutes are described in Fig. 1. We had
three types of physics runs depending on the detector
positions, and each run lasted 30 minutes if there wasno critical issue. This is the result when the beamheight is 24 mm below the center of TS. The stabilityof beam polarization can be conﬁrmed by ϵN , which
was conserved within 10% error. If the polarization
direction was changed or the ratio of polarized beam
decreased, the amplitude of ϵN would approach to zero.
Neutrons are dominant in the shower-triggered events.
The signs of ϵN show good agreement with the negative
sign of AN for very forward neutron production.5) ϵN
at TL is more diluted than at TS because the gamma
contamination is more enhanced at TL than at TS.
The production cross-section asymmetry of very forward particles was studied using the RHICf detector
with scaler bits. However, because there is a limit in
the reconstruction of particle information with it, further issues remain in the detector data analysis.
References(1)

as the second beam-polarization monitor, where N ↑(N ↓ ) is the number of scaler counts with the proton
polarized up (down) when a shower is generated at theRHICf detector. Events for which three successive layers at one of the towers were greater than 45 MeV werechosen based on a Monte-Carlo study. This conditionis called shower trigger hereafter. R is a correction
factor that compensates for the diﬀerence in the number of collisions between up and down polarization.
∗1RHICf raw asymmetry by scaler counts (> 20 min)Raw asymmetry

A new experiment, RHIC forward1) (RHICf), hasmeasured the transverse single spin asymmetry,AN ,√of very forward particle productions in s = 510 GeV
polarized proton-proton collisions at the Relativistic
Heavy Ion Collider (RHIC) in June, 2017.2) AN is deﬁned as a left-right asymmetry of the production crosssection to beam polarization. It plays an importantrole in the study of the production mechanism of very
forward particles, particularly from the view points ofdiﬀractive and non-diﬀractive interactions. The AN ofmainly very forward neutrons, π 0 , and γ with a transverse momentum (pT ) up to 1 GeV/c can be studied
in detail in the RHICf experiment.
In order to measure the AN of very forward particles
precisely, a new electromagnetic calorimeter (RHICfdetector), which had been originally developed for theLHCf experiment at CERN,3) was installed in frontof a hadron calorimeter4) (ZDC) at STAR to improve
the position resolution of the detected particles. The
RHICf detector consists of small (TS) and large (TL)
towers. Each tower is composed of 17 layers of tungsten plates, 16 layers of GSO plates, and 4 positionsensitive layers of thin GSO bars. ZDC has been used
as a polarization monitor by calculating the raw asymmetry (ϵN ) of neutron-like events using a STAR scaler
board. One scaler board is composed of 32 bits, andeach bit has a ﬂag of 0 or 1 for every entry dependingon whether an event satisﬁes a speciﬁc condition. Finally, it gives the number of counts for each of 232 typesof events. However, because the ZDC was screened by
the RHICf detector at operation, there was no guarantee that the ZDC monitor works as well as it used
to. Owing to the non-negligible interaction length ofthe RHICf detector, a hadronic shower was expected
to be generated in 30% of the neutrons. Therefore, weconnected two scaler bits to each tower of the RHICfdetector and measured the ϵN of each by

RIKEN Nishina Center
Department of Physics, Korea University- 114 -1) RHICf Collaboration: LOI, arXiv:1401.1004; RHICf
proposal: arXiv:1409.4860.
2) J. S. Park for the RHICf Collaboration, in this report.
3) LHCf Collaboration, JINST 3, S08006 (2008).
4) C. Adler et al., Nucl. Instrum. Meth. A 470, 488 (2001).
5) A. Adare et al., Phys. Rev. D 88, 032006 (2013).