RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

Expected energy resolution and its trigger dependence for very√
forward neutron measurements in s = 510 GeV polarized
proton-proton collisions at the RHICf experiment
M. H. Kim∗1,∗2 for the RHICf collaboration↑AN =↓

dσ − dσdσ ↑ + dσ ↓(1)

∗2(b)

Trigger efficiency
E resolution (Only RHICf)

90

E resolution (Both)

90

80

E resolution (ZDC)

80

70

70

60

60

505040403030

2020

RIKEN Nishina Center
Department of Physics, Korea University- 220 -00

Trigger efficiencyE resolution (RHICf only)
350 GeV proton (RHICf only)
E resolution (Both)E resolution (ZDC only)

100(%)

100

10for yields observed with proton polarization, wheredσ ↑ (dσ ↓ ) is the production cross section when the protons are polarized up (down). To measure the ANof very forward neutrons precisely, an electromagnetic
calorimeter was installed in front of the ZDC hadroncalorimeter1) at the STAR experiment to improve thelimited position resolution of ZDC (∼ 1 cm). Thisdetector was originally developed for the LHCf experiment2) at CERN and will be called the RHICf detector3) hereafter. RHICf is not designed to provide
good energy resolution to neutrons with an interaction
length of 1.6 λ. However, its energy resolution can bewell recovered by compensating for the shower leakage of RHICf caused by ZDC, the interaction lengthof which is 2 λ. Here, we report the result of a MonteCarlo (MC) study of the expected performance of neutron energy measurement when both RHICf and ZDC
are used together as well as its trigger dependence withrespect to the threshold energy.
In the MC study, 106 neutrons with energies of 100,150, 200, 250, and 350 GeV were generated as it passed
through the center point of the ﬁrst GSO plate andtheir energies were reconstructed using the correlation
between energy to energy deposit (edep) sum of RHICf
and the total number of photoelectrons (Npe) of ZDC.
Energy resolution was evaluated in the following threecases: 1) RHICf only, 2) ZDC only, and 3) the combined system.
Simulation results for the threshold energy dependence of neutron energy resolution and the ratio of theselected to the total events at 250 GeV are shown inFig. 1 (a). A higher threshold energy results in lower
trigger eﬃciency and better energy resolution. The
red dashed line indicates the intrinsic ZDC resolution(18.2%). The energy resolution becomes as good as
the intrinsic resolution of ZDC even though the shower
leakage of RHICf is measured by ZDC and combinedlater.
∗1(a)(%)

The single spin asymmetry, AN , of very forward neutrons in polarized proton-proton collisions plays an important role in the study of the production mechanismof very forward neutrons, particularly diﬀractive andnon-diﬀractive interactions. AN is a left-right asymmetry,

10102030405060708090100

Trigger threshold energy (MeV)

050100150200250300350

400

Neutron energy (GeV)

Fig. 1. (a) Trigger-threshold-energy dependence of the expected neutron energy resolution and the trigger eﬃciency. (b) Incident-neutron-energy dependence at atrigger threshold energy of 90.6 MeV.

The energy resolution at diﬀerent neutron energies isplotted in Fig. 1 (b). The threshold energy of the trigger was set at 90.6 MeV in order to compare with a previous LHCf test result of 350-GeV protons shown as a
blue open triangle. However, it will be optimized laterfor only the RHICf experiment. The MC prediction,
37.5 %, for 350-GeV neutrons shows good consistency
with the LHCf result, 36.0 %, within statistical error.
A better energy resolution is obtained with a lower
neutron energy because the same threshold energy wasapplied to the trigger. When only ZDC is used, the resolution at 100 GeV also shows good agreement with aprevious
experimental result for polarized pp collision√at s = 200 GeV.4) Finally, because the simulation result corresponds with experimental results in both the1) RHICf only and 2) ZDC only cases, for neutrons
with an energy of 100-250 GeV in the RHICf experiment, we expect an energy resolution of approximately
20% with a threshold of 90.6 MeV when both detectors
are used.
References
1) C. Adler et al., Nucl. Instrum. Meth. A470, 488 (2001).
2) Y. Itow, H. Menjo, T. Sako, N. Sakurai et al.,arXiv:1409.4860 [physics.ins-det].
3) K. Kawade et al., arXiv:1312.5950v1 [physics.ins-det].
4) A. Adare et al., Phys. Rev. D 88, 032006 (2013).