Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 49 (2016)

RHICf experiment to measure cross section and asymmetry in very
forward neutral particle production at RHIC
T. Sako∗1 for the RHICf Collaboration
The origin of cosmic rays is a long-standing mysteryin astrophysics. To determine the energy and particle type of the primary cosmic ray from the observed
air shower particle distributions, we rely on the Monte
Carlo simulation of air showers. However, the lack ofknowledge in hadronic interaction modeling results inuncertainty in this interpretation. To understand thisprocess, the Large Hadron Collider forward (LHCf)
experiment1) measured forward particle production atthe LHC up to the √particle energy in a ﬁxed target setting of 9×1016 eV ( s=13 TeV). LHCf
reported a scal√
ing of π 0 production spectra at s=2.76, and 7 TeV
proton-proton collisions2) ; in addition, it continues analyzing 13 TeV data. To conﬁrm the scaling or its violation in a wider energy range and to extrapolate theknowledge beyond the LHC energy range, the RHICfcollaboration was launched.
Another mystery in forward particle production, single transverse-spin asymmetry (SSA), was found in theRHIC experiments3) . It is believed that the SSA inneutron production originates from the interference between the amplitudes of spin-ﬂip pion exchange andnon spin-ﬂip meson exchange. SSA can shed light on
the fundamental process of forward particle production
and improve our understanding of air shower development. The excellent position resolution of the RHICf
detector improves the measurement accuracy of SSA
and enables us to test the proposed mechanism4) .
For RHICf, an LHCf detector is installed in front of
the zero degree calorimeter of the STAR experiment,Fig. 2. Expected photon spectra observed in 12h operation
for three diﬀerent hadronic interaction models.

as shown in Fig.15) . The proton-proton collisions with√
s =510 GeV at the RHIC enable RHICf to study particle production at the cosmic-ray equivalent energy of
1.4×1014 eV. Figure 2 shows the expected photon spectra observed in RHICf operation for 12h. It is foundthat the statistical errors are negligibly small with respect to the diﬀerence between predictions by threemajor hadronic interaction models. RHICf will alsomeasure spectra, or diﬀerential cross sections, of π 0 ’s
and neutrons. In neutron measurement, RHICf has aposition determination resolution better than 1 mm6) ,while the former SSA measurements were performed
with 10 mm resolution.
Although the original plan for RHICf was to operate it in the PHENIX site7) , according to the long-termplan of RHIC, RHICf was ﬁnally approved by PAC in2015 to be operated in 2017 with STAR. The detailed
plan of installation, commissioning, and data collection were intensively discussed with the STAR team in2015. The STAR and RHICf teams have exchanged an
MOU in early 2016.
References

Fig. 1. CAD drawing of the RHICf installation. TheRHICf detector can ﬁt in the narrow slot of the radiation shield near the STAR interaction point.
∗1

Institute for Space-Earth Environmental Research /Kobayashi-Maskawa Institute for the Origin of Particles andthe Universe, Nagoya University- 112 -

1) LHCf Technical Design Report, CERN-LHCC-2006004.
2) O. Adriani et al.: PRD received, arXiv:1507.08764 [hepex].
3) Y. Fukao, M. Togawa, et al.: Phys. Lett. B 650,325 (2007).; PHENIX Collaboration, Phys. Rev. D 88,032006 (2013).
4) B. Z. Kopeliovich, I. K. Potashnikova, I. Schmidt, andJ. Soﬀer: Phys. Rev. D 84, 114012 (2011).
5) RHICf Collaboration, LOI, arXiv:1401.1004; proposal,arXiv:1409.4860.
6) K. Kawade et al.: JINST, 9 P03016 (2014).
7) Y. Goto for the RHICf Collaboration: RIKEN Accel.
Prog. Rep. 48, 107 (2015).