Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 48 (2015)

Cross section and asymmetry measurement of very forward neutral
particle production at RHIC
Y. Goto∗1 for the RHICf Collaboration
Although air shower observations at the surface ofthe earth have been carried out to understand the origin of the ultrahigh energy cosmic rays, these observations have uncertainties in the interpretation of theobserved data from the present phenomenological nuclear collision models. The LHCf experiment has been
performed at LHC for understanding cosmic ray generation from the collider experiment data1–3) . Precision
measurements of the very forward particle productionin the collider experiments improve the understandingof particle production processes in the nuclear collisions, and largely affect the interpretation of the observed data and the origin of the cosmic rays.
A large 10% single transverse-spin asymmetry(SSA) in neutron production from transversely polarized proton collisions was found at RHIC in 20024) .
This provides an important clue to study elementary
processes in air shower generation because the large
SSA indicates that there are dominant contributionsfrom processes that strongly interfere with each other.
Because the SSA measurement of neutron production
provides interference measurement with one pion exchange with a spin flip, it is sensitive to a mesonexchange without spin flip5) . Although the SSA ofvery forward neutrons at collision energies of 62 GeV,
200 GeV, and 500 GeV was measured at RHIC, thetransverse-momentum resolution of the data was limited.
We will perform a new collider experiment at RHIC,
the so called RHICf experiment, which uses a LHCf detector (which is called the RHICf detector) with a highresolution and wide coverage of transverse-momentum
measurements in order to improve the studies performed at RHIC and LHC. RHIC is a dedicated machine for QCD physics and it allows flexible operationto achieve our physics goals. In the RHICf experiment, we will have an improved transverse-momentum
resolution using the RHICf detector. The LHCf experiment has measured very forward neutral particle
cross section at collision energies of 0.9 TeV and 7 TeV.
We will obtain precision measurements of very forward
neutral particles (neutron, photon, neutral pion) at acollision energy of 510 GeV at RHIC. These data ofvarious collision energies will provide an understanding of air shower generation and the limitations in theorigin of the ultrahigh energy cosmic rays.
The RHICf detector will be located in front of theZero-Degree Calorimeter (ZDC) in the north side, 18m from the PHENIX collision point. In this area, allcharged particles are swept out by the last dipole mag∗1Fig. 1. Schematic view of the RHICf detector installationand the structure of the calorimeter.

net for colliding beams at RHIC, and only neutral particles are detected. The RHICf detector consists of
two sampling EM (electromagnetic) calorimeters with
44 interaction-length tungsten, 16 layers of scintillators, and 8 layers of silicon strip detectors; one with
25 mm × 25 mm and another with 32 mm × 32 mm
cross sectional area. The ZDC detector is a sampling
hadron calorimeter composed of Cu-W alloy absorbers
with PMMA (Polymethyl methacrylate)-based opticalfibers. The detection, identification, and energy measurement of photons and neutrons are performed using
EM and hadron calorimeters. A schematic view of theRHICf detector installation and the structure of thecalorimeter are shown in Fig. 1.
We have proposed to perform a dedicated run for
1 week in 2016 with 510 GeV polarized proton collisions6) , and we are preparing for it. In the dedicated
RHICf run, we will use the normal accelerator condition of the RHIC, except the beta function value
at the PHENIX collision point, β ∗ = 10 m, in orderto obtain parallel beam collisions. We need 12 h toachieve sufficient luminosity for cross section measurements of photons, neutrons, and neutral pions including the transverse-momentum scan. The SSA can bemeasured with the same data set with higher statistics
than the measurements in the past.
References
1) O. Adriani et al., Phys. Lett. B 715, 298 (2012).
2) O. Adriani et al., Phys. Rev. D 86, 092001 (2012).
3) O. Adriani et al., Phys. Lett. B 703, 128 (2011).
4) Y. Fukao, M. Togawa et al., Phys. Lett. B 650, 325(2007).
5) B. Z. Kopeliovich, I. K. Potashnikova, I. Schmidt andJ. Soffer, Phys. Rev. D 84, 114012 (2011).
6) Y. Itow et al. [RHICf Collaboration], arXiv:1409.4860[physics.ins-det].

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