Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 53

Development of zero-degree calorimeter for EIC
Y. Goto,∗1 T. Chujo,∗2 Y. Itow,∗3 J. H. Lee,∗4 H. Menjo,∗5 Y. Miyachi,∗6 I. Nakagawa,∗1 K. Nakano,∗1,∗7
T. Sako,∗1,∗8 R. Seidl,∗1 T. -A. Shibata,∗1,∗7,∗9 K. Tanida,∗1,∗10 and Y. Yamazaki∗11EIC, physics programs require the tagging of forward
neutrons as spectators to identify the target nucleon.
It constrains kinematics for studies of the short-range
correlation (SRC). The SRC is a nucleon-nucleon interaction at a very short distance. It shows how nucleons form a nucleus, and has a deep connection to
how the quark-gluon structure of a nucleon in a nucleus is modiﬁed, which is a phenomenon known as
the EMC eﬀect.3) Experiments have shown it is universal that ∼20% of nucleons are in SRC pairs. These
SRC pairs have high momentum and are spatially very
close to each other. Almost all of these SRC pairs arefound to be similar to a quasi-deuteron at its highmomentum tail. In addition to the SRC study in electron + nucleus collisions, we will be able to understandthe deuteron as a baseline of the SRC pair in electron+ deuteron collisions at zero degree.
In electron + proton collisions, the leading protonand neutron productions in DIS were measured, andtheir production mechanisms were studied at HERA
by comparison with the fragmentation process andone-pion exchange (OPE) process. The results support that the OPE process dominates the production,
but there are still tension in detailed understanding of
the mechanism and comparison between ZEUS4) andH15) data. In addition to the production cross section measurement, the spin asymmetry measurementof the leading baryons in polarized electron + proton
collisions will give us useful additional information forthe study of the production mechanism. The data
will also be used to understand the energy ﬂow anddevelopment of the event generator, and applied forunderstanding the air shower evolution of high-energy
cosmic rays and the neutrino interaction.
We will conduct the following studies in the future;1) a photon detector study at a low energy <300 MeV,
2) a prototype study of ZDC with position sensitivity,and 3) a radiation hardness study of scintillators. We
would like to maximize the physics capability by studying detector design, development, and simulation.

We propose the development of zero-degree apparatus in the Electron-Ion Collider (EIC) experiment.
Zero-degree detectors play critical roles in a numberof important physics topics at EIC.1) We will study
the requirements and technologies of zero-degree detectors and mainly develop a position-sensitive zerodegree calorimeter (ZDC).
In electron + nucleus collisions, exclusive vector meson production in a diﬀractive process is one of thekey measurements at EIC. For the coherent process
in which the nucleus remains intact, the cross section dependent on the momentum transfer (t) can betranslated to the transverse spatial distribution of gluons in the nucleus and is thus considered to be directly sensitive to the gluon saturation. This requires
accurate determination of the coherence of the reaction, which must be determined by identifying thebreakup of the excited nucleus. Evaporated neutrons
from the breakup in the diﬀraction process can be used
to separate the coherence with the highest probability(∼90%). In addition, photons from the de-excitationof the exited nucleus signal incoherence in the absenceof evaporated neutrons. This leads to a requirement tomeasure neutrons and photons precisely at near zero
degree to complete the coverage of coherence tagging
in a wide t range.
Collision geometry is an important measure in electron + nucleus collisions for an event-by-event characterization. It has been proposed that collision geometries can be tagged through forward neutron multiplicities emitted at near zero degree.2) Constraining
collision geometry quantities such as the “travelinglength” of the struck parton in the nucleus, which iscorrelated with the impact parameter of the collision,is very meaningful in studies of nuclear medium effects. With the determination of collision geometry inthese measurements, higher-precision constraints canbe achieved to enhance our understanding of the nuclear structure.
In electron + deuteron and helium-3 collisions at∗1

RIKEN Nishina Center
Tomonaga Center for the History of the Universe, Universityof Tsukuba∗3 Institute for Space-Earth Environmental Research, NagoyaUniversity∗4 Physics Department, Brookhaven National Laboratory∗5 Graduate school of Science, Nagoya University∗6 Department of Physics, Yamagata University∗7 Department of Physics, Tokyo Institute of Technology∗8 Institute for Cosmic Ray Research, University of Tokyo∗9 College of Science and Technology, Nihon University∗10 Advanced Science Research Center, Japan Atomic EnergyAgency∗11 Graduate School of Science, Kobe University

References∗21) A. Accardi et al., Eur. Phys. J. A 52, 268 (2016)[arXiv:1212.1701 [nucl-ex]].
2) L. Zheng, E. C. Aschenauer, J. H. Lee, Eur. Phys. J. A
50, 189 (2014) [arXiv:1407.8055 [hep-ex]].
3) B. Schmookler et al., [CLAS Collaboration], Nature566, 354 (2019).
4) S. Chekanov et al., [ZEUS Collaboration], Nucl. Phys.
B 776, 1 (2007) [hep-ex/0702028].
5) F. D. Aaron et al., [H1 Collaboration], Eur. Phys. J. C68, 381 (2010) [arXiv:1001.0532 [hep-ex]].

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