Research Group for Hadron Nuclear Physics
Group Leader : TAMURA Hirokazu
Members : SAKO Hiroyuki, HASEGAWA Shoichi, TANIDA Kiyoshi, SATO Susumu,
ICHIKAWA Yudai, HASHIMOTO Tadashi, NARUKI Megumi, YAMAMOTO Takeshi, FUJITA
Manami, HAYAKAWA Shuhei, NANAMURA Takuya, UJIIE Ryosuke, NAKASUGA Satomi,
ASHIKAGA Sakiko, HINO Yota, TAKAURA Yudai, MARUYAMA Toshiki, Philipp Gubler,SUZUKI Kei, MIAKE Yasuo, TANAKA Kazuhiro, IMAI KenIchi, NAGAMIYA Shoji, ESUMIShinIchi, OYAMA Ken

The research objectives of the Hadron Nuclear Physics Groupare experimental studies of 1) exotic hadrons and nuclei withstrange and charm quarks and 2) hot and dense partonic/hadronicmatter at J-PARC, BNL-RHIC, and Belle (II). Through these
topics, we investigate many-body problems of quarks andhadrons in relation to Quantum Chromo Dynamics (QCD).
Hadron experimental studies at J-PARC
In J-PARC E07 experiment (studies of double Λ hypernuclei),−
we observed an event for Ξ− nucleus bound system (Ξ + 14N→Ξ 15C) named “IBUKI-event” as shown in Fig. 1 [1]. The event
was uniquely interpreted as a 15ΞC hypernucleus decaying into105−
ΛBe and ΛHe. The binding energy of Ξ was obtained to be 1.27
− bound in the nuclear 1p state. This±0.21 MeV, suggesting a Ξis the first precise determination of the Ξ− binding energy of Ξhypernuclei. For details, refer to the research highlight article inthis volume.
In preparation for the Hyperon Spectrometer for E42 (Hdibaryon search), E45 (baryon spectroscopy with πΝ → ππΝreactions), and E72 (search for a new Λ resonance),
commissioning runs for the water Cherenkov counter (WC) andHypTPC were performed at J-PARC K1.8 experimental area
during E40 beam time in June 2020 and Feb. 2021. As a result,the efficiencies of π, K, p with WC were evaluated to fulfil therequirement of E42. HypTPC also had good track reconstruction
performance. E42 beam time is scheduled in June-July 2021.

Fig.1: A hypernuclear event of Ξ- + 14N → Ξ15C.

For J-PARC E03 (Ξ atom X-ray spectroscopy), we tuned thesecondary kaon beamline (K1.8) in June 2020 and finished thedetector installation and tuning in Dec. 2020. We started thephysical run in Dec. 2020 but it was suspended in Feb. 2021 due
to a trouble in J-PARC Main Ring synchrotron. The experimentwill be resumed in Apr. 2021 to complete the data taking.

J-PARC E16 aims at studying in-medium modification of the
φ meson spectrum through φ decay into e+e- inside the nucleus.
The first and second commissioning runs of E16 were performedin June 2020 and Feb. 2021, with the proton beams transported in
the newly-constructed high-momentum beamline. We confirmed
performance of the proton beams such as the beam profiles. We
also successfully operated the tracking detectors (GEM Trackers
and Silicon Strip Detectors) and electron identification detectors(Hadron Blind Detectors and Lead Glass Calorimeters). In
addition, we tested prototype detectors for charged hadron
identification with the time-of-flight measurement: a high timing
resolution chamber (Multi-gap Resistive Plate Chamber) andstart timing counters for φ→K+K- measurement, and good timing
resolution of ~70ps was obtained in the time-of-flightmeasurements between these detectors. A Letter-of-Intent for theφ → K+K- measurement was submitted to J-PARC Program
Advisory Committee (PAC) in Dec. 2020.
Other research activities
We propose a future heavy-ion beam program at J-PARC (JPARC-HI) to create dense baryonic matter to search for a phaseboundary and the QCD critical point in the QCD phase diagram.
We organized the J-PARC-HI Working Group to aim atsubmitting the first experimental proposal to J-PARC PAC.
We found a large left-right asymmetry of neutral pions at very
forward angles in polarized proton–proton collisions at theRHICf experiment, which will change theoretical interpretationof the particle production mechanism [2]. We also participate in
Belle and Belle II experiments to study hadrons with heavyquarks. Belle II achieved the world’s highest luminosity in June
2020.
Reimei research programs
The Reimei research program “Systems with two strange quarks
at FAIR and J-PARC” was proposed to strengthen the cooperation
between scientists working at J-PARC and FAIR, the only two
facilities in the world for the two-strange quark systems, throughan international collaboration with Prof. J. Pochodzalla (Univ. ofMainz), organized by Mainz Univ. Helmholtz Lab., JAEA,Tohoku Univ., and Kyoto Univ. We held weekly seminars
between our Reimei research program and the THEIA program
funded by EU, including discussions on research plans to search
for X-rays from heavy hyperatoms for PANDA experiments at
FAIR and the J-PARC experiments E03, E42, E45, and E70.
References[1] S. H. Hayakawa, et al, Phys. Rev. Lett. 126, 062501 (2021).
[2] M. H. Kim, et al, Phys. Rev. Lett. 124, 252501 (2020).