Research Group for Hadron Physics
Group Leader : Ken-ichi Imai
Members : Toshiki Maruyama, Hiroyuki Sako, Susumu Sato, Shoichi Hasegawa, Kotaro Shirotori, Ryuta Kiuchi,Kiyoshi Tanida, Hitoshi Sugimura, Yudai Ichikawa, Minoru Okamoto

Experimental search for pentaquarks Θ+The Θ+ baryon has been searched for via the π−p → K−X
reaction using a 1.92 GeV/c π− beam at the J-PARC−K1.8 beamline. The K1.8 beam line spectrometer and the SKS spectrometershown in Fig. 1, both of which have an excellent momentum
resolution, were used. In the missing mass spectrum, noprominent peak has been observed (Fig. 2) in the Θ+ mass region[1]. The preliminary upper limit of the differential cross section
averaged over the scattering angle from 2 to 15 degrees at thelaboratory frame was less than 0.3 μb/sr at the 90% conﬁdence
level in the missing mass region of 1.51–1.55 GeV/c2.
Study of the exotic state with strangeness; H-dibaryon
One of the exotic hadronic states, composed of 6 quarks, isH-dibaryon (“uuddss” quark state). In order to observe such astate, a large acceptance tracking spectrometer is indispensable.
We have been doing R&D for a Time Projection Chamber(TPC), and Silicon Strip Detectors (SSDs) with Seoul National
University and Scintillating Fiber trackers (SciFi) with Tohoku
University as high-rate pion and kaon beam line trackers. A
beam test has been performed at RCNP in Nov. 2011, and goodperformance (at 106 of incident beam rate) of these detectors
was obtained (e.g., in TPC, a few 100s micrometer resolution
was achieved). Design of the spectrometer is progressing incollaborating with Pusan National University.
Study of heavier flavour (charm/bottom) in heavy ion
collision
We have developed and installed a vertex pixel detector incollaboration at BNL-RHIC PHENIX experiment, to extrapolate
the signal from particle with charmed and/or bottom ﬂavours. In
2011 and 2012, we have accumulated collision data of Au + Au
and Cu + Au.
Numerical study of inhomogeneous structures in nuclear
matterWe have numerically explored the pasta structures [3] andproperties of low-density nuclear matter without any assumption
on the geometry. We observed conventional pasta structures,
while a mixture of the pasta structures appeared as a metastablestate at some transient densities. Proton density distributionsof the ground states of symmetric matter are shown in Fig. 3.

Typical pasta structures were observed: (a) Spherical dropletsat baryon density of 0.01 fm −3. (b) Cylindrical rods with ahoneycomb crystalline structure at 0.024 fm −3. (c) Slabs at
0.05 fm−3. (d) Cylindrical tubes with a honeycomb crystalline
structure at 0.08 fm−3. (e) Spherical bubbles at 0.09 fm−3.

Group Activities

The research objectives of Hadron Physics Research Group
are: (i) empirical study of structure of nuclei and hadrons withstrangeness, with use of high intensity K(π)-beams at J-PARC,and (ii) theoretical study of nuclear matter, hadrons withstrangeness, and high density matter realized in neutron stars.
In JFY 2011, we have (1) searched for pentaquarks Θ+ at
J-PARC (E19) and established new upper limits, (2) proposed anexperiment for H-dibaryon at J-PARC (P42), and (3) developedand installed vertex pixel detector for charm/bottom tagging at
BNL−RHIC PHENIX experiment to study high energy heavy
ion collisions. Also we have studied (4) high density matters intheoretical simulations, in which phase transitions of baryon density
distribution has been found at the surface of the neutron star.

Fig. 1 K1.8 beam line spectrometer and the SKS spectrometer
at J-PARC.

Fig. 2 Missing mass spectrum distribution via π− p → K−X
reaction using a 1.92 GeV/c π− beam. The red histogramshows corresponding background.

Fig. 3 Proton density distributions of the ground states ofsymmetric nuclear matter. For details, see the text.
References[1] K. Shirotori et al., The Fifth Asia-Pacific Conference on
Few-Body Problems in Physics, APBF2011 (2011).
[2] Program Advisory Committee (PAC) for Nuclear andParticle Physics Experiments at the J-PARC 50 GeV Proton
Synchrotron: http://j-parc.jp/researcher/Hadron/en/PAC_for_NuclPart_e.html, J-PARC PAC (2011).
[3] T. Maruyama and T. Tatsumi, J. Phys. Conf. Ser. 312,042015 (2011).

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