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{\Large \bf Experimental Nuclear Physics Group}\\
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Professor          &:&Osamu~Hashimoto~and~Toshio~Kobayashi\\
Associate Professor&:&Hirokazu~Tamura,~Naohito~Iwasa~and~Satoshi Nakamura\\
Assistant Professor&:&Toshiyuki~Takahashi,~Yuu~Fujii~and~Hideaki~Otsu\\
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(http://lambda.phys.tohoku.ac.jp/)
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Our group is experimentally investigating 
new types of nuclei in extreme conditions,
{\bf hypernuclei} and {\bf proton/neutron-rich nuclei}. 
Hypernuclei are composed of hyperons (=baryons having ``strangeness'' 
quantum number) such as $\Lambda$ and $\Sigma$ particles
in addition to protons and neutrons.
Proton/neutron-rich nuclei are unstable nuclei having a large asymmetry 
in the proton/neutron numbers. 
We utilize accelerator 
facilities around the world use various beams of $\pi$/$K$ mesons,
electrons, and radioisotope nuclei.
We are also designing experiments and developing apparatus
for the J-PARC 50 GeV PS (proton synchrotron) in Tokai
and the Radio Isotope Beam Factory at RIKEN,
which will be available in near future.

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\noindent
{\bf 1. Hypernuclear Physics}

The goal of hypernuclear physics 
(or ``strangeness nuclear physics'') is
the fundamental understanding 
of many-body hadron systems from the quark level
through strangeness.
We can explore deeply inner regions of nuclei
with a hyperon which is free from Pauli principle. 
In addition, various modification of nuclear structure induced by
a hyperon often explodes our common knowledge in nuclear physics.
Structure of hypernuclei enables us to study
hyperon-nucleon interactions,
which extend our knowledge on nuclear force toward
unified understanding of baryon-baryon interactions.
The weak interaction between baryons can also be investigated 
through weak decays of hypernuclei. 


Our group is one of the major research groups
for strangeness nuclear physics in the world.
We are carrying out  
pioneering and unique experimental studies.
(1) Recently, we succeeded in high-resolution $\Lambda$ hypernuclear
spectroscopy by the $(e,e^{\prime}K^+)$ reaction 
for the first time at Jefferson Lab in U.S.
We are now constructing the High-resolution Kaon Spectrometer (HKS), 
with which improved experiments will start in 2004.
(2) We are studying structure and weak decays of $\Lambda$ hypernuclei
using $\pi$ meson beams and the Superconducting Kaon Spectrometer (SKS)
at KEK-PS.
(3) We recently constructed a large germanium
detector array (Hyperball) and measured
hypernuclear $\gamma$ rays for the first time.
With this new method,
precise structures of various $\Lambda$ hypernuclei
are being studied at KEK-PS and 
Brookhaven Lab in U.S. 
(4) In order to study mechanism of strangeness production
in electromagnetic interaction, we are investigating $K^0$
production by GeV $\gamma$-ray beams  
at Laboratory of Nuclear Science in Tohoku University.

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\noindent
{\bf 2. Exotic Nuclear Physics with RI Beams}

Another extreme form of nuclei is found at the extreme neutron/proton 
ratio. Those nuclei are far from the stability line 
and located near the neutron(proton) drip line.
Unlike many stable nuclei 
the main features of such exotic nuclei are governed by the small number of
active nucleons.  Interesting phenomena are observed 
such as neutron halos (skins),
soft giant dipole resonances, changes of the magic numbers, etc.
Recently, it has become possible not only to produce such nuclei
but also to use them as high-intensity high-purity secondary nuclear
beams (RI beams).
The group conducts experiments of those exotic nuclei
using the accelerator facilities at RIKEN and National Institute 
of Radiological Sciences. We study
(1) nuclear radius from reaction cross sections,
(2) single particle states by nucleon knockout reactions,
(3) resonance states just outside of the drip lines,
(4) giant resonances by forward inelastic scattering of proton, and
(5) nucleosysthesis from cross sections of 
nuclear reactions important inside stars.

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