Research Activities - Experimental Nuclear and Hadronic Physics Laboratory
Properties of hadrons, such as masses , interactions and so on, are expected to change inside nuclei due to many body effects and the partical restoration of chiral symmetry. Such modifications of hadron properties are called medium effects. We are searching the phenomena of medium effects and their systematics using the elastic, inelastic scatteing of protons and (p,2p) reactions at intermediate energies.
Proton scattering at the incident energy of 300 MeV to 400MeV is suitable to probe the nuclear interior because the mean free path of protons in nuclei is longest in this energy region and direct processes are dominant in the nuclear reaction. We have developed the unique focal plane polarimeter, with which we can measure spin polarizations even at zero degrees, so that we have precisely investigated the spin excitation in nuclei on both sides of the interaction and the structure. Particularly, at zero degrees the total spin transfer can completely decompose the spin-flip and non-spin-flip excitation. We intend to make various modes of resonances clear microscopically by adding decay coincidence measurements.
Isospin flip excitations in nuclei are closely related with beta decays in weak interactions. For instance, Gamow-Teller transition, in which both the spin and the isospin are flipped and the angular momentum transfer is equal to zero, can be well investigated by charge exchange reactions at zero degrees. As both incident and outgoing particles are ions in such a reaction of (3He,t), the detection efficiency and the energy resolution are high enough. Then the coincidence measurements with decay protons or photons from resonance states are powerful tools for us to elucidate these microscopic structures. Moreover the transition strength determined by these reactions can give the calibration of neutrino detectors which utilize inverse beta decays. In order to study the excitation of beta+ side, the (t,3He) experiments have also performed using the secondary triton beam.
The nuclear deep hole state is one of the high excitation states in nuclei whose microscopic structure has not yet been well investigated as well as those of giant resonances. This is correspond to the state in which one nucleon of the deepest 1s state in the shell model is lacked. We have measured decay branching ratios of p,d,t,alpha particles from s-hole states of 11B and 15N which were excited by the 12C,16O(p,2p) reactions. In light nuclei, it is theoretically predicted that the decay scheme is different from the simple statistical decay due to the SU3 spatial symmetry. The deep hole state can give useful information on the decay scheme of hyper-nuclei that are produced by exchanging a nucleon with a Lambda particle and is also relevant to the measurement of nucleon decay in the large water cherencov system. Data analyses are in progress.
4He nucleus is expected to be have much higher density than the normal saturation nuclear density. In order to get information from the high density, we have performed the experiment of elastic proton scattering from 4He with 300 MeV polarized proton beam of RCNP ring cyclotron, and we have microscopically analyzed the data with a relativistic impulse approximation.
Nuclear incompressiblity is one of the key physical quantities to discern various nuclear models describing the fundamental nuclear properties and phenomena, such as nuclear binding energies, explosions of super nova and so on. Nevertheless experimental and theoretical values contradict each other depending on their models adopted. We have recently established a technique to obtain a clean energy spectra of inelastic alpha particles in the giant resonance region at extremely forward angles including zero degrees, and are trying to fix the value of nuclear incompressibilities from the splitting of the Giant Monopole Resonances(GMR) in deformed nuclei and isoscalar giant dipole resonances in addition to the ordinary GMR's.
We measure phi-meson decays in nuclear media, both in the mesonic and leptonic decay modes. By observing both decay modes simultaneously and by examining a possible change of the invariant mass specra and the branching ratio, mass shift of K and phi in nuclear media will be studied. The phenomena can be interpreted as a consequence of the partial restoration of the chiral symmetry in nuclear media.
When high energy hadrons collide with a heavy target nucleus, low energy complex fragments with A>6 are frequently emitted. This process is called target multifragmentation and is expected to provide information on the thermodynamic properties of nuclear system at high temperature, for example nuclear liquid-gas phase transition. We, Multi group, have been systematically investigating the target multifragmentation reactions by using GeV-energy hadron beams obtained from the proton synchrotron (PS) at KEK and the heavy ion synchrotron (HIMAC) at NIRS.
Study of nuclei with strangeness and hyperon-nucleon interaction is one of the frontier of nuclear physics today. We have developed novel experimental devices and methods such as scintillating fiber detector, new hybrid-emulsion method and Ge-ball (Hyper-ball) to search for new nuclei and quark matter and to determine the hyperon-nucleon interactions. We are carrying out several experiments at KEK and BNL. Experiments are in preparation at SPring8.
Quantum Chromodynamics (QCD) predicts that heavy nuclei collided at ultra relatavistic energies will undergo a phase transition from ordinary hadronic matter to a quark-gluon plasma (QGP). Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory is built to create QGP, a deconfined high-energy-density phase of matter. Besides the QGP detection, RHIC is also used for polarized proton collisions to uncover the secrets of the spin structure of the proton. Currently it is known that the three quarks do not carry all of the spin of the proton. The rest of the spin might be carried by the gluons, sea quarks or by some as yet undiscovered mechanism.
In order to exploit fully the capability of the magnetic spectrometer of Grand Raiden( momemtum resolution of 1/37000 ), we are constructing a new beam line for the dispersion matching and angular matcching in collaboration with a group in RCNP and the group of Osaka Univ. In April 2000 the new cource will begin to provide a clean, halo free beam for the giant resonance study as well as a high resolution beam.
In RIKEN a new facility for accelerating unstable nuclei up to 400MeV/A is under construction. By using the inverse kinematics we could measure 400 MeV proton elastic scattering from the unstable nuclei and obtain the information of density distribution for nuclear structure study. A preparatory work for the recoil particle spectrometer is on going.
A Laser-Electron-Photon facility which produce the multi-GeV polarized photons by inverse Compton scattering of the short wave-length laser from 8 GeV electrons is under construction at SPring-8. At the end of June, 1999, the first laser-electron-photons have successfully been obtained and some experiments of the quark-nuclear physics will be started from this autumn. We are mainly taking care of large drift chambers for the tracking of outgoing charged particles by the reaction and are checking them.
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