Subatomic Physics at RCNP
T. NAKANO1, M. FUKUDA1, N. AOI1, M. YOSOI1, A. SATO1,2, S. UMEHARA1 AND A. HOSAKA1
1 RESEARCH CENTER FOR NUCLEAR PHYSICS, OSAKA UNIVERSITY, JAPAN
2 DEPARTMENT OF PHYSICS, FACULTY OF SCIENCE, OSAKA UNIVERSITY, JAPAN
The Research Center for Nuclear Physics (RCNP) was founded in 1971 at Osaka University as a national research center with a small AVF cyclotron (K=140 MeV) for nuclear physics. The current RCNP is a laboratory complex consisting of an upgraded cyclotron facility for nuclear physics, a laser-electron photon facility for hadron physics, a DC muon beamline for muon science, underground laboratories for neutrino physics, and a supercomputer for theoretical and computational physics. The facilities are open to international users and many active research programs are ongoing as reported in the following sections.
FACILITIES AND ACTIVITIES
The cyclotron facility at RCNP has been dedicated to pioneering nuclear physics research since 1976. A K140 AVF cyclotron and a K400 ring cyclotron  provide a wide variety of ions and neutrons for various experiments in not only nuclear physics but also radiochemistry, nuclear medicine and application fields such as semiconductor devise testing. The AVF and ring cyclotrons cover an acceleration energy region of up to 420 MeV for proton and 100 AMeV for heavy ions. Recently a DC muon beam has become available. The ring cyclotron produces ultrahigh quality beams with an energy spread ΔE/E of an order of 10-4, which is achieved by a flat-top acceleration system using the third harmonic acceleration voltage and a high-precision temperature control system for the cooling water of the cyclotron main coils. A bird-eye view of the cyclotron facility is shown in Fig. 1.
Fig. 1: A bird-eye view of the cyclotron facility.
Light-ion beams, such as proton, deuteron, 3He and alpha beams, are of the highest quality in the world (low emittance with halo-free). These high quality beams, together with high resolution double arm spectrometers called 'Grand Raiden' and 'LAS', enables us to perform high resolution nuclear-reaction experiments including zero degree inelastic proton scatterings and charge exchange (3He, t) reactions. High-resolution (p, n) reactions are also performed using a 100 m neutron TOF tunnel. With these experimental capabilities, a variety of nuclear structure studies and reaction studies probed through spin/isospin response of nucleus are conducted.
Heavy ion beams are provided both directly from the AVF cyclotron and from the ring cyclotron. RI beams produced by projectile-fragmentation reactions or charge exchange reactions are available at typical energies from 10 to 100 AMeV, which are used for the structure study of unstable nuclei using particle and γ-ray measurements.
The intense proton beams are also suitable to produce secondary beams, such as fast quasi-monoenergetic and white neutrons, and muon beams, which are used for various application purposes, such as fundamental physics, engineering, nuclear chemistry, and nuclear medicine. A new project for realizing targeted RI therapy using 6 MeV alpha particles emitted from At-211 has been started through cooperation between the Graduate School of Medicine, the Graduate School of Science and RCNP at Osaka University in order to enhance the five-year relative survival rate in advanced cancer treatments.
LASER-ELECTRON PHOTON BEAM FACILITY (LEPS)
The LEPS group of the Research Center for Nuclear Physics has studied quark-nuclear physics with international collaborations, aiming at the experimental elucidation of the character of quantum systems composed of quarks and gluons, which are called 'hadrons', with use of the laser-electron photon (LEP) beam at SPring-8 . The LEP beam is a polarized GeV gamma-ray beam which is produced by backward Compton scattering of injected ultraviolet (UV) or deep-UV laser photons off circulating 8-GeV electrons.
The LEPS experiments have been performed since 2000. We have measured hadron photoproductions by irradiating the LEP beam to mainly liquid hydrogen and liquid deuterium targets and obtained many important results, such as a hint of the existence of a pentaquark. Until quite recently, only two groups of hadrons have been found and established, namely, the particle consisting of three quarks (baryon) and the particle consisting of a quark and an anti-quark (meson). Our group has observed a signal of an exotic baryon (named Θ+) that has an anti-strange quark for the first time in the world and has continued to search for it. This particle should have at least four quarks and an anti-quark. The proof of its existence and the clarification of its physical character are extremely important.
In 2013, the second LEP beamline (LEPS2) was constructed at SPring-8. The intensity upgrade is about one order of magnitude greater and the efficiency upgrade by the construction of large acceptance detectors are the key ingredients in the LEPS2 experiments. We are preparing a high statistics experiment for Θ+ as well as exotic hyperons like Λ(1405) at LEPS2 using a large solenoid spectrometer (Fig. 2).
Fig. 2: 1-T solenoid magnet for the LEPS2 large acceptance spectrometer.
A high resolution 4π-elecromagnetic calorimeter (called BGOegg), which was developed at the Research Center for Electron Photon Science, Tohoku University, has also been equipped at LEPS2 to detect photons decaying from π0, η, etc. It is known that the sum of the quark masses occupies only a few percent of a hadron mass. The hadron mass is thought to be generated dynamically from spontaneous breaking of a kind of symmetry (chiral symmetry). By some theoretical predictions, the meson masses are changed in the nuclear medium owing to the partial restoration of the symmetry in the finite density. We have started the experiment to study η´-mesons in nuclei using BGOegg as the first experiment at LEPS2.
Double polarization measurement for photo-reactions with a polarized target and polarized photon beam is a sensitive means to investigate processes with small amplitudes, such as a knockout of a hidden s-quark and anti-s-quark pair in φ-meson photoproduction. In order to realize double polarization experiments, we have been developing a polarized hydrogen-deuteride (HD) target. A long relaxation time of about three months and polarization of more than 40% for protons have been achieved so far and it will be used in the LEPS experiment in the near future.
DC MUON BEAMLINE (MuSIC)
A new DC muon beam facility, MuSIC (Muon Science Innovative Commission), has been built in the west experimental hall of RCNP .
A unique aspect of MuSIC is its pion/muon capture system. We adopted the world's first pion capture system, which enables MuSIC to provide an intense muon beam using a 400 W proton beam from the ring cyclotron. A 392 MeV-1µA proton beam is injected to a 20 cm long graphite target as a pion production target. The target is located under a 3.5 T solenoidal magnetic field created by a superconducting magnet. The pions and muons emitted from the target are captured by the field, then backward particles are transported through a transport solenoid channel, which can apply a 2 T solenoid field and an additional dipole field of up to 0.04 T. The charge and central momentum of the moun beam can be selected by changing the direction and magnitude of the dipole field. Fig. 3 shows a picture of the pion capture system. It was constructed in 2009 and its performance has been demonstrated by a series of beam tests. The measured muon intensity at the end of 36-degrees transport solenoid is 108 muons/sec for 400 W proton beam operations, which corresponds to more than 1000 times higher muon collection efficiency normalized by proton beam power than other muon facilities such as PSI and J-PARC-MLF. The successful demonstration of performance of the pion capture system of MuSIC will be followed by two more pion capture systems, COMET at J-PARC and Mu2e at FNAL, for more powerful proton beam applications.
In order to start muon science experiments with MuSIC, an 18 meter-long muon beam line, MuSIC-M1, was constructed as an extension of the MuSIC transport solenoid channel in 2013. It consists of dipole magnets, quadropole magnets, and a Wien filter, which can work as a DC separator and a spin rotator. They are normal-conducting magnets. The beamline was designed to provide 104~106 muons/sec with 400 W proton operation for both negative and positive muon beams. The available momentum range is 20~110 MeV/c. The beam line has been successfully commissioned and an early operation resulted in a positive muon yield of 7×105 muons/sec for 60 MeV/c for a 400 W proton operation. Further beam tuning is in progress toward an early start of operations for users' experiments.
Fig. 3: The pion/muon capture system of MuSIC.
DOUBLE BETA DECAY FACILITY (CANDLES)
Neutrino-less double beta decay is acquiring great interest after the confirmation of neutrino oscillation, for which this year's Nobel Prize in Physics honored Prof. T. Kajita and Prof. A. McDonald. Neutrino oscillation indicates the existence of neutrino mass. Measurement of the neutrino-less double beta decay gives an absolute scale of the effective neutrino mass.
The RCNP double beta decay project is the challenge to observe neutrino-less double beta decay . Neutrino-less double beta decay has a very long half-life, which is longer than 1024 years. In order to measure the decay, a detector system needs a large amount of double beta decay nuclei and low background conditions. Thus, we have developed the CANDLES III system (Fig. 4), which is installed at the Kamioka Observatory, Institute for Cosmic Ray Research (ICRR), University of Tokyo. In the CANDLES system, CaF2 scintillators, which are the main detectors, are immersed in a liquid scintillator. The liquid scintillator acts as a 4 π active shield to veto backgrounds. These scintillators are installed in a main tank.
Fig. 4: Photograph of CaF2 scintillators and photomultiplier-tubes. The CANDLES III system consists of 96 CaF2 scintillators (300 kg) and a liquid scintillator.
In 2014 and 2015, we began two upgrades to the CANDLES III system. The upgrades consisted of the installation of a cooling system and a shielding system. The cooling system makes light emission of the CaF2 scintillators increase, because CaF2 light emission increases at lower temperatures. The other upgrade, the shielding system, reduces the background events due to γ-rays from neutron captures. The upgrades will be completed in early 2016.
After the upgrades, we will start low background measurement for the study of double beta decay.
The theory group is run by three permanent staff members. In addition, there are two assistant professors, several postdoc researchers and graduate course students. A unique feature of the theory group activities is the drive to keep as much contact as possible with the experimental groups around RCNP, while being based on microscopic fundamentals of strongly interacting particles. The systems are fully quantum mechanical, where we also utilize the methods of field theories for many-body systems. The keywords shared by us are quarks, hadrons and nuclei from their microscopic dynamics to observed phenomena. To keep in touch with actual observations, how to solve and handle scattering equations are the common issues.
Kazuyuki Ogata conducts reaction dynamics for nuclear and astrophysics. The method is a QCD-based microscopic effective reaction theory (MERT) that has successfully been applied to various direct reaction processes . With MERT, nuclear structural information such as two nucleon correlations, the alpha cluster structure of nuclei, and the so-called shell evolution can unambiguously be extracted from experimental observables. On the other hand, reactions of loosely or quasi bound systems of unstable nuclei themselves are important in nuclear synthesis processes for the formation of the elements in the universe. He is also taking a part of the ImPACT project working for the socially important issue of nuclear transmutation to eventually resolve the nuclear waste problem.
Noriyoshi Ishii conducts lattice QCD studies for hadron interactions and structure. He plays a central role in the so called HALQCD project which is now well-known as they have derived the nuclear force from the first principle theory, QCD  (see also Fig. 5). The method has been extended to hyperon interactions to make predictions for the interactions which can not be accessible by experiments. The hyperon physics is very important for the long standing question of H-dibaryon, and also for the hyperon matter associated with the neutron star problem. Noriyoshi Ishii has been also working in hadron phenomenology using effective methods of QCD.
Fig. 5: 2+1 flavor QCD results of the central and the tensor forces at mπ=570 MeV.
Atsushi Hosaka conducts hadron phenomenology and in particular, the physics of exotic phenomena in hadron physics. Using effective models of QCD as a foundation, he has worked on the reaction dynamics of hadron production in various processes of photo and hadron induced reactions. Recently, he has led theory activities of exotic hadrons under the project of "new hadrons". Molecular dynamics for heavy meson molecules have been emphasized . Based on the achievements of this project, he is now working on heavy baryons and pushing forward the physics of charmed baryons at J-PARC . By exploring heavy quark systems, it is expected to study more on non-perturbative dynamics of light quark systems.
Hiroyuki Kamano, one of the two young assistant professors, is working on the dynamics of meson production. The method elaborates the most realistic scattering amplitudes to be compared with experiments and extract properties of various hadron resonances in a reliable manner. Kosho Minomo, another assistant professor, is working on nuclear reactions, starting from a realistic nuclear force and by using microscopic models. He is also working for the IMPACT project, aiming at the construction of a reliable reaction model which enables explanations and predictions of various nuclear reactions.
In conclusion, we emphasize that we are interested in expanding our research interests by collaborating with researchers. In fact, in addition to the above regular members, we have many visiting researchers from all over the world. We welcome potential visitors who wish to discuss, give seminars and collaborate at any time.
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Takashi Nakano is the director of the Research Center for Nuclear Physics (RCNP). He received a doctorate degree at Kyoto University in 1991, and joined a K decay experiment at Brookhaven National Laboratory (BNL) as a postdoc of the University of Alberta. After he moved to Osaka University, he led the Laser Electron Photon Experiment at SPring-8 (LEPS). He has been serving as the director of RCNP since 2013. His major research interest is in hadron physics.
Mitsuhiro Fukuda is a professor at the Research Center for Nuclear Physics (RCNP), Osaka University. After receiving his PhD from Osaka University, he worked at the Takasaki Radiation Chemistry Research Establishment, the Japan Atomic Energy Research Institute (currently, the Japan Atomic Energy Agency) and moved to RCNP in 2006. He is a researcher in accelerator physics and technology.
Nori Aoi is a professor at the Research Center for Nuclear Physics (RCNP), Osaka University. He received his PhD from the University of Tokyo and worked at Rikkyo University, the University of Tokyo, and RIKEN before moving to RCNP in 2011. His research field is experimental nuclear structure physics.
Masaru Yosoi is a professor at the Research Center for Nuclear Physics (RCNP), Osaka University. He received his D.Sci from Kyoto University. He worked at the Tokyo Institute of Technology, Kyoto University and RCNP, and has been a professor since 2013 as the leader of the Laser-Electron Photon (LEPS) group. His research field is experimental nuclear/hadron physics and his interests include the search for exotic states of hadrons, polarization phenomena in nuclear and hadron physics, and the development of new detectors.
Akira Sato is an assistant professor of the Graduate School of Science, Osaka University. After he received his PhD in science from the Science University of Tokyo, Japan, in 2001, he moved to Osaka University to join an experimental particle physics group for muon physics. He also holds an assistant professorship at RCNP, and is the leader of the MuSIC muon facility of RCNP. His research field is muon related science, and includes the development of intense muon beams, the wide range of applications of muons, and the search for charged lepton flavor violation.
Saori Umehara is a specially appointed assistant professor at the Research Center for Nuclear Physics (RCNP), Osaka University. She has been a member of the CANDLES collaboration, double beta decay experiment. Her research interests include neutrino and dark matter physics in nuclear and particle physics. She currently approaches the research with underground experiments at the Kamioka observatory.
Atsushi Hosaka is a professor in the theory group of RCNP. He received his Doctor of Science degree at Tokyo Metropolitan University in 1987. After that he worked as a postdoc at INS (University of Tokyo), the University of Regensburg (Humboldt fellow), at the University of Pennsylvania, and at TRIUMF (Canada), before he became a faculty member at Numazu College of Technology from 1993 till 2001. He then joined the faculty at RCNP, Osaka University. His research interest is in hadron physics theory, especially phenomenology, working with experimentalists.