Mass Measurement of Short-lived Nuclei at HIRFL-CSR

Four campaigns of mass measurements for short-lived nuclei have been conducted using an isochronous mass spectrometry (IMS) technique at HIRFL-CSR (Cooler Storage Ring) in Lanzhou. The radioactive nuclei were produced by projectile fragmentation and injected into the experimental storage ring CSRe. Revolution times of the ions stored in the CSRe were measured from which masses of 78Kr, 58Ni, 86Kr and 112Sn fragments have been determined with a relative uncertainty of about 10-6-10-7. The experimental results are presented and their impacts on nucleosynthesis in the rp process and nuclear structure are discussed.

International collaboration is vital for enhancing productivity of scientific activities, and is particularly so for the fields of Big Science such as nuclear physics, where harmonic organization of the best intelligence and expertise over the world is demanded so as to fully exploit the rare opportunities provided by a limited number of leading edge facilities available. Bearing such spirits in mind contemporary China-Japan Cooperation on nuclear and accelerator physics has been pursued intensively for the last few decades. The cooperation effectively started in 1990 when the 1 st Joint Symposium on Accelerator Science was held in Atami. Ever since, the collaboration between the two countries has been developing steadily on both nuclear physics and accelerator science. Nowadays a wide variety of cooperative programs are active on the sites of daily research activities on experimental and theoretical nuclear physics.
In pursuing such collaboration one rather seeks for long-term merits which would gradually build up over a time span extending towards prosperity in far future. To accomplish such a long-term endeavor, one often needs to be patient by consistently Six campaigns of mass measurements for short-lived nuclei have been conducted at HIRFL-CSR (Cooler Storage Ring) in Lanzhou. The radioactive nuclei were produced by projectile fragmentation and injected into the experimental storage ring CSRe, which was set as an isochronous mass spectrometry (IMS). Revolution times of the ions stored in the CSRe were measured, and from which masses of 78Kr, 58Ni, 86Kr , 112Sn and 36Ar fragments have been determined with a relative uncertainty of about 10ˆ-6-10ˆ-7.
In the latest experiment the IMS with two time-of-flight detectors have been realized. In this way the velocity of each stored ion can be measured in addition to its revolution time, thus the resolving power of the IMS could be significantly improved. Some of the experimental results will be presented and their impacts on nucleosynthesis in the rp process and nuclear structure will discussed. [10:00-10:30]

SHE research at RIKEN/GARIS
Presenter: Kosuke Morita (Kyushu University/RIKEN) A cold heavy-ion fusion reactions have been used for producing nuclei of the heaviest elements at RIKEN with use of a gas-filled type recoil ion separator GARIS. The heaviest nuclei which we studied were an isotope of the 113th element, 278113, produced in the 209Bi(70Zn, n) reaction [1][2][3]. Unambiguous identification of an atomic number and an atomic mass-number of the isotope based on the genetic correlation was done. Before performing the experiment on element 113, we have done a series of experiment to confirm the experimental results performed at GSI on the atomic numbers 108 (Hs) [4], 110 (Ds) [5], 111 (Rg) [6], and 112 (Cn) [7,8] by using 58Fe, 64Ni, and 70Zn beams on to 208Pb, and 209Bi targets. The results provide firm confirmation to the results obtained at GSI. Some of new spectroscopic information on the nuclei were obtained also. Study of an actinide based hot fusion reaction are also started. The reaction in study is 248Cm + 48Ca. Similar decay chains to the ones previously observed at FLNR/GFRS and at GSI/SHIP were observed in two different beam energies. The corresponding excitation energies of the compound nucleus 296Lv were 37.5 ± 1.5 MeV and 41.5 ± 1.5 MeV. We plan to measure one more point at higher excitation energy to obtain an excitation function of the reaction. A 50Ti beam is now in development by MIVOC method [9] for the future study of the 248Cm + 50Ti → 298118* reaction.
properties, and spin properties of psd-shell nuclei. The differential cross sections of 11Be + p, 11Be + d elastic scattering were also measured at the same experiment to obtain the reliable optical potential parameters of the entrance channel 11Be + d and the proton percent in CD2 target by comparing these two elastic scattering data sets. The X-CDCC method, including the core-excitation of 11Be was employed to describe the angular distributions of the break-up reactions to 10Be + n continuums of the 11Be on protons and deuterons, and the results indicated that the core-excitations could not be ignored in the description of the break-up data. [15:00-15:30]

Two-proton emission from proton-rich nuclei-22 Mg and 23 Al
Presenter: Deqing Fang (Shanghai Institute of Applied Physics) The decay of proton-rich nuclei, especially the two-proton (2p) radioactivity, is an interesting process that may be observed in nuclei beyond or close to the proton dripline.
One and two-proton emission from 23 Al and 22 Mg have been measured experimentally by the nuclear reaction method and also the beta-delayed decay method. The energy spectrum for one proton emission and the half-life for beta-decay were obtained. From the spectrum of relative momentum and open angle between two protons, we observed strong component of 2  A container picture is proposed for understanding cluster dynamics where the clusters make nonlocalized motion characterized by the size parameter in the THSR (Tohsaki-Horiuchi-Schuck-Röpke) wave function. Quite recently, we studied some cluster states of 12C using an extended 2+ THSR wave function. Some resonance cluster states and 2 correlation problems in 12C are discussed in the container picture. [16:30-17:00]

Alpha inelastic scattering and cluster structures in 24 Mg
Presenter: Takahiro Kawabata (Kyoto University) Alpha particle clustering is an important concept in nuclear physics. The most famous α cluster state is the 0 + 2 state in 12 C which locates at an excitation energy higher than the 3α-decay threshold by 0.39 MeV. This 0 + 2 state is theoretically described by introducing a novel concept of the nuclear structure, i.e., this state has a dilute-gas-like structure where three α clusters are weakly interacting and are condensed into the lowest s-orbit.
The next natural question addressed is whether such α condensed states exist in heavier self-conjugate 4n nuclei.
Such α condensed states are theoretically predicted up to n = 10. The energy of the nα condensed state from the nα-decay threshold increases with n due to the short-range nature of the attractive force between α clusters and the long-rage nature of the Coulomb repulsion, and the nα condensed state becomes unstable beyond n = 10.
A recent theoretical work proposed a new conformation of the α condensed state where α clusters are condensed into the lowest s-orbit around a core nucleus. Attractive potential for α clusters provided by the core nucleus stabilizes the α condensed state around the core nucleus even in heavier nuclei. Thus, such α condensed states around core nuclei are expected to appear at lower excitation energies than the corresponding cluster-decay threshold energies.
In our previous work, it was demonstrated that spatially well-developed cluster states in light nuclei, whose spins and parities are the same as in the ground state, are strongly excited by isoscalar monopole transitions. The alpha inelastic scattering at intermediate energies and at forward angles is one of the most useful probes to measure the isoscalar monopole strengths because its reaction mechanism is simple. It has a selectivity for isoscalar natural-parity transitions and there is a good linear relation between the reaction cross sections and the relevant nuclear transition matrix elements.
Recently, we performed a high-resolution measurement of α inelastic scattering from 24 Mg to search for the α condensed states with the configuration of 6α or 2α+ 16 O. In the present talk, we will present the experimental details and results on the cluster structures in 24 Mg. In this report, we will discuss the monopole transition of light unstable nuclei in connection to the chemical-bonding structures. The monopole transition is considered to be an important tool to identify the cluster degrees of freedom. We employ the Generalized Two-center ClusterModel (GTCM), which is possible to describe the formation of the MO and AO structures in general two-center systems.
We will show the feature of the isoscalar and isovector monopole-transitions in the 12 Be nucleus. The former transition mainly excites the α-α relative excitation, while the latter transition strongly responds to the excitation of the excess neutrons. We will demonstrate that the excitation degrees of freedom can be identified by combing the isoscalar and isovector transitions.
The GTCM calculations are also applied to the mirror systems of 10 Be-10 C, and their mirror symmetry is investigated. We have found that the Coulomb shift is clearly suppressed in the cluster states. Moreover, we have confirmed that a prominent breaking of the mirror symmetry occurs in the monopole transition. We will discuss the important and interesting aspects in the analysis of the mirror systems.
Monday, Nov. 9 Session 5 (9:00-10:30) A newly constructed facility, SLOWRI at RIKEN RIBF, will be able to provide high-purity, low-energy or trapped radioactive nuclei of almost all elements, including short-lived nuclei. SLOWRI consists of two gas catcher devices: one is the RFC gas cell for universal ion beams using an rf-carpet ion guide and the other is the PALIS gas cell for parasitic RI beams using resonant re-ionization in the gas cell. Thus obtained RI beams will be transported to the SLOWRI experimental area where such experimental devices as a collinear fast beam apparatus, a multi-reflection time-of-flight mass spectrograph, and decay spectrometers are under preparation.
In order to emphasize the low-energy nuclear physics research activity using SLOWRI and KISS (KEK Isotope Separator System), an international collaboration group, SSRI-pns (stopped and slow RI beams for precise nuclear spectroscopy), has recently been organized. Within a few years, these facilities will be ready for users from universities and international laboratories, and comprehensive precision measurements of nuclear properties will be performed.

Status of the deuteron stripping reaction theories
Presenter: Danyang Pang (Beihang University) For more than 60 years deuteron stripping reactions have been one of the main tools to extract spectroscopic information, such as spectroscopic factors and asymptotic normalization coefficients, that are important for nuclear structure, nuclear astrophysics, and applied physics. In this talk we examine the main features of the theories for such a reaction, starting from the plane-wave Born approximation developed in early 50's of the last century to the Faddeev method which is still under development. Problems of nonlocality and inconsistency in the nucleon potentials and their effects on the transfer reaction cross sections will also be discussed. [11:30-12:00]

Microscopic reaction theory for many-body nuclear reactions
Presenter: Kosho Minomo (RCNP) Microscopic description of many-body nuclear reactions is the fundamental subject in nuclear physics. Based on multiple scattering theory, elastic scattering, which is one of most basic processes, is described by multistep processes with a nucleon-nucleon effective interaction. In practice, a g-matrix interaction is used as effective interactions.
We aim for fully microscopic understanding of many-body nuclear reactions with the nucleon-nucleon effective interactions.
In this talk, I introduce a new microscopic reaction theory starting from two-and three-nucleon forces based on chiral effective field theory (Ch-EFT). We first construct a g-matrix with the nuclear forces based on Ch-EFT using Brueckner-Hartree-Fock Theory, in which the three-nucleon force effects are represented through the density dependence of the g-matrix. Then, the folding model and microscopic coupled-channels method with the g-matrix are applied to nucleon-nucleus and nucleus-nucleus scattering at intermediate incident energies. This new microscopic reaction theory well describes the elastic and inelastic cross sections with no ad-hoc parameters. Furthermore, I show an application of the microscopic reaction theory to knockout and breakup reactions. [12:00-12:30]

Isovectoror Reorientation of Deuteron in the Field of Heavy Target
Presenter: Zhigang Xiao (Tsinghua University) The difference of nuclear force experienced by the proton and the neutron, known as isovector potential, is relevant to the reaction dynamics induced by exotic nuclei and to the evolution process and static properties of neutron star. Represented in the density dependence of the nuclear symmetry energy, it has been extensively discussed in the past decade. Some effective probes have been identified to probe the symmetry energy and some convergent constraints have been achieved near the saturation density. In this talk, I will describe the reorientation effect of deuteron attributed to isovector interaction in the nuclear field of heavy target nuclei within the framework of the performances of rare-RI ring (R3) were verified. We succeeded in injecting a particle, which was randomly produced from a DC beam from cyclotrons, into the R3 individually with a fast kicker system, and in extracting the particle from the R3 1 ms after the injection. We measured TOF of the 78 Kr particles between the entrance and the exit of the R3 to check the isochronism. Through the first-order adjustment with trim-coils fixed on the dipole magnets of the R3, the isochronism on the 10-ppm order was achieved for the momentum spread of ±0.2%. Higher-order adjustment employed in future will lead us to the isochronism on the order of ppm. In addition, we confirmed that a resonance-type Schottky pick-up successfully acquired the frequency information of one particle in a storage mode. In this conference, the technical aspects of the R3 and some results of the beam commissioning will be presented. proposed by Wollnik more than 20 years ago [1], have begun to make an impact in nuclear physics as isobar separators [2,3] and for use in precision mass measurements [4][5][6]. We hope to be able to use the MRTOF-MS to eventually change from a paradigm of identifying transactinide isotopes by α-decay chain to one of identification by mass determination. Doing so will reduce uncertainty in identification, especially in the region of hot-fusion where α-decay chains terminate in spontaneous fission before reaching well-known nuclei, which has bottlenecked acceptance by IUPAC of super heavy elements (SHE) with Z=113, 115, 117 and beyond [7].
The MRTOF-MS is the ideal tool for such a paradigm shift, being well-suited for low-yield, heavy, and short-lived nuclei. It achieves mass resolving powers R m > 100,000 with flight times shorter than 10 ms for even the heaviest nuclei [8].
Additionally, it is a true spectrograph ¥textemdash ãs opposed to a spectrometermaking it capable of mass determinations with, in principle, as few as one detected ion.
Owing to its spectrographic nature, the MRTOF-MS is able to simultaneously measure several species with high-precision. This capability has until now been limited to storage rings. As we have demonstrated [5,8], the MRTOF-MS allows for a considerably less complex data analysis than that used for storage rings.
As the first step toward mass spectrographic identification of SHE, we have installed a gas cell connected to an MRTOF-MS after the gas-filled recoil ion separator GARIS-II [9]. The system is described in some detail elsewhere [10]. We have used this system to initially perform mass measurements with fusion-evaporation reaction products lighter than Uranium, the masses of some of which have not previously been directly measured.
In these measurements we demonstrated the ability of the MRTOF-MS to precisely determine the masses of several isotopes simultaneously and to do so in some cases with less than 10 detected ions. This provides the first steps towards a paradigm shift in which SHE will eventually be identified by mass spectroscopy rather than decay spectroscopy -an absolute necessity if the "island of stability" is to be identified.
References: In the study of strongly correlated many-particle systems, a fundamental challenge is to find basic properties of a variety of elementary modes of excitation, and to identify the degrees of freedom that are suitable for describing the collective phenomena. This leads to deep insights into the basic concepts of quantum many-body physics.
The energy density functional (EDF) models for nuclei are currently a leading theory for describing properties of heavy nuclei. A single EDF is capable of quantitative description of almost all the nuclei, including infinite nuclear matter. The concept is very similar to the density functional theory (DFT) in electronic systems, utilized in atomic, molecular, and solid state physics. Major conceptual difference is that, for the isolated finite nucleus, the nuclear EDF models are designed to reproduce the intrinsic ground state. Namely, the self-consistent solution produces a density distribution which spontaneously violates symmetries of the system, such as translational, rotational, and gauge symmetries. Nevertheless, these features can be rigorously treated in the DFT theorems for "wave-packet" states.
An extension of the DFT to the time-dependent DFT (TDDFT) provides a feasible description of many-body dynamics, which contains information on excited states in addition to the ground state. According to the spontaneous violation of the symmetry, the nuclear EDF has advantages and disadvantages which I want to clarify in this talk. I also present selected recent results of the TDDFT activities (mainly in Japan) and future perspectives. [16:40-17:10]

Direct effect of tensor force by (p,d) reaction
Presenter: Satoru Terashima (Beihang University) The tensor forces are the major components of the nucleon-nucleon interactions that provide the attractive forces in atomic nuclei. The tensor forces are essential in theoretical calculations to reproduce the binding energies of deuteron and alpha particles. Despite the importance, there are few experimental signatures for tensor forces until now. Recently we started to search for experimental signatures of tensor forces via highly momentum miss-matched neutron transfer (p,d) reaction on 16O using 200 1200 MeV proton beam at RCNP and GSI. These measurements will cover high momentum transfer up to and beyond 2 fm-1 where the effect of the tensor forces is expected to be dominant. We found a strong energy dependence of the ratio of differential cross sections of the excited 5/2+ state and the ground 1/2− states in 15O, which is a possible signature of tensor forces. Extensive study on the scattering-angle dependence to understand/exclude effect of the reaction mechanism is ongoing. Further study of tensor effect at high momentum transfer region by measuring correlated nucleons via the (p,dp) and (p,dn) reactions has been proposed and approved at RCNP.
Detailed study of such correlated nucleons with high momentum beyond the Fermi momentum is expected to provide a clear signature of the tensor force. New experiment using inverse kinematics via the (p,d) reaction is also planned at future RI-beam facilities. In this meeting, we would like to report our recent measurements and future plans, and discuss possible strategies for the experimental studies of tensor forces Tuesday, Nov. 10 Session 9 (9:00-10:30) keV and that of splitting energies of the doublet were measured by gamma-ray spectroscopy using NaI counters to be 280 keV. These energy differences were already excluded of the mass difference of core nuclei. Such large differences for both 0 + and 1 + were not theoretically explained. However, the statistics of the gamma-ray data of 4 Λ He(1 + → 0 + ) to be 1.15 ± 0.04 MeV seems not enough. In 2015 April, gamma-ray spectroscopy of 4 Λ He via the 4 He(K -,π -) reaction was performed at J-PARC Hadron Experimentaly Facility (J-PARC E13). The gamma-ray energy were measured by a germanium detector array, Hyperball-J. We successfully observed and assigned the 4 Λ He(1 + → 0 + ) transition to be 1.406 MeV. Therefore, the previous result of 1.15 MeV was denied.
Combining previous data and our new result, the energy difference of 1 + was obtained to be almost zero (0.03 ± 0.05 MeV). The fact implies that the charge symmetry breaking effect in ΛN interaction is spin dependent. In this paper, we will present over view of the J-PARC E13 experiment and the results. ?' To answer this question, it is necessary to look at the energy spectra of 6 6 He, was studied to explore the excited states above the first 2 + state [3]. However, clear evidence of the second 2 + state was not obtained. In 2012, in Ref. [4], the transfer reaction experiment p( 8 He,t) 6 He shows an indication of the second 2 + state of 6 He as a resonant state at E x = 2.6 ± 0.3 (Γ = 1.6 ± 0.4) MeV. When a Λ particle is added to such a resonant state, due to a gluelike role of Λ particle, it is likely to result in narrower resonant states of 3/2 + 2 and 5/2 + 2 of 7 Λ He. The prediction of energies of second 3/2 + and 5/2 + states and decay widths would encourage further experimental investigation of 7 Λ He at JLab. (3) This observation stimulated us to study neutron-rich Λ hypernuclei because in light nuclei near the neutron drip line, interesting phenomena concerning neutron halos have been observed. When a Λ particle is added to such nuclei, it is expected that the resultant hypernuclei will become more stable against neutron decays due to the attraction of ΛN interaction and the fact that there is no Pauli exclusion effect between nucleons and a Λ particle. This phenomenon is one of the 'gluelike' roles of Λ particle.
In the symposium, I will report the above three issues within the framework of α + Λ + n + n four-body problem.
References:     We have constructed the KEK Isotope Separation System (KISS) at RIKEN RIBF facility to produce, separate and measure the nuclear properties of the neutron-rich nuclei around the neutron magic number of 126, which will be produced by the MNT reaction. KISS consists of an argon gas cell based laser ion source and an isotope separation on-line (ISOL), to produce pure low-energy beams of neutron-rich isotopes around N = 126 and to study their beta-decay properties.
We adopted the reaction system of 136 Xe + 198 Pt, which is considered to be one of the best candidates to efficiently produce the nuclei of interest. In order to investigate the feasibility of the nuclear production of the system, we have studied the collisions between 136 Xe and 198 Pt at the laboratory energy of 8 MeV/A by using the large acceptance magnetic spectrometer VAMOS++ at GANIL. In this presentation, we will show the isotopic distributions of projectile-like fragments (PLFs) detected by the spectrometer and isotopic distributions of target-like fragments (TLFs) deduced from the detected PLFs. We will discuss about the production of N = 126 TLFs. We will also introduce the KISS project and its present status. In the talk, recent results and future plans of the RIBF facility will be presented. [14:30-15:00]

Laser driven plasma collider for nuclear studies
Presenter: Changbo Fu (Shanghai Jiaotong University) A mini version of nuclear collider was realized for the first time by using laser-induced plasma. We studied the Deuteron-Deuteron (DD) nuclear reactions by using head-on collision plasma currents with lasers of 1 ns pulse width and 2000 J total laser energy.
The experimental results show that this mini version plasma collider can produce much higher low energy flux than the traditional accelerators. And also, due to the head-on collision, the nuclei's center-of-mass energy is doubled, and therefore can significantly enhance the reaction products, which is especially important for reactions in sub-coulomb barrier energy ranges.We observed up to 7.6 × 10 5 DD neutron products in the experiments, which was much larger than that of the non-collision cases. The possibilities of using this plasma version collider to study low energy nuclear reactions will be discussed. The tensor-force-driven shell evolution has been proposed in December, 2005. There have been many notable developments in experimental (e.g. observation of N=34 magic number) and theoretical sides, and I will overview it with an introduction of basic concepts. In 2010, the shell evolution due to the Fujita-Miyazawa three-nucleon force was proposed, and I shall review it as well. In recent years, the group in Tokyo has developed a new theory of effective interaction for multi-shell configurations, which are essential to exotic nuclei but cannot be treated in conventional theories. I will present some highlights from this new work. At the end, I will introduce Type II Shell Evolution in connection to shape coexistence phenomena, for instance in 68Ni and 186Pb. This leads us to a picture of Dual Quantum Liquid, which may be related further to the spontaneous fission and the island of stability. [16:40-17:10]

Neutron-proton asymmetry dependence of spectroscopic factors
Presenter: Jenny Lee (University of Hong Kong) Spectroscopic factors are fundamental quantities in nuclear physics. They have been extensively used in understanding the single particle properties and nucleon correlations in nuclei. The inconsistent spectroscopic factors obtained in transfer [1] and knockout reaction measurements [2] pose an intriguing question about the reaction mechanisms as well as the nature of nucleon correlations in nuclei with extreme isospin asymmetry. To solve the long-standing puzzle and establish reliable probes for extracting spectroscopic factors, we performed one-nucleon knockout reaction measurements of 30Ne at 250 MeV/u at RIKEN [3] and 14O at 60 MeV/u at RCNP [4] respectively for examining the knockout reaction mechanisms in details. In addition, we recently completed the (p,d) transfer measurements of 34,46Ar at 70 MeV/u at NSCL [5] for investigating the energy dependence in the reaction mechanism.
In this talk, the setup of these experiments, preliminary results and physics conclusions will be discussed.
We We report our microscopic investigation on the single-particle properties and the EOS of isospin asymmetric nuclear matter within the framework of the Brueckner theory extended to include a microscopic three-body force. We pay special attention to the discussion of the three-body force effect and the comparison of our results with the predictions by other ab initio approaches. Three-body force is shown to be necessary for reproducing the empirical saturation properties of symmetric nuclear matter within nonrelativistic microscopic frameworks, and also for extending the hole-line expansion to a wide density range. The three-body force effect on nuclear symmetry energy is repulsive, and it leads to a significant stiffening of the density dependence of symmetry energy at supra-saturation densities. Within the Brueckner approach, the three-body force affects the nucleon s.p. potentials primarily via its rearrangement contribution which is strongly repulsive and momentum-dependent at high densities and high momenta. Both the rearrangement contribution induced by the three-body force and the effect of ground state correlations are crucial for predicting reliably the single-particle properties within the Brueckner framework. [10:00-10:30]

Status of Kpp search experiments
Presenter: Tomofumi Nagae (Kyoto University) Kaonic nuclei are a new type of hadron many-body system with strangeness degrees of freedom, if existed. It is a bound system of meson and baryons instead of baryon many-body systems such as hypernuclei. Among them, so-called "Kpp" system which is composed of a Kand two protons is in strong focus as the simplest case. There is a possibility that the system has a large binding energy of > 60 MeV which could reach a high nuclear density close to twice the normal nuclear matter density. Thus, the experimental confirmation of the existence of kaonic nuclei or "Kpp" is an urgent task in this field.
A lot of experimental searches are recently carried out or on-going in various facilities in the world. In SPring-8/LEPS, a search for the "Kpp" was carried out in the γ + d → K + π -+ X reaction at E γ = 1.5−2.4 GeV . Because of a large background from K + Λ(1520) and K + π -πΛΣ processes, they were only able to put the upper limits of the production cross section of the order of 10-20% of typical hyperon production cross sections, when the decay width is assumed to be Γ=100 MeV.
HADES collaboration has reported their partial wave analysis result on the reaction of p(3.5 GeV) + p → pK + Λ to search for the "Kpp". They also put the upper limit for the "Kpp" production cross section of about 4 µb, while the Λ(1405) production cross section at this energy is about 10µb.
At J-PARC, there are two experiments, E15 and E27, on the "Kpp" search. The E15 collaboration just reported a semi-inclusive spectrum of the 3 He(K -,n) reaction at 1 GeV/c with a preliminary data. They estimated the upper limit of the "Kpp" production cross section to be 100-270 µb/sr in the case of Γ=100 MeV, which is about 5% of the quasi-elastic K -N cross section. In E27 experiment, the "Kpp" search was carried out in the d(π + ,K + ) reaction at 1.69 GeV/c. In order to enhance the signal to background ratio, high-momentum proton (≥250 MeV/c) coincidence in the large scattering angles was requested. Such proton coincidence probability showed a large bump structure centered at around 2.27 GeV/c 2 for the "Kpp" mass, in addition to the rather sharp structure of ΣN → ΛN cusp and conversion process near 2.13 GeV/c 2 . With two-proton coincidence condition, the decay modes of the "Kpp" into Λp, Σ 0 p, and πYp were separated in the missing energy. From the mass distribution obtained for the Σ 0 p decay mode, the mass and width of the "Kpp" were obtained to be 2275 +17 -18 (stat) +21 -30 (syst) MeV/c 2 and 162 +87 -45 (stat) +66 -78 (syst) MeV, respectively. It corresponds to the binding energy of 95 +18 -17 (stat) +30 -21 (syst) MeV. In this talk, the above experimental data and their implications will be discussed.