Search

The WASA-FRS experiment aims to reveal the nature of light Λ hypernuclei with heavy-ion beams. The lifetimes of hypernuclei are measured precisely from their decay lengths and kinematics. To reconstruct a π - track emitted from hypernuclear decay, track finding is an important issue. In this study, a machine learning analysis method with a graph neural network (GNN), which is a powerful tool for deducing the connection between data nodes, was developed to obtain track associations from numerous combinations of hit information provided in detectors based on a Monte Carlo simulation. An efficiency of 98% was achieved for tracking π - mesons using the developed GNN model. The GNN model can also estimate the charge and momentum of the particles of interest. More than 99.9% of the negative charged particles were correctly identified with a momentum accuracy of 6.3%. [ABSTRACT FROM AUTHOR]

Alpha-decay events in a nuclear emulsion are standard calibration sources for the relation between the track length and the kinetic energy in each emulsion sheet. We developed an efficient classifier that sorts such alpha-decay events from various vertex-like objects in an emulsion using a convolutional neural network (CNN). We trained the CNN using 15885 images of vertex-like objects, including 906 alpha-decay events, and tested it using a dataset of 46948 images including 255 alpha-decay events. The precision and recall scores of the classification using the previous method without a CNN for the same dataset were 0.081 ± 0.006 and 0.788 ± 0.056, respectively. In contrast, our trained models achieved an average precision score of 0.760 ± 0.006 for the test dataset, after extensively tuning the hyperparameters of the CNN. Moreover, for the model obtained, the discrimination threshold of the classification can be adjusted arbitrarily according to the trade-off between the precision and recall scores. Furthermore, the developed classifier obtained a precision of 0.571 ± 0.017 when the recall score was assigned a value of 0.788. Finally, the developed CNN method reduced the need for additional human visual inspection, required after classification, by a factor of approximately 1/7, compared to the former method without a CNN, proving the feasibility of the proposed classifier. [ABSTRACT FROM AUTHOR]

We revisit previous measurements for the hypertriton binding energy with nuclear emulsion which were published in 1968 and 1973. Using Monte Carlo simulations, the systematic error of the hypertriton binding energy in emulsion measurements has been estimated to be approximately 28 keV. We corroborate the recent works that re-evaluate the hypertriton binding energy by using the former emulsion measurements in the present work, and the ambiguities and difficulties of the reevaluation are observed. Considering the need of new precise measurements with a well-controlled systematic error, we introduce a new approach by analyzing the existing nuclear emulsion data from the J-PARC E07 experiment, from which the binding energy of hypertriton could be determined, with both statistical and systematic errors, to be approximately 30 keV with 400 events. [ABSTRACT FROM AUTHOR]

We have developed a compact detector for measuring beam particles using plastic scintillators readout through Multi-Pixel Photon Counters, which is employed for hypernuclear measurements in the WASA-FRS experiment at GSI. The Time-of-Flight resolution of the newly-developed detector has been investigated in relation to the overvoltage with respect to the breakdown voltage, a maximum counting rate of approximately 3 × 1 0 6 /s per segment, and a maximum beam charge of Z = 6. The evaluated Time-of-Flight resolutions between the neighboring segments of the detector range from 44. 6 ± 1. 3 ps to 100. 3 ± 3. 6 ps (σ) depending on the segment, overvoltage values, and beam intensity. It is also observed that the Time-of-Flight resolution is inversely correlated to the beam atomic charge (Z). [ABSTRACT FROM AUTHOR]

A novel technique to study bound states of exotic hadrons in subatomic nuclei, such as hypernuclei and mesic nuclei, has been developed by employing the Fragment Separator FRS and the WASA central detector at GSI. Two experiments, S447 for studying light hypernuclei, especially hypertriton and a Λ nn bound state, and S490 for searching for η ' mesic-nuclei, were recently performed. Data analyses are currently in progress, and light charged particles such as protons and π ± are clearly observed and identified in the both experiments. For S447, light nuclear fragments that can also be residual nuclei from decays of hypernuclei of interests have been analysed by the FRS, and a momentum resolution, Δ p / p , of 5 × 1 0 − 4 has been achieved. Further data analyses are to be completed. The WASA-FRS project will be continued and extended with the FRS at FAIR Phase 0, and upgrading of the WASA magnet and detectors is currently in progress. Furthermore, construction of a larger detector system with the Super-FRS at FAIR Phase 1 is also under consideration. [ABSTRACT FROM AUTHOR]

The hypernuclei, which are nuclei that contain the quark s , have been studied for more than 50 years. Notwithstanding, the recent experiments using high-energy heavy-ion induced reactions have challenged their current understanding. The high multiplicity of particles generated in the reaction allows for the measurement of the interaction point of the primary beam with the target. Then, a micro-vertex detection system for the WASA-FRS Experiments has been developed. Several experimental tests have been performed with 90Sr and 207Bi β sources and a 10-MeV proton beam at the CMAM tandem accelerator, and their results are reported. [ABSTRACT FROM AUTHOR]

We plan to conduct an experimental search for η'-mesic nuclei in order to investigate in-medium properties of the η' meson. A 2.5 GeV proton beam is employed to produce η'-mesic 11C nuclei with the 12C(p,d)η' 11C reaction. Simultaneous measurements of the forward ejected deuterons and decay protons from η'-mesic nuclei will allow us to achieve high experimental sensitivity. The experiment will be performed at GSI by making full use of the fragment separator FRS and the WASA detector system. The plan of this proposed experiment is described. [ABSTRACT FROM AUTHOR]

We have developed a gas electron multiplier (GEM)-based time projection chamber (TPC) for the H-dibaryon search experiment at J-PARC. High-rate K − beam particles enter a TPC gas volume of approximately 0.2 m3, in a direction perpendicular to the electric field. A long-rectangular hollow section is located inside the TPC volume to accommodate a diamond target. We commissioned the TPC using 230-MeV protons with beam rates of up to 1 MHz. The TPC data acquisition system collected 5768 pad signals in full readout mode, with almost 100% efficiency, at a preset trigger rate of 230 Hz. We operated the TPC in an Ar/CH 4 gas mixture (90/10) without a magnetic field. The spatial resolutions on the pad plane are measured to be 400– 700 μ m , which correspond to 230– 300 μ m in a magnetic field of 1 T. We confirmed high tracking capability at beam rates of up to 1 MHz. [ABSTRACT FROM AUTHOR]

This study developed a novel method for detecting hypernuclear events recorded in nuclear emulsion sheets using machine learning techniques. The artificial neural network-based object detection model was trained on surrogate images created through Monte Carlo simulations and image-style transformations using generative adversarial networks. The performance of the proposed model was evaluated using α -decay events obtained from the J-PARC E07 emulsion data. The model achieved approximately twice the detection efficiency of conventional image processing and reduced the time spent on manual visual inspection by approximately 1/17. The established method was successfully applied to the detection of hypernuclear events. This approach is a state-of-the-art tool for discovering rare events recorded in nuclear emulsion sheets without any real data for training. [ABSTRACT FROM AUTHOR]

We have developed a time-of-flight (ToF) array consisting of 24 plastic scintillators for triggering and timing measurements in a series of planned experiments to study double-strangeness hadronic and nuclear systems at the K1.8 beamline of J-PARC. Each ToF scintillator is 180 cm long with a cross-section of 8 × 3 cm 2 , and is read out by H1949-50 photomultiplier tubes at both ends. The timing and attenuation properties of the scintillators are measured using cosmic-ray muons and β rays from 90 Sr. The cosmic-ray muon events are triggered by three pairs of 120-cm-long plastic scintillators, sandwiching the ToF array. The intrinsic time resolutions of the scintillators are estimated to be in the range 60–100 ps, which is adequate for reliable separation between π and K at 1.2 GeV / c . The attenuation length is measured to be approximately 210 cm, which is half that for a bulk scintillator. [ABSTRACT FROM AUTHOR]

The pentaquark Θ+ has been searched for via the π- p → K- X reaction with beam momenta of 1.92 and 2.01 GeV/c at J-PARC. A missing mass resolution of 2 MeV (FWHM) was achieved but no sharp peak structure was observed. The upper limits on the production cross section averaged over the scattering angle from 2° to 15° in the laboratory frame were found to be less than 0.28 µb/sr at the 90% confidence level for both the 1.92- and 2.01-GeV/c data. The systematic uncertainty of the upper limits was controlled within 10%. Constraints on the Θ+ decay width were also evaluated with a theoretical calculation using an effective Lagrangian. The present result implies that the width should be less than 0.36 and 1.9 MeV for the spin-parity of 1/2+ and 1/2-, respectively. [ABSTRACT FROM AUTHOR]

Abstract: We have been searching for the pentaquark via the reaction at the J-PARC hadron facility. No peak structure was observed in the missing mass spectrum obtained at beam momentum. The upper limit for the production cross section averaged over the scattering angle from 2° to 15° in the laboratory frame was derived to be . In order to make a more stringent constraint we have also performed a measurement at . Present analysis status of this new data is reported. [Copyright &y& Elsevier]

The first experiment at the J-PARC hadron facility, the J-PARC E19 experiment, aims at searching for the Θ pentaquark in the hadronic reaction π p → K X using the missing-mass technique. Based on a superconducting magnet excited at 2.5 T, the spectrometer achieved the high mass resolution of 1.4 MeV/c for the Θ production process. The first data taking was performed in the autumn of 2010. No significant structure was observed in the missing-mass spectrum. The upper limit obtained for the differential cross section is 0.26 μb/sr in the laboratory frame at a 90 % CL. [ABSTRACT FROM AUTHOR]

In the J-PARC E27 experiment, we search for a K pp bound state via the d( π, K) reaction at 1.7 GeV/ c at the K1.8 beam line. The binding energy and decay width of the K pp bound state can be obtained in the missing mass measurement with a good energy resolution of 2 MeV/ c. A range counter array (RCA) was constructed to detect the two high-momentum protons from the K pp decay and to reduce the background such as quasi-free hyperon production. Recently, we have carried out a pilot run in June, 2012. The d( π, K) missing-mass spectrum has been obtained for the first time. In this report, an overview of the E27 experiment and a preliminary result on this pilot run are presented. [ABSTRACT FROM AUTHOR]

The energy spacing between the spin-doublet bound state of 4λHe(1+,0+) was determined to be 1406±2±2 keV, by measuring γ rays for the 1+→0+ transition with a high efficiency germanium detector array in coincidence with the 4He(K-,π-)4λHe reaction at J-PARC. In comparison to the corresponding energy spacing in the mirror hypernucleus HΛ4, the present result clearly indicates the existence of charge symmetry breaking (CSB) in ΛN interaction. By combining the energy spacings with the known ground-state binding energies, it is also found that the CSB effect is large in the 0+ ground state but is vanishingly small in the 1+ excited state, demonstrating that the ΛN CSB interaction has spin dependence. [ABSTRACT FROM AUTHOR]