Search for H-dibaryon at J-PARC with a Large Acceptance TPC

H-dibaryon has been predicted as a stable 6-quark color-singlet state. It has been searched for by many experiments but has never been discovered. Recent lattice QCD calculations predict H-dibaryon as a weakly bound or a resonant state close to the  threshold. E224 and E522 experiments at KEK observed peaks in  invariant mass spectra near the threshold in (K, K) reactions, which were statistically not significant. Therefore, we proposed a new experiment E42 at J-PARC. It will measure decay products of  and p in a (K, K) reaction. We design a large acceptance spectrometer based on a Time Projection Chamber (TPC) immersed in a dipole magnetic field. The TPC surrounds a target to cover nearly 4 acceptance, and accepts K beams up to 10 counts per second. To suppress drift field distortion at high beam rates, we adopt Gas Electron Multipliers (GEMs) for electron amplification and a gating grid. We show an overview of the experiment, the design of the spectrometer, and the R&D status of the TPC prototype. a Corresponding author: hiroyuki.sako@j-parc.jp DOI: 10.1051/ C © Owned by the authors, published by EDP Sciences, 2014 , / 09015 (2014) 201 66 epjconf EPJ Web of Conferences 4660 015 9 This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20146609015

Faculty of Education, Gifu University, Gifu 501-1193, Japan 11 Research Center for Nuclear Physics (RCNP), Osaka University, Ibaraki, Osaka 567-0047, Japan Abstract. H-dibaryon has been predicted as a stable 6-quark color-singlet state. It has been searched for by many experiments but has never been discovered. Recent lattice QCD calculations predict H-dibaryon as a weakly bound or a resonant state close to the  threshold. E224 and E522 experiments at KEK observed peaks in  invariant mass spectra near the threshold in (K -, K + ) reactions, which were statistically not significant. Therefore, we proposed a new experiment E42 at J-PARC. It will measure decay products of  and   p in a (K -, K + ) reaction. We design a large acceptance spectrometer based on a Time Projection Chamber (TPC) immersed in a dipole magnetic field. The TPC surrounds a target to cover nearly 4 acceptance, and accepts Kbeams up to 10 6 counts per second. To suppress drift field distortion at high beam rates, we adopt Gas Electron Multipliers (GEMs) for electron amplification and a gating grid. We show an overview of the experiment, the design of the spectrometer, and the R&D status of the TPC prototype.

Introduction
The H-dibaryon has been predicted as the most stable color and SU(3) f single 6-quark state (uuddss) in 1977 [1]. Since then, it has been searched for in many reaction channels and decay modes, but it has never been discovered. In addition, the observation of the double- hypernucleus,  6 He seemed to have ruled out the H-dibaryon existence [2]. On the other hand, there were some indications of H-dibaryons in two experiments, E224 [3] and E522 [4] at KEK Proton Synchrotron. They observed peaks near the  threshold in  invariant mass spectra. However, their statistics was too low to draw definite conclusions whether they are due to H-dibaryons or not.
Recent Lattice QCD calculations succeeded in reproducing properties of nucleon-nucleon interactions [5]. HAL QCD [6] and NPLQCD [7] collaborations extended these Lattice QCD calculations to the H-dibaryon studies. They calculated binding energies of H-dibaryon turned out to be 7.4±2.1±5.8 MeV (the errors are statistical and systematic) and -13±14 MeV (the error is statistical only) [6][7][8], respectively. Since the binding energy is close to 0, H-dibaryon may be either a weakly bound state or an unbound resonance state.
The current status of experimental and theoretical studies of H-dibaryon is summarized as follows. The H-dibaryon in the mass region m  <2224 MeV/c 2 is excluded by the observation of Therefore, we proposed a new experiment E42 at J-PARC [9] to have a two-order of magnitude higher statistics data sample than the KEK experiments.   vertical drift electric field of 180 V/cm is applied, resulting in an electron drift velocity of ~ 5 cm/sec. A magnetic field of ~1 T is applied in parallel to the electric field in order to achieve good horizontal position resolutions. The target is installed in the middle of the drift volume, by inserting it from the top of the TPC into the vertical cylindrical target holder as shown in Fig. 2. A uniform electric field is generated by the field strips on the walls of the target folder and the field cage. Ionization electrons drift downwards to the bottom amplification section, which consists of a gating grid plane, GEMs (Gas Electron Multipliers), and a pad plane. In order to achieve a high gain of ~10 4 , we combine three GEM sheets of 50 m, 50 m, and 100 m thickness. Since the largest GEM in Japan is limited to 300 mm x 300 mm, we use 4 GEM sheets per layer to cover the entire area. Fig. 3 shows the pad arrangements. The pad dimensions are designed to be 2.1-2.7 x 9 mm 2 in the inner rings (yellow area) and 2.3-2.4 x 12.5 mm 2 in the outer rings to achieve the horizontal position resolution better than 300 m for the drift length larger than 10 cm. The simulated momentum resolution is 0.8-2.3 % for and 2.2-4.0 % for p. The expected  invariant mass resolution is ~1.5 MeV/c 2 at the N mass threshold. A simulated invariant mass spectrum in E42 experiment, corresponding to a 33 day period, is shown in Fig. 4. We assume a zero H dibaryon mass width and a production cross section of 1.0 b/sr. The spectrum includes 11000  events with 1440 H-dibaryon signals. This statistics is 120 times as high as that in the KEK E522.

HypTPC design
We have adopted GET (General Electronics for TPC) [10] for readout electronics. The system is designed for various kinds of TPCs with thousands of channels. It performs signal amplification, pulse shape digitization, and zero suppression. The data acquisition rate is expected to be ~ 1 kHz.

TPC prototype tests
We built a TPC prototype with a drift volume of 10x10x20 cm 3 , with 10x10 cm 2 GEMs (50 m + 50 m + 100 m), gating grid wires, and 4 kinds of pads with width of 2, 3, 4 and 6 mm and 10 mm length. We have measured a gain of 10 4 at the GEM voltage of 325 V for 50 m thick GEM and 488 V at 100 m thick GEM. We have also performed a long term operation test of GEMs with 1 MBq 90 Sr and 1 MBq 55 Fe sources for 1 month without any damage.

High rate beam test at RCNP
A test with 400 MeV protons beams was performed at Research Center for Nuclear Physics, in Nov. 2011. The horizontal position resolution with 4 mm width pads without magnetic field was 400 m. Detection efficiency per pad was better than 95% at beam rates up to 10 6 cps/cm 2 . The horizontal hit position shifts due to positive-ion feedback measured with reference silicon strip detectors in the gating grid operation were reduced to less than 0.1 mm at the beam rate up to 10 6 cps/cm 2 , while ~1 mm shifts are observed with the gating grid open even at the low beam rate of 10 4 cps/cm 2 .

Laser test under magnetic field
We have performed a TPC test with a YAG laser in magnetic field at J-PARC in Apr. 2013. We injected laser beam of 266 nm wave length horizontally in parallel to the pad length direction into the TPC drift volume through quartz windows. We applied vertical magnetic field, parallel to the drift electric field. As shown in Fig. 5, horizontal position resolution improved by 50-70% by increasing the magnetic field from 0 to 0.7 T. The resolution, however, is much better, namely only 60% compared to that for minimum ionizing particles. It turned out due to times more ionization electrons.

Conclusions
We have designed a TPC for H-dibaryon search experiment E42 at J-PARC. It has the unique feature that the target is enclosed inside the drift volume to maximize the H-decay event acceptance, while an extremely high-rate Kbeam must be directly injected to the drift volume. In order to cope with the high-rate beam, we adopted GEMs and a gating grid. We tested a TPC prototype with and without magnetic field and good performance on the spatial resolution, efficiency and reduction of position shifts were obtained.
We have started the construction of the final TPC. We are going to perform GEM tests in this fall. In parallel, we will construct the field cage and the target holder. Production of readout boards will be completed by this year. We expect the completion of the TPC by the end of 2014.