Solve the hypertriton lifetime puzzle

2017 Fiscal Year Annual Research Report

• Project/Area Number: 17H04842
• Research Institution: Institute of Physical and Chemical Research
• Principal Investigator: MA YUE 国立研究開発法人理化学研究所, 開拓研究本部, 専任研究員 (80639576)
• Project Period (FY): 2017-04-01 – 2021-03-31
• Keywords: Hypertriton lifetime / Cherenkov EM calorimeter

Outline of Annual Research Achievements

As the lightest hypernucleus, Hypertriton plays a similar role as deuteron for nuclear physics, which provides the foundation for our understanding for YN interaction. However, a big surprise came from recent experiments that the mesonic weak decay lifetime of Hypertriton is ~30% shorter than the previous value. We proposed an original method to measure the Hypertriton lifetime, which will shed light on this puzzle.

In our proposal, we will employ K-+He3->Hypertriton+pi0 reaction to populate Hypertriton. The high energy gamma rays decayed from pi0 will be measured by a forward EM calorimeter to identify the production of strangeness exchange reaction. The signal from Hypertriton can be identified by measuring the mono- energetic pi- decayed from the stopped Hypertriton. Our approach will provide important complementary measurement for the Hypertriton lifetime puzzle.

We have carried out a full Monte-Calo simulation to estimate the performance of our experiment and submitted a proposal to the J-PARC PAC committee to request for beam time. In the mean while, we have purchased 40 pieces of PbF2 crystal for the construction of Cherenkov based EM Calorimeter. The construction of a new cryogenic system for the liquid He3 target system has been completed and achieved the designed value successfully.

Current Status of Research Progress

3: Progress in research has been slightly delayed.

Reason

There are two factors caused the delay of this project:
1, as the only supplier of PbF2 crystals in the world, Shanghai Institute of Ceramics doesn't have enough stock of crystals as we originally proposed because of a large contract from Fermi Lab in the US. Therefore, we have to modify our original experimental setup with a smaller acceptance of detector. This task has been finished and a revised proposal has been submitted to J-PARC.
2, half year after the submission of our proposal to J-PARC, another collaboration led by INFN, Italy, proposed an experiment with similar Physics interest. According to the J-PARC PAC, only one of these two proposals will be granted for beam time. We will do our best to convince the PAC members but a further delay of beam time decision could happen.

Strategy for Future Research Activity

We will mainly invest our efforts in the following tasks:
1, construct and calibrate the Cherenkov EM calorimeter and perform the calibration with gamma-ray beam
2, refine the proposal and try to obtain a good result from the next J-PARC PAC meeting.