Annual Report 2020

We are pleased to release a new RCNP Annual Report on our activities in 2020. Because of the pandemic of COVID-19, our activity has been severely limited. However, there are still excellent progresses and achievements as reported in this report.

A series of quasi-free (p,α) experiments has been carried out to examine the surface α-clustering strength in ^{112,116,120,124}Sn and its isotopic dependence. The scattered protons and particles after the (p, pα) reaction were detected in coincidence by the Grand Raiden and LAS spectrometers. For each of the four tin isotopes, the missing-mass spectrum shows a clear peak located at the known α-separation energy, as expected for the quasi-free knockout of preformed α particles.

The LEPS group reported on R&D to increase the intensity of the laser electron-photon beam by synchronizing the timings of incident laser and electron beam bunches entering the inverse Compton scattering region. The intensity of the laser electron-photon beam has become several times higher than that of an asynchronous laser of the same output.

The high-momentum beamline (high-p beamline) has been newly constructed at the J-PARC Hadron Facility on May 2020. It delivers primary protons up to 30 GeV for hadron spectroscopy on Ξ, Ω and/or charm baryons. To utilize the beamline, it is essential to implement particle identification detector for beam particles in the momentum range of 5 to 20 GeV/c. A prototype detector of the RICH has been designed with the silica aerogel whose refractive index is 1.02 and tested at LEPS. The results showed that kaon identification with 50% efficiency under the 3% contamination of π in the beam momentum range of 5 to 8.5 GeV/c is possible.

The theoretical group reported progresses on Pc pentaquarks in a hybrid model of chiral and quark dynamics. A hybrid picture where the long-distance structure of hadronic molecule driven by the one pion exchange potential (OPEP) and the short-range structure of five quarks dominated by the color force mediated by the gluons were proposed. The role of the tensor component of the OPEP both in their mass ordering and decay widths was shown to be crucial for the determination of spin and parity of the observed Pc.

Finally, the progress of the upgrading work for the AVF cyclotron is updated. The beam will be back soon. Please stay tuned.

Sincerely yours,

Takashi Nakano
Director of Research Center for Nuclear Physics