Search for η' mesic nuclei.

The mass of the η' meson (958 MeV/c2) is significantly greater than that of other pseudoscalar mesons due to the influence of the UA(1) quantum anomaly. The Nambu-Jona-Lasinio (NJL) model and the linear sigma model predict that the UA(1) quantum anomaly will also be resolved as chiral symmetry partially recovers at high-density states, leading to a mass reduction of 80-150 MeV at nuclear density. This study aims to generate bound η' mesic nuclei in the C(γ,p)X reaction and search for peak structures in the missing mass distribution by detecting forward-angle protons. The signal to noise ratio is significantly improved by detecting η generated through nucleon absorption of bound η' mesons and imposing mechanical conditions. However, a statistically significant structure in the missing mass spectrum of protons was not observed, and an upper limit for the production cross section was determined. For more details, please refer to the following article.

Measurement of the differential cross-section and beam asymmetry of the neutral mesons.

The production of mesons from liquid hydrogen targets is suitable for investigating the excitation states of nucleons. When the mesons are produced in the S-channel, they can be considered as decay particles from excited nucleons that were excited by the high-energy photon beam. By detecting the decay particles with a BGO egg calorimeter that has a large solid angle, the angular distribution of the production cross-section can be precisely measured. By comparing this observation with theoretical calculations based on partial wave analysis, the properties of the excited states of nucleons can be explored. Additionally, at SPring-8/LEPS2, a photon beam is generated using inverse Compton scattering, which allows for the use of highly linearly polarized beams as a probe. We can measure the beam asymmetry of meson photo-production, which is also an important observable for probing the nature of the excited states of nucleons. We measured the differential cross section of π, ω, η, and f0. Each results were summarized in the articles.

In-medium effect of the spectral shape of η'

η' has an extremely large mass due to the UA(1) quantum anomaly. In addition, the mass and width of η' are expected to change as a result of partial restoration of chiral symmetry inside high-density condition. We invesitate this effect by accurately measuring the spectrum of η' produced from a carbon target, which has a large density. We obtain the invariant mass distribution by acquiring the two γs decay particles of η' with the BGO egg calorimeter. We confirmed that the spectrum shape changes dominantly when the momentum of η' is low, i.e. when the probability of decay inside the carbon nucleus is high. To further advance this measurement, we plan to investigate the spectral changes of η' by using a copper target with a larger nuclear radius. Furthermore, to suppress multi-&pi production events that serve as background events in this measurement, we installed the Fwd-γ calorimeter forwardly and use it as a VETO detector.
For more detail about the project, please refer the proposal.