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Production and decay of Hypernuclei by using an intense heavy-ion beam incidenting on a Proton target

Periodic Reporting for period 1 - ProHyPro (Production and decay of Hypernuclei by using an intense heavy-ion beam incidenting on a Proton target)

Reporting period: 2016-06-01 to 2018-05-31

Hyperons are expected to be an important building block of high-density matter such as neutron stars. However, the hyperon-nucleon (YN) and hyperon-hyperon (YY) interactions are rarely known because the short lifetime of hyperons (hundreds of ps) making it extremely difficult to perform scattering or capture experiments with hyperon beams. Hypernuclei, which contains at least one hyperon in addition to the normal protons and neutrons, provide an alternative and unique ground to study the YN and YY interactions. The study of hypernuclei sheds new light on the world of traditional nuclei by revealing new symmetries and new phenomena produced by the additional strangeness dimension. Hyperons, at variance with nucleons, are free from Pauli blocking and play a glue-like role if embodied in a nucleus. The measured binding energies and energy gap between the spin doublet give precise information on the YN in medium interaction. Study of YY interaction is expected by measuring the fine structure of double-hypernuclei. Furthermore, the first excited state of the lambda particle is 70 MeV, much less than the lowest excitation of the nucleons around 300 MeV, leading to stronger three-body force effects. In astrophysics, the presence of hyperons could either soften or stiffen the equation of state (EOS) of high-density nuclear matter depending on the detailed properties of the YY interaction and the YNN three-body interaction.

In the past decades, (K-, pi-), (pi+, K+) and (e, e’K+) reactions has been widely used to produce hypernuclei in the laboratories. In these reactions, stable targets are used, and therefore most of the studied hypernuclei are limited to on/near the beta-stability line. New techniques aiming to produce exotic hypernuclei (neutron-rich or proton-rich) are ongoing which is important to obtain knowledge of the YNN three-body interaction and the density dependence of the YN interaction. This project (ProHyPro) aims at investigating the production and decay of hypernuclei by using an intense heavy-ion beam interacting with a proton target. 
1. We performed theoretical calculations to study the production yields of hypernuclei in ion-ion collisions. This work is carried out in collaboration with Prof. Alexander Botvina from FIAS. The production cross sections were computed using a hybrid dynamical model: cascade coalescence followed by Fermi breakup. We compared the theoretical predictions in detail with available experimental data. The dependence of the production cross section on the beam energy, beam mass number, as well as different projectile-target combinations were investigated. Interestingly, we find that hydrogen target also leads to sizable hypernuclear yield even for exotic species. In some hypernuclear invariant mass studies, the hydrogen target gives better signal-over-background ratios than a more conventional carbon target. Dr. Sun has published the results in Physical Review C as first author [PRC 98, 024903 (2018)].

2. In the ion-beam induced reactions, the produced kaon mesons signal the strangeness production. However, coincidence measurement of hypernuclei with kaon mesons has never been successfully performed due to the difficulty in the kaon identification. During the project, a series of simulations have been performed for the kaon detection. A time projection chamber inside of the R3B-GLAD magnet was finally determined to be the best choice. The kaon meson can be identified using the Bρ-dEdx method. The development of the TPC is in line with the common interests among the R3B collaboration. The feasibility is still under investigation due to the highly non-uniform magnetic field of the GLAD magnet. Dr. Sun was involved in the R3B collaboration and the R3B-TPC working group. He is also performing the TPC simulation in collaboration with A. Corsi from CEA and A. P. Hector from USC.

3. Unfortunately, the HypHI experiment as well as the first R3B campaign both were not scheduled during the project due to the delay of the beam time at GSI. The experimental program at GSI can start only around February in 2019. Instead Dr. Sun grasped good opportunities to join two experiments performed at RIBF at RIKEN, Japan: SEASTAR campaign in 2017 (as a key member) and SAMURAI 18O campaign in 2018 (as the spokesperson), which were with similar setup as R3B. In particular, a TPC was used as a reaction vertex tracker in SEASTAR campaign in 2017, and Dr. Sun built valuable experience by taking care of the data analysis for one of the main reaction channels from this campaign (the data from the RIKEN experiments). The data analysis has been finalized and the manuscript is distributed among the collaborators.
In this project, Dr. Sun has built valuable experience on the hypernuclei physics. Collaborations with experts in hypernuclei physics from both theory and experiment greatly extended Dr. Sun’s knowledge and expertise. Dr. Sun and his supervisor (Alexandre Obertelli) have moved to TU Darmstadt recently and they would like to continue the hypernculei study. An application for the BMBF grant in Germany is under preparation. If it is approved, they plan to build the TPC for the kaon detection and scintillator fibers for the tracking of the decay particles in forward angles. In addition, the thick hydrogen target is key device to improve the luminosity. This technique is also being developed in the new group. There will be a PhD student available to work for this project with Dr. Sun.
Mesonic Decay of Hypernuclei