Speaker
Description
Nuclear spectroscopy with heavy ion beams and fixed targets has emerged as a powerful tool for studying sub-atomic nuclei with strangeness, known as hypernuclei. Recent experiments using high-energy heavy ion beams have challenged the current understanding of light hypernuclei [1,2,3,4,5,6,7,8], particularly the hypertriton which is a subject of ongoing debate in the field, often referred to as the "hypertriton puzzle". This perplexing situation originated in part from our first HypHI (Hypernuclei with Heavy Ion) experiment at GSI that was performed in 2009 with a 6Li beam bombarding a carbon target at 2 GeV/u.
Our experimental approach focused on measuring and studying hypernuclei produced in the projectile rapidity region of nuclear collisions. Such Λ-hypernuclei are formed by the coalescence between a Λ hyperon produced in the mid-rapidity region of the hot compressed region of the reaction and a projectile spectator fragment. The HypHI collaboration demonstrated the feasibility of this approach through successful observations of the Λ hyperon and 3ΛH and 4ΛH hypernuclei via invariant mass spectroscopy [2]. Our results reported a shorter lifetime of 3ΛH and 4ΛH compared to their structure models [2,9]. Additional finding suggested a possible bound state of two neutrons and one Λ hyperon (nnΛ) [10], which challenges our current comprehension of the formation of light hypernuclei bound or resonance state. The implications of these results would have implications on the theoretical models of the hypernuclear structure, and therefore prompted other collaborations including STAR at RHIC [1,4,5,7], ALICE at CERN [3,5,6,8] and experiments using electron beams at the JLab [11] to investigate those two puzzles in depth.
To further investigate these puzzles, we conducted a new HypHI experiment, the HypHI WASA-FRS experiment, which utilized heavy ion beams at 2 GeV/u on a fixed carbon target at GSI-FAIR during the first quarter of 2022 [12]. This experiment employed the WASA central detector for pion measurement combined with the high-resolution fragment separator, FRS, for measuring decay residues. The primary goals of this experiment are to measure 3ΛH and 4ΛH lifetime and to confirm the observation of the nnΛ with improved precision. Achieving those objectives will demonstrate that combining WASA solenoid detector system and the high-resolution fragment separator FRS enhance the precision of hypernuclear spectroscopy [13].
In this presentation, we will discuss our recent progress in studying light hypernuclei using heavy ion beams, including details of the past and current experiments, preliminary results, and these future perspectives at GSI-FAIR. We will highlight the first signs of hypernuclei observed in the data analysis of WASA-FRS data.
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[12] T. R. Saito et al., Nature Reviews Physics volume 3, pages 803-813 (2021).
[13] H. Ekawa et al., Eur. Phys. J. A 59 103 (2023).
