Thesis BELLE2-PTHESIS-2023-011

Development of Λ baryons reconstruction and its application to the search for a stable hexaquark at Belle II

Bianca Scavino ; Prof. Dr. Concettina Sfienti

Institute for Nuclear Physics Mainz

Abstract: QCD-motivated models for hadrons predict an assortment of exotic hadrons, with structures more complex than the quark-antiquark mesons and the three­quark baryons of the ordinary hadron classification. Among these, one of the most advertised cases refers to the H dibaryon, a six-quark state with the quark content of two A baryons first suggested by R. L. Jaffe in 1977. Jaffe's initial proposal was followed by several theoretical models about its nature, mass, and stability, and in recent years a suggestive hypothesis has been postulated about the possibility that a stable double strange hexaquark, if it exists, might represent a dark matter candidate. The search for a stable double strange hexaquark is part of the Belle II physics program. In the coming years, a fraction of the Belle II data taking period will be dedicated to run at energies different from the Υ(4S), and in particular an unprecedented statistics will be collected at the Υ(3S) resonance. Being located below the BB threshold, the decay of such a resonance in open flavor is forbidden. It decays primarily via three gluons, in which ss quark pairs are produced with roughly the same probability as uu and dd pairs, making it particularly well suited for searches for multiquark states with nonzero strangeness. This thesis describes a sensitivity study for the search for a stable double strange six-quark state S produced in Υ(3S) decays, Υ(3S) -> SΛΛnπ (with n = 0,2, 4, 6, and 8). The predictions obtained for the channel with n=O are compared with an existing result from the BaBar collaboration, while the other channels will represent a novel measurement. In order to obtain optimal results, an integral part of the work is the optimization of the Belle II tracking package for displaced vertices with a focus on A baryon reconstruction, key players in the stable hexaquark analysis. Furthermore, ample space is given to the discussion of beam background and its possible effect on performance deterioration in general and on the hexaquark analysis in particular. The work presented in this thesis is intended to prepare a measurement where the tools and the analysis procedures developed, as well as the critical points observed, will be used on the Belle II data to set a new limit on the possible observation of such an exotic state.

Note: Presented on 27 07 2022
Note: PhD

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 Record created 2023-05-23, last modified 2023-05-23

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