Dissertation at the Institute for Nuclear Physics on the topic of exotic particles

27 September 2022

We congratulate Ms. Bianca Savino on completing her dissertation titled

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

This thesis focuses on the search for a hypothetical particle composed of six quarks in the context of the particle physics experiment Belle II in Japan. The possible existence of such a state would help us to better understand the behaviour of matter under extreme circumstances.
The work can be divided into two main parts: the optimization of the Belle II software deployed in the reconstruction of tracks originating far from the main interaction point; and the subsequent sensitivity study for the search for the six-quark state, heavily relying on such tracks.

QCD-motivated models for hadrons predict a wide variety of exotic hadrons, with structures more complex than the quark-antiquark mesons and the three-quark baryons of the ordinary hadron classification. Among these, one of the most intriguing cases refers to the H dibaryon, an elusive six-quark state with the quark content of two Λ baryons first suggested by R. L. Jaffe in 1977. The search for a stable double strange hexaquark, which, in recent years, was put forward to also be a dark matter candidate, 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 running at the energy of the Y(3S) resonance. This resonance decays primarily via three gluons, in which ss quark pairs are produced with roughly the same probability as and pairs, making it particularly well suited for searches for multiquark states with nonzero strangeness.
The thesis describes a feasibility study for the search for a stable double strange six-quark state S produced in Y(3S) decays, Y(3S) → S Λ Λ nπ (with n = 0, 2, 4, 6, and 8). The predictions obtained for the channel with n=0 are compared with an existing result from the BaBar collaboration, while the other channels 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 Λ baryon reconstruction, a key element in the stable hexaquark analysis.
After the optimization, the algorithms and selections developed for Λ baryons were extensively tested using Belle II data collected at the Y(4S) resonance.
Finally, a look at the future is taken. Given that the beam background level measured at Belle II in the first data-taking periods turned out to be higher than initial estimates, the possible effect of increased background on performance deterioration, both on tracking in general and on the hexaquark analysis in particular, is assessed and discussed in detail.
The work presented in the 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 Y(3S) Belle II data to set a new limit on the possible observation of such an exotic state.