Dissertation: Electromagnetic and Spin Properties of Hyperons

  • Date:
  • Location: Ångströmlaboratoriet, Lägerhyddsvägen 1 Häggsalen
  • Doctoral student: Elisabetta Perotti
  • Contact person: Elisabetta Perotti
  • Disputation

Elisabetta Perotti defends her doctoral thesis. Opponent: Dr. Luis Alvarez Ruso, Facultad de Ciencias Físicas, Universidad de Valencia. Supervisor: Prof. Stefan Leupold, Nuclear Physics, Department of Physics and Astronomy. The defence is held in English.

The study of hyperons improves our understanding of nature in multiple ways. The intrinsic properties of hadrons are not fully understood at a fundamental level. Hence, the electromagnetic properties of hyperons provide complementary information to that of the nucleon and therefore allow for a broader investigation of the structure of matter. Electromagnetic form factors have been extensively studied for the nucleon, and are now addressed also in the hyperon sector. In this thesis, the electromagnetic Σ-Λ hyperon transition form factors have been determined at low energies, in a pioneering framework that makes use of dispersion relations to combine theoretical and experimental input. Subsequently this analysis has been extended to the decuplet spin-3/2 Σ∗-Λ transition. Assuming that these transition form factors are saturated by a two-pion inelasticity, their imaginary part can be written in terms of the pion vector form factor and a pion-hyperon scattering amplitude. Chiral perturbation theory at next-to-leading order has been used to calculate the latter, while the pion vector form factor and the related pion phase-shift are known from measurements.

The spin properties of hyperons can be resolved through their weak decays. The angular distribution of their decay products displays both polarization parameters and decay asymmetry parameters. These originate from processes characterized by an interplay of partial waves andare useful for different purposes. The polarization is sensitive to the production mechanism; in the p¯p case, it can be used to probe the strong interaction in the non-perturbative regime while in the e+e− case it contains the relative phase between complex electromagnetic form factors. Decay asymmetry parameters, on the other hand, are used to construct observables that test CP violation. Additional sources of CP violation must be found to solve the problem of the baryon asymmetry in our universe. This work provides a systematic method to retrieve both polarization and decay asymmetry parameters directly from the angular distribution of the hyperon decay products.

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