Half-time PhD seminar: Hyperons, spin and electromagnetism

Half-time PhD seminar in theoretical nuclear physics


Hyperons are baryons containing one or more strange quarks. Rich experimental programs at BESIII and PANDA are devoted to investigate their properties and structure.
For example, form factors quantify the deviation from a point-like particle and depend on the invariant mass of the virtual photon (q²) probing the hadron. Ideally one would like to have a complete picture of the hyperon form factors both in the space- and time-like regions. I will focus on the neutral Sigma-Lambda transition and present our predictions for the space-like electromagnetic transition form factors, which are not easily accessible experimentally as compared to the nucleon case. Our method combines chiral perturbation theory with dispersion relations.
Studying the decay chain: Sigma to Lambda photon followed by Lambda to proton pion and comparing with the corresponding antiparticle chain, one can constrain CP violation - implemented in our work by a QCD theta term. I will present upper limits for the angular asymmetry (alpha_Sigma) and for an observable that tests CP symmetry. These are obtained from the present upper limit on the neutron electric dipole moment, exploiting flavor SU(3) symmetry.
The hyperons have one more interesting feature due to their weak, parity violating decay: the angular distribution of the decay products gives direct access to the polarization, which is in turn related to the phase difference between the electric and magnetic form factors, which are complex in the time-like region. Polarisation observables are an excellent tool to discriminate between different theoretical models since they are more sensitive to the underlying physics than e.g. cross sections. I have contributed to the development of a formalism that allows their extraction in positron-electron annihilation to baryon-antibaryon pair.