Electromagnetic transition form factors and Dalitz decays of hyperons

Dalitz decays of a hyperon resonance to a ground-state hyperon and an electron-positron pair can give access to some information about the composite structure of hyperons. We present expressions for the multi-differential decay rates in terms of general transition form factors for spin-parity combinations $$J^P = \frac{1}{2}^\pm , \frac{3}{2}^\pm $$ J P = 1 2 ± , 3 2 ± of the hyperon resonance. Even if the spin of the initial hyperon resonance is not measured, the self-analyzing weak decay of the “final” ground-state hyperon contains information about the relative phase between combinations of transition form factors. This relative phase is non-vanishing because of the unstable nature of the hyperon resonance. If all form factor combinations in the differential decay formulae are replaced by their respective values at the photon point, one obtains a QED type approximation, which might be interpreted as characterizing hypothetical hyperons with point-like structure. We compare the QED type approximation to a more realistic form factor scenario for the lowest-lying singly-strange hyperon resonances. In this way we explore which accuracy in the measurements of the differential Dalitz decay rates is required in order to distinguish the composite-structure case from the pointlike case. Based on the QED type approximation we obtain as a by-product a rough prediction for the ratio between the Dalitz decay width and the corresponding photon decay width.

EXCITED HYPERONS PRODUCED IN PROTON-PROTON COLLISIONS WITH ANKE AT COSY

Excited neutral hyperons Y0* produced in the pp → pK+Y0* reaction with a COSY beam momentum of 3.65 MeV/c have masses below 1540 MeV/c2. The ANKE spectrometer allows the simultaneous observation of different decay modes: Y0* → π0Σ0, π∓Σ±, π0Λ, K-p by measuring kaons and pions of either charge in coincidence with protons. We have found indications for a neutral excited hyperon resonance Y0* with a mass of M(Y0*) = (1480 ± 15) MeV / c 2 and a width of Γ(Y0*) = (60±15) MeV / c 2. The cross section for Y0* is of the order of few hundred nanobarns. It can be either a Σ0 or a Λ hyperon and on the basis of existing data no conclusion could be made whether it is a three–quark baryon or an exotic state. Missing- and invariant–mass techniques have been used to identify the Λ(1405) resonance decaying via Σ0π0. The cross section for Λ(1405) production is equal to (4.5 ± 0.9 stat ± 1.8 syst ) µb. The shape and position of the Λ(1405) distribution are similar to those found from other decay modes, so no support is given to the two-pole model.