Experimental determination of the complete spin structure forp¯p→Λ¯Λat

Abstract

The reactionpp →ΛΛ →pπ + pπ − has been measured with high statistics at a beam momentum of pp = 1.637 GeV/c. The use of a transversely-polarized frozen-spin target combined with the self-analyzing property of Λ/Λ decay allows access to unprecedented information on the spin structure of the interaction. The most general spin-scattering matrix can be written in terms of eleven real parameters for each bin of scattering angle, each of these parameters is determined with reasonable precision. From these results all conceivable spin-correlations are determined with inherent self-consistency. Good agreement is found with the few previously existing measurements of spin observables inpp →ΛΛ near this energy. Existing theoretical models do not give good predictions for those spin-observables that had not been previously measured.

The electromagnetic form factors of the Λ in the timelike region

“…Indeed, the only constraints we have for theΛΛ force come from the FSI in the reactionpp →ΛΛ. This reaction has been thoroughly studies in the PS185 experiment and data are available for total and differential cross-sections but also for polarization (P) and spin-correlation parameters (C i j ) [26,27,29], down to energies very close to the reaction threshold, see also the review in Ref. [28].…”

“…The only constraints one has for that interaction come from studies of FSI effects performed for another reaction, namelypp →ΛΛ. This particularpp annihilation channel has been thoroughly investigated in the PS185 experiment at LEAR and data are available (down to energies very close to the reaction threshold) for total and differential cross-sections but, thanks to the aforementioned self-analyzing weak Λ decay, also for spin-dependent observables [26,27,28,29]. In the present investigation we will resort toΛΛ models that have been developed for the analysis of those PS185 data by us in the past [30,31,32,33].…”

Spin structure of $\bar{p}p\rightarrow\bar{\Lambda}\Lambda$

“…PS185 used a 'frozen spin' target to make a complete determination of spin observables for a beam lab M. Alberg (B) Department of Physics, Seattle University, Seattle, WA, USA e-mail: alberg@seattleu.edu [3]. Depolarization, D nn ≡ Q[n p , n Λ ], is shown on the left, and spin transfer, K nn ≡ Q[n p , nΛ], is shown on the right momentum of 1.637 GeV/c [3]. The measured observables disagreed with both model predictions.…”

Polarization experiments with antiproton beam

“…In the simplest cases, such as Λ→pπ -and Λ→pπ + decays, the hyperon polarization is derived at one step from the measured anisotropy of decay proton or antiproton emission. Quite comprehensive studies of reaction pp →ΛΛ have been performed at LEAR [26]. At PANDA experiment, with its close to 4π-acceptance and excellent detection capabilities for charged and neutral particles with precise measurements of their characteristics, the more complex analysis of cascade decays would be feasible.…”

“…It will operate at the High Energy Storage Ring (HESR), utilizing the high-intensity and low emittance antiproton beam 5 of momenta from 1.5 to 15 GeV/c. Regarding specifically the pp-collisions, PANDA will cover center of mass energies of up to √s=5.5 GeV but overlap with the range explored by LEAR experiment [26] of CERN at the low energy end. The list of topics to be addressed by PANDA experiment includes a wide range of issues in hadron spectroscopy and hadron dynamics.…”