High-statistics measurements of thep¯p→Λ¯

Abstract

We report on precision measurements of the cross sections for the reactions p p→⌳ ⌳, p p→⌳ ⌺ 0 , and p p→⌺ 0 ⌳ in the region below 5 MeV excess kinetic energy above reaction threshold. For the p p→⌳ ⌳ measurement, the data represent a third and definitive scan through the threshold region by this experiment. Improvements in the detector system and analysis techniques, coupled with greatly increased statistics, have negated the question of the existence of a near-threshold resonance as had been indicated in our earlier, lower-statistics studies. The high statistics permit the first extraction of the polarization information at these low energies. We also report the first measurement of the cross section for the complementary reactions p p →⌳ ⌺ 0 ϩc.c. at excess energies up to 2.5 MeV. This allows a comparison of the onset of ⌳ ⌳ production to that of ⌳ ⌺ 0 and ⌺ 0 ⌳. The data show no evidence for near-threshold resonance structures in either reaction channel. Further, the p p→⌳ ⌺ 0 ϩc.c. reactions exhibit a larger P-wave fraction close to threshold compared to p p→⌳ ⌳.

Antinucleon–nucleon interaction at low energy: scattering and protonium

“…However, more recent studies, reported for instance at the LEAP98 Conference [339], indicated a smooth behaviour of the cross-section in this energy range. The nal analysis of the low-energy region [340] conrmed this result, as can be clearly seen in Fig. 4.…”

The electromagnetic form factors of the Λ in the timelike region

“…Interestingly, initial data forpp →ΛΛ suggested that there could be a near threshold resonance in the 3 S 1Λ Λ state [43]. However, a later high-statistics measurement by Barnes et al [27] ruled that out convincingly. This experiment scrutinized the first 5 MeV above theΛΛ threshold with an unprecedented resolution of 0.2 MeV.…”

“…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 $\bar{p}p\rightarrow\bar{\Lambda}\Lambda$

“…It has been studied theoretically with two types of models: meson exchange or quark-gluon interactions. The first two generations of experiments [1] by PS185 were carried out at LEAR, the Low Energy Antiproton Ring at CERN, using unpolarized targets. The resulting cross sections and spin observables were described equally well by either theory.…”