Unexpected features of e⁺e^- $\rightarrow$ p$\overline{{p}}$ and e⁺e^- $\rightarrow$ $\Lambda$ $\overline{{\Lambda}}$ cross-sections near threshold

Abstract

Unexpected features of the BaBar data on e ⁺ e ^- $\rightarrow$ B $\overline{{B}}$ cross-sections (B stands for baryon) are discussed. These data have been collected, with unprecedented accuracy, by means of the initial-state radiation technique, which is particularly suitable in giving good acceptance and energy resolution at threshold. A striking feature observed in the BaBar data is the non-vanishing cross-section at threshold for all these processes. This is the expectation due to the Coulomb enhancement factor acting on a charged fermion pair. In the case of e ⁺ e ^- $\rightarrow$ p $\overline{{p}}$ it is found that Coulomb final-state interactions largely dominate the cross-section and the form factor is | G ^p(4M ² _p)| ∼ 1 , which could be a general feature for baryons. In the case of neutral baryons an interpretation of the non-vanishing cross-section at threshold is suggested, based on quark electromagnetic interaction and taking into account the asymmetry between attractive and repulsive Coulomb factors. Besides strange baryon cross-sections are compared to U -spin invariance predictions.