Unexpected features of e+e- and e+e - → ∧ ∧̄ cross-sections near threshold

Abstract

Unexpected features of the BaBar data on e + e - → BB̄ 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 - → pp̄ it is found that Coulomb final-state interactions largely dominate the cross-section and the form factor is | G p(4M 2p)| ∼ 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. © 2009 SIF, Springer-Verlag Berlin Heidelberg.