Unexpected features of e~+e~-( )pp and e~+e~-( )ΛΛ cross-sections near threshold

摘要

Unexpected features of the BaBar data on e~+e~-( )BB 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~- ( ) pp it is found that Coulomb final-state interactions largely dominate the cross-section and the form factor is|G~p(4M_p~2)|( )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.