Eur. Phys. J. A (2009) 39: 315-321
https://doi.org/10.1140/epja/i2008-10716-1

Regular Article - Theoretical Physics

Unexpected features of e⁺e^- p and e⁺e^- cross-sections near threshold

¹ Museo Storico della Fisica e Centro Studi e Ricerche, E. Fermi, Rome, Italy
² Laboratori Nazionali di Frascati, INFN, Frascati, Italy
³ INFN and Department of Physics, University of Bologna, Bologna, Italy
⁴ CERN, Geneva, Switzerland

^* e-mail: simone.pacetti@lnf.infn.it

Received: 22 September 2008
Revised: 27 December 2008
Accepted: 28 December 2008
Published online: 16 February 2009

Abstract

Unexpected features of the BaBar data on e ⁺ e ^- 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 ^- 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.